PNNL

Abstract

Aqueous solvation of cations involves a competition between water molecules and anions in the first shell. The emergence of cation-anion species, or contact ion pairs, is fundamental to understanding the structure, thermodynamics, and electrical transport of aqueous solutions when moving from the ideal, low-concentration limit to the manifestly non-ideal limits of either very high solute concentration or very high constituent ion activity. We focus here on Zn halide solutions, and especially on the ZnCl2 system, both as a model system and also as an exemplar of the practical importance of issues spanning from (i) electrical energy storage via the paradigm of water in salt electrolyte (WiSE) to (ii) the physical chemistry of brines in geochemistry to (iii) the long-standing problem of nucleation. Using valence-to-core x-ray emission spectroscopy (VTC-XES), x-ray absorption near edge structure (XANES), and a combination of theoretical methods, we bring new insight to the key question of the structure and composition of the first coordination shell for Zn halide (Cl-, Br-) solutions. In particular, the hyper-local sensitivity of VTC-XES is combined with time-dependent density functional theory to quantify the halide coordination number NX (X= Cl- or Br-1) of Zn+2 ions as a function of Zn halide salt concentration and as a function of added halide content at fixed Zn concentration. Unlike prior work, our experimental analysis makes no embedded use of theoretical equilibrium constants. We find that NX saturates at a value of 2 for the pure salt solutions (ZnCl2 or ZnBr2) above a few molal concentration and that our results strongly suggest that saturation occurs at NX ~ 4 for very high anion activity. The inferred concentration dependence of NX show generally good agreement with theory as regards the fast onset of ion pairing and the saturation at NX ~ 2 for the pure salt solutions, but we find a persistent discrepancy in which classical molecular dynamics strongly favors NX ~ 3 at high anion activity. This work lays the groundwork for better understanding of the effects of ion coordination in concentrated electrolytes, serves as a foundation for additional study of second- and higher-shell effects on electrolyte properties, and provides a pathway for future VTC-XES directions in study of the physical chemistry of solutions.