  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 t 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
t observed in computer and real experiments. In particular, anomalous behavior,
i.e. the increase in mobility with density(pressure), is obtained for D and t
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. [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.
