Molecular dynamics simulations of acetonitrile/dimethyl sulfoxide liquid mixtures

Binary liquid mixtures of dimethyl sulfoxide and acetonitrile at the three molar fractions 0.25, 0.50, and 0.75 have been investigated by molecular dynamics computer simulations. Thermodynamic states corresponding to liquid-vapor coexistence at a temperature of 298 K were considered. Intermolecular interactions were described by potential models of the site-site (12-6) Lennard-Jones plus Coulomb type that have been developed for the description of the pure liquids. Dimethyl sulfoxide has been represented by four interactions sites and acetonitrile by a three- as well as a six-site potential model. We have calculated thermodynamic properties and the intermolecular pair distribution functions. Intermolecular interaction energies indicate deviations from the behavior of ideal mixtures. The local mole fraction analysis demonstrates that dimethyl sulfoxide is preferentially solvated by acetonitrile and that the first solvation shell surrounding acetonitrile molecules is significantly enriched by dimethyl sulfoxide. The nonideal behavior in the mixtures is not affected by the choice of the three- or the six-site potential model for acetonitrile. Orientational correlations of dipole vectors within the first solvation shells indicate that the relative molecular orientations found in pure acetonitrile and dimethyl sulfoxide are maintained in the mixtures. Parallel and antiparallel dipole-dipole configurations determine first shell acetonitrile-dimethyl sulfoxide configurations. Dynamical features of the mixtures are discussed in terms of diffusion constants and orientational correlation times as obtained from the time correlation functions for linear velocities and molecular dipole moments, respectively. Computed relaxation times indicate faster reorientational motion for dimethyl sulfoxide if acetonitrile is added. In contrast, the orientational dynamics of acetonitrile becomes stronger correlated upon dilution with dimethyl sulfoxide. The diffusion coefficients for both compounds follow this tendency.