Colloids and Surfaces A: Physicochemical and Engineering Aspects 149 (1999) 131–140 Forces in foam films containing polyelectrolyte and surfactant R.v. Klitzing *, A. Espert, A. Asnacios, T. Hellweg, A. Colin, D. Langevin Centre de Recherche Paul Pascal (CNRS), Av. Dr Schweitzer, F-33600, Pessac, France Received 30 August 1997; accepted 2 November 1997 Abstract Foam films of semi-dilute anionic polyelectrolyte solutions containing additionally the cationic surfactant dodecyltri- methylammonium bromide are investigated. The polyelectrolytes used are polystyrene sulfonate (PSS ) and acrylamide- acrylamidesulfonate (AAS ) as examples of strongly and weakly charged polyions, respectively. The disjoining pressure isotherm results in film stratification well below the critical micelle concentration (CMC ) and the critical aggregate concentration (CAC ). These pressure oscillations are related to a network of polyelectrolyte chains in the film bulk, and their period to the mesh size. The network can be described by the theory for semi-dilute polyelectrolyte solutions, which is different for the two polyelectrolytes because of different charge densities along the chain. The surfactant has no observable influence on the network. To obtain information about the film interface surface tension measurements at the water–air interface are carried out. In comparison with the pure polyelectrolyte and the surfactant, respectively, the surface tension is much reduced for the mixed systems as a result of the formation of complexes between the surfactant and the polyelectrolyte. The two polymers show different behaviour in saturation of the surface coverage, which leads us to the conclusion that the complexes are different, although the behaviour of the film bulk is not influenced by changes in the interface. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Disjoining pressure; Polyelectrolytes; Stratification; Surface tension; Thin films 1. Introduction Pure foam films were the subject of many studies ([1] and references cited therein). Because of the molecular interactions between the two film inter- Foams have many industrial applications such faces a pressure appears, either positive (disjoining) as enhanced oil recovery, the decontamination of or negative (conjoining) in nature. polluted surfaces or use in the cosmetic industry. The two most important contributions are the The research of foam also presents an important electrostatic forces which stabilize the film if they part of colloid and surface science. For the compre- are repulsive, and the van der Waals’ forces which hension, control and manipulation of the macro- are always attractive and, therefore, destabilizing. scopic properties (e.g. stability of foam) it is They constitute the DLVO theory [2]. necessary to understand the behaviour of the build- Investigating films made from surfactant solutions ing blocks i.e. the single liquid films. The model above the critical micelle concentration (CMC ) a system of the foam is presented as gas/film/gas. stepwise thinning is observed which cannot be explained by the DLVO theory. This oscillation of the disjoining pressure is due to additional stratifi- * Corresponding author. Tel: + 49 303 142 6774; fax: 49 303 142 6602; e-mail: klitzing@chem.tu-berlin.dc cation forces within the film [1,3]. This phenome- 0927-7757/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII S0927-7757(98)00307-0