Microcalorimetric investigations of anion and cation adsorption at well-ordered Au and Pt surfaces - On the influence of hydration and solvent structure on the entropy of formation of the electrochemical double layer

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Electrochemical reactions are strongly dependent on the structure and composition of the electrochemical double layer. However, details, e.g. on the surface excess concentration of the adsorbed ions, their hydration or on the solvent structure are difficult to obtain and such information known only for few systems. We plan to measure the heat upon charging of the double layer as a function of the potential. From this we determine the reaction entropy of the charging process. Together with the charge balance, we obtain information on the ions, which are involved in the charging process, their hydration, their surface excess concentrations and accompanying restructuring of the solvent water. In the previous phase of the project we showed for Ag and Cu underpotential deposition on Au(111) that we can measure the reaction entropy of the deposition process dependent on the potential, in parallel to the current-potential-relation of the electrochemical system. From this we identified the electrochemically induced adsorption processes. The sensitivity we reached with the newly developed calorimeter is sufficient to detect heat effects upon double layer charging, that is, in the absence of electrochemical charge transfer reactions. In the new project, we plan first to investigate the heat effects upon specific and non-specific ion adsorption on Au surfaces. Then we will measure the entropy of OH- and H-adsorption on Pt surfaces as a function of the potential. We will determine the entropic contribution to the stabilization of these adlayers and hope to find hints on the restructuring of the solvent at the interface. We then study the reaction entropy of non-covalently bound anions on OH-covered Pt electrodes and compare our results with the model proposed by Markovic et al. for those systems.