American Institute of Physics Acta Medica Okayama 0021-9606 153 11 2020 Structure and phase behavior of high-density ice from molecular-dynamics simulations with the ReaxFF potential 114501 EN Yuji Adachi Graduate School of Natural Sciences, Okayama University Kenichiro Koga 2Department of Chemistry, Okayama University We report a molecular dynamics simulation study of dense ice modeled by the reactive force field (ReaxFF) potential, focusing on the possibility of phase changes between crystalline and plastic phases as observed in earlier simulation studies with rigid water models. It is demonstrated that the present model system exhibits phase transitions, or crossovers, among ice VII and two plastic ices with face-centered cubic (fcc) and body-centered cubic (bcc) lattice structures. The phase diagram derived from the ReaxFF potential is different from those of the rigid water models in that the bcc plastic phase lies on the high-pressure side of ice VII and does the fcc plastic phase on the low-pressure side of ice VII. The phase boundary between the fcc and bcc plastic phases on the pressure, temperature plane extends to the high-temperature region from the triple point of ice VII, fcc plastic, and bcc plastic phases. Proton hopping, i.e., delocalization of a proton, along between two neighboring oxygen atoms in dense ice is observed for the ReaxFF potential but only at pressures and temperatures both much higher than those at which ice VII–plastic ice transitions are observed. No potential conflict of interest relevant to this article was reported.

American Institute of Physics Acta Medica Okayama 0003-6951 117 10 2020 Super-chiral vibrational spectroscopy with metasurfaces for high-sensitive identification of alanine enantiomers 101103 EN Takumi Iida Department of Electrical and Electronic Engineering, Okayama University Atsushi Ishikawa Department of Electrical and Electronic Engineering, Okayama University Takuo Tanaka Metamaterials Laboratory, RIKEN Cluster for Pioneering Research Atsuya Muranaka Advanced Elements Chemistry Laboratory, RIKEN Cluster for Pioneering Research Masanobu Uchiyama Advanced Elements Chemistry Laboratory, RIKEN Cluster for Pioneering Research Yasuhiko Hayashi Department of Electrical and Electronic Engineering, Okayama University Kenji Tsuruta Department of Electrical and Electronic Engineering, Okayama University Chiral nature of an enantiomer can be characterized by circular dichroism (CD) spectroscopy, but such a technique usually suffers from weak signal even with a sophisticated optical instrument. Recent demonstrations of plasmonic metasurfaces showed that chiroptical interaction of molecules can be engineered, thereby greatly simplifying a measurement system with high sensing capability. Here, by exploiting super-chiral field in a metasurface, we experimentally demonstrate high-sensitive vibrational CD spectroscopy of alanine enantiomers, the smallest chiral amino acid. Under linearly polarized excitation, the metasurface consisting of an array of staggered Au nano-rods selectively produces the left- and right-handed super-chiral fields at 1600 cm�|1, which spectrally overlaps with the functional group vibrations of alanine. In the Fourier-transform infrared spectrometer measurements, the mirror symmetric CD spectra of D- and L-alanine are clearly observed depending on the handedness of the metasurface, realizing the reliable identification of small chiral molecules. The corresponding numerical simulations reveal the underlying resonant chiroptical interaction of plasmonic modes of the metasurface and vibrational modes of alanine. Our approach demonstrates a high-sensitive vibrational CD spectroscopic technique, opening up a reliable chiral sensing platform for advanced infrared inspection technologies. No potential conflict of interest relevant to this article was reported.

American Institute of Physics Acta Medica Okayama 2158-3226 9 12 2019 Magnetic characterization change by solvents of magnetic nanoparticles in liquid-phase magnetic immunoassay 125317 EN Katsuya Jinno Graduate School of Interdisciplinary Science and Engineering in Health Systems, Okayama University Bunta Hiramatsu Graduate School of Interdisciplinary Science and Engineering in Health Systems, Okayama University Kenta Tsunashima Graduate School of Interdisciplinary Science and Engineering in Health Systems, Okayama University Kayo Fujimoto Graduate School of Interdisciplinary Science and Engineering in Health Systems, Okayama University Kenji Sakai Graduate School of Interdisciplinary Science and Engineering in Health Systems, Okayama University Toshihiko Kiwa Graduate School of Interdisciplinary Science and Engineering in Health Systems, Okayama University Keiji Tsukada Graduate School of Interdisciplinary Science and Engineering in Health Systems, Okayama University Liquid-phase magnetic immunoassay (MIA) using magnetic nano-particles (MNPs) has been studied as a more rapid method compared to optical methods for inspecting proteins and viruses. MIA can estimate the number of conjugated antibodies without being washed differently from conventional optical immunoassay. However, in the case of the liquid phase, it is considered that the magnetic properties of MNPs are affected by physical properties such as viscosity and impurity substances such as biological substances contained in the blood. In this study, the effect of sodium chloride (NaCl) in buffer and serum solution was evaluated to reveal the effect of serum because the sodium (Na+) and chloride (Cl-) ions in the serum dominate ion balance of blood. The measurement results of AC magnetic susceptibility and a dynamic light scattering (DLS) showed that the aggregation of MNPs was largely affected by the concentration of NaCl. This effect of the NaCl could be explained by shielding of the surface charge of MNPs by ions in the solution. Although the concentrations of NaCl in the buffer and serum solution were almost same, we found that MNPs were aggregated more in their size for those in the serum solution because of other impurities, such as proteins. These results suggest evaluation of effects of the contaminants in serum and optimization of polymer coatings of MNPs could be important factors to realize measurements of magnetic immunoassay with high accuracy. (C) 2019 Author(s). No potential conflict of interest relevant to this article was reported.

American Institute of Physics Acta Medica Okayama 00219606 150 16 2019 Three-phase equilibria in density-functional theory: Interfacial tensions 164701 EN Kenichiro Koga Research Institute for Interdisciplinary Science, Okayama University Joseph O. Indekeu Institute for Theoretical Physics, KU Leuven 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. No potential conflict of interest relevant to this article was reported.

American Institute of Physics Acta Medica Okayama 00219606 144 22 2016 A reference-modified density functional theory: An application to solvation free-energy calculations for a Lennard-Jones solution 224104 EN Tomonari Sumi Department of Chemistry, Faculty of Science, Okayama University Yutaka Maruyama Co-Design Team, Exascale Computing Project, RIKEN Advanced Institute for Computational Science Ayori Mitsutake Co-Design Team, Exascale Computing Project, RIKEN Advanced Institute for Computational Science Kenichiro Koga Department of Chemistry, Faculty of Science, Okayama University In the conventional classical density functional theory (DFT) for simple fluids, an ideal gas is usually chosen as the reference system because there is a one-to-one correspondence between the external field and the density distribution function, and the exact intrinsic free-energy functional is available for the ideal gas. In this case, the second-order density functional Taylor series expansion of the excess intrinsic free-energy functional provides the hypernetted-chain (HNC) approximation. Recently, it has been shown that the HNC approximation significantly overestimates the solvation free energy (SFE) for an infinitely dilute Lennard-Jones (LJ) solution, especially when the solute particles are several times larger than the solvent particles [T. Miyata and J. Thapa, Chem. Phys. Lett. 604, 122 (2014)]. In the present study, we propose a reference-modified density functional theory as a systematic approach to improve the SFE functional as well as the pair distribution functions. The second-order density functional Taylor series expansion for the excess part of the intrinsic free-energy functional in which a hard-sphere fluid is introduced as the reference system instead of an ideal gas is applied to the LJ pure and infinitely dilute solution systems and is proved to remarkably improve the drawbacks of the HNC approximation. Furthermore, the third-order density functional expansion approximation in which a factorization approximation is applied to the triplet direct correlation function is examined for the LJ systems. We also show that the third-order contribution can yield further refinements for both the pair distribution function and the excess chemical potential for the pure LJ liquids. 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 Institute of Physics Acta Medica Okayama 0003-6951 111 24 2017 Robust plasmonic hot-spots in a metamaterial lattice for enhanced sensitivity of infrared molecular detection 243106 EN Atsushi Ishikawa Department of Electrical and Electronic Engineering, Okayama University Shuhei Hara Department of Electrical and Electronic Engineering, Okayama University Takuo Tanaka Metamaterials Laboratory, RIKEN Xiang Zhan NSF Nanoscale Science and Engineering Center, University of California Kenji Tsuruta Department of Electrical and Electronic Engineering, Okayama University High-density and long-lived plasmonic hot-spots are an ideal system for high-sensitive surface-enhanced infrared absorption (SEIRA), but these conditions arc usually incompatible due to unwanted near-field coupling between the adjacent unit structures. Here, by fully controlling plasmonic interference in a metamaterial lattice, we experimentally demonstrate densely packed long-lived quadrupole plasmons for high-sensitive SEIRA. The metamaterial consists of a strongly coupled array of super-and sub-radiant plasmonic elements to exhibit an electromagnetic transparency mode at 1730 cm(-1), which spectrally overlaps with the C=O vibrational mode. In the SEIRA measurement, the C=O mode of poly(methyl methacrylate) molecules is clearly observed as a distinct dip within a transmission peak of the metamaterial. The corresponding numerical simulations reveal that constructive interference uniformly forms coherent quadrupole plasmons over the metamaterial lattice, leading to a stronger molecular signal from the system. Our metamaterial approach provides a robust way to construct ideal hot-spots over the sample, paving the way toward a reliable sensing platform of advanced infrared inspection technologies. Published by AIP Publishing. No potential conflict of interest relevant to this article was reported.

American Institute of Physics Acta Medica Okayama 0003-6951 111 18 2017 Selective electroless plating of 3D-printed plastic structures for three-dimensional microwave metamaterials 183102 EN Atsushi Ishikawa Department of Electrical and Electronic Engineering, Okayama University Taiki Kato Department of Electrical and Electronic Engineering, Okayama University Nobuyuki Takeyasu Department of Chemistry, Okayama University Kazuhiro Fujimori Department of Electrical and Electronic Engineering, Okayama University Kenji Tsuruta Department of Electrical and Electronic Engineering, Okayama University A technique of selective electroless plating onto PLA-ABS (Polylactic Acid-Acrylonitrile Butadiene Styrene) composite structures fabricated by three-dimensional (3D) printing is demonstrated to construct 3D microwave metamaterials. The reducing activity of the PLA surface is selectively enhanced by the chemical modification involving Sn2+ in a simple wet process, thereby forming a highly conductive Ag-plated membrane only onto the PLA surface. The fabricated metamaterial composed of Ag-plated PLA and non-plated ABS parts is characterized experimentally and numerically to demonstrate the important bi-anisotropic microwave responses arising from the 3D nature of metallodielectric structures. Our approach based on a simple wet chemical process allows for the creation of highly complex 3D metal-insulator structures, thus paving the way toward the sophisticated microwave applications of the 3D printing technology. Published by AIP Publishing. No potential conflict of interest relevant to this article was reported.

American Institute of Physics Acta Medica Okayama 0094-243X 1707 2016 Mathematical Formulation and Numerical Simulation of Bird Flu Infection Process within a Poultry Farm 050013 EN Arrival Rince Putri Graduate School Environmental and Life Science, Okayama University Tertia Delia Nova Andalas University Masaji Watanabe Graduate School Environmental and Life Science, Okayama University Bird flu infection processes within a poultry farm are formulated mathematically. A spatial effect is taken into account for the virus concentration with a diffusive term. An infection process is represented in terms of a traveling wave solutions. For a small removal rate, a singular perturbation analysis lead to existence of traveling wave solutions, that correspond to progressive infection in one direction. No potential conflict of interest relevant to this article was reported.

American Institute of Physics Acta Medica Okayama 0094-243X 1707 2016 Numerical study on anaerobic digestion of fruit and vegetable waste: Biogas generation 050017 EN Puteri Kusuma Wardhani Graduate School of Environmental and Life Science, Okayama University Masaji Watanabe Graduate School of Environmental and Life Science, Okayama University The study provides experimental results and numerical results concerning anaerobic digestion of fruit and vegetable waste. Experiments were carried out by using batch floating drum type digester without mixing and temperature setting. The retention time was 30 days. Numerical results based on Monod type model with influence of temperature is introduced. Initial value problems were analyzed numerically, while kinetic parameters were analyzed by using trial error methods. The numerical results for the first five days seems appropriate in comparison with the experimental outcomes. However, numerical results shows that the model is inappropriate for 30 days of fermentation. This leads to the conclusion that Monod type model is not suitable for describe the mixture degradation of fruit and vegetable waste and horse dung. No potential conflict of interest relevant to this article was reported.

American Institute of Physics. Acta Medica Okayama 0003-6951 100 25 2012 Enhanced thermoelectric properties by Ir doping of PtSb₂ with pyrite structure 252104-1 252104-3 EN Yoshihiro Nishikubo Seiya Nakano Kazutaka Kudo Minoru Nohara The effects of Ir doping on the thermoelectric properties of Pt1-xIrxSb2 (x = 0, 0.01, 0.03, and 0.1) with pyrite structure were studied. Measurements of electrical resistivity rho, Seebeck coefficient S, and thermal conductivity kappa were conducted. The results showed an abrupt change from semiconducting behavior without Ir (x = 0) to metallic behavior at x = 0.01. The sample with x = 0.01 exhibited large S and low rho, resulting in a maximum power factor (S-2/rho) of 43 mu W/cmK(2) at 400 K. The peculiar "pudding mold"-type electronic band dispersion could explain the enhanced thermoelectric properties in the metallic state. No potential conflict of interest relevant to this article was reported.

American Institute of Physics. Acta Medica Okayama 0003-6951 98 8 2011 Multiple phosphorus chemical sites in heavily phosphorus-doped diamond 082107-1 082107-3 EN Hiroyuki Okazaki Rikiya Yoshida Takayuki Muro Tetsuya Nakamura Takanori Wakita Yuji Muraoka Masaaki Hirai Hiromitsu Kato Satoshi Yamasaki Yoshihiko Takano Satoshi Ishii Tamio Oguchi Takayoshi Yokoya We have performed high-resolution core level photoemission spectroscopy on a heavily phosphorus (P)-doped diamond film in order to elucidate the chemical sites of doped-phosphorus atoms in diamond. P 2p core level study shows two bulk components, providing spectroscopic evidence for multiple chemical sites of doped-phosphorus atoms. This indicates that only a part of doped-phosphorus atoms contribute to the formation of carriers. From a comparison with band calculations, possible origins for the chemical sites are discussed. No potential conflict of interest relevant to this article was reported.

American Institute of Physics. Acta Medica Okayama 0021-8979 109 4 2011 Spectroscopic evidence of the formation of (V,Ti)O₂ solid solution in VO₂ thinner films grown on TiO₂(001) substrates 043702-1 043702-6 EN Y. Muraoka K. Saeki R. Eguchi T. Wakita M. Hirai T. Yokoya S. Shin We have prepared VO2 thin films epitaxially grown on TiO2(001) substrates with thickness systematically varied from 2.5 to 13 nm using a pulsed laser deposition method, and studied the transport property and electronic states of the films by means of resistivity and in situ synchrotron photoemission spectroscopy (SRPES). In resistivity measurements, the 13-nm-thick film exhibits a metal-insulator transition at around 290 K on cooling with change of three orders of magnitudes in resistivity. As the film thickness decreases, the metal-insulator transition broadens and the transition temperature increases. Below 4 nm, the films do not show the transition and become insulators. In situ SRPES measurements of near the Fermi level valence band find that the electronic state of the 2.5-nm-thick film is different than that of the temperature-induced insulator phase of VO2 itself although these two states are insulating. Ti 2p core-level photoemission measurements reveal that Ti ions exist near the interface between the films and TiO2 substrates, with a chemical state similar to that in (V,Ti)O-2 solid solution. These results indicate that insulating (V,Ti)O-2 solid solution is formed in the thinner films. We propose a simple growth model of a VO2 thin film on a TiO2(001) substrate. Near the interface, insulating (V,Ti) O-2 solid solution is formed due to the diffusion of Ti ions from the TiO2 substrate into the VO2 film. The concentration of Ti in (V,Ti) O-2 is relatively high near the interface and decreases toward the surface of the film. Beyond a certain film thickness (about 7 nm in the case of the present 13-nm-thick film), the VO2 thin film without any Ti ions starts to grow. Our work suggests that developing a technique for preparing the sharp interface between the VO2 thin films and TiO2 substrates is a key issue to study the physical property of an ultrathin film of "pure" VO2, especially to examine the presence of the novel electronic state called a semi-Dirac point phase predicted by calculations. No potential conflict of interest relevant to this article was reported.

American Institute of Physics. Acta Medica Okayama 0021-8979 108 4 2010 Bulk and surface physical properties of a CrO₂ thin film prepared from a Cr₈O₂₁ precursor 043916-1 043916-4 EN K. Iwai Y. Muraoka T. Wakita M. Hirai T. Yokoya Y. Kato T. Muro Y. Tamenori We have prepared a CrO(2) thin film by chemical vapor deposition from a Cr(8)O(21) precursor and studied the bulk and surface physical properties. The CrO(2) thin film is grown on a TiO(2) (100) substrate by heating of a Cr(8)O(21) precursor and TiO(2) (100) substrate together in a sealed quartz tube. The prepared film is found from x-ray diffraction analysis to be an (100)-oriented single phase. The magnetization and resistivity measurements indicate that the film is a ferromagnetic metal with a Curie temperature of about 400 K. Cr 3s core-level and valence band photoelectron spectroscopy spectra reveal the presence of a metallic CrO(2) in the surface region of the film. Our work indicates that preparation from a Cr(8)O(21) precursor is promising for obtaining a CrO(2) thin film with the metallic surface. No potential conflict of interest relevant to this article was reported.

American Institute of Physics. Acta Medica Okayama 0021-8979 107 7 2010 Room temperature ferromagnetic behavior in the hollandite-type titanium oxide 073910-1 073910-6 EN K. Noami Y. Muraoka T. Wakita M. Hirai Y. Kato T. Muro Y. Tamenori T. Yokoya A hollandite-type K(x)Ti(8)O(16) polycrystalline sample has been prepared and studied by magnetization, resistivity and x-ray photoelectron spectroscopy (XPS). Room temperature ferromagnetic behavior is observed in the magnetic hysteresis measurement. The sample shows a semiconductive temperature dependence in the resistivity measurement. Analysis of the Ti 2p(3/2) core-level XPS spectrum indicates that the titanium ions have a mixed valence of Ti(4+) and Ti(3+). In addition, the valence band spectrum reveals that the 3d electrons tend to localize on Ti(3+) ions in the hollandite-type TiO(2) lattice. Also, analysis of the valence band spectrum shows that the prepared sample is a wide-gap oxide with a band gap of 3.6 eV. These results indicate that the present hollandite-type K(x)Ti(8)O(16) sample can be classified as a TiO(2)-based wide-gap semiconductor with Curie temperature above room temperature. Room temperature ferromagnetism (RTFM) decreases in the sample prepared under a strong reducing gas atmosphere, accompanied with the decrease in the resistivity. The results imply that the localized 3d electrons are responsible for the RTFM of the K(x)Ti(8)O(16) sample. 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 127 6 2007 Line and boundary tensions on approach to the wetting transition EN Kenichiro Koga B. Widom 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. 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 121 15 2004 Hydrophobic effect in the pressure-temperature plane 7304 7312 EN Kenichiro Koga The free energy of the hydrophobic hydration and the strength of the solvent-mediated attraction between hydrophobic solute molecules are calculated in the pressure-temperature plane. This is done in the framework of an exactly soluble model that is an extension of the lattice model proposed by Kolomeisky and Widom [A. B. Kolomeisky and B. Widom, Faraday Discuss. 112, 81 (1999)]. The model takes into account both the mechanism of the hydrophobic effect dominant at low temperatures and the opposite mechanism of solvation appearing at high temperatures and has the pressure as a second thermodynamic variable. With this model, two boundaries are identified in the pressure-temperature plane: the first one within which the solubility, or the Ostwald absorption coefficient, decreases with increasing temperature at fixed pressure and the second one within which the strength of solvent-mediated attraction increases with increasing temperature. The two are nearly linear and parallel to each other, and the second boundary lies in the low-temperature and low-pressure side of the first boundary. It is found that a single, near-linear relation between the hydration free energy and the strength of the hydrophobic attraction holds over the entire area within the second boundary in the pressure-temperature plane. (C) 2004 American Institute of Physics. 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.