Study on the first-step adsorption of dodecyltrimethylammonium bromide solutions on silica wafer surfaces by ultramicroelectrode voltammetry

Ultramicroelectrode (UME) voltammetry is introduced to study the first-step adsorption of dodecyltrimethylammonium bromide (DTAB) solutions on silica wafer surfaces. This method is based on the exchange reaction of the surfactant molecules with hydrogen ions (H+) on the surfaces. In the first-step adsorption process, when a surfactant molecule is adsorbed to the hydroxylated silica surfaces, a H+ will be displaced. Therefore, H+ concentration will change with the adsorption process until it reaches saturation of the first-step adsorption. The molar adsorption amount of DTAB (mol m−2) before critical micelle concentration (CMC) can be calculated from the change in H+ concentration. The following adsorption process at higher surfactant concentrations is dominated by hydrophobic forces. Consequently, the H+ concentrations do not change with the adsorption process any more, which makes the measurement uninfluenced by the following hydrophobic adsorption process. The adsorption isotherms of DTAB on silica wafer surfaces under different pH are measured with this method. It is found that all the adsorption isotherms exhibit asymptote (L) shape and the equilibrium molar adsorption amounts increase with increasing the pH of the solution. These results indicate that H+ not only change the surface charge but also compete with surfactant for adsorption at higher proton concentrations.