Monolayers of cationic surfactants at the air-water and oil-water interfaces
Monolayers of the cationic surfactant hexadecyltrimethylammonium halide (CTAX, where X = F¯, Cl¯, Br¯, and I¯) have been studied at the air-water and oilwater interfaces. At the air-water interface, the effects of the halide counterion and the addition of counterion were investigated. Sum-frequency spectroscopy (SFS), ellipsometry, and surface tensiometry indicated that the counterion changed the efficiency and effectiveness of the surfactant, both decreasing in the order of Br¯> Cl¯>F¯. The addition of salt in the form of 0.1 M KX was found to reduce the cmc but had little effect on the limiting area per molecule attained at the cmc, which increased from 44 Å2 for CTAB to 65 Å2 for CTAC and ca. 94 Å2 for CTAF. Neither SFS nor ellipsometry provided any firm evidence for specific effects of the halide ions on the structure of the surfactant monolayers. For CTAB monolayers in the absence of excess electrolyte, the effect of area per molecule on the sum-frequency (SF) spectra was studied. Mixed monolayers of CTAB and tetradecane at the air-water interface exhibit a first-order phase transition from a conformationally disordered to a conformationally ordered state as the temperature is lowered. The phase transition occurs ca. 11 °C above the bulk melting point of tetradecane. A new experimental arrangement is described for acquiring SF spectra from surfactants at the oil-water interface. The key features of this approach are the stabilisation of a thin oil film between a sapphire prism and an aqueous phase, and the use of total internal reflection to enhance the total signal and discriminate against signals from other interfaces in the system. With this new methodology, the first SF vibrational spectra of surfactant monolayers at an alkane-water interface were obtained. Surface tensiometry was used to characterise the monolayers further. The structure of CTAB monolayers at the hexadecane-water interface was determined by SFS and compared with monolayers of CTAB at the air-water interface. At low concentrations, CTAB/hexadecane showed the expected features in the C-H stretching region, characteristic of a conformationally disordered monolayer. As the bulk concentration approached the critical micelle concentration, the spectra changed to one characteristic of a more ordered, upright conformation. Ellipsometric measurements supported this conclusion. This qualitative structural change is not observed in analogous monolayers at the air-water interface or CCl4-water interface, or in surfactant solutions in contact with a hydrophobic solid surface.