Drainage and critical thickness of foam films stabilized by n-dodecyl-β-D-maltoside

Abstract

The kinetic stability of foam films stabilized with n-dodecyl-β-D-maltoside (C12G2) in the presence of electrolyte (NaCl) has been investigated. The surface charge, type of the film and its electrostatic stabilisation were controlled by changing of the surfactant (from 0.01 to 1CMC) and electrolyte (0.1 and 0.001M) concentration. The drainage coefficient, critical thickness and life time to the critical state of films with different radii (0.05, 0.1 and 0.15mm) were measured interferometrically using a new procedure suggested by us for determination of the reflected light intensity from a video recording of the film evolution. It was found that the films from solutions of C12G2 up to CMC (even at CMC) with 0.001M NaCl are ``common (CF) electrostatically stabilized films'', while at the same C12G2 concentrations they were unstable films (UF) or stable Newtonian black films (NBF), when the added electrolyte was 0.1M. The values of film critical thickness obtained at a high electrolyte concentration were similar even when the concentration of C12G2 was varied from 0.1 to 1CMC. It means that in a wide surfactant concentration range the electrostatic disjoining pressure is screened and the experimental data of critical thickness are appropriate for theoretical analysis. The disjoining pressure acting at different steps of the film evolution was evaluated based on experimental data of the film drainage. A non-DLVO attractive force for films with non covered surfaces and Πel=0 was detected. It was expressed by a power law in the same form as the van-der-Waals force, which makes it possible to directly compare these two forces by means of the force constant K232 and the Hamaker constant. The non-DLVO force was found to be considerably stronger than the Lifshitz – van-der-Waals forces.The obtained new experimental data for critical thickness were compared with those calculated by us with different equations (Radoev–Sheludko–Manev, Vrij, and Coons). In some of these cases we used the experimentally obtained value of the drainage coefficient instead of the velocity of the film thinning, calculated theoretically. It was found that the calculated hcr are noticeably lower than experimentally measured ones. Good agreement between measured and calculated hcr was achieved with a non-DLVO attraction force taken into account, when the films were obtained from solutions with low (C12G2) and high (NaCl) concentrations.