Molecular Dynamics Examination of Microheterogeneity in Liquids

R. D. Mountain

Objective: To develop models that describe the size of microheterogeneous regions (on molecular length scales) in supercritical fluids and in aqueous mixtures.

Problem: Reactions involving solutes can be significantly influenced by the presence of void regions (supercritical fluids) or by compositional heterogeneities (aqueous mixtures) if the solutes are nonuniformly distributed in the fluid. Knowledge of when and where such nonuniformities occur and how different solutes are partitioned by such environments is lacking and limits our ability to predict and optimize reactions.

Approach: Molecular dynamics simulations of water, carbon dioxide, acetonitrile, and water-acetonitrile mixtures have demonstrated that existing model potentials for these fluids provide good descriptions of thermal properties. Simulations are being used to determine the size of clusters and voids in supercritical water and in supercritical carbon dioxide. Simulations are also being used to determine the structure of compositional heterogeneities in water-acetonitrile liquid mixtures over a wide range of compostions and temperatures.

Results and Future Plans: Now that the conditions where microheterogeneity occurs for these fluids are known, it is a straightforward task to introduce various solutes into the system.

The simulations are being used to examine the solvation and degree of association of ions in water for ambient and supercritical conditions. Related studies of ions in water-acetonitrile mixtures will be made. Also, the solvation of polar and nonpolar solutes will be examined for the mixtures and for supercritical fluids that relate to experiments underway in the Division.

Publications:

Mountain, R. D., "Voids and clusters in expanded water," J. Chem. Phys. 110, 2109 (1999).