(660h) Supramolecular Oscillatory Structural Forces and Stratification in Micellar Freestanding Films

Freestanding films of soft matter containing micelles, nanoparticles, polyelectrolyte-surfactant complexes, bilayers and smectic liquid crystals exhibit stratification. Stepwise thinning and coexisting thick-thin regions associated with drainage viastratification are attributed to the confinement-induced structuring and layering of supramolecular structures, that contribute supramolecular oscillatory structural forces. In freestanding micellar films, formed by a solution of an ionic surfactant above its critical micelle concentration, both interfacial adsorption and the micelle size and shape, are determined by the concentration of surfactant and of added electrolytes. Although the influence of surfactant concentration on stratification has been investigated before, the influence of added salt, at concentrations typically found in water used on daily basis, has not been investigated yet. In this contribution, we elucidate how the addition of salt affects step-wise thinning: step size, number of steps, as well as the shape and size of nanoscopic non-flat structures like mesas in micellar foam films formed with aqueous solutions of anionic surfactants: sodium dodecyl sulfate (SDS) and sodium naphthenates (NaN), and a cationic surfactant, CTAB . The nanoscopic thickness variations and transitions are visualized and analyzed using IDIOM (Interferometry Digital Imaging Optical Microscopy) protocols with exquisite spatio-temporal resolution (thickness ~ 1 nm, time < 1ms). In contrast to nanoparticle dispersions that show no influence of salt on step size, we find that the addition of salt to micellar freestanding films of SDS solutions results in a decrease in step size as well as the number of step-wise transitions, in addition to changes in nucleation and growth of mesas, all driven by the corresponding change in supramolecular oscillatory structural forces. Finally, by contrasting between different surfactant systems, we determine the influence of surfactant type on stratification dynamics and supramolecular structural forces.