Stepwise Thinning and Nanoscopic Thickness Variations in Foam Films Formed by Aqueous Sodium Naphthenate Solutions

Sodium Naphthenates found in crude oils can act as surfactants and self-assemble in aqueous solutions to form micelles and liquid crystals. Understanding and controlling the drainage kinetics of thin films is an important problem that underlies the stability, lifetime and rheology of petroleum foams and emulsions. Here, we show that foam films formed by aqueous solutions of sodium naphthenates exhibit step-wise thinning or stratification, due to the influence of non-DLVO forces, including supramolecular oscillatory structural forces. We utilize Interferometry, Digital, Imaging, Optical Microscopy protocols, previously developed by our group, to investigate the drainage and stratification in micellar foam films (< 100 nm) with high spatial (thickness < 10 nm) and temporal resolution (< 1 ms). We determine how the concentration of added sodium naphthenates influences the nanoscopic topography, stratification kinetics and step size of foam films, and contrast the results with behavior observed with stratifying foams made with sodium dodecyl sulfate (SDS) solutions. We span a relatively wide concentration range, such that micelle shape and size vary, as is revealed by complementary small angle x-ray scattering experiments.