Influence of Ionic Surfactants on the Drainage Velocity of Thin Liquid Films

Abstract

The process of thinning of liquid films in the presence of ionic surfactants and background electrolytes is studied. The lubrication approximation is used to obtain the influence of spatial dynamic distribution of the different ions and the electric potential on the mobility of film interfaces and on the additional nonequilibrium part of the electrostatic disjoining pressure. The material properties of the interfaces (Gibbs elasticity and surface viscosity) are taken into account. The bulk and surface diffusivities of ionic species tend to restore the equilibrium and suppress the role of the Gibbs elasticity. The problem is linearized under the assumptions of small deviations of the adsorption and the concentration from their equilibrium values and of small Peclet number. In the case of plane parallel films an analytical solution is obtained. The correctness of the assumptions, widely used in the literature, of constant surface charge, of constant surface potential, and of the quasi-equilibrium approach is discussed. Numerical analysis of the governing equations shows that the ionic surfactants influence the film drainage in two ways. For small surfactants and background electrolyte concentrations the main effect is a reduction in surface mobility, which decelerates the drainage of the film. At high surfactant or salt concentrations the interfaces become tangentially immobile and then dynamic changes in the concentration, adsorption, electric charge, and middle plane potential affect the film thinning due to the change in the nonequilibrium part of the electrostatic disjoining pressure.