Micellar Size, Shape, and Interactions Determine Stratification in Foam Films

Thin foam films containing adsorbed surfactant and supramolecular structures like micelles in bulk undergo drainage via stratification. Understanding the role of anionic surfactants sodium dodecyl sulfate (SDS) and sodium naphthenate (NaN) on the stratification behavior of freely draining foam films is important for consumer product formulations, molecular engineering, and for controlling foaming in industrial processes like petroleum refining. Stratification behavior in draining foam films is due to the influence of non-DLVO forces, including supramolecular oscillatory structural forces. We investigate the drainage and stratification behavior in micellar foam films by analyzing nanoscopic thickness variations and transitions in stratifying micellar foam films (h < 100 nm) with Interferometry Digital Imaging Optical Microscopy (IDIOM) protocols, previously developed by our group, with high spatial (thickness ~ 1 nm, in-plane < 1 micron) and temporal (time < 1 ms) resolution. We elucidate how SDS, as well as NaN, concentration influences the nanoscopic topography, stratification kinetics, and step-size of foam films, and contrast the results with behavior from bulk solutions using small angle X-ray scattering. Moreover, we discuss the influence of surfactant concentration and type on foam film stability. We span a relatively wide concentration range, such that micelle shape and size vary.