Royal Society of Chemistry (RSC) Acta Medica Okayama 1463-9076 2026 Phase behaviour of liquid CO2 with an impurity of water: influence of CO2 hydrate EN Hideki Tanaka Research Institute for Interdisciplinary Science, Okayama University Masakazu Matsumoto Research Institute for Interdisciplinary Science, Okayama University Takuma Yagasaki Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University Munetaka Takeuchi Engineering Advancement Association of Japan Yoshihito Mori Ochanomizu University Takumi Kono Engineering Advancement Association of Japan The solubility of water in liquid CO2 coexisting with CO2 hydrate or liquid water is evaluated in order to investigate the thermodynamic conditions to avoid the formation of CO2 hydrate in the transportation processes of liquid CO2. To this end, theoretical calculations have been carried out to obtain the chemical potentials of water and CO2 in all the phases involved in their coexistence. The solubility of water in liquid CO2 coexisting with liquid water decreases with decreasing temperature over a wide range of temperature and pressure, except for in the vicinity of the critical point of CO2. The decrease in the solubility is further enhanced by the formation of hydrate. We estimate the Gibbs energy of hydrate formation, which is an important property for sequestration of CO2, for cases where the temperature or pressure of water-saturated liquid CO2 decreases. We also estimate the amount of water precipitated as hydrate during these processes, which has a direct bearing on flow assurance in CO2 transportation. The present study will contribute to the development of a low-energy, safe CO2 transport network aiming at achieving large-scale carbon neutrality. No potential conflict of interest relevant to this article was reported.

AIP Publishing Acta Medica Okayama 0021-9606 159 19 2023 Efficiency and energy balance for substitution of CH4 in clathrate hydrates with CO2 under multiple-phase coexisting conditions 194504 EN Hideki Tanaka Research Institute for Interdisciplinary Science, Okayama University Masakazu Matsumoto Research Institute for Interdisciplinary Science, Okayama University Takuma Yagasaki Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University Many experimental and theoretical studies on CH4–CO2 hydrates have been performed aiming at the extraction of CH4 as a relatively clean energy resource and concurrent sequestration of CO2. However, vague or insufficient characterization of the environmental conditions prevents us from a comprehensive understanding of even equilibrium properties of CH4–CO2 hydrates for this substitution. We propose possible reaction schemes for the substitution, paying special attention to the coexisting phases, the aqueous and/or the fluid, where CO2 is supplied from and CH4 is transferred to. We address the two schemes for the substitution operating in three-phase and two-phase coexistence. Advantages and efficiencies of extracting CH4 in the individual scheme are estimated from the chemical potentials of all the components in all the phases involved in the substitution on the basis of a statistical mechanical theory developed recently. It is found that although substitution is feasible in the three-phase coexistence, its working window in temperature–pressure space is much narrower compared to the two-phase coexistence condition. Despite that the substitution normally generates only a small amount of heat, a large endothermic substitution is suggested in the medium pressure range, caused by the vaporization of liquid CO2 due to mixing with a small amount of the released CH4. This study provides the first theoretical framework toward the practical use of hydrates replacing CH4 with CO2 and serves as a basis for quantitative planning. No potential conflict of interest relevant to this article was reported.

AIP Publishing Acta Medica Okayama 0021-9606 154 9 2021 Formation of hot ice caused by carbon nanobrushes. II. Dependency on the radius of nanotubes 094502 EN Masakazu Matsumoto Research Institute for Interdisciplinary Science, Okayama University Takuma Yagasaki Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University Hideki Tanaka Research Institute for Interdisciplinary Science, Okayama University Stable crystalline structures of confined water can be different from bulk ice. In Paper I [T. Yagasaki et al., J. Chem. Phys. 151, 064702 (2019)] of this study, it was shown, using molecular dynamics (MD) simulations, that a zeolite-like ice structure forms in nanobrushes consisting of (6,6) carbon nanotubes (CNTs) when the CNTs are located in a triangle arrangement. The melting temperature of the zeolite-like ice structure is much higher than the melting temperature of ice Ih when the distance between the surfaces of CNTs is ∼0.94 nm, which is the best spacing for the bilayer structure of water. In this paper, we perform MD simulations of nanobrushes of CNTs that are different from (6,6) CNTs in radius. Several new porous ice structures form spontaneously in the MD simulations. A stable porous ice forms when the radius of its cavities matches the radius of the CNTs well. All cylindrical porous ice structures found in this study can be decomposed into a small number of structural blocks. We provide a new protocol to classify cylindrical porous ice crystals on the basis of this decomposition. No potential conflict of interest relevant to this article was reported.

IOP Publishing Acta Medica Okayama 2632-3338 1 3 2020 On the Occurrence of Clathrate Hydrates in Extreme Conditions: Dissociation Pressures and Occupancies at Cryogenic Temperatures with Application to Planetary Systems 80 EN Hideki Tanaka Research Institute for Interdisciplinary Science, Okayama University Takuma Yagasaki Research Institute for Interdisciplinary Science, Okayama University Masakazu Matsumoto Research Institute for Interdisciplinary Science, Okayama University We investigate the thermodynamic stability of clathrate hydrates at cryogenic temperatures from the 0 K limit to 200 K in a wide range of pressures, covering the thermodynamic conditions of interstellar space and the surface of the hydrosphere in satellites. Our evaluation of the phase behaviors is performed by setting up quantum partition functions with variable pressures on the basis of a rigorous statistical mechanics theory that requires only the intermolecular interactions as input. Noble gases, hydrocarbons, nitrogen, and oxygen are chosen as the guest species, which are key components of the volatiles in such satellites. We explore the hydrate/water two-phase boundary of those clathrate hydrates in water-rich conditions and the hydrate/guest two-phase boundary in guest-rich conditions, either of which occurs on the surface or subsurface of icy satellites. The obtained phase diagrams indicate that clathrate hydrates can be in equilibrium with either water or the guest species over a wide range far distant from the three-phase coexistence condition and that the stable pressure zone of each clathrate hydrate expands significantly on intense cooling. The implication of our findings for the stable form of water in Titan is that water on the surface exists only as clathrate hydrate with the atmosphere down to a shallow region of the crust, but clathrate hydrate in the remaining part of the crust can coexist with water ice. This is in sharp contrast to the surfaces of Europa and Ganymede, where the thin oxygen air coexists exclusively with pure ice. No potential conflict of interest relevant to this article was reported.

American Chemical Society Acta Medica Okayama 15499618 16 14 2020 Lennard-Jones Parameters Determined to Reproduce the Solubility of NaCl and KCl in SPC/E, TIP3P, and TIP4P/2005 Water 2460 2473 EN Takuma Yagasaki Research Institute for Interdisciplinary Science, Okayama University Masakazu Matsumoto Research Institute for Interdisciplinary Science, Okayama University Hideki Tanaka Research Institute for Interdisciplinary Science, Okayama University Most classical nonpolarizable ion potential models underestimate the solubility values of NaCl and KCl in water significantly. We determine Lennard-Jones parameters of Na+, K+, and Cl– that reproduce the solubility as well as the hydration free energy in dilute aqueous solutions for three water potential models, SPC/E, TIP3P, and TIP4P/2005. The ion–oxygen distance in the solution and the cation–anion distance in salt are also considered in the parametrization. In addition to the target properties, the hydration enthalpy, hydration entropy, self-diffusion coefficient, coordination number, lattice energy, enthalpy of solution, density, viscosity, and number of contact ion pairs are calculated for comparison with 17 frequently used or recently developed ion potential models. The overall performance of each ion model is represented by a global score using a scheme that was originally developed for comparison of water potential models. The global score is better for our models than for the other 17 models not only because of the quite good prediction for the solubility but also because of the relatively small deviation from the experimental value for many of the other properties. No potential conflict of interest relevant to this article was reported.

American Institute of Physics Acta Medica Okayama 00219606 150 4 2019 Phase diagram of ice polymorphs under negative pressure considering the limits of mechanical stability 041102 EN Takahiro Matsui Graduate School of Natural Science and Technology, Okayama University Takuma Yagasaki Research Institute for Interdisciplinary Science, Okayama University Masakazu Matsumoto Research Institute for Interdisciplinary Science, Okayama University Hideki Tanaka Research Institute for Interdisciplinary Science, Okayama University Thermodynamic and mechanical stabilities of various ultralow-density ices are examined using computer simulations to construct the phase diagram of ice under negative pressure. Some ultralow-density ices, which were predicted to be thermodynamically metastable under negative pressures on the basis of the quasi-harmonic approximation, can exist only in a narrow pressure range at very low temperatures because they are mechanically fragile due to the large distortion in the hydrogen bonding network. By contrast, relatively dense ices such as ice Ih and ice XVI withstand large negative pressure. Consequently, various ices appear one after another in the phase diagram. The phase diagram of ice under negative pressure exhibits a different complexity from that of positive pressure because of the mechanical instability. No potential conflict of interest relevant to this article was reported.

American Institute of Physics Acta Medica Okayama 00219606 150 21 2019 Liquid-liquid separation of aqueous solutions: A molecular dynamics study 214506 EN Takuma Yagasaki Research Institute for Interdisciplinary Science, Okayama University Masakazu Matsumoto Research Institute for Interdisciplinary Science, Okayama University Hideki Tanaka Research Institute for Interdisciplinary Science, Okayama University In the liquid-liquid phase transition scenario, supercooled water separates into the high density liquid (HDL) and low density liquid (LDL) phases at temperatures lower than the second critical point. We investigate the effects of hydrophilic and hydrophobic solutes on the liquid-liquid phase transition using molecular dynamics simulations. It is found that a supercooled aqueous NaCl solution separates into solute-rich HDL and solute-poor LDL parts at low pressures. By contrast, a supercooled aqueous Ne solution separates into solute-rich LDL and solute-poor HDL parts at high pressures. Both the solutes increase the high temperature limit of the liquid-liquid separation. The degree of separation is quantified using the local density of solute particles to determine the liquid-liquid coexistence region in the pressure-temperature phase diagram. The effects of NaCl and Ne on the phase diagram of supercooled water are explained in terms of preferential solvation of ions in HDL and that of small hydrophobic particles in LDL, respectively. No potential conflict of interest relevant to this article was reported.

American Institute of Physics Acta Medica Okayama 00219606 151 6 2019 Formation of Hot Ice Caused by Carbon Nanobrushes 064702 EN Takuma Yagasaki Research Institute for Interdisciplinary Science, Okayama University Masaru Yamasaki Graduate School of Natural Science and Technology, Okayama University Masakazu Matsumoto Research Institute for Interdisciplinary Science, Okayama University Hideki Tanaka Research Institute for Interdisciplinary Science, Okayama University Confinement in nanoscaled porous materials changes properties of water significantly. We perform molecular dynamics simulations of water in a model of a nanobrush made of carbon nanotubes. Water crystallizes into a novel structure called dtc in the nanobrush when (6,6) nanotubes are located in a triangular arrangement, and there is a space that can accommodate two layers of water molecules between the tubes. The mechanism of the solidification is analogous to formation of gas hydrates: hydrophobic molecules promote crystallization when their arrangement matches ordered structures of water. This is supported by a statistical mechanical calculation, which bears resemblance to the theory on the clathrate hydrate stability. No potential conflict of interest relevant to this article was reported.

American Institute of Physics Acta Medica Okayama 0021-9606 150 21 2019 A Bayesian approach for identification of ice Ih, ice Ic, high density, and low density liquid water with a torsional order parameter 214504 EN Masakazu Matsumoto Research Institute for Interdisciplinary Science, Okayama University Takuma Yagasaki Research Institute for Interdisciplinary Science, Okayama University Hideki Tanaka Research Institute for Interdisciplinary Science, Okayama University An order parameter is proposed to classify the local structures of liquid and solid water. The order parameter, which is calculated from the O–O–O–O dihedral angles, can distinguish ice Ih, ice Ic, high density, and low density liquid water. Three coloring schemes are proposed to visualize each of the coexisting phases in a system using the order parameter on the basis of Bayesian decision theory. The schemes are applied to a molecular dynamics trajectory in which ice nucleation occurs following spontaneous liquid-liquid separation in the deeply supercooled region as a demonstration. No potential conflict of interest relevant to this article was reported.

American Physical Society Acta Medica Okayama 0031-9007 115 19 2015 Chiral Ordering in Supercooled Liquid Water and Amorphous Ice 197801 EN Masakazu Matsumoto Takuma Yagasaki Hideki Tanaka The emergence of homochiral domains in supercooled liquid water is presented using molecular dynamics simulations. An individual water molecule possesses neither a chiral center nor a twisted conformation that can cause spontaneous chiral resolution. However, an aggregation of water molecules will naturally give rise to a collective chirality. Such homochiral domains possess obvious topological and geometrical orders and are energetically more stable than the average. However, homochiral domains cannot grow into macroscopic homogeneous structures due to geometrical frustrations arising from their icosahedral local order. Homochiral domains are the major constituent of supercooled liquid water and the origin of heterogeneity in that substance, and are expected to be enhanced in low-density amorphous ice at lower temperatures. No potential conflict of interest relevant to this article was reported.

American Institute of Physics Acta Medica Okayama 0021-9606 123 9 2005 Formation of ice nanotube with hydrophobic guests inside carbon nanotube EN Hideki Tanaka Kenichiro Koga A composite ice nanotube inside a carbon nanotube has been explored by molecular dynamics and grandcanonical 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. No potential conflict of interest relevant to this article was reported.

American Institute of Physics Acta Medica Okayama 0021-9606 124 13 2006 Close-packed structures and phase diagram of soft spheres in cylindrical pores EN Kenichiro Koga Hideki Tanaka 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. No potential conflict of interest relevant to this article was reported.

American Institute of Physics Acta Medica Okayama 122 10 2005 Phase diagram of water between hydrophobic surfaces EN Kenichiro Koga Hideki Tanaka 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. No potential conflict of interest relevant to this article was reported.

American Institute of Physics Acta Medica Okayama 0021-9606 127 8 2007 Phase equilibria and interfacial tension of fluids confined in narrow pores EN Yoshinobu Hamada Kenichiro Koga Hideki Tanaka 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. No potential conflict of interest relevant to this article was reported.

American Institute of Physics Acta Medica Okayama 0021-9606 127 4 2007 On the thermodynamic stability of hydrogen clathrate hydrates EN Keisuke Katsumasa Kenichiro Koga Hideki Tanaka 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. No potential conflict of interest relevant to this article was reported.

American Institute of Physics Acta Medica Okayama 0021-9606 122 7 2005 On the thermodynamic stability and structural transition of clathrate hydrates EN Yuji Koyama Hideki Tanaka Kenichiro Koga 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. No potential conflict of interest relevant to this article was reported.

American Institute of Physics Acta Medica Okayama 0021-9606 121 11 2004 On the thermodynamic stability of clathrate hydrates IV: Double occupancy of cages 5488 5493 EN Hideki Tanaka Takeharu Nakatsuka Kenichiro Koga 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. No potential conflict of interest relevant to this article was reported.