Vertical free-standing films of amphiphilic associating polyelectrolytes

Hydrophobically modified poly(acrylic acid) sodium salt (HMPAANa) copolymers are known to provide a huge stabilization of oil in water macroemulsions. An interstitial HMPAANa film is formed between the oil droplets, thus creating repulsion between them. We present an x-ray reflectivity study of vertical free-standing films drawn from aqueous solutions of HMPAANa copolymers. The vertical HMPAANa films are model systems for the interstitial films between oil droplets and the description of their behavior provides information about the stabilization process. Their thickness was investigated as a function of various parameters such as the solution concentration, the degree of grafting, the length of the grafts, and the backbone molecular weight. Below a solution concentration threshold (C(t)), the film thickness scales like the square root of the molecular weight and is independent of the degree of grafting and the length of the grafts. Polyelectrolyte chains adopt a self-screened coil conformation within the films and the thickness is governed by the radius of gyration of the coils. Above C(t), a transition from a bimolecular film to a physical gel is observed and the thickness then increases with concentration. Finally, we propose an explanation for the stabilization of macroemulsions by HMPAANa copolymers.