Dynamic effects in thin liquid films containing ionic surfactants

Abstract

This paper is dedicated to studying dynamic effects in thin liquid films (TLF) containing ionic surfactants. The standard theory of TLF drainage has been developed without considering the electrical double layer (EDL) in the hydrodynamic equations, although EDL always exists. In addition, it has been found that this theory very well describes the drainage of TLF containing non-ionic surfactants in the presence of electrolytes. The inclusion of EDL into the hydrodynam ics of TLF complicates the theory , producing additional dynamic effects during film drainage. For example, a gradient of electrostatic disjoining pressure across the film arises, thus causing non-uniform electrostatic repulsion between the film surfaces. This paper analyzes the hydrodynamics of TLF with EDL. A new equation of drainage was derived. This equation accounts for the non-uniform distribution of surface charges during the films drainage, which is coupled with non-uniform electrostatic repulsion between the film surfaces and results in faster film drainage. The theory was tested with drainage experiments on TLF with ionic surfactants. Foam films containing sodium dodecyl sulfate (SDS) in the presence and in the absence of added electrolyte were studied and the experimental data compared to the theoretical predictions. The experimental results, however, disagree with the theory. For example, the kinetic equation predicted faster film drainage for foam films at low ionic strength; at high ionic strength the theory tends to “Reynolds drainage”. Inversely, the experiment exhibited slower drainage than predicted by the Reynolds equation in both cases of low and high ionic strengths. Numerical simulations yielded V / V Re < 1 . In addition, cases of “positive” and “negative” velocity of film surfaces were shown. Despite the sign of the velocity the dependence V / V Re < 1 remained. The analysis showed similarity between the experimental data and the prediction of the Manev–Tsekov–Radoev (MTR) drainage model at R < R c r . The analysis showed rather complex dynamics of the film drainage. The paper calls upon further development on this topic.