STRUCTURE OF INTERPHASES OF LOW MELTING-POINT SALTS AND PROTONIC HYDRATES WITH MERCURY AND GOLD IN RELATION TO DOUBLE-LAYER CAPACITANCE BEHAVIOR

In relation to the unusual potential dependence of the double-layer capacitance C at lead in molten alkali halide salts, observed by Ukshe, Bukun, Leikis and Frumkin, where the relation of C to potential was like that for the diffuse-layer capacitance in dilute aqueous solutions, comparative measurements are presented for the capacitance behaviour of mercury and gold in some low melting point salts and salt hydrates. The systems investigated are LiAlBr4+NaAlCl4+KAlCl4 (3:5:2) and NaCl+AlCl3 melts, and proton hydrate salts of CF3SO3- (H3O+,H5O2+,H7O5+,H9O4+). In no case was the unusual behaviour of molten alkali halides at lead exhibited. The results also enable the progressive transition in double-layer capacitance behaviour between a molten salt, H3O+.CF3SO3-, at 307 K and systems containing solvent molecules to be evaluated and discussed in terms of interphasial structures, double-layer dimensions and local dielectric permittivity. At gold there are systematic and substantial changes in capacitance with speciation of the proton hydrates from H3O+, through H9O4+, to H+ aq. in dilute solution. At mercury, the species-dependence of the capacitance is also significant but is less systematic in its variation.