Electrophoretic mobility study of dodecyltrimethylammonium bromide in aqueous solution and adsorption on microspheres

The adsorption of the cationic surfactant dodecyltrimethylammonium bromide (DTAB) on uniform microspheres (polystyrene latex (PSL) particles) of various sizes has been studied using electrophoretic light scattering, dynamic light scattering, viscosity, and conductivity measurements. The measurements were performed on PSL particles of different diameters at different surfactant concentrations in an aqueous solution of 0.001 M NaBr. Initially, the uncoated surfactant-free latexes are negatively charged. After adsorbing a trace amount of the positive DTAB ions, the electrophoretic mobility of the latexes changes from negative to positive. The electrophoretic mobilities of the DTAB-adsorbed latexes increase with increasing DTAB concentration until reaching a maximum and then flatten out. The different magnitudes and patterns for the electrophoretic mobilities of the PSL particles of different sizes indicate that there are different adsorption mechanisms for the adsorbed surfactant on the surfaces of different curvatures. This observation is similar to the previous investigation of the adsorption of block copolymer micelles on PSL particles, where a trend was found in which the adsorbed layer thickness is inversely proportional to the diameters of the PSL particles. Dynamic light scattering experiments have provided useful information on the adsorption only at very low DTAB concentrations, since the measured hydrodynamic radii are biased at high DTAB concentrations due to the effects from the change in the solution ionic strength and viscosity. On the basis of this observation, two adsorption models are proposed which account for the adsorption of the surfactant on large PSL particles and on small PSL particles, respectively.