Molecular-dynamics simulation of aqueous mixtures: Methanol, acetone, and ammonia

Effective pair-potential models, parametrized to the properties of the pure liquids, have been used in molecular-dynamics simulations of aqueous (binary) mixtures containing methanol, ammonia, or acetone. Results are reported for thermodynamic and structural properties, self-diffusion coefficients, and reorientational correlation times. There is fair agreement with a wide variety of experimental data. The pattern of hydrogen bonding and the distribution of hydrogen-bond lifetimes in the simulated mixtures have been investigated. The observed anomalous behavior of methanol and acetone solutions appears to be related to specific features of the hydrogen bonding—namely, the ability of these molecules to exhibit enhanced acceptor character. As a consequence of the assumed intermolecular potentials, the balance between the competing effects of hydrophobic hydration of methyl groups and hydrogen bonding to oxygen atoms is tipped towards the latter. A number of interesting structural effects have been noted. In particular, the distribution function g(R) for water oxygen atoms sharpens appreciably on addition of the second component. The structure of the water–ammonia system is discussed in detail. There is good accord with available x-ray data at low concentration, but there are no measurements available to test the predictions at high concentration.