Phase diagram of water under an applied electric field.

Simulations are used to investigate for the first time the anisotropy of the dielectric response and the effects of an applied electric field E(ex) on the phase diagram of water. In the presence of electric fields ice II disappears from the phase diagram. When E(ex) is applied in the direction perpendicular to the ac crystallographic plane the melting temperatures of ices III and V increase whereas that of ice Ih is hardly affected. Ice III also disappears as a stable phase when E(ex) is applied in the direction perpendicular to the ab plane. E(ex) increases by a small amount the critical temperature and reduces slightly the temperature of the maximum density of liquid water. The presence E(ex) modifies all phase transitions of water but its effect on solid-solid and solid-fluid transitions seems to be more important and different depending on the direction of E(ex).

Viscous water meniscus under nanoconfinement.

A dramatic transition in the mechanical properties of water is observed at the nanometer scale. For a water meniscus formed between two hydrophilic surfaces in the attractive region, with < or = 1 nm interfacial separation, the measured viscosity is 7 orders of magnitude greater than that of bulk water at room temperature. Grand canonical Monte Carlo simulations reveal enhancement in the tetrahedral structure and in the number of hydrogen bonds to the surfaces as a source for the high viscosity; this results from a cooperative effect of hydrogen bonding of water molecules to both hydrophilic surfaces.

Surface coverages of bonded-phase ligands on silica: a computational study.

A computational study of the packing of various bonded-phase ligands bound to chromatographic silica is presented. This is done with the intention of examing the type of surface structures that are typically found in real chromatographic systems. Utilizing the surface structure of the (111) face of the beta-cristobalite crystal, it is shown that the maximum surface coverages of dimethyloctylsilane, dimethyloctadecylsilane, triisopropylsilane, diisopropyloctylsilane, and diisopropyloctadecylsilane can be calculated that are in good agreement with experiment. The maximum surface coverages are also calculated for the (100) face of the beta-cristobalite crystal and for a set of random silica surfaces. The coverages for the latter two surfaces types are found to be significantly lower than the experimental values for chromatographic silica surfaces. These results further suggest that chromatographic silica surfaces may resemble crystalline surface sites similar to the (111) face of beta-cristobalite, as has been previously suggested in the literature. Hence, these structures can be reliably utilized in molecular simulations of bonded-phase chromatography where the atomic-level detail of the silica surface has been previously lacking.

Simulating the adsorption of alkanes in zeolites.

The configurational-bias Monte Carlo technique is applied to simulate the adsorption of long chain alkanes in zeolites. This simulation technique is several orders of magnitude more efficient than conventional methods that can be used to simulate the adsorption of long chain alkanes. The calculated heats of adsorption are found to be in excellent agreement with experimental data. The results show a surprising chain length dependence of the heats of adsorption. This dependence has a simple molecular explanation in terms of preferential siting of the long chain alkanes.