PNNL

Abstract

Only very soluble electrolytes can form concentrated solutions. Some salts are so soluble that there are less than four water molecules per ion in saturated solution. Ions usually form clusters or networks with more than one counter ion in their coordination sphere in these concentrated solutions. Do these ultra-concentrated solutions form because the counter ions have high affinity for each other in liquid, or because they have a poor affinity for each other in solids? Here we address this question using the Valence Matching Principal of the Bond Valence Model by comparing the charge density mismatch between counterions to their solubility for a series of alkali fluorides, carboxylates, and oxyanions. The solubilities were plotted against the characteristic average bond valence of the alkali, and the lowest solubilities were those where alkali and anion had matching bond valences. Conversely, the highest solubilities were those with poorly matching bond valences. Available ion-pairing constants indicate that the weakest ion-pairs are those with the largest bond valence mismatch, indicating that the large water solubilities occur despite weak ion-pairing rather than because of strong ion-pairing. Therefore, a key characteristic of highly water-soluble salts is that the counter ions have mis-matched charge densities.