On the specific adsorption of anions in the electrical double layer

Abstract
The main experimental facts which should be explained by any ionic isotherm are described. A critical survey of the existing isotherms is made and it is shown that none can explain these facts quantitatively. On the basis of Devanathan’s model of the electrical double layer, an isotherm is derived which satisfactorily explains quantitatively all the facts cited. It is further shown that in this isotherm all the parameters except that involving the noncoulombic part of the free energy of adsorption can be predicted from theory. An evaluation of this non-coulombic energy is made for chloride, bromide and iodide ions from the thermodynamic data of Devanathan & Peries. It is shown that it is independent of the charge of the interface and that it increases linearly with the percentage covalent character of the mercuryhalide bond. With the aid of the isotherm, the broad features of the differential capacity curves are interpreted, in particular the nature of the hump is explained. Pronounced humps are attributed to the specific area of the molecule which may be due to its molecular structure or due to its hydration structure. The latter type of humps called 'hydration humps’ which are observed with ions like nitrate or perchlorate should be temperature sensitive, whereas the former ones should be independent of temperature. It is shown that the inner layer parameters used by Devanathan have been independently confirmed, with the isotherm providing confirmation ofK1-2. The present status of the ionic double layer is summarized.

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