Authorship：Last author,　Corresponding author   Language：English   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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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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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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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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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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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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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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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

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

DOI： 10.1063/1.2172592

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

DOI： 10.1021/acsnano.3c07084

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Abstract

Alcohols and urea are widely used as effective protein denaturants. Among monohydric alcohols, 2,2,2‐trifluoroethanol (TFE) has large cosolvent effects as a helix stabilizer in proteins. In contrast, urea efficiently denatures ordered native structures, including helices, into coils. These opposing cosolvent effects of TFE and urea are well known, even though both preferentially bind to proteins; however, the underlying molecular mechanism remains controversial. Cosolvent‐dependent relative stability between native and denatured states is rigorously related to the difference in preferential binding parameters (PBPs) between these states. In this study, GCN4‐p1 with two‐stranded coiled coil helices was employed as a model protein, and molecular dynamics simulations for the helix dimer and isolated coil were conducted in aqueous solutions with 2 M TFE and urea. As 2 M cosolvent aqueous solutions did not exhibit clustering of cosolvent molecules, we were able to directly investigate the molecular origin of the excess PBP without considering the enhancement effect of PBPs arising from the concentration fluctuations. The calculated excess PBPs of TFE for the helices and those of urea for the coils were consistent with experimentally observed stabilization of helix by TFE and that of coil by urea. The former was caused by electrostatic interactions between TFE and side chains of the helices, while the latter was attributed to both electrostatic and dispersion interactions between urea and the main chains. Unexpectedly, reverse‐micelle‐like orientations of TFE molecules strengthened the electrostatic interactions between TFE and the side chains, resulting in strengthening of TFE solvation.

DOI： 10.1002/pro.4763

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Authorship：Last author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry (RSC)  

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

DOI： 10.1039/d3fd00104k

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

DOI： 10.1039/d3cp03799a

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

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

DOI： 10.1103/PhysRevLett.129.224501

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

DOI： 10.1063/5.0097547

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DOI： 10.1021/acs.jpcb.1c08050

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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.1c03388

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

DOI： 10.1063/5.0015446

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DOI： 10.1063/1.5129160

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

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

DOI： 10.1002/jcc.25101

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Authorship：Last author   Language：English   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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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

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

DOI： 10.1063/1.4953191

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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.

DOI： 10.1039/c6cp01496h

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DOI： 10.1038/srep24657

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry  

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

DOI： 10.1039/c5cp07674a

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry  

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

DOI： 10.1039/c5cp02568k

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DOI： 10.1038/srep06596

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

DOI： 10.1016/j.molliq.2014.02.014

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DOI： 10.1063/1.4896236

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Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

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

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

DOI： 10.1063/1.4795498

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

DOI： 10.1021/jp311800p

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

DOI： 10.1143/jpsjs.81sa.sa021

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

DOI： 10.1143/jpsjs.81sa.sa022

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

DOI： 10.1143/jpsjs.81sa.sa018

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DOI： 10.1088/0953-8984/23/19/194101

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Authorship：Lead author   Publishing type：Research paper (scientific journal)   Publisher：Informa UK Limited  

DOI： 10.1080/00268976.2011.556095

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

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

DOI： 10.1039/c0cp01767a

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

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

DOI： 10.1103/PhysRevB.82.144105

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

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

DOI： 10.1103/PhysRevLett.104.115701

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

DOI： 10.1039/b925671g

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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]

DOI： 10.1063/1.3271341

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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.

DOI： 10.1016/j.physa.2009.02.034

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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.

DOI： 10.1063/1.2759926

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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.

DOI： 10.1063/1.2752156

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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.

DOI： 10.1063/1.2751168

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

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

DOI： 10.1080/08927020601059893

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Language：English   Publisher：CHEMICAL SOC JAPAN  

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

DOI： 10.1246/bcsj.79.1621

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

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

DOI： 10.1080/00268970600958574

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

DOI： 10.1063/1.2031127

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

DOI： 10.1063/1.1850904

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

DOI： 10.1063/1.1792571

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

DOI： 10.1063/1.1782471

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Language：English   Publisher：ROYAL SOC CHEMISTRY  

DOI： 10.1039/b407059n

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

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

DOI： 10.1080/0892702031000103158

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Ab initio 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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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

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

DOI： 10.1016/S0378-4371(02)01384-5

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

DOI： 10.1080/00268970210162899

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

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

DOI： 10.1016/S0378-4371(00)00579-3

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

DOI： 10.1063/1.1289554

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

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

DOI： 10.1103/PhysRevE.60.5833

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

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

DOI： 10.1016/S0009-2614(98)00035-9

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

DOI： 10.1021/jp970511g

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

DOI： 10.1021/jp953347a

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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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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Taylor and Francis Inc.  

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

DOI： 10.1080/08927029608024069

Scopus

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

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

Web of Science

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

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

Scopus

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Language：English   Publisher：The Polarographic Society of Japan  

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

DOI： 10.5189/revpolarography.56.3

CiNii Article

CiNii Books

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Other Link： https://jlc.jst.go.jp/DN/JALC/00352439800?from=CiNii

Language：Japanese  

CiNii Article

CiNii Books

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Other Link： http://dl.ndl.go.jp/info:ndljp/pid/10942694

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Language：Japanese   Presentation type：Public lecture, seminar, tutorial, course, or other speech  

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Language：English   Presentation type：Oral presentation (invited, special)  

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Presentation type：Public lecture, seminar, tutorial, course, or other speech  

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Presentation type：Public lecture, seminar, tutorial, course, or other speech  

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Presentation type：Oral presentation (invited, special)  

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Presentation type：Public lecture, seminar, tutorial, course, or other speech  

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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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Presentation type：Oral presentation (general)  

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Presentation type：Oral presentation (general)  

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Presentation type：Public lecture, seminar, tutorial, course, or other speech  

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Presentation type：Public lecture, seminar, tutorial, course, or other speech  

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Presentation type：Oral presentation (invited, special)  

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Presentation type：Oral presentation (keynote)  

Venue：St. Petersburg  

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Presentation type：Public lecture, seminar, tutorial, course, or other speech  

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Presentation type：Oral presentation (invited, special)  

Venue：Telluride, CO, USA  

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Presentation type：Oral presentation (general)  

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