(54i) Salt Weakens Intermicellar Interactions and Structuring in Bulk and Foam Films

Drainage via stratification in micellar foam films formulated with ionic surfactants shows dramatic changes on salt addition: both the step size and the number of steps in their stepwise thinning diminish. As the stratification process is governed by supramolecular oscillatory structural forces that arise due to confinement-induced structuring of micelles, it is apparent that salt addition reduces both the periodicity and decay length of the oscillatory forces. In this presentation, we characterize the changes in micellar size, shape, and interactions on salt addition in bulk using small angle X-ray scattering (SAXS) to understand and elucidate the influence of salt on stratification in micellar foam films and, more broadly, on the oscillatory structural forces. We infer that the increase in polydispersity of micelle size leads to a significant reduction in long-range correlations among them, resulting in a reduction of the periodicity of the oscillatory disjoining pressure leading to smaller step sizes and fewer steps during the film drainage. The influence of salt on oscillatory disjoining pressure is also shown to result in a breakdown of the analogy between step size in stratifying thin films and intermicellar distances in bulk solutions. We anticipate that our findings are significant for multicomponent soft and biological matter containing self-assembled supramolecular structures wherein screened Coulomb interactions govern the self-assembly, interfacial adsorption, interactions, dynamics, and stability.