Adsorption of ionic surfactants at Hg and Ag/AgCl/water interfaces

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Dewetting is an industrial drying process by adsorption of surfactants at the solid/liquid interface. From dewetting tests, it was found that glass, with a highly negative charged surface, is dewetted instantaneously by cationic surfactants while a less charged surface takes a longer time or may not be dewetted at all. This implies that eletrostatic forces play an important role in adsorption and it was studied by electrocapillarity using mercury drop times versus the applied potential using a modified Regula Falsi computer program. Amphoteric surfactant, Sarkosyl O, adsorbs at the dropping mercury eletrode at moderate cathodic and anodic potentials only. At more extreme potentials, there is an increase in competition from the much more prevalent water dipoles and the coulombically attracted supporting elecrolyte, eventually displacing the surfactant. Anionic surfactant, Aerosol OT, starts being adsorbed at a moderate cathodic potential. After reaching a maximum at around the point of zero charge, adsorption remains relatively constant as the potential is increased in the anodic branch. Desorption does not occur, as in the case of the amphoteric surfactant, because the coulombic attractive forces of the anionic surfactant and the supporting electrolyte are equal in magnitude. Cationic surfactant, didodecyldimethylammonium nitrate, behaves unexpectedly when compared to Aerosol OT as the interfacial pressure decreases back to zero in the extreme cathodic branch. Detailed study of a homologous series of quaternary ammonium surfactants show a condensed liquid film and a solid film are formed at the interface. Further study of the cationic surfactant, using the surface enhanced Raman scattering technique, shows that maximum intensity for peaks of the CH 2, stretches and scissoring were found at high negative potential. All these results indicate that cationic surfactants interact so strongly with negatively charged surfaces that the polar head group of the surfactant loses its lateral repulsion. At high negative potential, the formation of the surfactant monolayer (solid type) at the interface accounts for the decrease in interfacial pressure in electrocapillary experiments.