Foam and wetting films from amphiphilic block copolymers: Isoelectric state and stability

Foam and wetting films from PEO-PPO-PEO triblock copolymers Synperonic P85 and F108 are studied under the identical conditions, using microinterference method. The range of background electrolyte concentration, where DLVO (electrostatic and van der Waals) forces and non-DLVO (steric) forces act in the films, is determined. From the dependence of the film thickness on pH, it is unambiguously shown that electrostatic interactions (i.e., the potential and surface charge) in the foam and wetting films caused by the presence of nonionic polymer surfactants arise due to the preferential adsorption of OH⁻ ions at the solution-air interface. The films obtained below the critical pH values are sterically stabilized; i.e., a decrease in pH induces a transition from electrostatic to steric stabilization. Three-layer models are designed for both types of films that allow to calculate electrostatic disjoining pressure Π_el. The values of ϕ₀ potential of the foam film are used to calculate Π_el in wetting films. A relation between the isoelectric state of foam and wetting films and their stability is found to exist in the range of pH corresponding to electrostatic stabilization. Metastable films, film rupture, or the transition to sterically stabilized films were also found. The text was submitted by the authors in English.