Microscopic origin of breakdown of Stokes-Einstein relation in binary mixtures: Inherent structure analysis.

Abstract

Aqueous binary mixtures often exhibit dramatic departure from the predicted hydrodynamic behavior when transport properties are plotted against composition. We show by inherent structure (IS) analysis that this sharp composition dependent breakdown of the Stokes-Einstein relation can be attributed to the non-monotonic variation in the average inherent structure energy of these mixtures. Further IS analysis reveals the existence of a unique ground state, stabilized by both the formation of an optimum number of H-bonds and a favorable hydrophobic interaction at this composition. The surprisingly sharp turnaround behavior observed in the effective hydrodynamic radius also owes its origin to the same combination of these two factors. Interestingly, the temperature dependence of isothermal compressibility shows a minimum at the particular composition. Extensive studies on water-dimethyl sulfoxide and water-ethanol mixtures using two different force-fields of water reveal many features that are nearly universal. A justification of this quasi-universal behavior is provided in terms of a mode-coupling theory (MCT) of viscosity, which can serve as the starting point of a remarkable correlation observed with the nearest neighbor structure, as captured by the first peaks of the radial distribution function, and the slowdown in the intermediate scattering function at intermediate wavenumbers. Therefore, the formation of the local structure captured through IS analysis can be correlated with the MCT.