In situ analysis of alkyl phosphate surfactant adsorption at the alumina/aqueous solution interface

Both the extent of adsorption and spatial chain orientation of adsorbed alkyl phosphate surfactant molecules on alumina were investigated using an in situ attenuated total reflection-Fourier transform-infrared technique. The adsorption isotherm was characterized by three distinct regions, and adsorption continued to occur even at pH values higher than the isoelectric point of alumina. Upon adsorption of surfactant, the wettability of alumina decreased, followed by a slight increase in the wettability at concentrations greater than 10 ppm, indicating the formation of a second adsorption layer. It was also found that as the surfactant concentration increased, zeta potential reversal occurred, and in a 10 ppm surfactant solution, alumina exhibited negative zeta potentials across the whole pH range. The profile of zeta potential-pH curves was typical for systems involving chemisorption. The spatial alkyl chain orientation angles of the adsorbed surfactant molecules were found to be in the range 48-41 degrees from the normal to the alumina surface for 1-100 ppm surfactant solutions.