Oscillatory Forces from Polyelectrolyte Solutions Confined in Thin Liquid Films

Abstract

Several combinations of polyelectrolyte−surfactant mixtures are studied to probe the various structure−property relationships these systems produce in thin liquid foam films. We show that either bulk solution properties or surface specific adsorption of the polymer−surfactant complexes formed can dominate the thin-film behavior. Above the polymer overlap concentration, Cp*, oscillatory force profiles are observed. In agreement with previous studies, we find that the oscillation period, Δh, scales as the negative square root of the polymer concentration, Δh ∼ Cp-1/2. In addition, by comparing directly the bulk polymer correlation length, ξ, to the oscillation period, we establish for the first time that Δh = ξ for these systems. These findings are compared to oscillatory force behavior seen with micellar solutions which display a scaling relation with micelle concentration, Cm, which follows Δh ∼ Cm-1/3. The different scaling exponents are explained by the local cylindrical and spherical symmetry of the molecular structures seen in the corresponding systems. Finally, it is also shown that the adsorption of polymer and/or complexes to the interface can govern the thin-film properties and swamp out the influence of bulk structural effects in certain systems.