Updated on 2021/12/27

写真a

 
KOGA Kenichiro
 
Organization
Research Institute for Interdisciplinary Science Professor
Position
Professor
External link

Degree

  • 博士 ( 京都大学 )

Research Interests

  • 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

  • Hydrophobic effect

Research Areas

  • Nanotechnology/Materials / Fundamental physical chemistry

  • Natural Science / Biophysics, chemical physics and soft matter physics

Education

  • Kyoto University   工学研究科   高分子化学

    - 1996

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    Country: Japan

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  • Kyoto University    

    - 1996

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  • Kyoto University   工学研究科   分子工学専攻

    - 1993

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    Country: Japan

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  • Kyoto University    

    - 1993

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  • Osaka University   基礎工学部   化学工学科

    - 1991

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    Country: Japan

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  • Osaka University    

    - 1991

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Research History

  • ルーヴェン・カトリック大学 客員教授

    2010

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  • Visiting Professor,KU Leuven

    2010

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  • Okayama University   The Graduate School of Natural Science and Technology   Professor

    2009.6 - 2016.3

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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

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  • Cornell University   Department of Chemistry   Visiting Scientist

    2001.10 - 2003.6

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    Country:United States

    Notes:JSPS Fellowship for Research Abroad

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Professional Memberships

Committee Memberships

  • 分子シミュレーション学会   幹事  

    2020.1   

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    Committee type:Academic society

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  • 分子シミュレーション研究会   幹事  

    2006 - 2008   

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    Committee type:Academic society

    分子シミュレーション研究会

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Papers

  • Hydrophobicity Varying with Temperature, Pressure, and Salt Concentration Invited Reviewed

    K. Koga, N. Yamamoto

    Journal of Physical Chemistry B   122 ( 13 )   3655 - 3665   2018.4

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    Authorship:Lead author, Corresponding author   Language:English   Publishing type:Research paper (scientific journal)   Publisher: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 Reviewed

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

    The Journal of Physical Chemistry B   120 ( 47 )   12127 - 12134   2016.12

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    Authorship:Last author, Corresponding author   Publishing type:Research paper (scientific journal)   Publisher: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 Invited Reviewed

    K Koga, B Widom

    Journal of Physics: Condensed Matter   28 ( 24 )   244016 - 244016   2016.6

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    Authorship:Lead author, Corresponding author   Publishing type:Research paper (scientific journal)   Publisher:IOP Publishing  

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

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    Other Link: 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 Invited Reviewed

    Kiharu Abe, Tomonari Sumi, Kenichiro Koga

    The Journal of Physical Chemistry B   120 ( 8 )   2012 - 2019   2016.3

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    Authorship:Last author, Corresponding author   Publishing type:Research paper (scientific journal)   Publisher: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 Reviewed

    K. Koga, Vincent Holten, B. Widom

    The Journal of Physical Chemistry B   119 ( 42 )   13391 - 13397   2015.10

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    Authorship:Lead author, Corresponding author   Publishing type:Research paper (scientific journal)   Publisher:American Chemical Society (ACS)  

    DOI: 10.1021/acs.jpcb.5b07685

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  • Solid−liquid critical behavior of water in nanopores Reviewed

    Kenji Mochizuki, Kenichiro Koga

    Proceedings of the National Academy of Sciences   112 ( 27 )   8221 - 8226   2015.7

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    Authorship:Last author, Corresponding author   Publishing type:Research paper (scientific journal)   Publisher:Proceedings of the 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 of nanoconfined 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 are determined 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 the presence 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 changes occur 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 Reviewed

    K. Koga

    The Journal of Physical Chemistry B   117 ( 41 )   12619 - 12624   2013.10

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    Authorship:Lead author, Corresponding author   Language:English   Publishing type:Research paper (scientific journal)   Publisher:American Chemical Society (ACS)  

    DOI: 10.1021/jp4085298

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  • Solvation of hydrophobes in water and simple liquids Reviewed

    Kenichiro Koga

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

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    Language:English   Publishing type:Research paper (scientific journal)   Publisher: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   2010.1

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    Language:English   Publishing type:Research paper (scientific journal)   Publisher: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   2008.5

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    Language:English   Publishing type:Research paper (scientific journal)   Publisher: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   2008.3

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    Language:English   Publishing type:Research paper (scientific journal)   Publisher: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   2008.1

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    Language:English   Publishing type:Research paper (scientific journal)   Publisher: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 )   2006.4

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    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 )   2005.3

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    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   2003.5

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    Language:English   Publishing type:Research paper (scientific journal)   Publisher: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

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    Language:English   Publisher: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   2002.6

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    Language:English   Publishing type:Research paper (scientific journal)   Publisher: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   2001.8

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    Language:English   Publishing type:Research paper (scientific journal)   Publisher: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   2000.11

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    Language:English   Publishing type:Research paper (scientific journal)   Publisher: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   1999.2

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    Language:English   Publishing type:Research paper (scientific journal)   Publisher: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 Reviewed

    Koga Kenichiro, X. C. Zeng, Tanaka Hideki

    Physical Review Letters   79 ( 26 )   5262 - 5265   1997.1

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    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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  • Theory of Gas Solubility and Hydrophobic Interaction in Aqueous Electrolyte Solutions Invited Reviewed

    Ryuichi Okamoto, Kenichiro Koga

    The Journal of Physical Chemistry B   125 ( 46 )   12820 - 12831   2021.11

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    Authorship:Last author   Publishing type:Research paper (scientific journal)   Publisher: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 Reviewed

    Hiroyuki Katsuto, Ryuichi Okamoto, Tomonari Sumi, Kenichiro Koga

    The Journal of Physical Chemistry B   125 ( 23 )   6296 - 6305   2021.6

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    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 Reviewed

    Yuji Adachi, Kenichiro Koga

    The Journal of Chemical Physics   153 ( 11 )   114501 - 114501   2020.9

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    Authorship:Last author, Corresponding author   Publishing type:Research paper (scientific journal)   Publisher:AIP Publishing  

    DOI: 10.1063/5.0016565

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

    Ryuichi Okamoto, Kenichiro Koga, Akira Onuki

    The Journal of Chemical Physics   153 ( 7 )   074503 - 074503   2020.8

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    Language:English   Publishing type:Research paper (scientific journal)   Publisher: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 Reviewed

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

    The Journal of Chemical Physics   151 ( 24 )   244903 - 244903   2019.12

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    DOI: 10.1063/1.5129160

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

    Kenichiro Koga, Joseph O. Indekeu

    JOURNAL OF CHEMICAL PHYSICS   150 ( 16 )   2019.4

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    Authorship:Lead author, Corresponding author   Language:English   Publishing type:Research paper (scientific journal)   Publisher: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 Reviewed

    Tomonari Sumi, Kenichiro Koga

    SCIENTIFIC REPORTS   9   2019.3

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    Language:English   Publishing type:Research paper (scientific journal)   Publisher: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 Reviewed

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

    Journal of Computational Chemistry   39 ( 4 )   202 - 217   2018.2

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    Authorship:Last author   Publishing type:Research paper (scientific journal)   Publisher:Wiley  

    DOI: 10.1002/jcc.25101

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    Other Link: https://onlinelibrary.wiley.com/doi/full-xml/10.1002/jcc.25101

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

    Kenji Mochizuki, Tomonari Sumi, Kenichiro Koga

    Physical Chemistry Chemical Physics   19 ( 35 )   23915 - 23918   2017.8

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    Authorship:Last author   Publishing type:Research paper (scientific journal)   Publisher: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 Invited Reviewed

    Jesper Koning, Kenichiro Koga, Joseph. O. Indekeu

    The European Physical Journal Special Topics   226 ( 3 )   427 - 431   2017.2

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    Publishing type:Research paper (scientific journal)   Publisher:Springer Science and Business Media LLC  

    DOI: 10.1140/epjst/e2016-60200-8

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    Other Link: 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 Reviewed

    Tomonari Sumi, Yutaka Maruyama, Ayori Mitsutake, Kenichiro Koga

    The Journal of Chemical Physics   144 ( 22 )   224104 - 224104   2016.6

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    DOI: 10.1063/1.4953191

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  • Cononsolvency behavior of hydrophobes in water + methanol mixtures Reviewed

    Kenji Mochizuki, Kenichiro Koga

    Physical Chemistry Chemical Physics   18 ( 24 )   16188 - 16195   2016.5

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    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.

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

    Kenji Mochizuki, Tomonari Sumi, Kenichiro Koga

    Scientific Reports   6 ( 1 )   2016.4

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    DOI: 10.1038/srep24657

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

    Kenji Mochizuki, Tomonari Sumi, Kenichiro Koga

    Physical Chemistry Chemical Physics   18 ( 6 )   4697 - 4703   2016.1

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    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 Reviewed

    Kenji Mochizuki, Kenichiro Koga

    Physical Chemistry Chemical Physics   17 ( 28 )   18437 - 18442   2015.7

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    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 Reviewed

    Yuchi He, Gang Sun, Kenichiro Koga, Limei Xu

    Scientific Reports   4 ( 1 )   2015.5

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    DOI: 10.1038/srep06596

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

    K. Abe, K. Koga

    Journal of Molecular Liquids   200   7 - 11   2014.12

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    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 Reviewed

    Kiharu Abe, Tomonari Sumi, Kenichiro Koga

    The Journal of Chemical Physics   141 ( 18 )   18C516 - 18C516   2014.11

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    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 Reviewed

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

    Physical Review E   88 ( 2 )   2013.8

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    DOI: 10.1103/physreve.88.022122

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    Other Link: http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevE.88.022122/fulltext

  • Thermodynamic functions as correlation-function integrals Reviewed

    K. Koga, B. Widom

    The Journal of Chemical Physics   138 ( 11 )   114504 - 114504   2013.3

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    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 Reviewed

    B. Widom, K. Koga

    The Journal of Physical Chemistry B   117 ( 4 )   1151 - 1154   2013.1

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    DOI: 10.1021/jp311800p

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

    Kiharu Abe, Kenichiro Koga

    Journal of the Physical Society of Japan   81 ( Suppl.A )   SA021 - SA021   2012.1

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    DOI: 10.1143/jpsjs.81sa.sa021

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  • Diffusivity of Liquid Argon in Carbon Nanotubes Reviewed

    Hiroki Akiyoshi, Kenichiro Koga

    Journal of the Physical Society of Japan   81 ( Suppl.A )   SA022 - SA022   2012.1

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    DOI: 10.1143/jpsjs.81sa.sa022

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

    Lukman Hakim, Masakazu Matsumoto, Kenichiro Koga, Hideki Tanaka

    Journal of the Physical Society of Japan   81 ( Suppl.A )   SA018 - SA018   2012.1

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    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? Reviewed

    J O Indekeu, K Koga, B Widom

    Journal of Physics: Condensed Matter   23 ( 19 )   194101 - 194101   2011.5

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    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 Reviewed

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

    Molecular Physics   109 ( 7-10 )   1297 - 1311   2011.3

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    DOI: 10.1080/00268976.2011.556095

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  • Hydrophobicity in Lennard-Jones solutions Reviewed

    Mario Ishizaki, Hideki Tanaka, Kenichiro Koga

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

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    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.

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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   2010.10

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    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   2010.5

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    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   2010.3

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    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

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    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.

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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   2009.12

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    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   2009.6

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    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.

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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   2007.8

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    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.

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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   2007.8

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    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.

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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   2007.7

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    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.

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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   2007.1

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    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.

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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   2006.11

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    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.

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

    K. Koga, B. Widom

    MOLECULAR PHYSICS   104 ( 22-24 )   3469 - 3477   2006.11

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    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.

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  • Formation of ice nanotube with hydrophobic guests inside carbon nanotube

    H Tanaka, K Koga

    JOURNAL OF CHEMICAL PHYSICS   123 ( 9 )   2005.9

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    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.

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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 )   2005.2

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    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.

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  • Hydrophobic effect in the pressure-temperature plane

    Kenichiro Koga

    Journal of Chemical Physics   121 ( 15 )   7304 - 7312   2004.10

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    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.

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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   2004.9

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    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.

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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

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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   2003.10

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    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.

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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   2003.3

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    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   2003.3

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    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   2002.12

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    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.

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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   2002.11

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    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   2001.3

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    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   2001.3

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    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   2000.9

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    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].

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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   1999.11

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    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].

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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   1999.11

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    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   1999.5

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    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 Reviewed

    Kenichiro Koga, X. C. Zeng, Hideki Tanaka

    Chemical Physics Letters   285 ( 3-4 )   278 - 283   1998.3

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    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

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    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   1997.8

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    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.

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

    K Koga, XC Zeng, H Tanaka

    JOURNAL OF CHEMICAL PHYSICS   106 ( 23 )   9781 - 9792   1997.6

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    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   1997.1

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    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 Reviewed

    Kenichiro Koga, Hideki Tanaka, X. C. Zeng

    Journal of Physical Chemistry   100 ( 41 )   16711 - 16719   1996.10

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    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.

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

    K Koga, H Tanaka

    JOURNAL OF CHEMICAL PHYSICS   104 ( 1 )   263 - 272   1996.1

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    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 Reviewed

    Kenichiro Koga, Hideki Tanaka, Koichiro Nakanishi

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

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    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.

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  • STABILITY OF POLAR GUEST-ENCAGING CLATHRATE HYDRATES Reviewed

    K KOGA, H TANAKA, K NAKANISHI

    JOURNAL OF CHEMICAL PHYSICS   101 ( 4 )   3127 - 3134   1994.8

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    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 Reviewed

    Kenichiro Koga, Hideki Tanaka, Koichiro Nakanishi

    Molecular Simulation   12 ( 3-6 )   241 - 252   1994.3

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    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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Books

  • 化学系の統計力学入門

    化学同人  2005 

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  • Water, Steam, and Aqueous Solutions for Electric Power

    Maruzen  2005 

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  • Encyclopedia of Nanoscience and Nanotechnology

    Marcel Dekker  2004 

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

    Springer-Verlag  2003 

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  • New Kinds of Phase Transitions: Transformations in Disordered Substances

    Kluwer,Dordrecht  2002 

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  • Science and Technology of High-Pressure Research

    2000 

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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 Invited Reviewed

    望月建爾, 甲賀研一郎

    アンサンブル   2021.1

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  • Tribute to Benjamin Widom Reviewed

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

    JOURNAL OF PHYSICAL CHEMISTRY B   122 ( 13 )   3203 - 3205   2018.4

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    DOI: 10.1021/acs.jpcb.8b00129

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  • Preface

    Kenichiro Koga, Ryo Akiyama

    JOURNAL OF MOLECULAR LIQUIDS   200   1 - 1   2014.12

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    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

  • Preface

    Kenichiro Koga, Ryo Akiyama

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   81   2012.9

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  • Wetting transitions at fluid interfaces and related topics

    Kenichiro Koga

    Review of Polarography   56 ( 1 )   3 - 10   2010

  • How soft interfaces get wet

    甲賀研一郎

    物性研究   93 ( 3 )   301 - 301   2009

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

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

    Chemistry   1999

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Presentations

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

    Kenichiro Koga

    Physical Chemistry Seminar, Department of Chemistry, Purdue University  2021.11.10 

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

    Kenichiro Koga

    Theory Group Seminar, Department of Chemistry, Baker Laboratory, Cornell University  2021.11.6 

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  • 統計力学 Invited

    甲賀研一郎

    第15回分子シミュレーションスクール  2021.9.7 

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

    Kenichiro Koga

    IUPAC CCCE 2021  2021.8.13 

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  • 疎水効果の分子論,集中講義 Invited

    甲賀研一郎

    2020.11.11 

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  • 疎水性相互作用の温度・圧力・塩濃度依存性 Invited

    甲賀研一郎

    奈良女子大学理学部化学科講演会  2020.11.10 

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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 Invited

    甲賀 研一郎

    日本物理学会  2020.9.9 

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  • 統計力学 Invited

    甲賀研一郎

    第14回分子シミュレーションスクール  2020.9.4 

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  • 疎水性相互作用の温度応答・イオン種依存性 Invited

    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  2020.1.10 

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    Presentation type:Symposium, workshop panel (nominated)  

    Venue: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? International coauthorship International conference

    Okinawa Colloids 2019  2019.11.6 

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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  2019.9.10 

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  • 統計力学 Invited

    甲賀研一郎

    第13回分子シミュレーションスクール  2019.9.2 

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

    Kenichiro Koga

    Research Seminar at St. Petersburg State University  2019.7.9 

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

    Kenichiro Koga

    Soft Matter Physics: From the perspective of the essential heterogeneity  2018.12.11 

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

    Kenichiro Koga

    5th International Conference on Colloid Chemistry and Physicochemical Mechanics  2018.9.12 

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    Presentation type:Oral presentation (keynote)  

    Venue:St. Petersburg  

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  • 統計力学 Invited

    甲賀研一郎

    2018.9.4 

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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  2018.8.30 

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

    Kenichiro Koga

    Theory Group Seminar, Department of Chemistry, Baker Laboratory, Cornell University  2018.8.1 

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

    Kenichiro Koga

    TSRC Workshop, Hydrophobicity: From Theory, to Simulation, to Experiment, Telluride  2018.7.17 

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    Presentation type:Oral presentation (invited, special)  

    Venue: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  2017.11.30 

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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 

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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 

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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 

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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 

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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 

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

    The 4th International Conference on Molecular Simulation  2016 

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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 

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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 

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  • Solid-liquid critical behavior of water in hydrophobic nanotubes

    The 2015 International Chemical Congress of Pacific Basin Societies  2015 

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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 

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

    Density Functional Days in Tuebingen  2015 

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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 

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

    The 7th Mini-Symposium on Liquids  2013 

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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 

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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 

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  • Hydrophobes in Water/Water in Nanotubes

    Seminar  2012 

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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 

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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 

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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 

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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 

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  • A general view on Hydrophobicity

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

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

    5th Mini-Symposium on Liquids  2011 

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  • General View on Solvation in Water and Simple Liquids

    第1回ソフトマター研究会  2011 

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  • 疎水性水和と溶媒誘起相互作用

    第5回領域研究会  2011 

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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 

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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 

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  • Introduction to homogeneous and inhomogeneous liquids

    IITS Course in Theoretical Physics for PhD Students  2010 

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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 

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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 

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  • Introduction to homogeneous and inhomogeneous liquids

    IITS Course in Theoretical Physics for PhD Students  2010 

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  • 均一・不均一液体の統計力学

    集中講義  2010 

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  • 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 

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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 

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  • First, second, and higher-order wetting transitions

    International Symposium on Non-equilibrium Soft Matter 2010  2010 

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

    11th Granada* Seminar on Computational and Statistical Physics  2010 

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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 

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  • Hydrophobic Solutes in Water and Water in Hydrophobic Pores

    International Symposium between Shaanxi Normal University and Okayama University  2009 

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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 

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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 

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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 

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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 

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  • Wetting transitions and the line tension

    The 4th LSW Symposium on Soft & Wet Matter  2009 

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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 

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  • An infinite-order wetting transition

    100th Statistical Mechanics Meeting  2008 

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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 

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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 

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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 

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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 

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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 

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  • 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 

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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 

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  • 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 

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  • 不均一系の相転移

    液体化学夏の学校  2007 

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  • Phase Transitions of Confined Water and Simple Liquids

    Institute of Theoretical Physics Seminar  2007 

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  • 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 

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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 

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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 

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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 

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  • 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 

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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 

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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 

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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 

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  • 学術賞受賞講演

    第19回分子シミュレーション討論会  2005 

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  • 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 

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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 

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  • Phase transitions in confined water

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

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  • 疎水効果と水の相転移

    理論化学シンポジウム  2004 

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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 

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  • Correlation between Hydrophobic Attraction and the Free Energy Hydration

    第25回溶液化学シンポジウム  2002 

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  • Model of freezing in one-dimensional liquids

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

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

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

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  • How does water freeze inside carbon nanotubes?

    Horizons in Complex Systems,  2001 

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  • Phase behavior of water confined in a slit nanopore

    AIRAPT-17  1999 

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Awards

  • 花王研究奨励賞

    2007  

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    Country:Japan

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  • 日本物理学会若手奨励賞

    2007  

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    Country:Japan

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  • 分子シミュレーション研究会学術賞

    2005  

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    Country:Japan

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Research Projects

  • Theoretical Modeling of Surfactant Aggregation Behavior

    Grant number:20H02696  2020.04 - 2024.03

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)  Grant-in-Aid for Scientific Research (B)

    甲賀 研一郎

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    Grant amount:\15990000 ( Direct expense: \12300000 、 Indirect expense:\3690000 )

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  • Understanding solvent-mediated forces with diverse responses to ions, co-solvents, and temperature

    Grant number:18KK0151  2018.10 - 2023.03

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research Fund for the Promotion of Joint International Research (Fostering Joint International Research (B))  Fund for the Promotion of Joint International Research (Fostering Joint International Research (B))

    甲賀 研一郎, 岡本 隆一, 墨 智成, 望月 建爾

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    Grant amount:\17940000 ( Direct expense: \13800000 、 Indirect expense:\4140000 )

    (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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  • Phase Transitions of Water in Nanopores

    Grant number:15H05474  2015.04 - 2019.03

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research Grant-in-Aid for Young Scientists (A)  Grant-in-Aid for Young Scientists (A)

    Mochizuki Kenji, Koga Kenichiro, Sumi Tomonari, Matsumoto Masakazu, Ben-Amotz Dor, Molinero Valeria

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    Grant amount:\24180000 ( Direct expense: \18600000 、 Indirect expense:\5580000 )

    Using molecular dynamics simulations and theoretical methods, we have studied the influence of confined space and solute on the phase behavior of water. We showed the potential solid-liquid critical point of water in carbon nanotube and the formation of ice between a pair of antifreeze proteins even at room temperature. Furthermore, we investigated the mutual relationship between the structural change of macromolecule and its hydration shell, and the promotion/depression of freezing of water by macromolecules.

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  • Theoretical Study of the Hydrophobic Effect Under Varying Environments

    Grant number:26287099  2014.04 - 2018.03

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)  Grant-in-Aid for Scientific Research (B)

    Koga Kenichiro

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    Grant amount:\15730000 ( Direct expense: \12100000 、 Indirect expense:\3630000 )

    What we have learned from this project is summarized as follows. First, the mean-field approximation of liquids is capable to reproduce the solvation free energy, μ, and the hydrophobic interaction, w(r), and one can understand mechanisms of the temperature T, pressure p, salt concentration c dependences of μ and w(r). Second, in each mode of T, p, and c variations, there exist linear correlations between μ and w(rc) with rc the contact distance between hydrophobic solute molecules. Third, the mean-field approximation is extended to inhomogeneous fluid systems and temperature dependences of local μ(z) are now understood based on the mean-field approximation. Fourth, a model hydrophobic polymer, which we devised, undergoes upon heating a coil-to-globule conformation change in water near room temperature. The mechanism of the hydrophobic collapse is clarified.

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  • Theory of Surface Phase Transitions and Inhomogeneous Fluids

    Grant number:23340122  2011.04 - 2015.03

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)  Grant-in-Aid for Scientific Research (B)

    KOGA Kenichiro

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    Grant amount:\15730000 ( Direct expense: \12100000 、 Indirect expense:\3630000 )

    Various kinds of wetting phenomena are commonly observed. The origin of the variety, however, has long been unknown. Here we built an analytically solvable model of wetting transitions, solved it, and found the conditions under which the model gives the first-order, second-order, continuously varying order, and infinite-order wetting transitions. We also studied the effect of thermal fluctuations on wetting transitions.
    We proposed the method for calculating the second osmotic virial coefficient B of hydrophobic molecules in water. For an aqueous solution of methane, we found that B is positive at low temperatures, decreases with increasing temperature, and becomes very negative at high temperatures. This indicates that the hydrophobic molecules in water, overall, repel each other at low temperature and they attract each other at high temperature. We also studied how far the hydrophobic effect persists as a hydrophobic solute approaches the liquid-vapor interface of water.

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  • Hydration and transport phenomena of ions in nano pores and ion channels

    Grant number:21245006  2009 - 2012

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (A)  Grant-in-Aid for Scientific Research (A)

    TANAKA Hideki, MATSUMOTO Masakazu, KOGA Kenichiro, OHKUBO Takahiro, OHMINE Iwao

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    Grant amount:\27170000 ( Direct expense: \20900000 、 Indirect expense:\6270000 )

    We examine structure, phase behavior, and transport process in an ideal model of ion channel to establish a microscopic view of dynamics of ions. Anisotropic and limited hydrations of ions in a nano space are investigated by molecular dynamics simulations as well as peculiar phase behaviors of water in a nano space. In addition, we propose a strategic method to search for a new clathrate hydrate by investigating the selectivity of its crystalline structure, which is another type of confinement.

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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

    Grant number:21015021  2009 - 2010

    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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    Grant amount:\3700000 ( Direct expense: \3700000 )

    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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  • Phase transitions of inhomogeneous liquids in pores or at interfaces

    Grant number:20550017  2008 - 2010

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (C)  Grant-in-Aid for Scientific Research (C)

    KOGA Kenichiro, WIDOM Benjamin, JOSEPH O. INDEKEU

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    Grant amount:\4940000 ( Direct expense: \3800000 、 Indirect expense:\1140000 )

    Liquids confined in narrow pores (e.g., water in carbon nanotubes, oxygen in meso-porous materials, etc) and fluids at interfaces (e.g., interfaces between oil and water) exhibit phase transitions or complex phase behavior that would never be observed for bulk fluid phases. A goal of our theoretical study is to extract and understand some fundamental properties in confined and interfacial fluids. Statistical mechanics and computer simulation are the tools to study these problems. With these approaches we found several key factors determining phase behavior of argon in nanopores and revealed four kinds of wetting transitions in a standard mean-filed density functional model.

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  • Fluctuation and Chemical Reaction in Low-dimensional Condensed Phase

    Grant number:18066011  2006 - 2009

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research on Priority Areas  Grant-in-Aid for Scientific Research on Priority Areas

    TANAKA Hideki, KOGA Kenichiro, MISHIMA Osamu

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    Grant amount:\41300000 ( Direct expense: \41300000 )

    Phase transitions and phase behaviors of water in confined spaces have been investigated with a statistical mechanical theory and molecular simulations. Examined are a phase behavior of water confined in quasi one dimensional spaces and the thermodynamic stability of clathrate hydrates containing hydrogen on the basis of a newly developed theory and simulation methods.

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  • 疎水環境下の水の相転移および疎水効果

    Grant number:16685003  2004 - 2006

    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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    Grant amount:\23270000 ( Direct expense: \17900000 、 Indirect expense:\5370000 )

    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

    Grant number:12740322  2000 - 2001

    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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    Grant amount:\1500000 ( Direct expense: \1500000 )

    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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