Mesoscopic ordering of polyelectrolyte chains in foam films: Role of electrostatic forces

Recent studies using a thin film balance of free-standing thin aqueous films (foam films) containing polyelectrolytes resulted in jump-like discontinuities in the film thickness with increasing outer pressure. These jumps in film thickness correspond to an oscillation of the disjoining pressure. The oscillation period of the disjoining pressure scales as c(-0.5) with the polyelectrolyte concentration c. A mesoscopic ordering of the chains in the film similar to that found in aqueous semidilute polyelectrolyte solutions is assumed. In experiments presented in this work, the electrostatic effect is investigated by adding low molecular salt and varying the degree of charge of the polyelectrolyte chains. The studies reveal that the amplitudes of the disjoining pressure oscillations are reduced with increasing ionic strength and decreasing degree of charge. The results show that the jumps in film thickness and therefore the structuring are due to the electrostatic repulsions between the polyelectrolyte chains.