Water-in-Salt: Fast Dynamics, Structure, Thermodynamics, and Bulk Properties

Abstract

We present concentration-dependent dynamics of highly concentrated LiBr solutions and LiCl temperature-dependent dynamics for two high concentrations and compare the results to those of prior LiCl concentration-dependent data. The dynamical data are obtained using ultrafast optical heterodynedetected optical Kerr effect (OHD-OKE). The OHD-OKE decays are composed of two pairs of biexponentials, i.e., tetraexponentials. The fastest decay (t 1 ) is the same as pure water's at all concentrations within error, while the second component (t 2 ) slows slightly with concentration. The slower components (t 3 and t 4 ), not present in pure water, slow substantially, and their contributions to the decays increase significantly with increasing concentration, similar to LiCl solutions. Simulations of LiCl solutions from the literature show that the slow components arise from large ion/water clusters, while the fast components are from ion/water structures that are not part of large clusters. Temperaturedependent studies (15−95 °C) of two high LiCl concentrations show that decreasing the temperature is equivalent to increasing the room temperature concentration. The LiBr and LiCl concentration dependences and the two LiCl concentrations' temperature dependences all have bulk viscosities that are linearly dependent on τ c slow , the correlation time of the slow dynamics (weighted averages of t 3 and t 4 ). Remarkably, all four viscosity vs 1/τ C slow plots fall on the same line. Application of transition state theory to the temperature-dependent data yields the activation enthalpies and entropies for the dynamics of the large ion/water clusters, which underpin the bulk viscosity.

Fast Structural Dynamics in Concentrated HCl Solutions: From Proton Hopping to the Bulk Viscosity

“…Recent OHD-OKE studies found that the bulk viscosities of LiCl and LiBr solutions are linearly related to the correlation time of the slow decay components of the polarizability–polarizability time correlation function, defined as the weighted sum of the time constants not present in pure water. , In these studies, the concentration dependences of LiCl and LiBr solutions were investigated, and the temperature dependences of two concentrations of LiCl were studied . Similar to NaCl studied here, the two fast decays, t 1 and t 2 , were water-like in the sense that their values approached the water values when the concentration went to zero and they did not change a great deal with concentration.…”