(473g) Foam Film Stratification Studies Probe Intermicellar Interactions

Ultrathin foam films, containing supramolecular structures like micelles in bulk and adsorbed surfactant at liquid-air interface, undergo drainage via stratification. For a fixed surfactant concentration, stratification studies of micellar films yield a characteristic step-size, associated with a stepwise decrease in average film thickness. By visualizing and analyzing nanoscopic thickness variations and transitions in stratifying foam films using Interferometry Digital Imaging Optical Microscopy (IDIOM) protocols that we developed, we find that step-size also describes the quantized thickness difference between coexisting thick-thin flat regions. It is well-established that step-size is inversely proportional to the cubic root of surfactant concentration, and cannot be estimated by adding micelle size to Debye length, as the latter is inversely proportional to square root of surfactant concentration. Even though the claim intermicellar distance equals step-size is repeated in studies published by many group in the last thirty years, including our own, fascinatingly, we found no reports of a close comparison between the two length scales. In this contribution, we utilize small angle x-ray scattering to obtain measurements of variation in intermicellar distance as well as size, charge, and aggregation number of micelles as a function of surfactant type and concentration. We present a close comparison between the variation in step-size with concentration-dependent changes in intermicellar distance, and elucidate the role played by intermicellar interactions and the thermodynamics of self-assembly in dictating the hydrodynamics and stability of stratifying foam films.