Solvation response of polar liquid mixtures: Water-dimethylsulfoxide

The solvation dynamics following the instantaneous creation of a positive o r negative electronic charge in a previously neutral solute immersed in dif ferent water-dimethyl sulfoxide (DMSO) mixtures, spanning the entire compos ition range, is analyzed by molecular dynamics simulations. The solvation r esponses are strongly dependent on the sign of the solute charge, being con siderably faster in the presence of cations for all mixtures considered. In terms of the composition dependence, the mixtures' solvation response to t he creation of the anion departs substantially from the pure solvents', whe reas for the cation, the mixtures' responses are close to those exhibited b y pure DMSO. In the case of anions, the mixture overall solvation time, def ined as the time integral of the nonequilibrium response, can be as large a s ten times the solvation time in pure DMSO, the slowest of the two cosolve nts. The DMSO contribution to the mixtures' solvation response may present an intriguing negative branch in the rotational-diffusion regime which pers ists for times much longer than the time scales typically found in other po lar liquids and mixtures. This negative portion is nearly cancelled by an e qually long-lasting positive contribution from water, resulting in a fast-d ecaying, total response curve which is typical of many polar liquid environ ments. This behavior is rationalized in terms of the time evolution of the first solvation shell around each type of solute. (C) 1999 American Institu te of Physics. [S0021-9606(99)51325-1].