Disjoining pressure study of formamide foam films stabilized by surfactants.

A thin film pressure balance was used to investigate the disjoining pressure Pi as a function of the film thickness h of surfactant-stabilized formamide foam films. Nonionic (alkylpoly(ethylene glycol)s) and cationic surfactant (alkyltrimethylammonium bromides (C(n)TAB) with n = 14 and 16) solutions were studied in the absence and presence of electrolyte. The resulting Pi-h curves were fitted with the DLVO theory from which we extracted surface charge densities q(0) and surface potentials Psi(0). Investigating formamide foam films is of interest for studying the electrostatic component of the stabilizing forces in foam films. We know that the aqueous foam films are stabilized via electrostatic forces. In this case the self-dissociation of water contributes to the charges in the foam film. As formamide has a dissociation constant which is about 2 orders of magnitude lower than that of water, the number of charges in the solution due to self-dissociation is much smaller, which, in turn, should lead to lower electrostatic forces. Indeed, we found that formamide solutions of nonionic surfactants did not form stable foam films at concentrations below the critical micelle concentration. Regarding the cationic surfactants, the main difference between the formamide and the aqueous foam films is the fact that the concentration of ionic surfactants to form stable foam films is about 2 orders of magnitude higher compared to water. Consequently, the screening length for the electrostatic interaction and thus the film thickness are much smaller compared to films formed by the respective aqueous solutions.