Desorption Behavior of Surfactant Mixtures at the Alumina-Water Interface

The desorption behavior of cationic-nonionic surfactant mixtures, tetradecyltrimethylammonium chloride (TTAC) and pentadecylethoxylated nonyl phenol (NP-15), at the alumina-water interface was studied. It has been found that while nonionic NP-15 itself does not adsorb at alumina-water interface, it will do so with cationic TTAC preadsorbed at the interface. During the desorption process, however, the presence of NP-15 in the system was discovered to cause desorption of TTAC. This is attributed to changes in equilibrium among surfactant monomers, mixed micelles, hemimicelles, and solloids upon the addition of NP-15: The cationic TTAC species at the interface is solubilized in NP-15 rich micelles in the bulk and perturb the equilibrium. This in turn is proposed to cause the desorption of NP-15 significant. The desorption behavior in the mixed surfactant system depends on the mixing ratio of surfactants in the mixtures. In nonionic NP-15 rich mixtures, negative hysteresis is observed for the desorption of both TTAC and NP-15. In cationic TTAC rich mixtures, the effect of NP-15 on the desorption of TTAC is limited and the desorption of TTAC is similar to that in single TTAC system; in this case some positive hysteresis is observed for both TTAC and NP-15.