A new perspective on aqueous electrolyte solutions.

Aqueous electrolyte solutions are central to many natural phenomena and industrial applications leading to continuous development of increasingly complex analytical models. These are based on an atomistic description of electrostatic interactions between ions, along with mean-field approaches for the dielectric response of water. Despite many achievements, such concepts often fall short in quantitatively describing scenarios where ion-ion correlations and specific solvation effects become relevant, particularly in concentrated electrolyte solutions. Here, we propose a shift in perspective, by introducing a statistical, coarse-grained approach to describe the average thermodynamic properties of aqueous electrolyte solutions. This method eliminates the need to define ion pairs or ion complexes and does not require any prior knowledge on specific solvation. We base our concept on separating the solution into a spherical observation volume whose size and average composition are uniquely determined by the solution parameters, and its environment, which consists of the remaining solution. This separation allows us to express the volume-environment interaction in terms of a generalized multipole expansion, in a convenient, additive way. We applied this approach to 135 electrolytes including some notoriously complex species, such as LiCl or ZnCl over their full solubility ranges. This paves the road toward understanding super-saturated and water-in-salt solutions and electrolyte nucleation.