Non-equilibrium solvation dynamics in water-DMSO binary mixture: Composition dependence of non-linear relaxation

Because of a larger number of intermolecular interactions and configurations available to them, aqueous binary mixtures exhibit a variety of dynamics that are not seen in pure liquids, often hard to understand or predict, and have attracted considerable interest recently. Among all such solutions, mixtures of water and dimethyl sulfoxide (DMSO) stand out for their unique role in chemistry and biology. The low DMSO concentration regime of the water-dimethyl sulfoxide (DMSO) mixture is relevant in wide ranging chemical and biological processes. Interestingly, this low concentration regime is known to display a string of yet unexplained anomalies. We probe these anomalies in atomistic simulations by studying (i) equilibrium solvation dynamics both in the ground and the excited states of the probe separately and (ii) the non-equilibrium solvation dynamics subsequent to excitation at time t = 0 and then following the solvation process. The latter needed a large number of simulations to obtain a reliable average. We carried out such studies across a large number of compositions of the water-DMSO mixture. We find that the usually employed linear response approximation breaks down at those concentrations where binary mixtures display other anomalies. The non-linearity is reflected in substantially different solvent responses in the ground and in the excited states of the solute probe indole and also in non-equilibrium solvation. The difference is maximum near 20%-35% of the DMSO concentration regime.