Study of anomalous mobility of polar molecular solutions by means of the site-site memory equation formalism

In this work, the memory equation approach is applied for theoretical study of dynamics of polar molecular liquids described by the interaction site model. The study includes the temperature-density (pressure) dependence of the translational diffusion coefficients D and orientational relaxation times τ for infinitely dilute solutions of acetonitrile and methanol in water, and methanol in acetonitrile. Calculations are performed over the range of temperatures and densities employing the SPC/E model for water and optimized site-site potentials for acetonitrile and methanol. Despite an approximate character of the model potentials and closure relation used, the theory is able to reproduce qualitatively all main features of temperature and density dependences of D and τ observed in computer and real experiments. In particular, anomalous behavior, i.e., the increase in mobility with density (pressure), is obtained for D and τ of methanol in water, while acetonitrile in water or methanol in acetonitrile do not show deviations from the usual. The observed enhancement in the molecular mobility is interpreted in accordance with the concept by Yamaguchi et al. [T. Yamaguchi, S.-H. Chong, F. Hirata, J. Chem. Phys. 119 (2003) 1021.], i.e., in terms of two competing origins of friction, which interplay with each other as density increases: the collisional and dielectric frictions that have tendency, respectively, to strengthen and weaken with increasing density.