Dynamic heterogeneity in aqueous ionic solutions

It is well known that supercooled liquids have heterogeneous dynamics, but it is still unclear whether dynamic heterogeneity also exists in aqueous ionic solutions at room or even higher temperatures. In this work, taking KSCN aqueous solutions as an example, we identify by molecular dynamics simulation that dynamics of ionic solutions at a finite concentration are heterogeneous at room and even higher temperatures. Our results indicate that thermal movements of K+ and SCN- deviate from the Gaussian distribution in time and space, as demonstrated by a non-Gaussian parameter and the self-van Hove function. The dynamic susceptibility is nonzero at intermediate times for both K+ and SCN-. The self-intermediate scattering function of ions decays in a stretched exponential way with an exponent smaller than one. The dynamics of the solution are more homogeneous at a higher temperature. Since transient ion clusters of different sizes decay with different lifetimes and exponents, we propose that the dynamic heterogeneity is introduced by transient cluster formation and dissociation in ionic solutions, which leads to a mixed relaxation scenario. Variants of the Stokes-Einstein relation are found to break down into a fractional form analogous to supercooled liquids, but the original Stokes-Einstein relation is indeed valid if taking into account the temperature dependence of the effective hydrodynamic radius. Overall, despite some quantitative differences, the dynamic heterogeneity in aqueous ionic solutions at room or higher temperatures is qualitatively analogous to that in supercooled liquids at a much lower temperature.