Electrochemistry / Calorimetry / Underpotential Deposition / Peltier Heat / Entropy of Electrode ReactionsWe measured heat effects upon underpotential deposition or dissolution of a fraction of a monolayer of Cu on a Au surface. Temperature changes due to electrochemical reactions were pyroelectrically detected at the backside of a 100 µm thin polycrystalline Au electrode. Pulsed electrochemical deposition or dissolution during 10 ms intervals prevented significant loss of heat into the electrolyte on that time scale. The combination of both, i.e., pulsed electrochemical reactions at a thin electrode with low heat capacity, leads to unprecedented sensitivity. In addition, we compare the experimental temperature transients with the temperature evolution simulated for our experimental setup. Interferences stemming from mechanical deformation of the electrode foil due to potential-induced surface stress are also discussed.
The aggregation of amphiphilic molecules (e.g., the formation of micelles or membranes) is usually entropy-driven. We use electrochemical microcalorimetry to directly determine the entropy change a dodecyl sulfate molecule experiences upon potential-induced adsorption from aqueous solution into a surface aggregate. From measurements of the heat, which is reversibly exchanged during the adsorption or desorption process, we determined a value of 37 ± 9 J/(mol K) for the aggregation entropy. This value is in accordance with entropies of micellization of dodecyl sulfate in solution. A comparison with estimates of the entropy of aggregation of dodecane in aqueous solutions reveals that the aggregation is driven by the entropic contribution of the hydrophobic hydrocarbon tail, in accordance with general models for the aggregation of amphiphilic molecules.
For the independent and simultaneous characterization of electrochemical processes, we employ surface plasmon resonance (SPR) detection together with standard electrochemical methods. As a first test system we studied sulfate adsorption on Au(111). After careful calibration of the SPR signal we obtained submonolayer sensitivity on a millisecond time scale. The experimental data are simulated within the framework of a layer stack model of the optical interface. The sulfate adsorbate is simply described, employing the optical properties of sulfuric acid. The obtained potential dependent sulfate coverage is in reasonable agreement with independent results as presented in the literature.
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