Stepwise Thinning and Nanoscopic Thickness Variations in Foam Films formed by Aqueous Sodium Napthenate Solutions

Sodium Napthenates 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 pertroleum foams and emulsions. Here we show that foam films formed by aqueous solutions of sodium napthenates 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 napthenates 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.