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 ion-ion electrostatic interactions combined with mean-field approaches for the dielectric response of water. Despite many achievements, these concepts fail to quantitatively describe situations where ion-ion correlation and specific solvation become relevant, such as for concentrated electrolyte solutions. Here, we propose a change of perspective, by introducing a statistical, coarse-grained view to describe the average thermodynamic properties of aqueous electrolyte solutions. This approach bypasses 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 droplet whose size and average composition are uniquely determined by the solution parameters, and its environment consisting of the remaining solution. This allows us to express the droplet-environment interaction in terms of a generalized multipole expansion, i.e. in a convenient, additive way. We applied this approach to 135 electrolytes including some notoriously complex electrolytes, such as LiCl or ZnCl$_2$ over the full solubility range. This paves the road toward understanding super-saturated and water-in-salt solutions and electrolyte nucleation.