Foam and wetting films from aqueous cetyltrimethylammonium bromide solutions: Electrostatic stability

Foam films and wetting films on quartz formed from aqueous solutions of cet yltrimethylammonium bromide (CTAB) are investigated in a wide range of surf actant concentrations in the presence of background electrolyte (5 x 10(-4) mol dm(-3) NaCl). Foam and wetting films are convenient models for the stu dy of symmetric (free thin liquid films) and asymmetric (thin liquid films on solid substrate) films with the same air/solution interface, Microinterf erometric methods of assessment of foam and wetting films are used which al low precise determination of the film thickness. Determined are the values of the potential phi (0) of the diffuse electrical layer at the solution/ai r interface (applying the method of "equilibrium foam films") and the poten tial phi (1) at the solution/quartz interface (applying the method of capil lary electrokinetics). These values are used to analyze the stability of th e films studied in terms of the DLVO theory. A conclusion drawn is that bot h kinds of films studied are stabilized by electrostatic interaction forces . It is shown that with increasing CTAB concentration, a charge reversal oc curs at both the solution/air and solution/quartz interfaces which determin es the stability/instability conditions of the foam and wetting films. Conc entration ranges where both kinds of films produce stable (equilibrium) fil ms are found. There are also concentration ranges where the films either ru pture or are metastable (quasi-equilibrium). The CTAB concentration ranges, which provide the formation of unstable (rupturing and metastable) and sta ble films, are different for symmetric (foam) and asymmetric (wetting) thin liquid films. It is only at high CTAB concentrations (higher that >2 x 10( -4) mol dm(-3)) that both cases render formation of stable equilibrium film s. These studies give direct experimental indications that the electrostati c interactions between identical or different interfaces can differ when th e surfactant concentration is varied.