The dispersion-stability diagram of boehmite nanoparticles in aqueous AOT solutions

The dispersion stability of hydrophilic boehmite nanoparticles in aqueous sodium bis(2-ethylhexyl) sulfosuccinate (AOT) solutions was studied in a wide range of particle and surfactant concentrations. The two experimental parameters significantly influence the dispersion stability and span a stability diagram. With increasing surfactant concentration and decreasing particle concentration, the system changes from stable via moderately stable to unstable and back. In concentrated AOT solutions, fully redispersed particles are present, stabilized by a surfactant bilayer or admicelles on the surface. The redispersion can be reversibly induced by dilution or concentration of the samples. The positions of two transitions, namely for complete precipitation and for beginning redispersion, can be fitted accurately using a simple model based on H-type adsorption and including the specific surface area of the particle and the molar area of the surfactant. The transitions are controlled by the concentration of free surfactant molecules in solution as well as the saturation surface coverage and were corroborated by turbidity measurements.