(421c) Influences of Surfactant and Salt on Micellar Assemblies and Foam Film Stability

Ultrathin foam films, containing interfacially-adsorbed amphiphilic surfactant molecules at liquid-air interfaces and supramolecular structures like micelles in bulk solutions, undergo drainage via stratification. Understanding the role of interfacially- and self-assembled surfactants in dictating the stability and lifetime of foams and emulsions is essential towards molecular engineering of consumer product formulations as well as controlling foaming and emulsification in industrial processes. In this presentation, we will elucidate trends in the structure of surfactant self-assembly and the nanoscopic foam film thickness of ultrathin foam films containing a model surfactant sodium dodecyl sulfate (SDS) by utilizing a combination of small-angle X-ray scattering and stratification studies. We will first explicate the influence of surfactant concentrations on micellar structure evolution and interparticle interactions of aqueous SDS solutions. These trends will be compared with nanoscopic topography and thickness transitions of freely draining SDS foam films, elucidating the striking similarity on stepwise decreases between intermicellar separations in bulk solutions and stratification step sizes in thin films. Furthermore, we will highlight that increasing the ionic strength of SDS solutions results in a breakdown of the agreement between the separation distances in bulk solutions and in thin films, emerging from weakening of the micellar layering in the films and an increasing influence of the amphiphiles assembled at the air-water interfaces on the disjoining pressure in the films. Our studies will demonstrate that stratification in a foam film, driven by confinement induced layering of micelles between the liquid-air interfaces, provides a sensitive probe of non-DLVO oscillatory forces and micellar interactions.