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Electrochemical and spectroelectrochemical characterization of the adsorption of octadecanol on Au(111) Yang, Yanguo

Abstract

An in-situ elastically scattered light setup has been developed and combined with electrochemical techniques to investigate the potential dependent adsorption/desorption of octadecanol onto/from Au(lll) electrode surface. Octadecanol has been found to adsorb on the electrode surface at potentials close to the potential of zero charge and desorb at more negative potentials. The concentration of the supporting electrolyte solution has been found to have a significant influence on the adsorption/desorption process. The potential-induced phase transitions shown as peaks in the CV's and capacitance measurements are shifted in the negative direction by about 100 mV as the electrolyte concentration decreases from 100 mM to 5 mM. A thermodynamically stable aggregated form is postulated to exist when the surfactant molecules are desorbed from the electrode surface. The changes in the optical measurement, used to measure the characteristics of the desorbed species, or aggregates, were also found to be affected similarly. An intermediate adsorbed state (adsorbed aggregate) has been found to exist during the adsorption/desorption process. Electrolyte concentration most strongly affects the stability and form of the intermediate state. The elastically scattered light signal for the intermediate adsorbed state was more prevalent for higher electrolyte concentration. The lower electrolyte concentration displayed a larger potential region where this intermediate adsorbed state was stable, but the intensity of the elastically scattered light was much lower. Detailed analysis of the elastically scattered light signal and electrochemical responses makes us believe that the pre-desorption configurations in different electrolyte concentrations are quite different, implying a different conformation or structure of the aggregates. Under certain simplifying conditions, this implies that the average size of the predesorption state is larger for the higher electrolyte concentration. Potential sweep rate also has an influence on the scattering characteristics of the desorbed species, suggesting a possible method for measuring the kinetics of the adsorption/desorption process, or for controlling the character of the desorbed species. These changes were explained in terms of mechanism for the wetting or de-wetting of a solid surface by an organic phase. The influence of electrolyte concentration provides another opportunity for investigating the dynamics of this adsorption/desorption process.

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