Salt Weakens Intermicellar Interactions and Structuring in Bulk Solutions and Foam Films

Ultrathin foam films containing supramolecular structures like micelles in bulk and adsorbed surfactant at the liquid-air interface undergo drainage via stratification. At a fixed surfactant concentration, the stepwise decrease in the average film thickness of a stratifying micellar film yields a characteristic step size. In this presentation, we will contrast these step sizes obtained from the analysis of nanoscopic thickness variations in thin foam films using interferometry protocols with the intermicellar distances in bulk surfactant solutions obtained using small-angle X-ray scattering (SAXS) measurements. In the absence of added salt, the step size in thinning films equals the intermicellar distance in bulk solutions obtained from SAXS experiments. However, this analogy will be shown to break down with the addition of salt. Adding salt will be shown to result in a significant reduction in long-range correlations between micelles and smaller intermicellar distances. Weakened long-range correlations also diminish the magnitude, periodicity, and decay length of the oscillatory disjoining pressure leading to smaller step sizes, fewer steps, and rich nanoscopic topography. We infer that the coupling between free surface shape, flows, and structuring within stratified foam films and changes in micellar shape, size, and interactions upon salt addition influence the step size in stratified films and result in the breakdown of the step size - intermicellar distance analogy.