The structure and dynamics of liquid water at the interface with three solid surfaces has been investigated via molecular dynamics simulation. The three surfaces include a flat hydrophobic surface, an atomically rough hydrophobic surface, and a contrasting, hydrophilic, fully hydroxylated silica surface. The results of analysis show that, as expected, the solvent near each of the two hydrophobic surfaces behaves essentially equivalently, with loss of hydrogen bonding at the interface. For the hydroxylated surface, surface–solvent hydrogen bonding is stronger than interactions in the bulk solvent, with the nearest solvent layer interacting specifically with up to three surface hydroxyl groups. Nevertheless, distinct structural perturbation of the solvent extends in every case no more than about 10 Å from the surface, and the perturbation is only strong in the immediate solvation layer. Furthermore, the corresponding dynamical perturbation of the solvent, as measured by the diffusion rates and reorientation times in comparison to the bulk, is always relatively small. For the hydrophilic case, it is largest, but even here it is less than a factor of 5 at the immediate interface and less than a factor of 2 in the second hydration layer. The residence time for solvent at the interface is found to be insensitive to the hydrophilicity of the surface. Calculated nuclear magnetic resonance (NMR) order parameters for the solvent are found to reflect solvent orientational ordering, but are shown not to be distinctive of the nature of that order.

We present results of computer simulations of the mobilities of the alkali metal ions (Li + , Na + , K + , Rb + , and Cs + ) and the halides (F -, Cl -, Br -, and I -) at 25°C using the SPC/E model for water and ion-water parameters fitted to the binding energies of small clusters of ions. A simple truncation of the ion-water and waterwater potentials was used, and the mobilities calculated from the mean square displacement and the velocity autocorrelation functions, respectively, were found to be in good agreement with each other. The calculations demonstrate, for the first time, cation and anion mobilities that fall on separate curves, as functions of ion size, with distinct maxima. This is in complete accord with experimental trends observed in water at 25°C. The cation mobilities are also in better agreement with the measured values than the calculations done earlier (J. Chem. Phys. 1994, 101, 6964) using the TIP4P model. The mobilities of the halides calculated here for the SPC/E model are however slightly lower than the experimental results. The residence times of water in the hydration shells around an ion are found to decrease dramatically with its size. Stereoscopic pictures show that the structure of the solvent cage around an ion is qualitatively different for the larger ions, implicating both solvent dynamics and structure as important factors in explaining ion mobility in aqueous systems.

We describe a series of molecular dynamics simulations performed on model cation-water systems at 25 °C representing the behavior of Li+, Na+, K+, Rb+, and Cs+ in an electric field of 1.0 V/nm and in its absence. The TIP4P model was used for water and TIPS potentials were adapted for the ion-water interactions. The structure of the surrounding water molecules around the cations was found to be independent of the applied electric field. Some of the dynamic properties, such as the velocity and force autocorrelation functions of the cations, are also field independent. However, the mean-square displacements of the cations, their average drift velocities, and the distances traveled by them are field dependent. The mobilities of the cations calculated directly from the drift velocity or the distance traveled by the ion are in good agreement with each other and they are in satisfactory agreement with the mobilities determined from the mean-square displacement and the velocity autocorrelation function in the absence of the field. They also show the same trends with ionic radii that are observed experimentally; the magnitudes are, however, smaller than the experimental values in real water by almost a factor of 2. It is found that the water molecules in the first solvation shell around the small Li+ ion are stuck to the ion and move with it as an entity for about 190 ps, while the water molecules around the Na+ ion remain for 35 ps, and those around the large cations stay for 8–11 ps before significant exchange with the surroundings occurs. The picture emerging from this analysis is that of a solvated cation whose mobility is determined by its size as well as the static and dynamic properties of its solvation sheath and the surrounding water. The classical solventberg model describes the mobility of Li+ ions in water adequately but not those of the other ions.

We report on the development of force-field parameters for accurately modeling lithium and hydrox-ide ions in ethanol in solution. Based on quantum calculations of small molecular clusters mimicking the solvent structure of individual ions as well as the solvated LiOH dimer, significant improvements of off-the-shelf force-fields are obtained. The quality of our model is demonstrated by comparison to ab initio molecular dynamics of the bulk solution and to experimental data available for ethanol/water mixtures.

The friction and diffusion coefficients of a massive Brownian particle in a mesoscopic solvent are computed from the force and the velocity autocorrelation functions. The mesoscopic solvent is described in terms of free streaming of the solvent molecules, interrupted at discrete time intervals by multiparticle collisions that conserve mass, momentum, and energy. The Brownian particle interacts with the solvent molecules through repulsive Lennard-Jones forces. The decays of the force and velocity autocorrelation functions are analyzed in the microcanonical ensemble as a function of the number N of solvent molecules and Brownian particle mass and diameter. The simulations are carried out for large system sizes and long times to assess the N dependence of the friction coefficient. The decay rates of these correlations are confirmed to vary as N(-1) in accord with earlier predictions. Hydrodynamic effects on the velocity autocorrelation function and diffusion coefficient are studied as a function of Brownian particle mass and diameter.