Surface configuration of sorbed hexadecyltrimethylammonium on kaolinite as indicated by surfactant and counterion sorption, cation desorption, and FTIR

Sorption of cationic surfactants on clay minerals was studied extensively in the past 15 years, but most was focused on smectite clays because of their larger surface area and higher cation exchange capacity. For smectite, due to its swelling property and surfactant intercalation, it is difficult to differentiate amount of surfactant sorbed on mineral surfaces from that in interlayer spaces. Kaolinite is a 1:1 layered clay mineral showing no swelling in the presence of quaternary ammonium compounds, thus, making it ideal for the study of surfactant sorption on external mineral surfaces. Sorption of hexadecyltrimethylammonium (HDTMA) bromide on two reference kaolinites (KGa-1b and KGa-2) was studied as a function of initial HDTMA concentration, contact time, accumulative cation desorption, co-adsorption of counterion, and FTIR. When the equilibrium HDTMA concentration is below its critical micelle concentration, sorbed surfactant molecules primarily form monolayer formation. At the HDTMA sorption maximum, the ratios of HDTMA sorbed to counterion sorbed, HDTMA sorbed to cation desorbed, and counterion sorbed to cation desorbed all indicated an overbalanced admicelle sorption on kaolinite contrast to a bilayer HDTMA formation. A consistent shift of C-C symmetric and asymmetric vibration from high to low frequencies further supported a transition from adsorbed monomers to admicelles as the HDTMA surface coverage increased. The ratio of HDTMA sorbed via hydrophobic interaction to total HDTMA sorbed on kaolinite was 0.64 determined from HDTMA sorption and bromide co-adsorption data and 0.7-0.8 determined from the HDTMA sorption and cation desorption data. Compared to cation exchange, hydrophobic interaction played a more important role in HDTMA sorption.