Ion-Specific Effects on Bubble Coalescence: A Cohesive Pressure Approach to SDS Foam Stability

This work investigates the effect of specific electrolytes (NaCl, KCl, MgCl2, and CaCl2) on bubble coalescence and the stability of the resulting foam caps in aqueous sodium dodecyl sulfate (SDS) solutions. At low concentrations, we observe that all electrolytes studied exhibited a similar effect on bubble coalescence, driven primarily by the electrostatic screening of the repulsion between bubbles. However, a distinct divergence in behavior emerges at higher concentrations: KCl and CaCl2 destabilize the foam, while NaCl and MgCl2 act as foam boosters. By applying Pitzer’s theory and the concept of cohesive pressure, we propose that foam stability is governed by the hydration compatibility between the surfactant head groups and the salt ions. We describe a mechanism in which the “cohesive pressure” of the water matrix determines the efficiency of the SDS adsorption layer. Specifically, strong kosmotropic ions (e.g., Mg2+) preserve the hydration shell of the sulfate head groups, thereby preventing film collapse, whereas ions with lower dehydration energy (e.g., Ca2+) are proposed to facilitate the formation of contact ion pairs, thereby promoting rapid coalescence.