Entropy Changes upon Double Layer Charging at a (111)-Textured Au Film in Pure 1-Butyl-1-Methylpyrrolidinium Bis[(trifluoromethyl)sulfonyl]imide Ionic Liquid

We investigated the entropy of the electrical double layer (EDL) formation at a (111)-textured Au film in the ionic liquid 1-butyl-1-methylpyrrolidinium bis[(trifluoromethypsulfonyl]imide by electrochemical microcalorimetry. We found reversible heat exchange upon EDL charging in the potential range of -0.6 to 1 V versus Fc/Fc(+). Dependent on the potential, the heat evolution continued for 10 to 200 ms after the current flow, indicating slow reorganization of the EDL on this time scale. The partial molar entropy of the formation of the EDL Delta S-R(form)., was derived from the reversibly exchanged heat upon EDL charging. It varied about linearly with the potential from 27 J K-1 moL(-1) at -0.6 V to -79 J K-1 mol(-1) at 1 V versus Fc/Fc(+). Delta S-R(form) crosses zero close to the potential of zero charge of the system. The entropy values are much higher than those arising from purely configurational entropy of a 2D lattice gas, thus pointing to strong, potential dependent immobilization of the constituents of the EDL, where the immobilization is correlated with the strength of the electric field in the EDL. In addition, from our entropy values, we derive an apparent contradiction with experimentally determined temperature coefficients of the EDL capacitance, which implies a strongly varying activity of the ions participating in charging of the EDL with temperature.