Influence of surfactant choice on stratification of freestanding micellar 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 average film thickness of a stratifying micellar film yields a characteristic step-size that 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 SDS concentration, and cannot be estimated by adding micelle size to Debye length, as the latter is inversely proportional to the square root of SDS concentration. Recently we contrasted the step-size obtained from the analysis of nanoscopic thickness variations and transitions in stratifying SDS micellar foam films using Interferometry Digital Imaging Optical Microscopy (IDIOM) protocols (that we developed) with the intermicellar distance obtained using small-angle X-ray scattering. We found that step-size equals to intermicellar distance obtained using scattering from bulk solutions, and stratification driven by the confinement-induced layering of micelles within the liquid-air interfaces of a foam film provides a sensitive probe of non-DLVO oscillatory forces and micellar interactions. In this contribution, we examine the concentration dependency of step-size and layer number for anionic surfactants, including SDS, SDBS, and NaN, and contrast these with observations for nonionic surfactants.