Physicochemical vs. Chemical Pathways of Foam Inhibition: The Role of Cohesive Pressure and Specific Ion-Pairing

This study investigates the inhibitory effects of alkali metal chlorides lithium chloride, sodium chloride and potassium chloride (LiCl, NaCl, and KCl) on sodium dodecyl sulfate (SDS) foams, focusing on the transition from interfacial to bulk-driven destabilization mechanisms. The research demonstrates that foam collapse at high electrolyte concentrations is governed by a massive increase in bulk cohesive pressure and specific ion-pairing (SIP), which leads to interfacial dehydration and the mechanical decoupling of the surface from the bulk phase. It is shown that while surface adsorption reaches a plateau, the thermodynamic state of the solvent becomes the primary driver for film drainage. The results indicate that KCl acts as the most potent defoamer due to its optimal matching of water affinities with the surfactant head groups. These findings provide a new theoretical framework for understanding foam stability in concentrated electrolytic environments, emphasizing the role of bulk cohesive stress over traditional interfacial elasticity.