On the origin of electrostatic interaction in foam films from ABA triblock copolymers

Abstract

The effect of pH on the equilibrium thickness and the diffuse double layer potential of microscopic foam films from two non-ionic ABA triblock copolymers of ethylene oxide (A) and propylene oxide (B) - Synperonic P85 and Synperonic F108 - is studied by the microinterferometric method of Scheludko-Exerowa. A strong effect of pH (within the range 2.0–6.0) on the film thickness at constant ionic strength and capillary pressure is found. The film thickness decreases with decreasing pH. With Synperonic P85 at the critical value, pH_CR (=3.8), a transition to black films (thickness 15 nm) occurs. With Synperonic F108 dark gray films (thickness 40 nm) are observed below pH_CR.ST (=3.2). Further decreasing of pH has no influence on the film thickness. Above the critical pH values the films are electrostatically stabilized. The values of the diffuse double layer potential, ϕ ₀, and the surface charge density, σ, are estimated from the DLVO theory. Both ϕ ₀ and σ diminish to zero when approaching pH_CR. These results corroborate our hypothesis that electrostatic interaction (i.e. ϕ ₀ and σ) in foam films from non-ionic surfactants arises from preferential adsorption of OH⁻ ions at the solution/air interface. The films obtained below the critical pH values are sterically stabilized, i.e. decreasing the pH induces a transition from electrostatic to steric stabilization. The parameter pH_CR.ST characterizes the transition to thicker than black, sterically stabilized films.