The applicability of two frequently used interaction potentials for water, the five-site ST2 model and the four-site TIP4P model, is investigated in computer simulations of water droplets of varying cluster size from N=64 to N=512. The orientation of the water molecules in the surface region is investigated for the both models. Surface properties, such as work of cluster formation, local density profiles, kinetic and total energy profiles, and pressure profiles as a function of the droplet size, obtained using the two models are compared. Moreover, the surface potential and the electric potential profiles are calculated. Surface tension is calculated and its dependence on the cluster size is investigated. It is found that surface properties are very sensitive to the used potential models. For example, the water molecules are found to lie differently in the inner region of the surface layer, the ST2 molecules being predominantly perpendicular to the surface, while the TIP4P molecules lie mainly parallel to the surface. In the outer surface region, the molecular planes are perpendicular to the surface for the both models. The TIP4P model allows a calculation of the surface tension, giving a value 54 mN/m which is somewhat lower than the experimental value for the plane surface, while for the ST2 model, larger clusters are needed than those treated in the present study.

The critical properties of the Lennard-Jones fluid in slitlike pores of different widths have been studied by the Gibbs ensemble Monte Carlo method and the lattice gas model. Graphite pores up to 10 molecular diameters in width and similar pores with weaker solid−fluid interactions have been considered. Strong layering of the adsorbate in the graphite pores confirmed the applicability of the lattice gas model to such systems. The vapor−liquid phase diagrams for the confined fluids obtained with the two methods are in reasonable agreement with each other. Linear dependence of the critical temperature on the inverse pore width was found. It is demonstrated that the critical temperature may depend strongly on the strength of the sold−fluid interactions. The lattice gas model showed nonmonotonic dependence of the critical density on the pore width.

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Clustering of water molecules on charged particles has been studied using the method of molecular dynamics simulations. A selected set of model metal and halogen ions, carrying both positive and negative charges, is chosen as nucleation centers for water molecules. The influence of the ion charge, its size, and short-range interactions on the local structure, and kinetic characteristics are investigated for the ion-centered clusters of 20 and 30 water molecules at 200 and 300 K, respectively. It is shown, based on radial densities, energy, polarization profiles, and orientational distribution functions, that the local water structure in the clusters becomes perturbed to a larger degree around negative ions compared to ions carrying a corresponding positive charge. The electric field of an anion is more effectively screened by the first hydration shell, resulting in a weaker dependence of the relaxation processes on the ion field in the second hydration shell. The dependence of the work of cluster formation on the ion radius is more pronounced in the case of negative ions. The dependence of the properties on the cluster size are investigated. It was found that for the water–alkali ion system potentials used, the dependence of the work of cluster formation on the number of water molecules has a minimum at about N=30. The obtained work of cluster formation for the anions was found to be consistently less than that for the cations. Unfortunately, this work of formation does not alone provide an answer to the still unsolved problem of sign preference connected to water condensation on charged particles in atmospheric conditions.

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Water clusters, formed on the aqueous ions H+, OH-, or H3O+ are investigated using the molecular dynamics simulation method. Temperature dependence of various cluster properties is studied from 150 to 300 K. The dependence of both the cluster structure and its thermodynamic characteristics on the cluster size (N) is analyzed at 300 K. It is found, based on the energy criteria, that the cluster around the hydronium (H3O+) ion is more stable for all cluster sizes, while the work-of-formation criteria predicted the water cluster on the protonated ion to be more stable when N > 30. On the bases of work-of-cluster-formation and its size dependent, the nucleation barrier was found to be lower for clusters around hydronium ion than those formed around the hydroxyl ions.

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