THEORY OF ION SOLVATION DYNAMICS IN MIXED DIPOLAR SOLVENTS

Time dependent density functional theory in its ''extended linear'' or ''surrogate'' form is used to investigate the dynamics of selective i on solvation in binary dipolar solvents. It is shown that simple analy tical approximations that trap the basic physics of the solvation proc ess can be obtained. In particular, it is found that the relaxation of :the solvent number densities about a charged solute is governed by tw o distinct modes clearly associated with electrostriction and redistri bution processes. This is consistent with the physical picture suggest ed by molecular dynamics (MD) simulations. The solvent polarization re laxation is also dominated by two modes associated with the-two rotati onal diffusion constants of the binary solvent. In addition to the ana lytical approximations, full numerical solutions of the extended linea r theory are obtained and the dependence of the relaxation on solvent density and solute charge is discussed. Detailed comparisons of the th eory with MD simulations for a closely related model indicate that the theory is qualitatively correct, but quantitatively poor generally pr edicting relaxation rates which are too fast. This is due mainly to th e neglect of inertial or non-Markovian effects in the theoretical appr oach. (C) 1998 American Institute of Physics. [S0021-9606(98)50132-8]