Behavior of soap films stabilized by a cationic dimeric surfactant

We report a study on thin liquid films made of a cationic dimeric surfactant referred to as 12-2-12. This kind of surfactant enables one to form charged wormlike micelles whereas classical systems such as CTAB or CPClO3 require added salt to form only screened wormlike micelles. The disjoining pressure has been measured as a function of film thickness at various surfactant and salt concentrations. In a dilute surfactant regime, we observed a stable common black film which undergoes a transition to a Newton black film by addition of salt. This behavior, classical with anionic surfactant, is shown for the first time with a cationic surfactant. A comparison with the monomeric surfactant DTAB is presented showing the important role of the twin structure on film stability. In the concentrated regime or overlap regime, pressure oscillations have been measured. They correspond to a structuring effect in the film. The number of oscillations increases with c, the concentration, whereas the period decreases as c(-1/2). By comparison with the theory developed for charged chains, we propose to relate the observed stratification to the correlation length of semidilute solutions. SANS experiments have been performed to determine the peak position dependence on concentration and temperature.