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 via stratification are attributed to the confinement-induced structuring
and layering of supramolecular structures, which 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 a daily basis, has not been investigated
yet. In this contribution, we elucidate how the addition of salt affects
stepwise thinning: step size, number of steps, as well as the shape
and size of nanoscopic nonflat structures such as mesas in micellar
foam films formed with aqueous solutions of anionic surfactant (sodium
dodecyl sulfate (SDS)). The nanoscopic thickness variations and transitions
are visualized and analyzed using IDIOM (Interferometry Digital Imaging
Optical Microscopy) protocols with exquisite spatiotemporal resolution
(thickness ∼1 nm, time <1 ms). 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 results in a decrease
in step size as well as the number of stepwise transitions, in addition
to changes in nucleation and growth of mesas, all driven by the corresponding
change in supramolecular oscillatory structural forces.