Ion-specific coalescence of bubbles in mixed electrolyte solutions

Ion-specificity is at the heart of many biological processes and is important in all soft-matter systems at high electrolyte concentrations. The effect of electrolytes on bubble coalescence in water is a simple example of ion-specificity. Single electrolytes inhibit coalescence or have no effect, as predicted by ion-combining rules based on empirical assignments (α or β). The mechanism behind electrolyte inhibition, as well as the salt differentiation, is not understood. We here report that mixed electrolytes also follow the ion assignments. In addition, inhibition is consistent with the hypothesis that electrolyte effects depend upon ion separation within the interfacial region. We demonstrate that Gibbs elasticity is not the mechanism by which coalescence is inhibited; rather we propose a mechanism for coalescence inhibition in which some electrolyte combinations modify the hydrodynamic boundary condition at the air-water interface, which explains the manifestation of ion-specificity in long-range effects even at high electrolyte concentrations.