Separation of Temperature Effects on Double-Layer and Charge-Transfer Processes for Platinum|Solution Interphases. Entropy of Formation of the Double Layer and Absolute Molar Entropy of Adsorbed Hydrogen and OH on Pt(111)

The present work combines results from pulsed-laser experiments and a thermodynamic analysis, to separate temperature effects on double-layer and charge-transfer processes. The entropy of formation of the double layer for Pt(111) in 0.1 M HClO4 is evaluated and compared with the corresponding entropy of formation of the overall interphase. The difference between the two quantities is due to the entropy change associated with charge-transfer processes, thus allowing for the evaluation of the absolute molar entropy of adsorbed hydrogen and OH on Pt(111). The results indicate that adsorbed hydrogen on Pt(111) is very mobile, whereas adsorbed OH species are rather immobile. Finally, values of the entropy of formation of the double layer are compared to data for mercury electrodes. It is concluded that the entropy of the interfacial water network on Pt(111) exhibits a nearly symmetrical dependence on charge density. This result contrasts with the behavior observed for mercury electrodes, for which the state of maximum entropy is achieved at clearly negative charge densities.