Salt Weakens Intermicellar Interactions and Structuring in Bulk Solutions 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 the magnitude, periodicity, and decay length of the oscillatory forces. In this contribution, we characterize the changes in micellar size, shape, and interactions on salt addition in bulk solutions 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. Adding salt leads to a significant reduction in long-range correlations between micelles and smaller intermicellar distances. These effects manifest as a weakening of the primary peak of the structure factor, ascertained from SAXS spectra, accompanied by its shift to higher wave vectors. Weakened long-range correlations diminish the magnitude and periodicity of the oscillatory disjoining pressure leading to smaller step sizes, fewer steps, and a rich nanoscopic topography, due to the influence of disjoining pressure on the deformable interfaces. The step sizes in stratifying thin films and intermicellar distances in bulk solutions present incongruous values, implying an imperfect analogy with studies on charged nanoparticles with matched and salt concentration-independent values of measured interparticle distances that equal the periodicity of force–distance curves. 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