Acta Chimica 128. (1991)

2. szám - Szabó Kálmán–Mika József: Temperature-dependence of the capacitance of electric double layer at the gallium electrode

SZABÓ, MIKA: TEMPERATURE-DEPENDENCE OF THE CAPACITANCE 201 where er is the relative permettivity of H20 in the Helmholtz layer, [ix the component of the dipole moment of H20, perpendicular to the surface, N1 the number of water dipoles at 1 cm2 of the surface; K = e2kT . 2 16я2^2^ ’ fi2 is the component of the H20 dipole moment perpendicular to the surface, N2 the number of chemisorbed molecules at 1 cm2 surface, if gM = 0. respectively. Damaskin presumed in the calculation of the capacitance of the Hg/NaF solution, in the calculation of the СH — дм curve that y1 *** y2 8 juC/cm2, Kx = 40 jitF/cm2 and K2 = 200 /iF/cm2. In similar calculations in the case of aqueous Cd/NaF solution he modified the value of K2 to 50 /iF/cm2, presuming that if the energy of chemisorption increases, then N2 increases too, and thereby, the value of K2 decreases. The С и — 9m curves calculated with the parameters above, agree well with the respective curves obtained experimentally [17]. When Eq. (2) was applied in the case of the gallium electrode, the value of K2 (similarly as was done in the case of cadmium) was taken as 50 juF/cm2, which seems realistic, because the hydrophility of Cd and Ga is practically identical, and therefore, the bondenergy of the chemisorbed H20 molecules is near identical. The values of the other parameters were left unchanged: Ki = 40 /iF/cm2 and у = 8 /iC/cm2, at T = 273 K. The value of K1 was not changed, because capacitance arising from H20 dipoles does not depend on the hydrophility of the metal. The dependence of у on temperature was calculated on the bases of the relation Го273 . In the case of Kx it has been assumed that it does depend not only directly on temperature, but the value of Nx changes also with changing T. In the calculation of the temperature dependence of Nx it was assumed that not only associates dipoles are present at the electrode surface, but according to the Bockris model H20 monomers and (H20)2 dimers [8]. The dipole moment of the latter is zero, therefore, Nx represents only the number of H20 mono­mers at the electrode surface. The number of monomers and its change with temperature can be calculated, when the enthalpy change of the equilibrium we have l/Kjip = —— sech2 ~~~ ; l/ICCh = exp ^ -. Kx and K2 are K1 У1 K2 y2 quantities of capacitance character, while y1 and y2 of charge character: K = e2kT 1 16л2 [il Nx ’ erkT srkT Vi = -------- and У2 = --------> Acta Chim. Hung. 128, 1991

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