Hiroki Nada

An intermolecular potential model for the simulation of ice and water near the melting point: A six-site model of H2O

An intermolecular potential model of H2O with six interaction sites is proposed. The model is developed for the simulation of ice and water near the melting point. Parameters in the potential are determined to reproduce the real melting point of ice, and densities of ice and water near the melting point, which are predicted by calculating derivatives of the free energies and volumes of ice and water against potential parameters. Free energy calculations are carried out for several ice structures and water, and the results are compared with those obtained in four- and five-site models, which are currently in use. It is shown that, only in the present six-site model, the proton-disordered hexagonal ice is the stable structure at the melting point, as in real ice. The melting point of the proton-disordered hexagonal ice at 1 atm is estimated to be 271±9 K in the present model, which is in good agreement with the real melting point of 273.15 K. Moreover, results of Monte Carlo simulations of ice and water sho...

Growth Inhibition Mechanism of an Ice-Water Interface by a Mutant of Winter Flounder Antifreeze Protein : A Molecular Dynamics Study

Molecular dynamics (MD) simulations of a growing ice-water interface of a pyramidal {2021} plane in the presence of a mutant of winter flounder antifreeze protein (AFP) were conducted. Simulation results indicated that the AFP was partially surrounded by ice grown at the pyramidal interface. The AFP stably bound to the interface only when AFP hydrophobic residues bound to ice. Simulation results also indicated a drastic decrease in the growth velocity of the ice surrounding the stably bound AFP, in agreement with ice growth inhibition processes that have been observed in real systems. We confirmed that the decrease in the growth velocity of ice was attributable to the melting point depression caused by the Gibbs-Thomson effect. Simulation results suggested that the growth of ice surrounding the AFP is needed to promote stable AFP binding to the interface and subsequent ice growth inhibition. MD simulations of a growing ice-water interface of a prismatic {10_10} plane were also conducted. Neither the stable binding of the AFP to the interface nor the decrease in the growth velocity occurred for the prismatic plane. These results agree with the fact that AFPs inhibit the growth of ice only on the pyramidal planes in real systems.

Anisotropic Properties of Ice/Water Interface: A Molecular Dynamics Study

Ice/water interface structures were studied using a molecular dynamics method. The simulations were carried out for both ice {0001}/water and ice {100}/water interfaces. The results strongly suggest that each interface has a diffuse structure and that the physical properties of the interfacial region differ from each other between the interfaces. The anisotropy in the structures and dynamic properties of ice/water interfaces, that is, the difference in the structures and dynamic properties between the interfaces were observed for the first time in this molecular dynamics simulation.

Anisotropy in Growth Kinetics of Tetrahydrofuran Clathrate Hydrate : A Molecular Dynamics Study

The growth kinetics of a tetrahydrofuran (THF) clathrate hydrate at the interface between the clathrate and an aqueous THF solution were investigated by means of a molecular dynamic simulation. The simulation was carried out for the interface of both the {100} and {111} planes of the THF clathrate. The simulation indicated the same anisotropic growth as that observed in real systems: the growth of the THF clathrate was much slower at the {111} interface than at the {100} interface. When the THF clathrate grew, THF molecules that were dissolved in the solution first were arranged at both large and small cage sites on the interface. Subsequently, the formation of cages by H(2)O molecules occurred in regions surrounded or sandwiched by those arranged THF molecules. As the formation of cages progressed, the THF molecules that had once been arranged at small cage sites gradually moved away from the sites, and finally the structure of the clathrate was completely formed. Simulation results strongly suggested that the rate-determining process for clathrate growth was the rearrangement of THF molecules at the interface from a disordered state to a state in which THF molecules were ideally arranged at large cage sites only. This rearrangement occurred much more slowly at the {111} interface than at the {100} interface, owing to the formation of a modified structure in which large and small cages were formed at opposite positions of the {111} interface. The anisotropic growth kinetics of the THF clathrate, which were obtained in this study, are consistent with the fact that growth shapes of THF clathrates in real systems are octahedral with flat {111} planes.

Tuning the Stability of CaCO3 Crystals with Magnesium Ions for the Formation of Aragonite Thin Films on Organic Polymer Templates

Thin-film growth of aragonite CaCO3 on annealed poly(vinyl alcohol) (PVA) matrices is induced by adding Mg(2+) into a supersaturated solution of CaCO3. Both the growth rate and surface morphology of the aragonite thin films depend upon the concentration of Mg(2+) in the mineralization solution. In the absence of PVA matrices, no thin films are formed, despite the presence of Mg(2+). Molecular dynamics simulation of the CaCO3 precursor suggests that the transition of amorphous calcium carbonate to crystals is suppressed in the presence of Mg(2+). The role for ionic additives in the crystallization of CaCO3 on organic templates obtained in this study may provide useful information for the development of functional hybrid materials.

Effects of magnesium ions and water molecules on the structure of amorphous calcium carbonate: a molecular dynamics study.

Molecular dynamics simulations were conducted to elucidate the effects of Mg(2+) and H2O additives on the structure of amorphous calcium carbonate (ACC). New potential parameters for Mg(2+) ions were developed. The distribution function of the angle formed by three nearest-neighbor atoms was introduced to analyze the short-range local structure of ACC. The simulation indicated that ACC had a weakly ordered local structure resembling the local structure of a CaCO3 crystal. The local structure of pure ACC resembled that of vaterite. The formation of the vaterite-like local structure was hindered by Mg(2+) ions, whereas H2O molecules did not significantly influence the structure of ACC when the fraction of H2O molecules was low. However, when the fraction of H2O was high, the formation of a monohydrocalcite-like local structure was promoted. The effects of the additives on the structure of ACC were verified using the size of the additives and the interaction between the additives and CaCO3. The simulated structure of ACC was compared with the structure of CaCO3 crystals nucleated through the formation of ACC particles in real systems.

Development of Nanostructured Water Treatment Membranes Based on Thermotropic Liquid Crystals: Molecular Design of Sub‐Nanoporous Materials

Abstract Supply of safe fresh water is currently one of the most important global issues. Membranes technologies are essential to treat water efficiently with low costs and energy consumption. Here, the development of self‐organized nanostructured water treatment membranes based on ionic liquid crystals composed of ammonium, imidazolium, and pyridinium moieties is reported. Membranes with preserved 1D or 3D self‐organized sub‐nanopores are obtained by photopolymerization of ionic columnar or bicontinuous cubic liquid crystals. These membranes show salt rejection ability, ion selectivity, and excellent water permeability. The relationships between the structures and the transport properties of water molecules and ionic solutes in the sub‐nanopores in the membranes are examined by molecular dynamics simulations. The results suggest that the volume of vacant space in the nanochannel greatly affects the water and ion permeability.

Anisotropy in geometrically rough structure of ice prismatic plane interface during growth: Development of a modified six-site model of H2O and a molecular dynamics simulation

This paper presents a modified version of the six-site model of H2O [H. Nada and J. P. J. M. van der Eerden, J. Chem. Phys. 118, 7401 (2003)]. Although the original six-site model was optimized by assuming the cut-off of the Coulomb interaction at an intermolecular distance of 10 Å, the modified model is optimized by using the Ewald method for estimating the Coulomb interaction. Molecular dynamics (MD) simulations of an ice-water interface suggest that the melting point of ice at 1 atm in the modified model is approximately 274.5 K, in good agreement with the real melting point of 273.15 K. MD simulations of bulk ice and water suggest that the modified model reproduces not only the structures and density curves of ice and water, but also the diffusion coefficient of water molecules in water near the melting point at 1 atm. Using the modified model, a large-scale MD simulation of the growth at an ice-water interface of the prismatic plane is performed to elucidate the anisotropy in the interface structure during growth. Simulation results indicate that the geometrical roughness of the ice growth front at the interface is greater in the c-axis direction than in the direction normal to the c-axis when it is analyzed along the axes parallel to the prismatic plane. In addition, during the growth at the interface, the transient appearance of specific crystallographic planes, such as a {202¯1} pyramidal plane, occurs preferentially at the ice growth front. The effect of different ensembles with different simulation systems on the anisotropy in the interface structure is also investigated.