Transition from electrostatic to steric stabilization in foam films from ABA triblock copolymers of poly(ethylene oxide) and poly(propylene oxide)

The equilibrium thickness of microscopic foam films obtained from aqueous solutions of two different poly(ethylene oxide)/poly(propylene oxide)/poly(ethylene oxide) triblock copolymers (Synperonic PE P85 and F108) is experimentally determined. The amphiphilic triblock copolymers adsorb at the air/solution interface as a brush layer. The foam film may be stabilized by electrostatic and/or steric repulsion. At a given copolymer concentration (slightly lower than the CMC) increasing the electrolyte concentration induces a transition from electrostatic to steric stabilization. The thicker foam films are electrostatically stabilized. The thinnest foam films are sterically stabilized, but their thickness is unexpectedly larger than twice the brush layer thickness. While the thickness h of these films strongly depends on the degree of polymerization N, its scaling structure is essentially the same: two brush layers (h(1) similar to N) and an aqueous core (h(2) similar to N-3/5). The microscopic foam film is a suitable tool for exploring the transition from electrostatic to steric stabilization.