Anomalous orientational relaxation of solute probes in binary mixtures

Abstract

The orientation of a solute probe in a binary mixture often exhibits multiple relaxation times at the same solvent viscosity but different compositions [Beddard et al., Nature (London) 294, 145 (1981)]. In order to understand this interesting observation, we have carried out (NPT) molecular dynamics simulation study of rotation of prolate ellipsoids in binary mixtures. The simulations show that for a broad range of model parameters the experimental behavior can be reproduced. The plot of orientational relaxation time versus the solvent viscosity, the latter changed by varying the composition, shows a nonmonotonic viscosity dependence. The nature of the plot is found to depend on the system parameters, especially on the solute-solvent interactions. A mode coupling theoretical analysis of this complex dependence of the rotational relaxation time τ_R on the binary viscosity η is presented. The theory can qualitatively explain the origin of the multiple relaxation time at the same viscosity.