The problem of structure and properties of the metal/salt melt interface has recently attracted the attention of numerous investigators 1,2. Significant information on this question may be obtained in two different ways : (a) by measuring the capacitance when the nature and composition of the metal and salt phases, temperature and electrode potential are varied but the a.c. frequency remains fixed ; (b) by studying the effect of frequency on the electrode capacitance and resistance.
We have previously investigated the fixed-frequency capacitances of the stationary liquid and solid electrodes in molten alkali and alkaline-earth metal halides 3-11. The results of our work may be summarized as follows :
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The capacitance of the electrode/alkali halide interface depends significantly on a.c. frequency 3. However for sufficiently large frequencies (above 20 kHz) this dependence becomes small enough and thus this measured capacitance may be considered to differ insignificantly from the double-layer capacitance.
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The capacitance of the electrode/alkali (or alkaline-earth) metal halides interface depends on the electrode potential. In most cases C-E curves are close to parabolic with a pronounced minimum, but for some systems (Pb/LiCI~CsCI, Pb/NaC1-CaC12, Ag/NaC1-KC1, etc.) the symmetry of the capacitance curves is broken and a step or even a second minimum appears on the cathodic branch 7-9.
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The potentials of the minimum capacitance (Emin) have been found to be very close to the p.z.c. (electrocapillary maximum) of metals in LiC1-KC1 and NaC1-KC1 molten mixtures.
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The minimum capacitance (C~,~n) rises significantly and C-E curves become narrower as the temperature increases. The dependence of Cmi, on temperature however is not linear and practically disappears as the temperature approaches the melting poinΒ’ Β°'11. The absolute values of Cmi n for the lead electrode were 20-70 #F cm -2, depending on temperature and the nature of the melt 7.
To explain these high capacitance values it has been suggested that the melt side of the double layer has the alternating sign structure. In the terms of this hypothesis the effect of temperature, d.c. potential and the nature of melt on the capacitance can be qualitatively inferred 7'12-14
In 1964-69 Delimarskij and Kikhno 15'16 undertook the study of the capacitance of some solid metals which had not been investigated before. They have practically confirmed all the regularities enumerated a'.
* Recently Alekseeva and Kuznetsov (Elektrokhimiya 4 (1968) 95, 1351) have found systems where the potential of Cm~ n does not coincide with the p.z.c, and the value of Cml n is not independent on the nature of the metal.