2024/02/02 更新

写真a

コウガ ケンイチロウ
甲賀 研一郎
KOGA Kenichiro
所属
異分野基礎科学研究所 教授
職名
教授
外部リンク

学位

  • 博士 ( 京都大学 )

研究キーワード

  • 熱力学

  • 計算機シミュレーション

  • liquids

  • interfaces

  • phase transitions

  • 統計力学

  • Statistical mechanics

  • thermodynamics

  • computer simulation

  • 液体

  • 相転移

  • 界面

  • 疎水効果

研究分野

  • ナノテク・材料 / 基礎物理化学

  • 自然科学一般 / 生物物理、化学物理、ソフトマターの物理

学歴

  • 京都大学    

    - 1996年

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    国名: 日本国

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  • 京都大学   Graduate School, Division of Engineering  

    - 1996年

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  • 京都大学    

    - 1993年

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    国名: 日本国

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  • 京都大学   Graduate School, Division of Engineering  

    - 1993年

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  • 大阪大学   School of Engineering Science Direct Affiliates  

    - 1991年

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    国名: 日本国

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  • 大阪大学   Faculty of Engineering Science  

    - 1991年

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▼全件表示

経歴

  • ルーヴェン・カトリック大学 客員教授

    2010年

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  • Visiting Professor,KU Leuven

    2010年

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  • 岡山大学   大学院自然科学研究科   教授

    2009年6月 - 2016年3月

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  • - 岡山大学自然科学研究科機能分子化学専攻 教授

    2009年

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  • コーネル大学   化学部   客員研究員

    2001年10月 - 2003年6月

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    国名:アメリカ合衆国

    備考:日本学術振興会海外特別研究員

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所属学協会

委員歴

  • 分子シミュレーション学会   幹事  

    2020年1月 - 現在   

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    団体区分:学協会

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  • 分子シミュレーション研究会   幹事  

    2006年 - 2008年   

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    団体区分:学協会

    分子シミュレーション研究会

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論文

  • Hydrophobicity Varying with Temperature, Pressure, and Salt Concentration 招待 査読

    K. Koga, N. Yamamoto

    Journal of Physical Chemistry B   122 ( 13 )   3655 - 3665   2018年4月

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    担当区分:筆頭著者, 責任著者   記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Chemical Society  

    Temperature-, pressure-, and salt-concentration-induced variations in the solubility of small nonpolar solutes in aqueous solution and the corresponding variations in the solvent-induced pair attraction between such solute molecules are investigated. The variations in the solvation free energy of a solute and those in the solvent-induced pair attraction are well reproduced by a mean-field approximation in which the repulsive cores of solute molecules are treated as hard spheres and the mean-field energy of a solute molecule is taken to be the average potential energy that the solute molecule feels in solution. The mechanisms of variation in the solvation free energy and those of variation in the solvent-induced pair potential, with increasing temperature, pressure, and salt concentration, are clarified. Correlations between the solvation free energy and the solvent-induced pair potential at a contact distance in temperature, pressure, and salt concentration variations are near linear in any mode of variation, but the slope of the linear relation is dependent on the mode of variation and is determined by a ratio of the solvation thermodynamic quantities characteristic of each mode of variation.

    DOI: 10.1021/acs.jpcb.7b12193

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  • Hydrophobic Polymer Chain in Water That Undergoes a Coil-to-Globule Transition Near Room Temperature 査読

    I. Hatano, K. Mochizuki, T. Sumi, K. Koga

    The Journal of Physical Chemistry B   120 ( 47 )   12127 - 12134   2016年12月

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    担当区分:最終著者, 責任著者   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Chemical Society (ACS)  

    DOI: 10.1021/acs.jpcb.6b08347

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  • Density functional models of the interfacial tensions near the critical endpoints and tricritical point of three-phase equilibria 招待 査読

    K Koga, B Widom

    Journal of Physics: Condensed Matter   28 ( 24 )   244016 - 244016   2016年6月

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    担当区分:筆頭著者, 責任著者   掲載種別:研究論文(学術雑誌)   出版者・発行元:IOP Publishing  

    DOI: 10.1088/0953-8984/28/24/244016

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    その他リンク: http://stacks.iop.org/0953-8984/28/i=24/a=244016?key=crossref.f153fd1716b384446b7fa41b5de9dca9

  • Mean-Field Approximation to the Hydrophobic Hydration in the Liquid–Vapor Interface of Water 招待 査読

    Kiharu Abe, Tomonari Sumi, Kenichiro Koga

    The Journal of Physical Chemistry B   120 ( 8 )   2012 - 2019   2016年3月

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    担当区分:最終著者, 責任著者   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Chemical Society (ACS)  

    DOI: 10.1021/acs.jpcb.5b10169

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  • Deriving Second Osmotic Virial Coefficients from Equations of State and from Experiment 査読

    K. Koga, Vincent Holten, B. Widom

    The Journal of Physical Chemistry B   119 ( 42 )   13391 - 13397   2015年10月

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    担当区分:筆頭著者, 責任著者   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Chemical Society (ACS)  

    DOI: 10.1021/acs.jpcb.5b07685

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  • Solid-liquid critical behavior of water in nanopores 査読

    Mochizuki Kenji, Koga Kenichiro

    Proceedings of the National Academy of Sciences of the United States of America   112 ( 27 )   8221 - 8226   2015年7月

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    担当区分:最終著者, 責任著者   記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:National Academy of Sciences.  

    Nanoconfined liquid water can transform into low-dimensional ices whose crystalline structures are dissimilar to any bulk ices and whose melting point may significantly rise with reducing the pore size, as revealed by computer simulation and confirmed by experiment. One of the intriguing, and as yet unresolved, questions concerns the observation that the liquid water may transform into a low-dimensional ice either via a first-order phase change or without any discontinuity in thermodynamic and dynamic properties, which suggests the existence of solid−liquid critical points in this class ofnanoconfined systems. Here we explore the phase behavior of a model of water in carbon nanotubes in the temperature−pressure−diameter space by molecular dynamics simulation and provide unambiguous evidence to support solid−liquid critical phenomena of nanoconfined water. Solid−liquid first-order phase boundaries aredetermined by tracing spontaneous phase separation at various temperatures. All of the boundaries eventually cease to exist at the critical points and there appear loci of response function maxima, or the Widom lines, extending to the supercritical region. The finite-size scaling analysis of the density distribution supports thepresence of both first-order and continuous phase changes between solid and liquid. At around the Widom line, there are microscopic domains of two phases, and continuous solid−liquid phase changesoccur in such a way that the domains of one phase grow and those of the other evanesce as the thermodynamic state departs from the Widom line.

    DOI: 10.1073/pnas.1422829112

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  • Osmotic Second Virial Coefficient of Methane in Water 査読

    K. Koga

    The Journal of Physical Chemistry B   117 ( 41 )   12619 - 12624   2013年10月

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    担当区分:筆頭著者, 責任著者   記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Chemical Society (ACS)  

    DOI: 10.1021/jp4085298

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  • Solvation of hydrophobes in water and simple liquids 査読

    Kenichiro Koga

    PHYSICAL CHEMISTRY CHEMICAL PHYSICS   13 ( 44 )   19749 - 19758   2011年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ROYAL SOC CHEMISTRY  

    The solvation of nonpolar molecules in water and that in simple liquids are compared and contrasted. First, solvation thermodynamics is reviewed in a way that focuses on how the enthalpy and entropy of solvation depend on the choice of microscopic volume change v in the solvation process-including special choices v being zero (fixed-volume condition) and v being the partial molecular volume of a solute molecule (fixed-pressure condition)-and how the solvation quantities are related with temperature derivatives of the solvation free energy. Second, the solvation free energy and the solvation enthalpy of a Lennard-Jones (LJ) atom in model water are calculated in the parameter space representing the solute size and the strength of the solute-solvent interaction, and the results are compared with those for an LJ atom in the LJ solvent. The solvation diagrams showing domains of different types of solvation in the parameter space are obtained both for the constant-volume condition and for the constant-pressure condition. Similarities between water and the simple liquid are found when the constant-volume solvation is considered while a significant difference manifests itself in the fixed-pressure solvation. The domain of solvation of hydrophobic character in the parameter space is large in the constant-volume solvation both for water and for the simple liquid. When switched to the constant-pressure condition accompanying a microscopic volume change, the hydrophobic domain remains large in water but it becomes significantly small in the simple liquid. The contrasting results are due to the smallness of the thermal pressure coefficient of water at low temperatures.

    DOI: 10.1039/c1cp22344e

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  • Infinite-Order Transitions in Density-Functional Models of Wetting

    K. Koga, J. O. Indekeu, B. Widom

    PHYSICAL REVIEW LETTERS   104 ( 3 )   036101   2010年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    A class of density-functional models for wetting transitions is defined. A necessary condition for the transitions to be of higher than first order is derived. A locus of wetting transitions in the plane of two model field variables is determined on which there are states of first-order and of higher-order, including infinite-order, transitions. The observed behavior is rationalized via a different but related, analytically soluble model.

    DOI: 10.1103/PhysRevLett.104.036101

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  • A plastic phase of water from computer simulation

    Yoshio Takii, Kenichiro Koga, Hideki Tanaka

    JOURNAL OF CHEMICAL PHYSICS   128 ( 20 )   204501   2008年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER INST PHYSICS  

    We report a member of ices called plastic or rotator phase, in which individual water molecules make facile rotations as in liquid state but are held tightly in an ordered structure. Molecular dynamics simulations of three classical models of water show that a plastic ice phase appears at a temperature when ice VII is heated or liquid water is cooled at high pressures above several gigapascals. A large amount of latent heat is absorbed when ice VII is transformed to the rotator phase at 590 K and 10 GPa, which is a typical characteristic of the plastic transitions for nearly spherical molecules. In addition to the spontaneous formation of plastic phase in the simulations, its existence is supported by robustness of plastic phase for hypothetical water with varying degrees of Coulombic interactions. (C) 2008 American Institute of Physics.

    DOI: 10.1063/1.2927255

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  • Mean-field density-functional model of a second-order wetting transition

    K. Koga, B. Widom

    JOURNAL OF CHEMICAL PHYSICS   128 ( 11 )   114716   2008年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER INST PHYSICS  

    First- and second-order wetting transitions are contrasted. A mean-field density-functional model that leads to a second-order transition is introduced. The way in which it differs from an earlier, otherwise similar model in which the transition is first order is noted. The interfacial and line tensions in the model are obtained numerically and their behavior on approach to the transition is determined. The spatial variation of the model's densities in the neighborhood of the contact line near the wetting transition is also found and seen to be characteristically different at a second-order transition from what it is at a first-order transition. The results for the line tension and for the spatial variation of the densities are in accord with those from an earlier interface-displacement model of the same phenomena.

    DOI: 10.1063/1.2895748

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  • Phase diagram of water in carbon nanotubes

    Daisuke Takaiwa, Itaru Hatano, Kenichiro Koga, Hideki Tanaka

    PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA   105 ( 1 )   39 - 43   2008年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:NATL ACAD SCIENCES  

    A phase diagram of water in single-walled carbon nanotubes at atmospheric pressure is proposed, which summarizes ice structures and their melting points as a function of the tube diameter up to 1.7 nm. The investigation is based on extensive molecular dynamics simulations over numerous thermodynamic states on the temperature-diameter plane. Spontaneous freezing of water in the simulations and the analysis of ice structures at 0 K suggest that there exist at least nine ice phases in the cylindrical space, including those reported by x-ray diffraction studies and those unreported by simulation or experiment. Each ice has a structure that maximizes the number of hydrogen bonds under the cylindrical confinement. The results show that the melting curve has many local maxima, each corresponding to the highest melting point for each ice form. The global maximum in the melting curve is located at approximate to 11 angstrom, where water freezes in a square ice nanotube.

    DOI: 10.1073/pnas.0707917105

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  • Close-packed structures and phase diagram of soft spheres in cylindrical pores

    K Koga, H Tanaka

    JOURNAL OF CHEMICAL PHYSICS   124 ( 13 )   2006年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER INST PHYSICS  

    It is shown for a model system consisting of spherical particles confined in cylindrical pores that the first ten close-packed phases are in one-to-one correspondence with the first ten ways of folding a triangular lattice, each being characterized by a roll-up vector like the single-walled carbon nanotube. Phase diagrams in pressure-diameter and temperature-diameter planes are obtained by inherent-structure calculation and molecular dynamics simulation. The phase boundaries dividing two adjacent phases are infinitely sharp in the low-temperature limit but are blurred as temperature is increased. Existence of such phase boundaries explains rich, diameter-sensitive phase behavior unique for cylindrically confined systems. (c) 2006 American Institute of Physics.

    DOI: 10.1063/1.2172592

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  • Phase diagram of water between hydrophobic surfaces

    Kenichiro Koga, Hideki Tanaka

    Journal of Chemical Physics   122 ( 10 )   2005年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    Molecular dynamics simulations demonstrate that there are at least two classes of quasi-two-dimensional solid water into which liquid water confined between hydrophobic surfaces freezes spontaneously and whose hydrogen-bond networks are as fully connected as those of bulk ice. One of them is the monolayer ice and the other is the bilayer solid which takes either a crystalline or an amorphous form. Here we present the phase transformations among liquid, bilayer amorphous (or crystalline) ice, and monolayer ice phases at various thermodynamic conditions, then determine curves of melting, freezing, and solid-solid structural change on the isostress planes where temperature and intersurface distance are variable, and finally we propose a phase diagram of the confined water in the temperature-pressure-distance space. © 2005 American Institute of Physics.

    DOI: 10.1063/1.1861879

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  • Freezing in one-dimensional liquids

    K Koga

    JOURNAL OF CHEMICAL PHYSICS   118 ( 17 )   7973 - 7980   2003年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER INST PHYSICS  

    Freezing of liquids in one dimension is studied by a lattice model that is an extension of the model solvent of the hydrophobic attraction. The model in one dimension, which is exactly solvable, exhibits a continuous phase change between a high-temperature disordered "liquid" state and a low-temperature ordered "solid" state but also does exhibit a first-order freezing transition at some finite temperature with either one of the two model parameters taken to be infinite. In this theoretical framework the sharpness of the freezing in one dimension is expressed by a simple function of the microscopic model parameters and thus is related with other macroscopic properties of the substance. These results may account for continuity and discontinuity of the liquid and solid reported for different one-dimensional substances. (C) 2003 American Institute of Physics.

    DOI: 10.1063/1.1564049

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  • The hydrophobic effect

    B Widom, P Bhimalapuram, K Koga

    PHYSICAL CHEMISTRY CHEMICAL PHYSICS   5 ( 15 )   3085 - 3093   2003年

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    記述言語:英語   出版者・発行元:ROYAL SOC CHEMISTRY  

    The thermodynamics of the hydrophobic effect, as measured primarily through the temperature dependence of solubility, is reviewed, and then a class of models that incorporate the basic mechanism of hydrophobicity is described. These models predict a quantitative relation between the free energy of hydrophobic hydration and the strength of the solvent-mediated attraction between pairs of solute molecules. It is remarked that the free energy of attraction being just of the order of the thermal energy kT may be important for the effective operation of the hydrophobic effect in proteins. Deviations from pairwise additivity of hydrophobic forces are also briefly discussed.

    DOI: 10.1039/b304038k

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  • Solvation forces and liquid-solid phase equilibria for water confined between hydrophobic surfaces

    K Koga

    JOURNAL OF CHEMICAL PHYSICS   116 ( 24 )   10882 - 10889   2002年6月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER INST PHYSICS  

    Solvation force and phase behavior of water confined between hydrophobic surfaces at nanoscale distances have been studied by molecular dynamics simulation of the TIP4P model water. Freezing and melting of confined water are observed at certain intersurface separations in bringing one surface to the other at a fixed temperature and a fixed lateral or bulk pressure. Solvation force curves are found to be discontinuous upon freezing and melting of confined water and exhibit strong hystereses, implying a peculiar manifestation of the hydrophobic effect. The thermodynamics for a confined system at fixed surface separation, temperature, and lateral or bulk pressure is applied for examining the liquid-solid equilibria of confined water. (C) 2002 American Institute of Physics.

    DOI: 10.1063/1.1480855

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  • Formation of ordered ice nanotubes inside carbon nanotubes

    K Koga, GT Gao, H Tanaka, XC Zeng

    NATURE   412 ( 6849 )   802 - 805   2001年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:MACMILLAN PUBLISHERS LTD  

    Following their discovery(1), carbon nanotubes have attracted interest not only for their unusual electrical and mechanical properties, but also because their hollow interior can serve as a nanometre-sized capillary(2-7), mould(8-11) or template(12-14) in material fabrication. The ability to encapsulate a material in a nanotube also offers new possibilities for investigating dimensionally confined phase transitions(15). Particularly intriguing is the conjecture(16) that matter within the narrow confines of a carbon nanotube might exhibit a solid-liquid critical point(17) beyond which the distinction between solid and liquid phases disappears. This unusual feature, which cannot occur in bulk material, would allow for the direct and continuous transformation of liquid matter into a solid. Here we report simulations of the behaviour of water encapsulated in carbon nanotubes that suggest the existence of a variety of new ice phases not seen in bulk ice, and of a solid-liquid critical point. Using carbon nanotubes with diameters ranging from 1.1 nm to 1.4 nm and applied axial pressures of 50 MPa to 500 MPa, we rnd that water can exhibit a first-order freezing transition to hexagonal and heptagonal ice nanotubes, and a continuous phase transformation into solid-like square or pentagonal ice nanotubes.

    DOI: 10.1038/35090532

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  • First-order transition in confined water between high-density liquid and low-density amorphous phases

    K Koga, H Tanaka, XC Zeng

    NATURE   408 ( 6812 )   564 - 567   2000年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:NATURE PUBLISHING GROUP  

    Supercooled water and amorphous ice have a rich metastable phase behaviour. In addition to transitions between high- and low-density amorphous solids(1,2), and between high- and low-density liquids(3-8), a fragile-to-strong liquid transition has recently been proposed(9,10), and supported by evidence from the behaviour of deeply supercooled bilayer water confined in hydrophilic slit pores(11). Here we report evidence from molecular dynamics simulations for another type of first-order phase transition-a liquid-to-bilayer amorphous transition-above the freezing temperature of bulk water at atmospheric pressure. This transition occurs only when water is confined in a hydrophobic slit pore(12-14) with a width of less than one nanometre. On cooling, the confined water, which has an imperfect random hydrogen-bonded network, transforms into a bilayer amorphous phase with a perfect network (owing to the formation of various hydrogen-bonded polygons) but no long-range order. The transition shares some characteristics with those observed in tetrahedrally coordinated substances such as liquid silicon(15,16), liquid carbon(17) and liquid phosphorus(18).

    DOI: 10.1038/35046035

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  • Thermodynamic expansion of nucleation free-energy barrier and size of critical nucleus near the vapor-liquid coexistence

    K Koga, XC Zeng

    JOURNAL OF CHEMICAL PHYSICS   110 ( 7 )   3466 - 3471   1999年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER INST PHYSICS  

    Nucleation free-energy barrier height and size of the critical nucleus are expanded in powers of the chemical potential difference between the supersaturated vapor (or expanded liquid) in the metastable state and the saturated vapor-liquid system in the stable equilibrium state at the same temperature. The coefficients in the expansions are expressed in terms of the thermodynamic properties at the stable equilibrium state. Comparisons with the results obtained from the density-functional calculation for nucleation of the Lennard-Jones fluid show that systematic improvement in predicting properties of the critical nucleus, either liquid droplet or vapor cavity, is achieved by adding the higher order terms in the expansions. The scaling relations proposed by McGraw and Laaksonen are found to be good approximations to the general expansion; in particular, the barrier height displacement appearing in these scaling relations is naturally given as the second order coefficient in the expansion of the barrier height. (C) 1999 American Institute of Physics. [0021-9606(99)50707-1].

    DOI: 10.1063/1.478214

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  • Freezing of confined water: A bilayer ice phase in hydrophobic nanopores 査読

    Koga Kenichiro, X. C. Zeng, Tanaka Hideki

    Physical Review Letters   79 ( 26 )   5262 - 5265   1997年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    Molecular dynamics simulations were performed to study the phase behavior of a thin film of water confined to a slit nanopore with smooth walls. A first-order water-to-ice freezing transition has been observed. The resulting ice, which is a crystal of bilayer consisting of rows of distorted hexagons, does not resemble any ice crystals found so far. The confined water contracts upon freezing when the confinement load is low (0.5 kbar) and expands when the load is high (10 kbar). The residual entropy of the bilayer ice can be calculated exactly, which is about half of the entropy of the bulk ice. © 1997 The American Physical Society.

    DOI: 10.1103/PhysRevLett.79.5262

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  • Close-Packed Ices in Nanopores. 国際誌

    Kenji Mochizuki, Yuji Adachi, Kenichiro Koga

    ACS nano   2023年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    Water molecules in any of the ice polymorphs organize themselves into a perfect four-coordinated hydrogen-bond network at the expense of dense packing. Even at high pressures, there seems to be no way to reconcile the ice rules with the close packing. Here, we report several close-packed ice phases in carbon nanotubes obtained from molecular dynamics simulations of two different water models. Typically they are in plastic states at high temperatures and are transformed into the hydrogen-ordered ice, keeping their close-packed structures at lower temperatures. The close-packed structures of water molecules in carbon nanotubes are identified with those of spheres in a cylinder. We present design principles of hydrogen-ordered, close-packed structures of ice in nanotubes, which suggest many possible dense ice forms with or without nonzero polarization. In fact, some of the simulated ices are found to exhibit ferroelectric ordering upon cooling.

    DOI: 10.1021/acsnano.3c07084

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  • Molecular mechanism of the common and opposing co-solvent effects of fluorinated alcohol and urea on a coiled coil protein 査読

    Noa Nakata, Ryuichi Okamoto, Tomonari Sumi, Kenichiro Koga, Takeshi Morita, Hiroshi Imamura

    Protein Science   32 ( 10 )   2023年9月

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    掲載種別:研究論文(学術雑誌)  

    DOI: 10.1002/pro.4763

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  • How do water-mediated interactions and osmotic second virial coefficients vary with particle size? 招待 査読

    Hidefumi Naito, Tomonari Sumi, Kenichiro Koga

    Faraday Discussions   2023年

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    担当区分:最終著者, 責任著者   掲載種別:研究論文(学術雑誌)   出版者・発行元:Royal Society of Chemistry (RSC)  

    We examine quantitatively the solute-size dependences of the effective interactions between nonpolar solutes in water and in a simple liquid. The potential w(r) of mean force and the osmotic second...

    DOI: 10.1039/d3fd00104k

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  • Solvation free energies of alcohols in water: temperature and pressure dependences

    Aoi Taira, Ryuichi Okamoto, Tomonari Sumi, Kenichiro Koga

    Physical Chemistry Chemical Physics   2023年

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    掲載種別:研究論文(学術雑誌)   出版者・発行元:Royal Society of Chemistry (RSC)  

    The three important contributions to the solvation free energy of alcohols in water are quantified as functions of temperature and pressure based the perturbation combining method and other step-wise methods.

    DOI: 10.1039/d3cp03799a

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  • Wetting and Nonwetting near a Tricritical Point 査読

    Joseph O. Indekeu, Kenichiro Koga

    PHYSICAL REVIEW LETTERS   129 ( 22 )   2022年11月

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    担当区分:責任著者   記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    The dihedral contact angles between interfaces in three-fluid-phase equilibria must be continuous functions of the bulk thermodynamic fields. This general argument, which we propose, predicts a nonwetting gap in the phase diagram, challenging the common belief in "critical-point wetting," even for short-range forces. A demonstration is provided by exact solution of a mean-field two-density functional theory for three-phase equilibria near a tricritical point (TCP). Complete wetting is found in a tiny vicinity of the TCP. Away from it, nonwetting prevails and no wetting transition takes place, not even when a critical endpoint is approached. Far from the TCP, reentrant wetting may occur, with a different wetting phase. These findings shed light on hitherto unexplained experiments on ternary H2O-oil-nonionic amphiphile mixtures in which nonwetting continues to exist as one approaches either one of the two critical endpoints.

    DOI: 10.1103/PhysRevLett.129.224501

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  • Osmotic second virial coefficients for hydrophobic interactions as a function of solute size

    Hidefumi Naito, Ryuichi Okamoto, Tomonari Sumi, Kenichiro Koga

    The Journal of Chemical Physics   156 ( 22 )   221104 - 221104   2022年6月

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    掲載種別:研究論文(学術雑誌)   出版者・発行元:AIP Publishing  

    To gain quantitative insight into how the overall strength of the hydrophobic interaction varies with the molecular size, we calculate osmotic second virial coefficients B for hydrophobic spherical molecules of different diameters σ in water based on molecular simulation with corrections to the finite-size and finite-concentration effects. It is shown that B (<0) changes by two orders of magnitude greater as σ increases twofold and its solute-size dependence is best fit by a power law B ∝ σ α with the exponent α ≃ 6, which contrasts with the cubic power law that the second virial coefficients of gases obey. It is also found that values of B for the solutes in a nonpolar solvent are positive but they obey the same power law as in water. A thermodynamic identity for B derived earlier [K. Koga, V. Holten, and B. Widom, J. Phys. Chem. B 119, 13391 (2015)] indicates that if B is asymptotically proportional to a power of σ, the exponent α must be equal to or greater than 6.

    DOI: 10.1063/5.0097547

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  • Theory of Gas Solubility and Hydrophobic Interaction in Aqueous Electrolyte Solutions 招待 査読

    Ryuichi Okamoto, Kenichiro Koga

    The Journal of Physical Chemistry B   125 ( 46 )   12820 - 12831   2021年11月

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    担当区分:最終著者   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Chemical Society (ACS)  

    DOI: 10.1021/acs.jpcb.1c08050

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  • Ion Size Dependences of the Salting-Out Effect: Reversed Order of Sodium and Lithium Ions 査読

    Hiroyuki Katsuto, Ryuichi Okamoto, Tomonari Sumi, Kenichiro Koga

    The Journal of Physical Chemistry B   125 ( 23 )   6296 - 6305   2021年6月

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    担当区分:最終著者, 責任著者   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Chemical Society (ACS)  

    DOI: 10.1021/acs.jpcb.1c03388

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  • Structure and phase behavior of high-density ice from molecular-dynamics simulations with the ReaxFF potential 査読

    Yuji Adachi, Kenichiro Koga

    The Journal of Chemical Physics   153 ( 11 )   114501 - 114501   2020年9月

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    担当区分:最終著者, 責任著者   掲載種別:研究論文(学術雑誌)   出版者・発行元:AIP Publishing  

    DOI: 10.1063/5.0016565

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  • Theory of electrolytes including steric, attractive, and hydration interactions 査読

    Ryuichi Okamoto, Kenichiro Koga, Akira Onuki

    The Journal of Chemical Physics   153 ( 7 )   074503 - 074503   2020年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AIP Publishing  

    DOI: 10.1063/5.0015446

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  • The effect of a finite number of monomers available for aggregation at nucleation and micellization in a fixed volume 査読

    A. K. Shchekin, K. Koga, N. A. Volkov

    The Journal of Chemical Physics   151 ( 24 )   244903 - 244903   2019年12月

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    掲載種別:研究論文(学術雑誌)   出版者・発行元:AIP Publishing  

    DOI: 10.1063/1.5129160

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  • Three-phase equilibria in density-functional theory: Interfacial tensions 査読

    Kenichiro Koga, Joseph O. Indekeu

    JOURNAL OF CHEMICAL PHYSICS   150 ( 16 )   2019年4月

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    担当区分:筆頭著者, 責任著者   記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER INST PHYSICS  

    A mean field density functional model for three-phase equilibria in fluids (or other soft condensed matter) with two spatially varying densities is analyzed analytically and numerically. The interfacial tension between any two out of three thermodynamically coexisting phases is found to be captured by a surprisingly simple analytic expression that has a geometric interpretation in the space of the two densities. The analytic expression is based on arguments involving symmetries and invariances. It is supported by numerical computations of high precision, and it agrees with earlier conjectures obtained for special cases in the same model. An application is presented to three-phase equilibria in the vicinity of a tricritical point. Using the interfacial tension expression and employing the field variables compatible with tricritical point scaling, the expected mean field critical exponent is derived for the vanishing of the critical interfacial tension as a function of the deviation of the noncritical interfacial tension from its limiting value, upon approach to a critical endpoint in the phase diagram. The analytic results are again confirmed by numerical computations of high precision. Published under license by AIP Publishing.

    DOI: 10.1063/1.5091599

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  • Theoretical analysis on thermodynamic stability of chignolin 査読

    Tomonari Sumi, Kenichiro Koga

    SCIENTIFIC REPORTS   9   2019年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:NATURE PUBLISHING GROUP  

    Understanding the dominant factor in thermodynamic stability of proteins remains an open challenge. Kauzmann's hydrophobic interaction hypothesis, which considers hydrophobic interactions between nonpolar groups as the dominant factor, has been widely accepted for about sixty years and attracted many scientists. The hypothesis, however, has not been verified or disproved because it is difficult, both theoretically and experimentally, to quantify the solvent effects on the free energy change in protein folding. Here, we developed a computational method for extracting the dominant factor behind thermodynamic stability of proteins and applied it to a small, designed protein, chignolin. The resulting free energy profile quantitatively agreed with the molecular dynamics simulations. Decomposition of the free energy profile indicated that intramolecular interactions predominantly stabilized collapsed conformations, whereas solvent-induced interactions, including hydrophobic ones, destabilized them. These results obtained for chignolin were consistent with the site-directed mutagenesis and calorimetry experiments for globular proteins with hydrophobic interior cores.

    DOI: 10.1038/s41598-019-41518-1

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  • Application of reference‐modified density functional theory: Temperature and pressure dependences of solvation free energy 査読

    Tomonari Sumi, Yutaka Maruyama, Ayori Mitsutake, Kenji Mochizuki, Kenichiro Koga

    Journal of Computational Chemistry   39 ( 4 )   202 - 217   2018年2月

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    担当区分:最終著者   記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Wiley  

    Recently, we proposed a reference-modified density functional theory (RMDFT) to calculate solvation free energy (SFE), in which a hard-sphere fluid was introduced as the reference system instead of an ideal molecular gas. Through the RMDFT, using an optimal diameter for the hard-sphere reference system, the values of the SFE calculated at room temperature and normal pressure were in good agreement with those for more than 500 small organic molecules in water as determined by experiments. In this study, we present an application of the RMDFT for calculating the temperature and pressure dependences of the SFE for solute molecules in water. We demonstrate that the RMDFT has high predictive ability for the temperature and pressure dependences of the SFE for small solute molecules in water when the optimal reference hard-sphere diameter determined for each thermodynamic condition is used. We also apply the RMDFT to investigate the temperature and pressure dependences of the thermodynamic stability of an artificial small protein, chignolin, and discuss the mechanism of high-temperature and high-pressure unfolding of the protein. (c) 2017 Wiley Periodicals, Inc.

    DOI: 10.1002/jcc.25101

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    その他リンク: https://onlinelibrary.wiley.com/doi/full-xml/10.1002/jcc.25101

  • Influence of co-non-solvency on hydrophobic molecules driven by excluded volume effect 査読

    Kenji Mochizuki, Tomonari Sumi, Kenichiro Koga

    Physical Chemistry Chemical Physics   19 ( 35 )   23915 - 23918   2017年8月

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    担当区分:最終著者   記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Royal Society of Chemistry (RSC)  

    <p>We demonstrate by molecular dynamics simulation that co-non-solvency of hydrophobic molecules arises from solvent-excluded volume effect.</p>

    DOI: 10.1039/c7cp04152g

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  • Efficiency at maximum power for an isothermal chemical engine with particle exchange at varying chemical potential 招待 査読

    Jesper Koning, Kenichiro Koga, Joseph. O. Indekeu

    The European Physical Journal Special Topics   226 ( 3 )   427 - 431   2017年2月

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    掲載種別:研究論文(学術雑誌)   出版者・発行元:Springer Science and Business Media LLC  

    DOI: 10.1140/epjst/e2016-60200-8

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    その他リンク: http://link.springer.com/article/10.1140/epjst/e2016-60200-8/fulltext.html

  • A reference-modified density functional theory: An application to solvation free-energy calculations for a Lennard-Jones solution 査読

    Tomonari Sumi, Yutaka Maruyama, Ayori Mitsutake, Kenichiro Koga

    The Journal of Chemical Physics   144 ( 22 )   224104 - 224104   2016年6月

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    担当区分:最終著者   掲載種別:研究論文(学術雑誌)   出版者・発行元:AIP Publishing  

    DOI: 10.1063/1.4953191

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  • Cononsolvency behavior of hydrophobes in water + methanol mixtures 査読

    Kenji Mochizuki, Kenichiro Koga

    Physical Chemistry Chemical Physics   18 ( 24 )   16188 - 16195   2016年5月

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    担当区分:最終著者   記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Royal Society of Chemistry  

    The molecular origin of cononsolvency behavior is explored using molecular dynamics simulations. Cononsolvency behavior in aggregations of methane molecules and conformational changes of those clusters dissolved in water + methanol mixtures are confirmed by re-entrant changes in the solvent-mediated interactions with increasing methanol concentration. The results indicate that the cononsolvency behavior arises from the solute-solute hydrophobic interactions rather than other interactions such as solute-solvent hydrophilic interactions. Furthermore, we show that even the van der Waals interaction is not necessary to induce the cononsolvency behavior by investigating the dimerization process of repulsive cavities. The non-monotonic change of the solvent-mediated interaction results from the difference in the concentration dependencies of excess chemical potentials between an isolated methane and methane clusters. The concentration dependencies of the excess chemical potentials are decomposed into contributions from various intermolecular effective interactions through the framework of the Kirkwood-Buff theory, and then we show that the change of the relative magnitude between hydrophobe-methanol and hydrophobe-water effective interactions with increasing methanol concentration is responsible for the cononsolvency behavior.

    DOI: 10.1039/c6cp01496h

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  • Liquid–liquid phase separation of N-isopropylpropionamide aqueous solutions above the lower critical solution temperature 査読

    Kenji Mochizuki, Tomonari Sumi, Kenichiro Koga

    Scientific Reports   6 ( 1 )   2016年4月

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    担当区分:最終著者   掲載種別:研究論文(学術雑誌)   出版者・発行元:Springer Science and Business Media LLC  

    DOI: 10.1038/srep24657

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    その他リンク: http://www.nature.com/articles/srep24657

  • Driving forces for the pressure-induced aggregation of poly(N-isopropylacrylamide) in water 査読

    Kenji Mochizuki, Tomonari Sumi, Kenichiro Koga

    Physical Chemistry Chemical Physics   18 ( 6 )   4697 - 4703   2016年1月

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    担当区分:最終著者   記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Royal Society of Chemistry  

    Driving forces for the pressure-induced aggregation of poly(N-isopropylacrylamide) (PNiPA) in water are investigated by performing extensive molecular dynamics simulations. First, we observe that the model short oligomer of PNiPA with a modified OPLS-AA force field in water shrinks with increasing pressure. At varying pressures, the potentials of mean force (PMFs) between a pair of N-isopropylpropionamide (NiPPA) molecules, the repeating unit of PNiPA, are obtained and decomposed into the nonpolar and Coulombic contributions. The nonpolar contribution is the PMF between the hypothetical nonpolar NiPPA molecules in the solvent, which is mainly due to the molecular volume effect. The attractive force between NiPPA molecules is enhanced at higher pressures in agreement with the behavior of PNiPA. This pressure dependence of the PMF is caused by the growing nonpolar contribution at higher pressures. In contrast, the Coulombic contribution to the PMF becomes higher overall, making the mean force less attractive or more repulsive, with increasing pressure. The strength of the aggregation and its pressure dependence of the nonpolar contribution in water are closely reproduced even in nonpolar solvents. The degree of the pressure dependence is explained by the isothermal compressibility or the tightness of the solvation shell around an isolated solute, without regard to the existence and variation of hydrogen bond networks in a solvent. The role of hydrogen bonds in the aggregation of NiPPA and PNiPA molecules is also discussed.

    DOI: 10.1039/c5cp07674a

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  • Solid-liquid critical behavior of a cylindrically confined Lennard-Jones fluid 査読

    Kenji Mochizuki, Kenichiro Koga

    Physical Chemistry Chemical Physics   17 ( 28 )   18437 - 18442   2015年7月

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    担当区分:最終著者, 責任著者   記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Royal Society of Chemistry  

    Extensive molecular dynamics simulations have been performed to study the phase behavior of Lennard-Jones particles confined in a quasi-one-dimensional hydrophobic nanopore. We provide unambiguous evidence for a solid-liquid critical point by investigating (i) isotherms in the pressure-volume plane, (ii) the spontaneous solid-liquid phase separation below a certain temperature, (iii) diverging heat capacity and isothermal compressibility as a certain point is approached, (iv) continuous change of dynamical and structural properties above the point, (v) the finite-size scaling analysis of the density distribution below and above the point. The result combined with earlier studies of confined water suggests that the solid-liquid critical point is not uncommon in quasi-one- and quasi-two-dimensional fluids.

    DOI: 10.1039/c5cp02568k

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  • Electrostatic field-exposed water in nanotube at constant axial pressure 査読

    Yuchi He, Gang Sun, Kenichiro Koga, Limei Xu

    Scientific Reports   4 ( 1 )   2015年5月

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    掲載種別:研究論文(学術雑誌)   出版者・発行元:Springer Science and Business Media LLC  

    DOI: 10.1038/srep06596

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    その他リンク: http://www.nature.com/articles/srep06596

  • Local solubility of nonpolar molecules in the liquid–vapor interfaces of water and simple liquids 査読

    K. Abe, K. Koga

    Journal of Molecular Liquids   200   7 - 11   2014年12月

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    担当区分:最終著者, 責任著者   掲載種別:研究論文(学術雑誌)   出版者・発行元:Elsevier BV  

    DOI: 10.1016/j.molliq.2014.02.014

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  • Temperature dependence of local solubility of hydrophobic molecules in the liquid-vapor interface of water 査読

    Kiharu Abe, Tomonari Sumi, Kenichiro Koga

    The Journal of Chemical Physics   141 ( 18 )   18C516 - 18C516   2014年11月

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    担当区分:最終著者, 責任著者   掲載種別:研究論文(学術雑誌)   出版者・発行元:AIP Publishing  

    DOI: 10.1063/1.4896236

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  • Renormalization group calculations for wetting transitions of infinite order and continuously varying order: Local interface Hamiltonian approach 査読

    J. O. Indekeu, K. Koga, H. Hooyberghs, A. O. Parry

    Physical Review E   88 ( 2 )   2013年8月

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    掲載種別:研究論文(学術雑誌)   出版者・発行元:American Physical Society (APS)  

    DOI: 10.1103/physreve.88.022122

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    その他リンク: http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevE.88.022122/fulltext

  • Thermodynamic functions as correlation-function integrals 査読

    K. Koga, B. Widom

    The Journal of Chemical Physics   138 ( 11 )   114504 - 114504   2013年3月

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    担当区分:筆頭著者, 責任著者   記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AIP Publishing  

    DOI: 10.1063/1.4795498

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  • Note on the Calculation of the Second Osmotic Virial Coefficient in Stable and Metastable Liquid States 査読

    B. Widom, K. Koga

    The Journal of Physical Chemistry B   117 ( 4 )   1151 - 1154   2013年1月

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    担当区分:最終著者   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Chemical Society (ACS)  

    DOI: 10.1021/jp311800p

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  • Model of Freezing Behavior of Liquid Monolayers Adsorbed in Cylindrical Pores 査読

    Abe Kiharu, Koga Kenichiro

    Journal of the Physical Society of Japan   81 ( Suppl.A )   SA021 - SA021   2012年1月

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    担当区分:最終著者, 責任著者   記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:The Physical Society of Japan  

    DOI: 10.1143/jpsjs.81sa.sa021

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  • Diffusivity of Liquid Argon in Carbon Nanotubes 査読

    Akiyoshi Hiroki, Koga Kenichiro

    Journal of the Physical Society of Japan   81 ( Suppl.A )   SA022 - SA022   2012年1月

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    担当区分:最終著者, 責任著者   記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:The Physical Society of Japan  

    DOI: 10.1143/jpsjs.81sa.sa022

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  • Inclusion of Neon Inside Ice Icand Its Influence to the Ice Structure 査読

    Hakim Lukman, Matsumoto Masakazu, Koga Kenichiro, Tanaka Hideki

    Journal of the Physical Society of Japan   81 ( Suppl.A )   SA018 - SA018   2012年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:The Physical Society of Japan  

    DOI: 10.1143/jpsjs.81sa.sa018

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  • How much does the core structure of a three-phase contact line contribute to the line tension near a wetting transition? 査読

    J O Indekeu, K Koga, B Widom

    Journal of Physics: Condensed Matter   23 ( 19 )   194101 - 194101   2011年5月

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    掲載種別:研究論文(学術雑誌)   出版者・発行元:IOP Publishing  

    DOI: 10.1088/0953-8984/23/19/194101

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  • Wetting transitions of continuously varying or infinite order from a mean-field density-functional theory 査読

    K. Koga, J.O. Indekeu, B. Widom

    Molecular Physics   109 ( 7-10 )   1297 - 1311   2011年3月

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    担当区分:筆頭著者   掲載種別:研究論文(学術雑誌)   出版者・発行元:Informa UK Limited  

    DOI: 10.1080/00268976.2011.556095

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  • Hydrophobicity in Lennard-Jones solutions 査読

    Mario Ishizaki, Hideki Tanaka, Kenichiro Koga

    PHYSICAL CHEMISTRY CHEMICAL PHYSICS   13 ( 6 )   2328 - 2334   2011年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ROYAL SOC CHEMISTRY  

    The analogue of the hydrophobic hydration is explored for Lennard-Jones solutions. The free energy of solvation and its temperature derivatives, both in the constant-pressure process and in the constant-volume process, are obtained numerically for a variety of the size and energy parameters for the solute-solvent Lennard-Jones potential. We identify in the parameter space a region in which the solvation is of hydrophobic character, with an understanding that hydrophobicity is characterized by both the solvation free energy being positive and the solvation process being exothermic. Such a region is found in each case of the isobaric and isochoric conditions and the region is seen to be much wider in the isochoric process than in the isobaric one. Its origin and implication are discussed.

    DOI: 10.1039/c0cp01767a

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  • Thermodynamic stability of hydrogen hydrates of ice I-c and II structures

    Lukman Hakim, Kenichiro Koga, Hideki Tanaka

    PHYSICAL REVIEW B   82 ( 14 )   114105   2010年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    The occupancy of hydrogen inside the voids of ice I-c and ice II, which gives two stable hydrogen hydrate compounds at high pressure and temperature, has been examined using a hybrid grand-canonical Monte Carlo simulation in wide ranges of pressure and temperature. The simulation reproduces the maximum hydrogen-to-water molar ratio and gives a detailed description on the hydrogen influence toward the stability of ice structures. A simple theoretical model, which reproduces the simulation results, provides a global phase diagram of two-component system in which the phase transitions between various phases can be predicted as a function of pressure, temperature, and chemical composition. A relevant thermodynamic potential and statistical-mechanical ensemble to describe the filled-ice compounds are discussed, from which one can derive two important properties of hydrogen hydrate compounds: the isothermal compressibility and the quantification of thermodynamic stability in term of the chemical potential.

    DOI: 10.1103/PhysRevB.82.144105

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  • Novel neon-hydrate of cubic ice structure

    Lukman Hakim, Kenichiro Koga, Hideki Tanaka

    PHYSICA A-STATISTICAL MECHANICS AND ITS APPLICATIONS   389 ( 9 )   1834 - 1838   2010年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER  

    The stability of ice I cubic (ice I-c) whose voids are occupied by neon particles is investigated using a hybrid type of isobaric grand-canonical Monte Carlo simulation. We show that the resulting neon hydrate is stable under high pressure and temperature where ice I-c alone is unstable, suggesting the existence of a novel neon hydrate of ice I-c. We also show through chemical potential calculation that the neon hydrate of the ice I-c structure is more favorable than the neon hydrate of the ice II structure, whose existence was proven from experiment under high pressure condition. (C) 2009 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physa.2009.12.021

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  • Phase Behavior of Different Forms of Ice Filled with Hydrogen Molecules

    Lukman Hakim, Kenichiro Koga, Hideki Tanaka

    PHYSICAL REVIEW LETTERS   104 ( 11 )   115701   2010年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    A hybrid grand-canonical Monte Carlo simulation has been performed to investigate the hydrogen hydrate compounds in which hydrogen molecules are stored in ice II and ice I(c). A simple theoretical model, which can reproduce the simulation results, provides the phase diagrams of the two-component system in the pressure-composition plane. Stability enhancement of the two ice forms by hydrogen is quantified by the chemical potential calculation of water. The phase transitions among various phases including the two hydrogen hydrates are predicted as functions of pressure, hydrogen occupancy, and temperature.

    DOI: 10.1103/PhysRevLett.104.115701

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  • First- and second-order wetting transitions at liquid-vapor interfaces

    K. Koga, J. O. Indekeu, B. Widom

    FARADAY DISCUSSIONS   146   217 - 222   2010年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ROYAL SOC CHEMISTRY  

    Wetting transitions, in which one liquid wets, or spreads at, the interface between a second liquid and their common vapor, are defined and first- and second-order transitions are distinguished. The mean-field density-functional models of fluid interfaces are recalled. A criterion is noted for determining when the wetting transitions in those models are required to be of first order or may be of second order. It is seen how two examples of such density-functional models that have been treated in the past, one leading to a first-order and the other to a second-order wetting transition, provide examples of the application of the criterion.

    DOI: 10.1039/b925671g

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  • Augmented stability of hydrogen clathrate hydrates by weakly polar molecules

    Takato Nakayama, Kenichiro Koga, Hideki Tanaka

    JOURNAL OF CHEMICAL PHYSICS   131 ( 21 )   214506   2009年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER INST PHYSICS  

    Thermodynamic stability of hydrogen clathrate hydrates has been examined in a wide range of pressure based solely on the intermolecular interactions involved. We show that the stability is indeed augmented by a second guest species (here acetone) called a promoter, a consequence of which is notable reduction in the dissociation pressure of the hydrates encaging hydrogen alone. This evaluation is made by extension of the van der Waals-Platteeuw theory combined with semi-grand-canonical Monte Carlo (GCMC) simulations where the number of hydrogen molecules is allowed to vary while those of host water and promoter acetone molecules are fixed. The GCMC simulations then provide various types of cage occupancies of hydrogen from single to quadruple, from which the chemical potential of water in the clathrate hydrate is obtained as a function of the cage occupancy by acetone and the pressure. These occupancies are used to calculate the chemical potential of water in the clathrate hydrate. The stability is estimated by comparison of the chemical potential of water in the clathrate hydrate with that in hexagonal ice. We show the extent to which the dissociation pressure is reduced with increasing the occupancy of the larger cages by acetone. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3271341]

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  • Phase behavior and fluid-solid surface tension of argon in slit pores and carbon nanotubes

    Yoshinobu Hamada, Kenichiro Koga, Hideki Tanaka

    PHYSICA A-STATISTICAL MECHANICS AND ITS APPLICATIONS   388 ( 12 )   2289 - 2298   2009年6月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Phase behaviors of argon in several types of cylindrical and slit pores are examined by grand-canonical Monte Carlo simulations. Condensation processes in single- and multiwalled carbon nanotubes along With those in hard-wall tubes are compared. Effects of the pore size oil pressure-tensor components. the fluid-wall surface tension. and the adsorption are also compared for the different fluid-pore interactions. The chemical potential at which the fluid begins to condense in the single-walled nanotube is greater than that in the multi-walled nanotube by all amount nearly equal to the difference in the potential-well depth of the fluid-pore interaction, and the adsorption isotherms overlap each other almost completely for narrow pores and partially for wider pores. Similar analyses are performed for slit pores of two different hydrocarbon models. (c) 2009 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physa.2009.02.034

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  • Line and boundary tensions on approach to the wetting transition

    K. Koga, B. Widom

    JOURNAL OF CHEMICAL PHYSICS   127 ( 6 )   064704   2007年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER INST PHYSICS  

    A mean-field density-functional model often used in the past in the study of line and boundary tensions at wetting and prewetting transitions is reanalyzed by extensive numerical calculations, approaching the wetting transition much more closely than had previously been possible. The results are what are now believed to be definitive for the model. They include strong numerical evidence for the presence of the logarithmic factors predicted by theory both in the mode of approach of the prewetting line to the triple-point line at the point of the first-order wetting transition and in the line tension itself on approach to that point. It is also demonstrated with convincing numerical precision that the boundary tension on the prewetting line and the line tension on the triple-point line have a common limiting value at the wetting transition, again as predicted by theory. As a by product of the calculations, in the model's symmetric three-phase state, far from wetting, it is found that certain properties of the model's line tension and densities are almost surely given by simple numbers arising from the symmetries, but proving that these are exact for the model remains a challenge to analytical theory. (c) 2007 American Institute of Physics.

    DOI: 10.1063/1.2752156

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  • Phase equilibria and interfacial tension of fluids confined in narrow pores

    Yoshinobu Hamada, Kenichiro Koga, Hideki Tanaka

    JOURNAL OF CHEMICAL PHYSICS   127 ( 8 )   84908   2007年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER INST PHYSICS  

    Correlation between phase behaviors of a Lennard-Jones fluid in and outside a pore is examined over wide thermodynamic conditions by grand canonical Monte Carlo simulations. A pressure tensor component of the confined fluid, a variable controllable in simulation but usually uncontrollable in experiment, is related with the pressure of a bulk homogeneous system in equilibrium with the confined system. Effects of the pore dimensionality, size, and attractive potential on the correlations between thermodynamic properties of the confined and bulk systems are clarified. A fluid-wall interfacial tension defined as an excess grand potential is evaluated as a function of the pore size. It is found that the tension decreases linearly with the inverse of the pore diameter or width. (c) 2007 American Institute of Physics.

    DOI: 10.1063/1.2759926

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  • On the thermodynamic stability of hydrogen clathrate hydrates

    Keisuke Katsumasa, Kenichiro Koga, Hideki Tanaka

    JOURNAL OF CHEMICAL PHYSICS   127 ( 4 )   44509   2007年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER INST PHYSICS  

    The cage occupancy of hydrogen clathrate hydrate has been examined by grand canonical Monte Carlo (GCMC) simulations for wide ranges of temperature and pressure. The simulations are carried out with a fixed number of water molecules and a fixed chemical potential of the guest species so that hydrogen molecules can be created or annihilated in the clathrate. Two types of the GCMC simulations are performed; in one the volume of the clathrate is fixed and in the other it is allowed to adjust itself under a preset pressure so as to take account of compression by a hydrostatic pressure and expansion due to multiple cage occupancy. It is found that the smaller cage in structure II is practically incapable of accommodating more than a single guest molecule even at pressures as high as 500 MPa, which agrees with the recent experimental investigations. The larger cage is found to encapsulate at most 4 hydrogen molecules, but its occupancy is dependent significantly on the pressure of hydrogen. (c) 2007 American Institute of Physics.

    DOI: 10.1063/1.2751168

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  • Structures of filled ice nanotubes inside carbon nanotubes

    D. Takaiwa, K. Koga, H. Tanaka

    MOLECULAR SIMULATION   33 ( 1-2 )   127 - 132   2007年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:TAYLOR & FRANCIS LTD  

    We report ordered structures of water in carbon nanotubes that are different from previously found n-gonal ice nanotubes and may be called filled ice nanotubes. Spontaneous formation of the filled ice nanotubes is observed in molecular dynamics (MD) simulations of water at fixed densities and a fixed temperature. Outer layers of filled ice nanotubes are characterized by a roll-up vector (n, m) while inner files of molecules do not have definite ordered structures. With this notation the filled ice nanotubes are of the (6, 0), (7, 0), (8, 0) and (8, 1) types, the last of which has a helical structure in its outer layer whereas the outer layers of the first three have the same achiral structures as the n-gonal ice nanotubes. Structure analysis is done for their hydrogen-bond networks and average dipole moments.

    DOI: 10.1080/08927020601059893

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  • Theoretical studies on the structure and dynamics of water, ice, and clathrate hydrate

    Hideki Tanaka, Kenichiro Koga

    BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN   79 ( 11 )   1621 - 1644   2006年11月

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    記述言語:英語   出版者・発行元:CHEMICAL SOC JAPAN  

    We have investigated various anomalous properties of water such as the divergent character of the thermodynamic functions and liquid-liquid transition in supercooled water, the phase behaviors of water and new ices in nanoscale confinement, the thermodynamic stability of clathrate hydrates over a wide range of pressure, and anomalous thermodynamic and structural properties of ices. These are studied by some theoretical calculations and Monte Carlo/molecular dynamics computer simulations. It is demonstrated that the potential energy surface and the connectivity of supercooled water are keys to understand why liquid-liquid transition can take place in deeply supercooled water. A tetrahedral coordination of water is preserved even in extreme confinements, forming tubule ice and bilayer crystalline (or amorphous) ice, although the heavy stress makes the bond angles and lengths different from the ideal values. Thermodynamic stability of clathrate hydrates, including double occupancy, is more accurately predicted by considering the host-guest coupling and other factors. The negative thermal expansivity and the change in slope of the Debye-Waller factor of ice are explained with a simple model of water.

    DOI: 10.1246/bcsj.79.1621

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  • Line adsorption in a mean-field density-functional model

    K. Koga, B. Widom

    MOLECULAR PHYSICS   104 ( 22-24 )   3469 - 3477   2006年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:TAYLOR & FRANCIS LTD  

    Recent ideas about the analogue for a three-phase contact line of the Gibbs adsorption equation for interfaces are illustrated in a mean-field density-functional model. With d tau the infinitesimal change in the line tension a that accompanies the infinitesimal changes d mu(i) in the thermodynamic field variables mu(i) and with Lambda(i) the line adsorptions, the sum d tau + Sigma Lambda(i)d mu(i), unlike its surface analogue, is not 0. An equivalent of this sum in the model system is evaluated numerically and analytically. A general line adsorption equation, which the model results illustrate, is derived.

    DOI: 10.1080/00268970600958574

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  • Formation of ice nanotube with hydrophobic guests inside carbon nanotube

    H Tanaka, K Koga

    JOURNAL OF CHEMICAL PHYSICS   123 ( 9 )   2005年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER INST PHYSICS  

    A composite ice nanotube inside a carbon nanotube has been explored by molecular-dynamics and grand canonical Monte Carlo simulations. It is made from an octagonal ice nanotube whose hollow space contains hydrophobic guest molecules such as neon, argon, and methane. It is shown that the attractive interaction of the guest molecules stabilizes the ice nanotube. The guest occupancy of the hollow space is calculated by the same method as applied to clathrate hydrates. (C) 2005 American Institute of Physics.

    DOI: 10.1063/1.2031127

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  • On the thermodynamic stability and structural transition of clathrate hydrates

    Y Koyama, H Tanaka, K Koga

    JOURNAL OF CHEMICAL PHYSICS   122 ( 7 )   2005年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER INST PHYSICS  

    Gas mixtures of methane and ethane form structure II clathrate hydrates despite the fact that each of pure methane and pure ethane gases forms the structure I hydrate. Optimization of the interaction potential parameters for methane and ethane is attempted so as to reproduce the dissociation pressures of each simple hydrate containing either methane or ethane alone. An account for the structural transitions between type I and type II hydrates upon changing the mole fraction of the gas mixture is given on the basis of the van der Waals and Platteeuw theory with these optimized potentials. Cage occupancies of the two kinds of hydrates are also calculated as functions of the mole fraction at the dissociation pressure and at a fixed pressure well above the dissociation pressure. (C) 2005 American Institute of Physics.

    DOI: 10.1063/1.1850904

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  • Hydrophobic effect in the pressure-temperature plane

    Kenichiro Koga

    Journal of Chemical Physics   121 ( 15 )   7304 - 7312   2004年10月

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    掲載種別:研究論文(学術雑誌)  

    The free energy of the hydrophobic hydration and the strength of the solvent-mediated attraction between hydrophobic solute molecules, in the pressure-temperature plane, were calculated. An exactly soluble model, that was an extension of the lattice model was proposed. The mechanism of the hydrophobic effect dominant at low temperature and the opposite mechanism of solvation appearing at high temperatures and has the pressure as a second thermodynamic variable, were taken into account by the model. The two boundaries were identified in the pressure-temperature plane, with this model. It was shown that the first one within which the solubility, or the Ostwald absorption coefficient, decreases with increasing temperature at fixed pressure.

    DOI: 10.1063/1.1792571

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  • On the thermodynamic stability of clathrate hydrates IV: Double occupancy of cages

    H Tanaka, T Nakatsuka, K Koga

    JOURNAL OF CHEMICAL PHYSICS   121 ( 11 )   5488 - 5493   2004年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER INST PHYSICS  

    We have extended the van der Waals and Platteeuw theory to treat multiple occupancy of a single cage of clathrate hydrates, which has not been taken into account in the original theory but has been experimentally confirmed as a real entity. We propose a simple way to calculate the free energy of multiple cage occupancy and apply it to argon clathrate structure II in which a larger cage can be occupied by two argon atoms. The chemical potential of argon is calculated treating it as an imperfect gas, which is crucial to predict accurate pressure dependence of double occupancy expected at high pressure. It is found that double occupancy dominates over single occupancy when the guest pressure in equilibrium with the clathrate hydrate exceeds 270 MPa. (C) 2004 American Institute of Physics.

    DOI: 10.1063/1.1782471

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  • Reply to 'Comment on "The hydrophobic effect" ' by G. Graziano, Phys. Chem. Chem. Phys., 2004, 6, DOI: 10.1039/b405824k

    B Widom, P Bhimalapuram, K Koga

    PHYSICAL CHEMISTRY CHEMICAL PHYSICS   6 ( 18 )   4529 - 4530   2004年

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    記述言語:英語   出版者・発行元:ROYAL SOC CHEMISTRY  

    DOI: 10.1039/b407059n

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  • Formation of quasi two-dimensional bilayer ice in hydrophobic slits: A possible candidate for ice XIII?

    J Bai, XC Zeng, K Koga, H Tanaka

    MOLECULAR SIMULATION   29 ( 10-11 )   619 - 626   2003年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:TAYLOR & FRANCIS LTD  

    We performed molecular dynamics simulations of water confined to hydrophobic slit nanopores. Two five-site potential models of water were employed: the ST2 model and the TIP5P model. The simulations confirm our previous simulation results on basis of the four-site TIP4P model of water, that is, upon cooling the confined liquid water may undergo a first-order phase transition to either a bilayer crystalline ice phase or to a bilayer amorphous ice. The selection of the bilayer crystalline phase occurs at the constant normal pressure condition whereas the selection of the bilayer amorphous ice phase is likely to occur at the fixed pore-width condition. This computer-simulation generated ice form, if confirmed by crystallographic or spectroscopic experiments, may be a candidate for "ice XIII", provided that purely quasi two-dimensional ice forms (metastable in vacuum) can be viewed as a stand-alone solid phase of ice.

    DOI: 10.1080/0892702031000103158

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  • Ab initio studies of quasi-one-dimensional pentagon and hexagon ice nanotubes

    J Bai, CR Su, RD Parra, XC Zeng, H Tanaka, K Koga, JM Li

    JOURNAL OF CHEMICAL PHYSICS   118 ( 9 )   3913 - 3916   2003年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER INST PHYSICS  

    Ab initio plane-wave total-energy calcuation is carried out to study the relative stability of the quasi-one-dimensional (Q1D) pentagon and hexagon ice nanotubes. Electronic structure calculations indicate the two Q1D ice nanotubes have nearly the same band structures and energy bandgap as those of proton-ordered bulk ice I-h. Ab initio molecular-orbital and density-functional theory calculations, as well as three classical potential models of water, are also employed to investigate the relative stability of the pentagon and hexagon water clusters (H2O)(30), (H2O)(60), and (H2O)(120). Clusters of this kind can serve to bridge the gap between the small polygonal water rings and the infinitely long Q1D polygon ice nanotubes. It is found that the polygon water prisms with the size (H2O)(120) begin to show the relative energetic behavior of the infinitely long polygon ice nanotubes. (C) 2003 American Institute of Physics.

    DOI: 10.1063/1.1555091

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  • Computer simulation of bilayer ice: structures and thermodynamics

    J Slovak, H Tanaka, K Koga, XC Zeng

    PHYSICA A-STATISTICAL MECHANICS AND ITS APPLICATIONS   319   163 - 174   2003年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    A series of molecular dynamics simulations is performed in order to examine in more detail the results of a previous simulation which shows that a thin film of water, when confined to a hydrophobic slit nanopore, freezes into a bilayer ice crystal composed of two layers of hexagonal rings. Three simulations are carried out and each starts with a different initial configuration but has the same number Of molecules and the area density. Using a previously introduced solid-like cluster definition, we monitor the dynamic process of crystallization. We find that only in one case the confined water completely freezes into perfect bilayer of ice whereas in other two cases, an imperfect crystalline structure consisting of hexagons of slightly different shapes is observed and this imperfection apparently hinders the growth of perfect bilayer of crystal. After adjusting the area density to match spatial arrangements of molecules, the latter two systems;ire able to crystallize completely. As a result, we obtain three forms of bilayer crystal differing in the area density and hexagonal rings alignment. Further analyses of these bilayer crystals provide more insightful explanation on the influence of the boundary condition and the simulation-cell size on the diversity of possible crystallographic structures. (C) 2002 Elsevier Science B.V. All rights reserved.

    DOI: 10.1016/S0378-4371(02)01384-5

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  • Correlation between hydrophobic hydrophobic attraction and the free energy of hydration

    K Koga, P Bhimalapuram, B Widom

    MOLECULAR PHYSICS   100 ( 24 )   3795 - 3801   2002年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:TAYLOR & FRANCIS LTD  

    We correlate the strength of the solvent-mediated attraction of a pair of hydrophobic solute molecules with the free energy of hydration of a single such molecule. This is done in the framework of a particular model but the results may be more general. When the model parameters are chosen to represent methane as the solute in water it is found that over the relevant temperature range the strength of the attraction, expressed as a multiple of the thermal energy kT, increases nearly linearly with increasing hydration free energy expressed in the same units. In the middle of the temperature range studied the strength of the attraction is roughly one-third of the hydration free energy.

    DOI: 10.1080/00268970210162899

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  • How does water freeze inside carbon nanotubes?

    K Koga, GT Gao, H Tanaka, XC Zeng

    PHYSICA A-STATISTICAL MECHANICS AND ITS APPLICATIONS   314 ( 1-4 )   462 - 469   2002年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Phase behavior of quasi-one-dimensional water confined inside a carbon nanotube is studied in the thermodynamic space of temperature, pressure, and diameter of the cylindrical container. Four kinds of solid-like ordered structures-ice nanotubes-form spontaneously from liquid-like disordered phases at low temperatures. In the model system that comprises of TIP4P water molecules interacting with each other via short-range Lennard-Jones and long-range Coulomb site-site potentials under a periodic boundary condition in the axial direction, the phase change occurs either discontinuously or continuously depending on the path in the thermodynamic space. That the isotherms for a given diameter are found to be similar to those around the liquid-gas critical point of fluids suggests existence of a phase boundary terminated by a critical point. The apparently-complex phase behavior is accounted for by noting that the phase boundaries are layered surfaces in the three-dimensional thermodynamic space and some of the surfaces are terminated by critical lines. (C) 2002 Elsevier Science B.V. All rights reserved.

    DOI: 10.1016/S0378-4371(02)01074-9

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  • Correlation functions in decorated lattice models

    Ispolatov, I, K Koga, B Widom

    PHYSICA A   291 ( 1-4 )   49 - 59   2001年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Occupation probabilities for primary-secondary-primary cell strings and correlation functions for primary sites of a decorated lattice model are expressed through the well-studied partition function and correlation functions of the Ising model. The results are analogous to those found in related lattice models of hydrophobic interactions and are interpreted in similar terms. (C) 2001 Elsevier Science B.V. All rights reserved.

    DOI: 10.1016/S0378-4371(00)00482-9

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  • Computer simulation of water-ice transition in hydrophobic nanopores

    J Slovak, H Tanaka, K Koga, XC Zeng

    PHYSICA A   292 ( 1-4 )   87 - 101   2001年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    A series of molecular dynamics simulations is performed in order to examine in more detail the results of a previous simulation which shows that a thin film of water, when confined in a hydrophobic nanopore, freezes into a bilayer ice crystal composed of two layers of hexagonal rings. Three simulations are carried out and each starts with a different initial configuration but has the same number of molecules and the area density. Using a previously introduced solid-like cluster definition, we monitor the dynamic process of crystallization We find that only in one case the confined water completely freezes into perfect bilayer ice whereas in other two cases, an imperfect crystalline structure consisting of hexagons of slightly different shapes is observed and this imperfection apparently hinders the growth of perfect bilayer crystal. After adjusting the area density to match spatial arrangements of molecules, the latter two systems are able to crystallize completely. As a result, we obtain three forms of bilayer crystal differing in the area density and hexagonal rings alignment. Further analyses of these bilayer crystals provide more insightful explanation on the influence of the boundary condition and the simulation-cell size on the diversity of possible crystallographic structures. (C) 2001 Elsevier Science B.V. All rights reserved.

    DOI: 10.1016/S0378-4371(00)00579-3

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  • Ice nanotube: What does the unit cell look like?

    K Koga, RD Parra, H Tanaka, XC Zeng

    JOURNAL OF CHEMICAL PHYSICS   113 ( 12 )   5037 - 5040   2000年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER INST PHYSICS  

    It is discovered that for an n-gonal ice nanotube built from stacking a single type of n-gonal rings of water, the unit cell consists of two stacked n-gonal rings. In one ring the O-H bonds of water molecules line up clockwise whereas in the other ring the O-H bonds line up counterclockwise. Among the n-gonal ice nanotubes examined, the pentagonal or hexagonal ice nanotube appears to be the most stable. (C) 2000 American Institute of Physics. [S0021-9606(00)51036-8].

    DOI: 10.1063/1.1289554

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  • Confined water in hydrophobic nanopores: Dynamics of freezing into bilayer ice

    J Slovak, K Koga, H Tanaka, XC Zeng

    PHYSICAL REVIEW E   60 ( 5 )   5833 - 5840   1999年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMERICAN PHYSICAL SOC  

    Molecular dynamics simulations for a thin film of water confined to a slit nanopore are performed in order to investigate the dynamic process of crystallization of the system. The system upon freezing creates a bilayer ice crystal composed of two layers of hexagonal rings. We perform one simulation at T = 257 K during which the system remains a supercooled liquid state, and another one at T = 253 K during which the system freezes. Many patterns of molecular arrangement are found upon freezing, and an account is given of the origin of multiple peaks in the distributions of binding energy and pair interaction energy. A definition of the solidlike cluster is introduced in order to analyze the time evolution of the clusters' population and their shapes. A large variety of shapes including highly nonspherical ones can be detected during simulations. A steady population of clusters is found at T = 257 K, whereas at T = 253 K a post-critical nucleus of the solid phase emerges within a few nanoseconds and continues to grow until the system freezes completely. [S1063-651X(99)13411-1].

    DOI: 10.1103/PhysRevE.60.5833

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  • Imaging point defects in a liquid environment: A model AFM study

    K Koga, XC Zeng

    PHYSICAL REVIEW B   60 ( 20 )   14328 - 14333   1999年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    Effects of the liquid on atomic force microscopy (AFM) imaging are examined for a model system consisting of a hexagonal flake of seven Lennard-Jones (LT) atoms as a multiatom tip, a monolayer of LJ crystal containing a single point defect as a substrate, and three-site model water as the liquid. A previous simulation [Koutsos et al., Europhys. Lett. 26, 103 (1994)] has shown that the true atomic resolution of a point defect cannot be achieved in vacuum by use of the multiatom tip. Hen we examine the feasibility of such atomic resolution when both the tip and substrate are immersed in a liquid. The liquid-induced interaction between the tip and the substrate is evaluated in the framework of the integral equation theory of molecular quids extended for the system consisting of an infinitely large crystal. It is found that the potential of mean force indeed manifests the point defect even when the tip and the substrate are a few Angstroms apart. This implies that in the liquid environment the point defect could be discerned in the constant-height mode of AFM measurement. For the constant-load made of AFM, a characteristic potential can be used to determine the stable, metastable, and unstable vertical position (height) of the tip at any lateral position. The contour surface of the stable heights reveals periodic features of the underlying lattice of the substrate except in the vicinity of the defect, provided certain load is applied. [S0163-1829(99)10543-5].

    DOI: 10.1103/PhysRevB.60.14328

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  • Can thin disk-like ice clusters be more stable than compact droplet-like ice clusters?

    H Tanaka, R Yamamoto, K Koga, XC Zeng

    CHEMICAL PHYSICS LETTERS   304 ( 5-6 )   378 - 384   1999年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    The relative stability of water clusters of two different morphologies has been examined with the size N ranging from 54 to 864; the first one is a circular fragment of bilayer ice (a disk-like cluster), and the second one is a spherical fragment of normal cubic ice (a droplet-like cluster). We found a crossover at N-c similar or equal to 1000, below which the disk-like cluster becomes more energetically favorable. The crossover arises because the potential energy (per molecule) for the droplet-like cluster decreases linearly with N-1/3 whereas it decreases by N-1/2 for the disk-like cluster. (C) 1999 Elsevier Science B.V. All rights reserved.

    DOI: 10.1016/S0009-2614(99)00293-6

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  • Effects of confinement on the phase behavior of supercooled water 査読

    Kenichiro Koga, X. C. Zeng, Hideki Tanaka

    Chemical Physics Letters   285 ( 3-4 )   278 - 283   1998年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Elsevier  

    Molecular dynamics simulations are performed to investigate phase behavior of a thin film (thickness d~2 nm) of supercooled water confined between two parallel planar walls. In contrast to bulk supercooled water, the confined water, under fixed normal pressures of 0.1 MPa and 500 MPa, does not exhibit the high density amorphous (HDA) to low density amorphous (LDA) liquid-liquid phase transition in the course of the cooling process. A possible explanation is that when the water is confined between two walls, the bulk HDA-LDA phase boundary and the associated critical point C′ shift to lower temperature or/and lower Pzz (confinement pressure) as decreasing the film thickness.

    DOI: 10.1016/S0009-2614(98)00035-9

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  • Validity of Tolman's equation: How large should a droplet be?

    Kenichiro Koga, X. C. Zeng, A. K. Shchekin

    Journal of Chemical Physics   109 ( 10 )   4063 - 4070   1998年

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    掲載種別:研究論文(学術雑誌)  

    Surface tension and the length δ (distance between the Gibbs surface of tension Rs and the equimolar surface Re) of simple liquid droplet (Lennard-Jones and Yukawa) are computed over a wide range of droplet sizes up to about 4×106 molecules. The study is based on the Gibbs theory of capillarity combined with the density-functional approach to gas-liquid nucleation. Since this method provides behavior of the surface tension fully consistent with the tension of the planner surface, the constant in Tolman's equation δ∞ can be determined unequivocally from the asymptotic behavior of σs. Comparison of the tension given by Tolman's equation against the result of exact thermodynamic relations reveals that Tolman's equation is valid only when the droplet holds more than 106 molecules for the simple fluid systems near their triple points, in contrast to the conventional wisdom that Tolman's equation may be applicable down to droplets holding a few hundreds of molecules. © 1998 American Institute of Physics.

    DOI: 10.1063/1.477006

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  • Large thermal expansivity of clathrate hydrates

    Hideki Tanaka, Yoshinori Tamai, Kenichiro Koga

    Journal of Physical Chemistry B   101 ( 33 )   6560 - 6565   1997年8月

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    掲載種別:研究論文(学術雑誌)  

    The free energies of clathrates are calculated over a wide range of temperatures in order to explain a large thermal expansivity of clathrate hydrates compared with that of ice. Several proton-disordered configurations for hexagonal ice and type I chathrate hydrates are generated. The free energy is approximated to the sum of the minimum potential energy, the harmonic free energy, and the configurational entropy arising from the disordered nature of protons. The free energy at a given temperature is minimized with respect to the volume of the system. This enables us to evaluate the thermal expansivity from only intermolecular interaction potentials. The larger thermal expansivity of clathrate hydrates than ice is successfully reproduced. It is found that the large expansivity of clathrate hydrate structure I stems from the existence of guest molecules and that a difference in oxygen atom arrangement between clathrate hydrates and ice plays a minor role. The effective potential energy surface of a guest molecule becomes harmonic with an increase in temperature. This seems to undermine the large difference in the thermal expansivity between clathrate hydrates and ice.

    DOI: 10.1021/jp970511g

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  • Solvent-induced interactions between hydrophobic and hydrophilic polyatomic sheets in water and hypothetical nonpolar water 査読

    K Koga, XC Zeng, H Tanaka

    JOURNAL OF CHEMICAL PHYSICS   106 ( 23 )   9781 - 9792   1997年6月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER INST PHYSICS  

    Hydrophobic and hydrophilic interactions are two major intermolecular forces between hydrophobic nonpolar and hydrophilic polar sites of macromolecules or materials surfaces in solvents. To further understand these two interactions at the microscopic level, an idealized polyatomic model is devised, which includes hydrophobic, hydrophilic, and partially hydrophilic polyatomic planar square molecular sheets. The hydrophobic molecular sheet is composed of the Lennard-Jones particles while the hydrophilic molecular sheet consists of positive and negative charge sites. In the framework of the extended reference interaction site model integral equation theory the solvent-induced interactions (or the potential of mean forces) between two parallel molecular sheets in water and in the hypothetical nonpolar water are investigated in a systematic fashion. Such a highly idealized model allows us to isolate and to explore the important effects of molecular size, relative intermolecular position (e.g., in- or out-of-registry configuration), and hydrophilic site distribution on the hydrophobic and hydrophilic interactions in both water and the hypothetical nonpolar water. Significant insight into these effects at the molecular level is obtained. For the hydrophobic planar molecules in water we find solvent separated hydrophobic interaction becomes less favored as sheet size increases.; Moreover, the contact hydrophobic interaction between two molecular sheets in the out-of-registry configuration is always most favorable. For the latter case we find it is the van der Waals attraction, rather than the hydrophobic attraction, that dominates the total interaction. We also find that in both water and the hypothetical nonpolar water the solvent-induced interaction between two hydrophobic sheets behaves similarly. One possible explanation is that the hydrophobic hydration originating from the hydrogen bonding network in water plays an insignificant role in the solvent-induced interaction, at least in the infinitely dilute aqueous solution. For hydrophilic planar molecular sheets in water, we find water-induced hydrophilic interaction is much more substantial compared with the hydrophobic one. In many cases, the hydrophilic interaction is found directly against the intermolecular force between two parallel molecular sheets in vacuum. Finally, for the partially hydrophilic planar molecules in water, a newly discovered feature is that a disperse hydrophilic site distribution gives rise to stronger solvent-induced interaction compared with the clustered hydrophilic site distribution. (C) 1997 American Institute of Physics.

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  • Scanning motions of an atomic force microscope tip in water

    Kenichiro Koga, X. C. Zeng

    Physical Review Letters   79 ( 5 )   853 - 856   1997年1月

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    掲載種別:研究論文(学術雑誌)  

    Integral equation techniques are used to study scanning motions of a single-atom tip of the atomic force microscope (AFM) over a rigid, hydrophobic monolayer substrate in water. The calculated force curve is found to be oscillatory, in agreement with recent AFM experiments, which can lead to multiple scanning trajectories for the tip under a constant load. The unique trajectory along which the system is thermodynamically stable is revealed. This study shows that the tip may take a hopping motion over a defect-free substrate due to layering of water molecules between the tip and substrate. © 1997 The American Physical Society.

    DOI: 10.1103/PhysRevLett.79.853

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  • RISM integral equation study of local solvation behavior of naphthalene in supercritical carbon dioxide 査読

    Kenichiro Koga, Hideki Tanaka, X. C. Zeng

    Journal of Physical Chemistry   100 ( 41 )   16711 - 16719   1996年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Chemical Society  

    We applied the extended RISM integral equation theory to investigate the local solvation behavior of a naphthalene solute in a supercritical carbon dioxide solvent. A ten-site model potential for naphthalene (by Sediawan, Gupta, and Mclaughlin) and a three-site potential for carbon dioxide (by Murthy, Singer, and McDonald) were used to elucidate local orientation of the carbon dioxide solvent molecules around the solute. Important physical effects of the quadrupole of carbon dioxide as well as molecular geometry of naphthalene are, therefore, taken into account. To gain insight into preferential orientation of carbon dioxide around a naphthalene molecule in a supercritical carbon dioxide solvent, we used a novel supermolecule approach by which potential of mean force surfaces of a carbon dioxide-naphthalene pair were calculated. Effects of molecular shape of solute and solute-solvent attractive interactions on solute partial molar volume were examined. © 1996 American Chemical Society.

    DOI: 10.1021/jp953347a

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  • Rearrangement dynamics of the hydrogen-bonded network of clathrate hydrates encaging polar guest 査読

    K Koga, H Tanaka

    JOURNAL OF CHEMICAL PHYSICS   104 ( 1 )   263 - 272   1996年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER INST PHYSICS  

    Mechanisms of rearrangement process in a defect-bearing solid hydrogen-bonded network system -a clathrate hydrate encaging polar guest-are examined in terms of the waiting time distribution functions for connectivity changes of water molecules. As previously found for the lifetime distributions of hydrogen bonds in supercooled water by Sciortino et al., a power-law-like behavior is observed for the waiting time distribution in short-time region in the clathrate hydrates; in long time region the distribution is close to an exponential form. It is revealed that the behavior is accounted for by assuming two kinds of processes of the connectivity changes: one is caused by thermal excitation in a perfect network region, which can be regarded as a Poisson process and another is induced by defects in the environmental network, which is responsible for the power-law-like behavior. Also the waiting time for the defect-induced connectivity changes of a molecule is related to how many other molecules are involved in local network rearrangement processes. By a simple model of single-axis rotator for reorientational jumps of a water molecule, we show that a power-law-like behavior appears in the dipole autocorrelation function when the waiting time for jump events is a strong power-law type distribution as observed in the defect-induced connectivity changes. (C) 1996 American Institute of Physics.

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  • Rearrangement of the hydrogen-bonded network of the clathrate hydrates encaging polar guest 査読

    Kenichiro Koga, Hideki Tanaka, Koichiro Nakanishi

    Molecular Simulation   16 ( 1-3 )   151 - 165   1996年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Taylor and Francis Inc.  

    Rearrangement process of the hydrogen-bonded network of clathrate hydrate of the polar guest ethylamine is examined by the molecular dynamics simulation. The hydrogen-bonded network rearrangements with reorientation of water or migration of water are observed in the 10 ns trajectories and analyzed in term of a representative connectivity pattern of a time zone longer than a time scale of vibrational motion of molecules. The most frequent rearrangement is the reorientation of single water molecule rotating 180° around its twofold axis in the network unlike Bjerrum's picture of molecular rotation in ice. Migration of water in the host lattice rarely occurs and very long time (several hundred pico second) is required to complete the rearrangement process since cooperative reorientation of many neighboring water is necessarily accompanied. The correlation of reorientational motion of water appears to decay not with the Debye type but rather with a power-law behavior.

    DOI: 10.1080/08927029608024069

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  • STABILITY OF POLAR GUEST-ENCAGING CLATHRATE HYDRATES 査読

    K KOGA, H TANAKA, K NAKANISHI

    JOURNAL OF CHEMICAL PHYSICS   101 ( 4 )   3127 - 3134   1994年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER INST PHYSICS  

    In order to account for the experimental observation that some amines form clathrate hydrates but that alcohols inhibit hydrate formation, we investigate the stability of clathrate hydrates which encage highly polar guest molecules by examining potential energy local minimum structures and also thermally excited structures. First, we examine the local minimum structure at which the total potential energy has a minimum value for amine and alcohol hydrates and inquire whether, in the absence of thermal fluctuations, the conditions for true-clathrate hydrates are satisfied. The local minimum structures of alcohol hydrates are distinguished from those of stable clathrate hydrates of structures I and II, while amine hydrates hold the host lattice of clathrate hydrates. We argue that the difference between the magnitude of the partial charge on the hydrogen atom of the hydroxyl and amino groups plays a much more significant role in the stability of both kinds of clathrate hydrates than the difference in molecular geometry does. Second, we examine kinetic stability by molecular dynamics simulation. Near room temperature the host lattice structure encaging amine remains intact. This is in contrast to the alcohol hydrate, which begins to melt easily with thermal excitation. Interesting is the finding that very long-time-scale (similar to 100 ps) fluctuations of the host potential energy is observed, as well as fast oscillations caused by temporary partial defects in the host network. Clathrate hydrates of polar guest molecules may experience an unstable state in which the host hydrogen-bonded network is partially broken and guest-host hydrogen-bond generation occurs but the stable state of their host lattices is again restored.

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  • On the stability of clathrate hydrates encaging polar guest molecules: Contrast in the hydrogen bonds of methylamine and methanol hydrates 査読

    Kenichiro Koga, Hideki Tanaka, Koichiro Nakanishi

    Molecular Simulation   12 ( 3-6 )   241 - 252   1994年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    The stability of clathrate hydrates encaging highly polar guests has been investigated in order to explain the experimental observation that some amines form clathrate hydrates but alcohols act as inhibitor to hydrate formation. We choose methylamine and methanol as guest species and examine the stable structure, at which the total potential energy has a minimum value. At the local minima of those two hydrates, the potential energies of water-water and guest-water, and their hydrogen bonded networks are compared. It is found that methanol does not retain the host lattice structure, while the host-network structure is kept in the presence of methylamine. It is shown that the difference in the magnitude of the partial charge on the hydrogen atom between the hydroxyl and amino groups plays a much more significant role on the stability of both clathrate hydrates than the difference in molecular geometry. This is supported from the result of a methylamine-like model that has the same partial charges on the atoms in the hydrophilic site as methanol. © 1994, Taylor &amp
    Francis Group, LLC. All rights reserved.

    DOI: 10.1080/08927029408023034

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書籍等出版物

  • 化学系の統計力学入門

    化学同人  2005年 

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  • Water, Steam, and Aqueous Solutions for Electric Power

    Maruzen  2005年 

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  • Encyclopedia of Nanoscience and Nanotechnology

    Marcel Dekker  2004年 

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  • H. Tanaka, *K. Koga, in V. Buch and J.P. Devlin

    Springer-Verlag  2003年 

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  • New Kinds of Phase Transitions: Transformations in Disordered Substances

    Kluwer,Dordrecht  2002年 

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  • Science and Technology of High-Pressure Research

    2000年 

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MISC

  • カーボンナノチューブの中に形成されるプラスチック氷相 招待 査読

    望月建爾, 甲賀研一郎

    アンサンブル   2021年1月

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    掲載種別:記事・総説・解説・論説等(その他)  

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  • Tribute to Benjamin Widom 査読

    Roger F. Loring, Kenichiro Koga, Dor Ben-Amotz

    JOURNAL OF PHYSICAL CHEMISTRY B   122 ( 13 )   3203 - 3205   2018年4月

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    記述言語:英語   出版者・発行元:AMER CHEMICAL SOC  

    DOI: 10.1021/acs.jpcb.8b00129

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  • Preface

    Kenichiro Koga, Ryo Akiyama

    JOURNAL OF MOLECULAR LIQUIDS   200   1 - 1   2014年12月

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    記述言語:英語   出版者・発行元:ELSEVIER SCIENCE BV  

    DOI: 10.1016/j.molliq.2014.11.026

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  • 定積・定圧・任意条件下における溶媒和熱力学量とその温度依存性

    甲賀研一郎

    アンサンブル   15 ( 2 )   92 - 96   2013年

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    記述言語:日本語   出版者・発行元:分子シミュレーション研究会  

    <tt>定積,定圧,または任意の微小体積変化をともなう溶媒和過程のいずれを選択するかにより,溶媒和自由エネルギーは変化しないが,溶媒和エンタルピーおよび溶媒和エントロピーは変化することを振りかえり,低温における疎水性水和に特徴的な負の溶媒和エンタルピーと正の溶媒和自由エネルギーが実現する条件について考察する.また,任意の溶媒和過程に対する溶媒和エンタルピーまたはエントロピーは,溶媒和自由エネルギーの温度微分</tt>—<tt>定圧,定積条件とは異なる温度微分</tt>—<tt>に関係づけられることを示す</tt><tt>. </tt>

    DOI: 10.11436/mssj.15.92

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  • Preface

    Kenichiro Koga, Ryo Akiyama

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   81   2012年9月

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    記述言語:英語   出版者・発行元:PHYSICAL SOC JAPAN  

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  • Wetting transitions at fluid interfaces and related topics

    Kenichiro Koga

    Review of Polarography   56 ( 1 )   3 - 10   2010年

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    記述言語:英語   出版者・発行元:日本ポーラログラフ学会  

    Wetting transitions, in which a fluid wets the interface between two fluid phases, are defined. There may be first, second, and higher order transitions. The line tension, an excess free energy per unit length of a three-phase contact line, and the boundary tension, another excess free energy associated with the boundary of two surface phases at the prewetting transition, are also defined. Mean-field density functional models of the fluid interfaces are introduced and their results on the wetting transitions, the line and boundary tensions, and the fluid structures in the three-phase contact region are presented.

    DOI: 10.5189/revpolarography.56.3

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  • How soft interfaces get wet

    甲賀研一郎

    物性研究   93 ( 3 )   301 - 301   2009年

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    記述言語:日本語   出版者・発行元:物性研究刊行会  

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    その他リンク: http://dl.ndl.go.jp/info:ndljp/pid/10942694

  • Discovery of two-dimensional ice - Peculiar structure predicted by computer simulation

    K. Koga, X.C. Zeng, H. Tanaka

    Chemistry   1999年

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講演・口頭発表等

  • Two Aspects of the Osmotic Second Virial Coefficient 招待

    Kenichiro Koga

    EMLG/JMLG  2022年9月16日  EMLG/JMLG

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    開催年月日: 2022年9月12日 - 2022年9月16日

    記述言語:英語   会議種別:口頭発表(基調)  

    開催地:The University of Barcelona   国名:スペイン  

  • 統計力学 招待

    甲賀研一郎

    第16回分子シミュレーションスクール  2022年9月5日 

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    開催年月日: 2022年9月5日 - 2022年9月7日

    会議種別:公開講演,セミナー,チュートリアル,講習,講義等  

    開催地:愛知県岡崎市   国名:日本国  

  • “Osmotic second virial coefficients for hydrophobic interactions: Ion-specific effects and solutesize dependences”

    Kenichiro Koga

    Hydrophobicity: From Theory, to Simulation, to Experiment  2022年6月29日  Telluride Intermediate School

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    開催年月日: 2022年6月26日 - 2022年6月30日

    記述言語:英語   会議種別:口頭発表(一般)  

    開催地:725 West Colorado Ave Telluride, CO 81435   国名:アメリカ合衆国  

  • Ion-Specific Effects on Hydrophobicity: Salting-out effect and Salt-Enhanced Association 招待

    Kenichiro Koga

    Physical Chemistry Seminar, Department of Chemistry, Purdue University  2021年11月10日 

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    記述言語:英語   会議種別:公開講演,セミナー,チュートリアル,講習,講義等  

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  • Ion-Specific Effects on Hydrophobicity: Salting-out effect and Salt-Enhanced Association 招待

    Kenichiro Koga

    Theory Group Seminar, Department of Chemistry, Baker Laboratory, Cornell University  2021年11月6日 

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    記述言語:英語   会議種別:公開講演,セミナー,チュートリアル,講習,講義等  

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  • 統計力学 招待

    甲賀研一郎

    第15回分子シミュレーションスクール  2021年9月7日 

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    記述言語:日本語   会議種別:公開講演,セミナー,チュートリアル,講習,講義等  

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  • Ion-size dependences of the salting-out effect 招待

    Kenichiro Koga

    IUPAC CCCE 2021  2021年8月13日 

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    記述言語:英語   会議種別:口頭発表(招待・特別)  

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  • 疎水効果の分子論,集中講義 招待

    甲賀研一郎

    2020年11月11日 

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    会議種別:公開講演,セミナー,チュートリアル,講習,講義等  

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  • 疎水性相互作用の温度・圧力・塩濃度依存性 招待

    甲賀研一郎

    奈良女子大学理学部化学科講演会  2020年11月10日 

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    会議種別:公開講演,セミナー,チュートリアル,講習,講義等  

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  • 細孔内準一次元物質のポリモルフィズム: 最密充填vs.水素結合 招待

    甲賀 研一郎

    日本物理学会  2020年9月9日 

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    会議種別:口頭発表(招待・特別)  

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  • 統計力学 招待

    甲賀研一郎

    第14回分子シミュレーションスクール  2020年9月4日 

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    会議種別:公開講演,セミナー,チュートリアル,講習,講義等  

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  • 疎水性相互作用の温度応答・イオン種依存性 招待

    甲賀 研一郎

    第9回 次世代の物質科学・ナノサイエンスを探る  2020年1月10日 

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    会議種別:シンポジウム・ワークショップ パネル(指名)  

    開催地:北海道大学北キャンパス地区シオノギ棟  

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  • Is the wetting transition inevitable near the critical endpoint and tricritical point? 国際共著 国際会議

    Kenichiro Koga

    2019年11月6日 

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    会議種別:口頭発表(一般)  

    開催地:沖縄  

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  • 密度汎関数理論による臨界終点および三重臨界点近傍の濡れ転移相図

    甲賀研一郎

    日本物理学会2019年秋季大会  2019年9月10日 

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    会議種別:口頭発表(一般)  

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  • 統計力学 招待

    甲賀研一郎

    第13回分子シミュレーションスクール  2019年9月2日 

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    会議種別:公開講演,セミナー,チュートリアル,講習,講義等  

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  • Interfacial tensions near the critical endpoints and the tricritical point of three-phase equilibria 招待

    Kenichiro Koga

    Research Seminar at St. Petersburg State University  2019年7月9日 

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    会議種別:公開講演,セミナー,チュートリアル,講習,講義等  

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  • Interfacial tensions near the critical endpoints; the tricritical point of three-phase equilibria;Mean-field density-functional model 招待

    2018年12月11日 

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    会議種別:口頭発表(招待・特別)  

    開催地:九州大学西新プラザ  

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  • SOLVATION OF HYDROPHOBIC PARTICLES IN AQUEOUS SOLUTION AND SOLVENT-MEDIATED INTERACTIONS VARYING WITH TEMPERATURE, PRESSURE, AND SALT CONCENTRATION 招待

    Kenichiro Koga

    5th International Conference on Colloid Chemistry and Physicochemical Mechanics  2018年9月12日 

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    会議種別:口頭発表(基調)  

    開催地:St. Petersburg  

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  • 統計力学 招待

    甲賀研一郎

    2018年9月4日 

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    会議種別:公開講演,セミナー,チュートリアル,講習,講義等  

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  • 疎水効果に対するイオン添加効果

    甲賀 研一郎

    液体の化学夏の学校  2018年8月30日 

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    会議種別:公開講演,セミナー,チュートリアル,講習,講義等  

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  • Hydrophobicity varying with T, P, and salt concentration 招待

    Kenichiro Koga

    Theory Group Seminar, Department of Chemistry, Baker Laboratory, Cornell University  2018年8月1日 

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  • Hydrophobicity (solubility and hydrophobic interaction) varying with T, P, & salt concentration 招待

    Kenichiro Koga

    TSRC Workshop, Hydrophobicity: From Theory, to Simulation, to Experiment, Telluride  2018年7月17日 

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    会議種別:口頭発表(招待・特別)  

    開催地:Telluride, CO, USA  

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  • 疎水性相互作用の温度,圧力,塩濃度依存 性

    甲賀 研一郎

    分子シミュレーション討論会  2017年11月30日 

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    会議種別:口頭発表(一般)  

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  • 統計力学

    第11回分子シミュレーションスクール  2017年 

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  • 疎水性相互作用における平均場近似の妥当性

    日本物理学会2017年秋季大会  2017年 

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  • 多様な環境下における疎水効果

    日本物理学会 第72回年次大会  2017年 

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  • 疎水性高分子鎖のコイル-グロビュール転移における水の役割

    日本物理学会第72回年次大会  2017年 

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  • 多様な環境下における疎水効果

    日本化学会第97回春季年会・特別企画講演 極限状態が織りなす新しい化学の世界  2017年 

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  • Coil-globule transformation of a simple polymer chain in water

    The 4th International Conference on Molecular Simulation  2016年 

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  • The solubility of nonpolar solutes at the liquid-vapor interface of water: A simple mean-field description

    Workshop "Hydrophobicity: From Theory, Simulation, to Experiment"  2016年 

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  • 統計力学

    第10回分子シミュレーションスクールー基礎から応用までー  2016年 

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  • DFT models of interfaces near critical endpoints and tricritical points: some preliminary results

    Density Functional Days in Tuebingen  2015年 

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  • Solid-liquid critical behavior of water in hydrophobic nanotubes

    The 2015 International Chemical Congress of Pacific Basin Societies  2015年 

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  • 疎水性相互作用に対する温度、圧力、塩濃度効果の定量化

    日本物理学会 第70回年次大会  2015年 

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  • 浸透第2ビリアル係数でみる疎水性相互作用

    日本物理学会2013年秋季大会  2013年 

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  • 疎水性相互作用の温度依存性について

    第3回ソフトマター研究会  2013年 

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  • The osmotic second virial coefficient of hydrophobic solutes in aqueous solution

    The 7th Mini-Symposium on Liquids  2013年 

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  • 濡れ転移の多様性について

    公益社団法人 新化学技術推進協会 先端科学・材料技術部会 コンピュータケミストリ分科会  2012年 

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  • Methane in cold and hot water

    The Third Symposium in Commemoration of International Exchange Agreement between Faculty of Science, University of Copenhagen, Denmark and Graduate School of Natural Science and Technology, OKayama Un  2012年 

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  • Hydrophobes in Water/Water in Nanotubes

    Seminar  2012年 

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  • Lennard-Jones溶液における疎水効果

    次世代ナノ統合シュミレーションソフトウェアの研究開発 第5回公開シンポジウム  2011年 

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  • 疎水性水和の微視的描像のアンサンブル依存性

    第63回コロイドおよび界面化学討論会  2011年 

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  • 疎水性水和の熱力学

    2011年日本化学会西日本大会  2011年 

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  • 疎水効果と4℃の水の関係

    第34回溶液化学シンポジウム  2011年 

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  • A general view on Hydrophobicity

    7th Congress of the International Society for Theoretical Chemical Physics  2011年 

     詳細を見る

  • A general view on solvation of apolar solutes in water and in simple liquids

    5th Mini-Symposium on Liquids  2011年 

     詳細を見る

  • General View on Solvation in Water and Simple Liquids

    第1回ソフトマター研究会  2011年 

     詳細を見る

  • 疎水性水和と溶媒誘起相互作用

    第5回領域研究会  2011年 

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  • 疎水性水和の特殊性と一般性

    理研シンポジウム 動的水和構造と分子過程Ⅳ  2010年 

     詳細を見る

  • Introduction to homogeneous and inhomogeneous liquids

    IITS Course in Theoretical Physics for PhD Students  2010年 

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  • 定圧過程と定積過程でみた疎水効果(疎溶媒効果)

    日本物理学会  2010年 

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  • 定圧及び定積条件でみた溶媒和熱力学量とその意味、新学術領域研究 水を主役としたATPエネルギー変換

    共溶媒効果に関する勉強会  2010年 

     詳細を見る

  • Introduction to homogeneous and inhomogeneous liquids

    IITS Course in Theoretical Physics for PhD Students  2010年 

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  • 均一・不均一液体の統計力学

    集中講義  2010年 

     詳細を見る

  • 疎水性水和と溶媒誘起相互作用

    第163回化学コロキウム  2010年 

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  • 界面における相転移

    「実在系の分子理論」成果とりまとめシンポジウム  2010年 

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  • First, second, and higher-order wetting transitions

    International Symposium on Non-equilibrium Soft Matter 2010  2010年 

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  • First, second, and higer-order wetting transitions at fluid interfaces

    11th Granada* Seminar on Computational and Statistical Physics  2010年 

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  • Wetting transitions and the line tension

    The 4th LSW Symposium on Soft & Wet Matter  2009年 

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  • ソフトな界面における連続濡れ転移

    日本物理学会  2009年 

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  • 疎水環境下における水の構造・相転移・ダイナミクス

    実在系の分子理論 研究交流会  2009年 

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  • ソフトな界面における連続濡れ転移

    日本物理学会  2009年 

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  • Hydrophobic Solutes in Water and Water in Hydrophobic Pores

    International Symposium between Shaanxi Normal University and Okayama University  2009年 

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  • 濡れ転移の密度汎関数モデルから何がわかるか

    九州大学物性基礎論コロキウム  2009年 

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  • ナノ細孔内液体の構造・吸着・固液相転移

    ナノ統合拠点分子科学WG物性科学WG共同連続研究会  2009年 

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  • 連続濡れ転移の特長と条件

    特定領域研究 ソフトマター物理 第4回領域研究会  2009年 

     詳細を見る

  • An infinite-order wetting transition

    100th Statistical Mechanics Meeting  2008年 

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  • カーボンナノチューブ内の水およびアルゴンの構造と固液相転移

    第1回界面科学研究会  2008年 

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  • 制約空間および界面における凝集相構造と相転移

    日本物理学会  2008年 

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  • Line tension on approach to wetting transitions

    Max Planck Institute for Metals Research Ringberg meeting Soft Matter at Interfaces 2008  2008年 

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  • 液体の統計力学理論と分子シミュレーションの基礎と応用

    分子科学若手の会夏の学校  2008年 

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  • 濡れ転移近傍における線張力

    第61回コロイドおよび界面化学討論会  2008年 

     詳細を見る

  • 不均一系の相転移

    液体化学夏の学校  2007年 

     詳細を見る

  • Phase Transitions of Confined Water and Simple Liquids

    Institute of Theoretical Physics Seminar  2007年 

     詳細を見る

  • 濡れ転移近傍における密度分布と線張力

    分子シミュレーション討論会  2007年 

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  • 不均一系の分子シミュレーション

    第3回分子・物質シミュレーション中核拠点形成事業人材育成講座「分子シミュレーションスクール -基礎から応用まで-」  2007年 

     詳細を見る

  • 物理・化学のおもしろさ:身近なことから最先端の話題まで

    進路啓発セミナー  2007年 

     詳細を見る

  • カーボンナノチューブ内の水および単純液体の相転移

    フラーレン・ナノチューブ総合シンポジウム  2007年 

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  • 界面における水の構造と相転移

    第9回花王研究奨励賞受賞記念講演  2007年 

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  • ナノチューブ内の単純液体の構造,相転移,相図

    日本物理学会第61回年次大会, シンポジウム『分子シミュレーションと熱力学量』  2006年 

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  • Line tension and adsorption equation for the three-phase contact

    日本物理学会秋期大会  2006年 

     詳細を見る

  • 三相接触線における線張力と吸着式

    第20回分子シミュレーション討論会  2006年 

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  • ナノチューブ中の水、水溶液:制約空間がもたらす新規な物性と相転移ダイナミクス

    次世代ナノ生体物質・次世代エネルギー合同グループ会議  2006年 

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  • 細孔内の水とアルゴンの相転移と相図

    自然科学研究機構計算科学研究センター スーパーコンピューターワークショップ 超高速シミュレーターが切り開く分子科学の諸相:若手研究者による理論・方法論展開とその展望  2006年 

     詳細を見る

  • ナノ細孔内の単純液体および水の相転移

    分子シミュレーション夏の学校  2005年 

     詳細を見る

  • 学術賞受賞講演

    第19回分子シミュレーション討論会  2005年 

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  • 疎水性細孔内の水の相転移 Phase transitions of water confined in hydrophobic pores

    第58回コロイドおよび界面化学討論会  2005年 

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  • 疎水効果の熱力学と微視的模型

    立命館大学理工学研究所プロジェクト研究シンポジウム  2004年 

     詳細を見る

  • Phase transitions in confined water

    14th International Conference on the Properties of Water and Steam (14th ICPWS)  2004年 

     詳細を見る

  • 疎水効果と水の相転移

    理論化学シンポジウム  2004年 

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  • Phase transitions of quasi-one-dimensional and quasi-two-dimensional water

    International Symposium on "Physicochemistry of Water and Dynamics of Materials and the Earth --- Structures and Behaviors of the Thin Film Water ---",  2003年 

     詳細を見る

  • Correlation between Hydrophobic Attraction and the Free Energy Hydration

    第25回溶液化学シンポジウム  2002年 

     詳細を見る

  • Model of freezing in one-dimensional liquids

    88th Statistical Mechanics Conference: A Celebration of Elliot Lieb's 70th Birthday.  2002年 

     詳細を見る

  • How does water freeze inside carbon nanotubes?

    Horizons in Complex Systems,  2001年 

     詳細を見る

  • Phase equilibria and phase transitions of water confined in Q2D and Q1D geometries

    NATO Advanced Research Workshop ---New kinds of phase transition---,  2001年 

     詳細を見る

  • Phase behavior of water confined in a slit nanopore

    AIRAPT-17  1999年 

     詳細を見る

▼全件表示

受賞

  • 花王研究奨励賞

    2007年  

     詳細を見る

    受賞国:日本国

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  • 日本物理学会若手奨励賞

    2007年  

     詳細を見る

    受賞国:日本国

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  • 分子シミュレーション研究会学術賞

    2005年  

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    受賞国:日本国

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共同研究・競争的資金等の研究

  • ミセル形成駆動力の解明に基づく界面活性剤凝集挙動の理論モデリング

    研究課題/領域番号:20H02696  2020年04月 - 2024年03月

    日本学術振興会  科学研究費助成事業 基盤研究(B)  基盤研究(B)

    甲賀 研一郎

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    配分額:15990000円 ( 直接経費:12300000円 、 間接経費:3690000円 )

    ミセル形成要因のひとつである疎水基同士の会合に関する理論研究を進展させた.疎水基部位の会合は,界面活性剤分子の疎水基部位集団が水溶液からの「分離」して,ミセルの核を形成することを意味する.この現象の駆動力は,疎水性分子の水への低溶解度(疎水効果)と疎水性分子間の引力相互作用(疎水性相互作用)の2つの効果である.本年度は,疎水性溶質の溶解度および疎水性相互作用に関する理論的研究を引き続き行った.まず第一に,水溶液中の疎水性溶質の溶媒和自由エネルギーに対するイオン添加効果の一般的性質の起源を解明した.イオンサイズが小さいほど,そのイオンによる塩析効果が大きい.前年度の研究では,イオン周囲の水和水の構造が塩析効果を決めていることまでを明らかにした.本年度は,イオン添加効果を定量化するために用いられるセチェノフ係数を与える一般式を導出し,それに基づく解析を行った.この式は溶質-水間相関関数積分と溶質-イオン間相関関数積分との差によって,セチェノフ係数が与えられるという簡単な式であるが,応用範囲は広く,今後実験結果の解釈に役立つものであると期待できる.また,イオン添加効果に限らず,様々な化学成分の添加効果も基本的に同じ式を用いて調べることができる.塩析効果に関して,特にカチオンサイズ依存性に注目した研究では,リチウムイオンの例外的挙動の解明に取り組んだ.分子動力学シミュレーションおよび自由エネルギー計算を実行し,水和殻の構造を解析し,リチウムイオンの場合,水分子の配向が強く制限されることにより,水和水の低密度化が起こることを突き止めた.この結果はJ. Phys. Chem. Bに発表された.また,複数のアルコール分子種について溶媒和自由エネルギーを効率よく計算する手法の開発を進めた.その結果,平均場近似を部分的に用いることにより高効率計算ができることがわかった.

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  • 多彩な環境応答を示す溶媒誘起力の解明:理論・実験融合研究

    研究課題/領域番号:18KK0151  2018年10月 - 2023年03月

    日本学術振興会  科学研究費助成事業 国際共同研究加速基金(国際共同研究強化(B))  国際共同研究加速基金(国際共同研究強化(B))

    甲賀 研一郎, 岡本 隆一, 墨 智成, 望月 建爾

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    配分額:17940000円 ( 直接経費:13800000円 、 間接経費:4140000円 )

    (1)水溶液中の疎水性溶質の溶媒和自由エネルギーに対するイオン添加効果について予備的な分子動力学シミュレーション計算を進め,今後の研究方針を固めた.第一に,電荷をもったイオンの場合,既存の分子間相互作用ポテンシャルがイオン水溶液の基礎物性をうまく再現することができないという点を確認し,ポテンシャルを改良することを試みた.その結果,イオンの電荷qをそのまま用いるのではなく,分子分極を考慮するために,一定の割合でqを減少させることにより,総合的に基礎物性がうまく再現できることを確認した.今後,このイオンポテンシャルを用いて,疎水性分子の溶媒和エネルギーに対するイオンの添加効果をイオン種毎に明らかにし,その機構を解明する.
    (2)溶質溶解度に対するイオン添加効果を定量化するために用いられるセチェノフ係数を溶液構造を反映する相関関数積分によって与える式を導出した.この式を用いて,イオン種および溶質分子種に依存するセチェノフ係数の大小を溶液の微視的構造から理解することができる.
    (3)シニョリンとよばれる人工タンパク質の水溶液中における安定性の機構を液体の密度汎関数理論とシミュレーションを組み合わせた方法により,解明した.その結果,従来の定説とは反対に,溶媒誘起力はタンパク質の折り畳み構造を不安定化させる方向に寄与しており,タンパク質分子内直接相互作用が折り畳みにおける駆動力であることが定量的に示された.この結果は,疎水効果が折りたたみ構造を安定化させるという既存仮説を否定するが,一般的なタンパク質に関する熱測定や部位突然変異体導入に基づく結論と一致する.

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  • ナノ細孔内の相転移:カーボンナノチューブ内の水の固液臨界現象を中心に

    研究課題/領域番号:15H05474  2015年04月 - 2019年03月

    日本学術振興会  科学研究費助成事業 若手研究(A)  若手研究(A)

    望月 建爾, 甲賀 研一郎, 墨 智成, 松本 正和

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    配分額:24180000円 ( 直接経費:18600000円 、 間接経費:5580000円 )

    分子シミュレーションや理論化学的手法を用いて、狭い空間や溶質が水の相転移に与える影響を分子レベルで解明した。カーボンナノチューブの内部の水が、バルクの水では見られない固液臨界点を持つ可能性や、タンパク質間に挟まれた水が常温でも氷構造を形成することを示した。さらに、高分子の構造変化に対する水和水の影響、逆に高分子やタンパク質が水の結晶化に与える影響を調べた。研究成果を複数の国際誌へ発表した。

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  • 多様な環境下における疎水効果の理論的研究

    研究課題/領域番号:26287099  2014年04月 - 2018年03月

    日本学術振興会  科学研究費助成事業 基盤研究(B)  基盤研究(B)

    甲賀 研一郎

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    配分額:15730000円 ( 直接経費:12100000円 、 間接経費:3630000円 )

    多様な環境下における疎水性溶質の溶媒和自由エネルギーおよび疎水性相互作用に関する理論研究を行った.小さな疎水性分子については溶媒和自由エネルギーμおよび疎水性相互作用に対して平均場近似が極めて有効であることがわかった.それにより,温度・圧力・塩に対する溶解度および疎水性相互作用の依存性を説明することができた.さらに,メタン水溶液の温度・圧力・塩濃度変化について,μと疎水性相互作用の強さとの間に線形相関が存在することを見出した.平均場近似を不均一系に拡張し,気液界面における疎水性分子のμの温度および局所密度依存性が統一的に理解できることを示した.モデル疎水性高分子の高温収縮機構を明らかにした.

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  • 界面相転移および不均一溶液の理論

    研究課題/領域番号:23340122  2011年04月 - 2015年03月

    日本学術振興会  科学研究費助成事業 基盤研究(B)  基盤研究(B)

    甲賀 研一郎

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    配分額:15730000円 ( 直接経費:12100000円 、 間接経費:3630000円 )

    界面の濡れは一般に広く知られているが、その多様性の起源については未解明であった。私たちは解析的に計算が可能なモデルを構築し、どのような条件でどのような濡れ転移が起こるのかを明らかにした。さらに濡れ転移に対する熱ゆらぎの影響についても検討した。
    水溶液中のメタン分子間に働く疎水性相互作用を精密に計算することに成功し、メタン分子の浸透第二ビリアル係数をこれまでにない精度で求めた。低温ではメタン分子間相互作用はむしろ斥力的であり、温度の上昇とともに引力的相互作用に変化することを明らかにした。その他、水の気液界面付近における疎水性分子の溶解度とその温度依存性を計算することに成功した。

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  • ナノ細孔とイオンチャンネル中のイオンの水和と輸送現象

    研究課題/領域番号:21245006  2009年 - 2012年

    日本学術振興会  科学研究費助成事業 基盤研究(A)  基盤研究(A)

    田中 秀樹, 松本 正和, 甲賀 研一郎, 大久保 貴広, 大峯 巌

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    配分額:27170000円 ( 直接経費:20900000円 、 間接経費:6270000円 )

    ナノサイズの空間における構造と相挙動および物質移動、イオンチャンネルを想定した水と種々のイオンのダイナミクスの微視的描像を確立することを目的とする。これまでに見出された制約空間における水の特異な相挙動に加えて、異方かつ限定的な水和によるイオンの輸送過程を、計算機シミュレーションにより明らかにした。ナノ細孔としてクラスレートハイドレートと呼ばれる水が作る籠状の空間について、ゲスト分子の種類により結晶構造はどのような影響を受けるかどうかについて詳細に調べた。

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  • ソフトな界面における一次および連続濡れ転移の発現機構の解明

    研究課題/領域番号:21015021  2009年 - 2010年

    日本学術振興会  科学研究費助成事業 特定領域研究  特定領域研究

    甲賀 研一郎

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    配分額:3700000円 ( 直接経費:3700000円 )

    1.前年度比引き続き,濡れ転移の微視的モデルである平均場密度汎関数モデルの研究を行った。これに対応する現実系は,炭化水素(液体)とその蒸気の界面をエタノール(液体)が濡らすようなものである。このような系は,炭化水素の種類により,一次濡れ転移と連続濡れ転移を示すが,濡れ転移の挙動を左右する因子については明らかになっていない。われわれのモデルは二つの熱力学的変数a,bをもつ。bは温度,圧力,化学ポテンシャル,あるいはそれらの組み合わせに対応する熱力学的場の変数であり,aは成分1と成分2の密度ブロファイルの減衰長の比ξ1/ξ2を表す。この2変数平面上における濡れ転移の相図を明らかにした。変数aが濡れ転移の次数を決定する因子であることを示し,さらにこれまで提案されてきた固体表面の濡れ転移,超伝導の濡れ転移,磁性体の濡れ転移のモデルとの関連を明らかにすることができた。
    2.濡れ転移が連続転移として起こるための必要条件を解析的に示すことに成功した。これは上のモデルに関して,2成分密度ρ1,ρ2平面における濡れ軌道と非濡れ軌道を界面を濡らす相の密度近傍で解析的に求め,それらが連続転移で相似であることを要求することにより得られた結果である。他の濡れ転移のモデル系および現実系における濡れ挙動を判定するための必要条件へと拡張することが今後の課題である。

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  • 細孔内及び界面における不均一液体・溶液の相転移現象の解明

    研究課題/領域番号:20550017  2008年 - 2010年

    日本学術振興会  科学研究費助成事業 基盤研究(C)  基盤研究(C)

    甲賀 研一郎

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    配分額:4940000円 ( 直接経費:3800000円 、 間接経費:1140000円 )

    分子数個分という非常に狭い空間に閉じ込められた液体,あるいは水と油の流体界面等における液体は,普通の液体にはみられないユニークな相転移あるいは非常に複雑な相挙動を示す。わたしたちは,統計力学理論およびコンピューター・シミュレーションにより,この興味深い液体が示す現象をできる限り系統的に分析し,そこに潜む法則を明らかにすることを目指した研究を行い,一定の成果を上げることができた。具体的には界面における相転移のひとつである「濡れ転移」および細孔のなかの準一次元・二次元流体の相挙動に関するいくつかの成果を上げた。

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  • 低次元系凝集系の揺らぎと化学反応

    研究課題/領域番号:18066011  2006年 - 2009年

    日本学術振興会  科学研究費助成事業 特定領域研究  特定領域研究

    田中 秀樹, 甲賀 研一郎, 三島 修

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    配分額:41300000円 ( 直接経費:41300000円 )

    統計力学理論と分子シミュレーションを用いて、制約空間における相転移と相平衡についての研究を行った。特に、新規なシミュレーション方法の開発と理論の展開により、準一次元系における水の構造と相挙動について、また水素を吸蔵するクラスレートハイドレートと氷の水素占有率と熱力学的安定性の関係について詳細に検討した。

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  • 疎水環境下の水の相転移および疎水効果

    研究課題/領域番号:16685003  2004年 - 2006年

    日本学術振興会  科学研究費助成事業 若手研究(A)  若手研究(A)

    甲賀 研一郎

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    配分額:23270000円 ( 直接経費:17900000円 、 間接経費:5370000円 )

    球形粒子系の最密充填構造と相図
    分子動力学シミュレーション及びエネルギー極小構造解析によって,不均一系の多様な相挙動は水などの複雑液体に限らず,円筒状細孔内に拘束されたアルゴンのような単純液体にも見られることを明らかにした.まず,シミュレーションによって得られた構造の解析から,最密充填構造は三角格子の折り畳み方法と一対一対応するという予想を提案した.これより,なぜ直径の変化とともに,様々な準一次元結晶構造および螺旋構造が現れるのかが説明できた.第二に,長距離相互作用が存在しなくても準一次元系において一次相転移的相変化が観測されることを示した.さらに圧力-直径面および温度-直径面の相図が正確に得られ,低温では鋭い相境界が存在し,高温になると境界が明確でなくなることを明らかにした.
    カーボンナノチューブ内の水の構造と相挙動
    カーボンナノチューブ内の水の分子動力学シミュレーション及びエネルギー極小構造解析によって,これまでに報告のない準一次元氷の構造を複数見いだし,それらの固相が係わる水の相転移挙動を明らかにした.今回見つかった氷の構造は外層を形成する水とその内部に存在する水からなり,Filled ice nanotubeと呼ぶことができる.外層の構造を開いて二次元にすると四角格子となる点は,以前我々が見いだしたN角形型アイスナノチューブと同様であるが,四角格子の巻き方により,結晶構造と螺旋構造が合われることが明らかになった.外層の内側に存在する水は秩序構造をとらないことが明らかになった.
    線張力の理論
    疎水性表面の濡れ現象と密接に関連する線張力の理論的計算を行った.ギブズの表面吸着式を単純に線吸着に拡張した式では,線張力が接触線の位置の選び方に依存するという結果をもたらす.この最近明らかになった事実を説明するために,新しい線吸着の式を導いた.また平均場密度汎関数モデルの数値計算を行い,以上の事実と新しい吸着式の妥当性を確認することができた.

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  • ナノスケール空間内の水の相挙動と相転移ダイナミクス

    研究課題/領域番号:12740322  2000年 - 2001年

    日本学術振興会  科学研究費助成事業 奨励研究(A)  奨励研究(A)

    甲賀 研一郎

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    配分額:1500000円 ( 直接経費:1500000円 )

    準2次元(スリット状)および準1次元(カーボンナノチューブ状)細孔内に拘束された水の相挙動を分子シミュレーションに基づき調べた。細孔壁が疎水性であり、かつ細孔サイズがナノメートル程度である場合、無秩序な水素結合ネットワークをもつ液相と完全な水素結合ネットワークをもつ固相との間の特異な相転移と相平衡が存在する可能性が明らかになった。
    1 スリット状細孔内(疎水壁間)の水
    壁に平行な方向の圧力テンソルを一定に保ち温度を徐々に下げていくと、液体からほぼ完全な水素結合をもつアモルファス相への相変化が起こることを示した。アモルファス相は二層の水分子からなり、層内では水素結合の6員環のみならず5,7,および8員環が存在し、先に見つかった6員環のみからなる準2次元の結晶相とは異なる構造をもつ。しかし水素結合ネットワークにはほとんど欠陥がなく、拡散係数もバルクの氷と同じ程度であることがわかった。この結果は水のPolyamorphic転移(無秩序相間の相転移)の存在を初めて明確に示したものである。
    2 カーボンナノチューブ内部の水
    カーボンナノチューブ内部に水を閉じこめ高温相(液体状態)から徐々に温度を下げていくと、低温相は4員環が一次元方向につながった氷(四角アイスナノチューブ)であることがわかった。カーボンナノチューブの直径を調整することにより、低温相が五角形、六角形、および七角形のアイスナノチューブとして存在することも明らかになった。液相と六角および七角形のアイスナノチューブ相の間の相転移は鋭い一次相転移であるが、液相と四角および五角形のアイスナノチューブ相のと間の相変化は温度および圧力を変化させる経路により連続的である場合と一次相転移である場合があることがわかった。等温曲線、自由エネルギー、および構造秩序変数の計算結果から、準1次元系において固液平衡の臨界点が存在する可能性を示した。

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