Comparison of different polymer-like structures in the confined geometry of foam films

Recent studies by a thin film balance of free standing thin liquid films containing polyelectrolytes result in jump-like discontinuities in the film thickness with increasing pressure. These jumps correspond to an oscillation of the disjoining pressure. They are correlated to a network-like chain structure in the film similar to that one found in aqueous semi-dilute polyelectrolyte solutions. The results presented in this paper show that the application of the theory for polyelectrolytes in bulk solutions to the behaviour in the confined geometry of foam films is generally valid. A surfactant which forms worm-like micelles and two different polyelectrolytes were studied. Though the systems are very different, in all cases the oscillation period of the disjoining pressure scales as 1/c with the concentration c. It is assumed that there is a network of polyelectrolyte chains and worm-like micelles respectively in the foam film and that the oscillation period corresponds to the meshsize (correlation length) of the network. The correlation length calculated by the theory for semi-dilute polyelectrolyte solutions corresponds to the measured jump size at a fixed polyelectrolyte concentration.