Acta Chimica 128. (1991)
2. szám - Szabó Kálmán–Mika József: Temperature-dependence of the capacitance of electric double layer at the gallium electrode
196 SZABÓ, MIKA: TEMPERATURE-DEPENDENCE OF THE CAPACITANCE and the sign of дСц/дТ will become positive only at a charge of qM ]> 0, in the case of the Pb and Ch electrodes the change in sign of the temperature coefficient of Ch occurs at surface charges of —7 and —10 pC/cm2, respectively [2, 3]. The differences between the temperature-dependence of the capacitance of the mercury electrode and of the lead and cadmium electrodes are explained by the authors [2, 3] with the greater hydrophility of lead and cadmium, as compared to mercury. According to the measurements of Bagoczkaja et al. on Ga—In alloy in Na2S04 solution, at charges of q = 0 to —5 fiC/cm2 Ch decreases with temperature, while at the gallium electrode at potentials more positive than zero charge potential (pzc), Cd slightly increases with temperature [4]. At the (110) silver crystal face in KPF6 solution, the capacitance decreases with increasing temperature in the potential interval —1.2 to —0.6 V [5]. At (210) gold crystal face, the temperature-dependence of Ch is similar to that found at the mercury electrode [6]. Using experimental data, obtainable by the investigation of the dependence of electrode—electrolyte boundary layer capacitance on temperature, models relevant to the structure of the double layer and to the arrangement of the solvent molecules at the electrode surface can be checked. When the temperature coefficient of the capacitance, дСц/дТ is known, the entropy of formation of the electrode—electrolyte boundary layer, ZlSM-S0lv, and its dependence on qм can be calculated. The function solv _ <jM gjves information on how the order of the solvent molecules in changing in the double layer with the value of [7]. The maximum of the zjisM-80lv .— qM curve appears according to the investigations carried out at the mercury electrode [7] at a value of qM = = —4-------6 pC/cm2, which corresponds in the given system to the highest disorder of water dipoles. The experimental finding that ASM~solv has not at zero charge potential its maximum, can be explained by the Hg—OH2 interaction, owing to which water dipoles are turned with their negative end towards the electrode surface, and therefore, the double layer has at a surface charge of q^ = 0 a more ordered structure, than at qM = —4-------6 /tC/cm2, where the repulsing effect of the negative surface charge just compensates the attraction arising from the Hg— OH2 interaction [7]. Of the double layer models known, the Bockris—Habib model [8] describes in conformity with the experiments in the case of mercury electrode the AS — qM relationship. The shape of Ch — ?m and the magnitude and sign of dC^/dT, calculated with Parsons’ model, agree with the experimental results, but with respect to the position of AS maximum there is no good agreement with the experimental data [9]. The Frumkin—Damaskin model [10], and its form modified by Damaskin [11] describe in the case of the Hg/NaF system in agreement with the experimental results the course of the Сн — </м curves, the dependence Ada Chim. Hung. 128, 1991