Double-layer capacity solvent dependence

The double-layer capacity depends strongly on the nature of the electrode material, even in cases where there is no specific adsorption of ions and solvent. It was therefore suggested, first by O.K. Rice, that the metal makes a direct contribution to the double-layer capacity. This idea was quantitatively pursued within the -> jel-lium model, in which the distribution of the electrons at the surface is affected by the double-layer field. In essence, the surface electrons form a highly polarizable medium, which enhances the capacity. In combination with the hard sphere electrolyte model, it gives the correct order of magnitude for the capacity at the -> point of zero charge also, it predicts correctly that the capacity of simple sp-metals should increase with the electronic density. An extension of the jellium-hard sphere elec-... 

Amokrane and Badiali proposed a semiempirical approach to the determination of the solvent contribution C, to the capacitance of the double layer in aqueous and nonaqueous " solutions. They used the relation C = Cf - C m, where Q is the experimentally determined capacity of the inner layer and Cm is the contribution of the metal. The plots ofC, vs. (Tm were presented for various solvents and correlated with their properties.However, the problem of the supporting electrolyte was entirely neglected in the quoted papers. It was shown recently that the height and position of the maximum on the C, vs. Gm plots depend on the type of the supporting electrolyte. Experimental differential capacity data obtained on the Hg electrode in methanol and ethanol containing various electrolytes with nonadsorbing anions (F , PFg, ClOi) indicate that the type as well as concentration of the electrolyte influences the position and the height of the maximum on the C, vs. plots (Fig. 13). 

The capacity of the double layer depends strongly on the solvent but there is a qualitative similarity in the shape of capacity-potential curves in all solvents including water. Early measurements seemed to indicate qualitative differences between water and such non-aqueous solvents as methanol, ethanol and ammonia which have featureless capacity curves in contrast to the characteristic humped curves found in water. However, more recent studies have shown that capacity humps occur commonly in solvents of widely differing types. They are found in solvents of high and medium dielectric constant and probably have a common origin in field reorientation of solvent dipoles. 

Although many models for the double layer have been published in the literature, there is no general model that can be used in all experimental situations. This is because the double-layer structure and its capacity depend on several parameters such as electrode material (metals, types of carbon, semiconductors, material porosity, the presence of layers of either oxides or polymeric films or other solid materials at the surface), type of solvent, type of supporting eleetrolyte, extent of specific adsorption of ions and molecules, and temperature. 

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