Neutron Reflectivity and Atomic Force Microscopy Studies on the Effect of pH and Solubilizate upon the Structure of Adsorbed Cationic Surfactant Films on Quartz

Full text: We have examined the adsorbed layer morphology of cationic surfactant, tetradecyltrimethylammonium bromide (TTAB), on the hydrophilic quartz surface using atomic force microscopy (AFM) and neutron reflectometry (NR). We report on the effect of varying the surface charge of quartz by adjusting the bulk solution pH upon the adsorbed surfactant film structure. AFM results show that number density of globular (or spherical) TTAB aggregates on quartz increases, as the pH of the bulk solution is increased, reflecting the increase in surface charge of quartz. NR results also verify the AFM observations. We also report on the effect of the incorporation of a non-adsorbing solubilisate, naphthol, into the adsorbed surfactant film. In the bulk solution the incorporation of naphthol molecules into surfactant micelles promotes in micellar growth. Similarly, in the adsorbed surfactant film a transition to from globular to cylindrical surfactant aggregates was also observed. (authors)