Study On The Surfactant Systems By Ultramicroelectrode Voltammetry

Ultamicroelectrode voltammetry is a new technique which has been used in the surfactant systems in recent years. It makes the character of the surfactant system much more clear. Based on its usage in the surfactant solutions, the ultramicroelectrode is used to investigate the systems: structure transition of the microemulsion; the adsorption of the cationic surfactant on the silica surfaces; the interaction mechanism and the coadsorption of the dihydroxybenzene and the surfactants. The main contents and the results are as follows:1. The structure transition of the microemulsion from w/o to o/w through bicontineous has been successfully detected by the ultramicroelectrode (UME) voltammetric measurement. The interaction mechanism between oil drops in o/w region is dissussed in detail for the first time. With increasing the water content, the change in diffusion coefficient has three transition points. The first two points are because of the change in the microemulsion structure as reported before. However, the other one is because of the forces between oil droplets.2. 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. The measurements are uninfluenced by the following hydrophobic adsorption process. The adsorption isotherms of DTAB on silica wafer surfaces under different pH are measured with this method. 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.3. The adsolubilization of dihydroxybenzenes (catechol and hydroquinone) into cetyltrimethylammonium bromide (CTAB) layers on silica particles have been investigated by dye method and UV spectrum. The adsolubilization amount of catechol reaches a maximum value at the critical micelle concentration (CMC) similar with the other adsolubilizates. For hydroquinone, the maximum adsolubilization amount reaches at critical surface aggregation concentration (CSAC) instead of CMC of the CTAB and then decreases to constant values. This phenomenon is interpreted by the interaction mechanism between dihydroxybenzenes and CTAB in the bulk solution measured by UME.4. The interaction mechanism between dihydroxybenzenes and surfactants has been investigated by measuring the diffusion coefficients of dihydroxybenzenes using UME voltammetry for the first time. The position of the substituted hydroxyl groups can affect the interaction mechanism. Catechol mainly interacts with DTAB micelles by hydrophobic forces. However, hydroquinone mainly interacts with DTAB monomers rather than DTAB micelles through electrostatic forces. On the other hand, the interaction mechanism is also dependent on the hydrocarbon chain length. Shorter hydrocarbon chains interact much easier with dihydroxybenzenes. The partition coefficients of dihydroxybenzenes in the surfactant solutions are also calculated. The surface tension of the system was measured to confirm the voltammetry results. The results obtained from surface tension measurement are well consistent with the voltammetry results.