Effect of sodium butanoate and sodium hexanoate on micelle formation of model cationic–nonionic and anionic–nonionic surfactant mixtures

The formation of mixed micelles in cationic–nonionic (Dodecylpyridinium chloride (DPC)–Triton X-100) and anionic–nonionic (Aerosol OT (AOT)–Triton X-100) aqueous surfactant solutions has been studied at 25 °C both in the presence and absence of sodium butanoate (NaBu) and sodium hexanoate (NaHx). The critical micelle concentration (cmc) and free energy of micellization have been estimated through surface tension measurements. Composition, activity coefficients and mutual interaction parameters of mixed micelles were evaluated in terms of different theoretical models like that of Clint, Rubingh, Rodenas, Maeda and Blankschtein, that explain binary system well. The analysis reveals that, in spite of synergism, very small mole fraction of the ionic surfactant is present in the mixed micelles. Blankschtein model predicts a continuous decrease in synergism for the DPC + TX-100 system both in NaBu and NaHx due to their salt effect. On the contrary, Rubinghʹs treatment reveals an increase in synergism in the presence of NaHx above 30 mM as against a continuous decrease in NaBu in the whole concentration range. For the AOT + TX-100 system, where Blankschtein model is not applicable, Rubinghʹs treatment results in a continuous decrease in synergism in the presence of NaBu as well as NaHx.