Surface charge density interpretation

The metals of the s block of the Periodic Table have properties that can be interpreted in terms of the trend in their ionic radii down each periodic column. There is a very strong tendency towards formation of M+ (for ns1 or alkali) and M2+ (for ns2 or alkaline earth) metal ions, and other oxidation states are not important. With relatively high surface charge density, the alkali and alkaline earth metal ions are hard Lewis acids and have a preference for small hard Lewis bases. They particularly like O-donors, but can also accommodate N-donors, especially when present as part of a molecule offering a mixed 0,N-donor set. The number of metal-donor bonding interactions varies a great deal, depending in one... 

The chemical interpretation of o-in measured by the Schofield method depends sensitively on the type and concentration of probe electrolyte used. If these properties are chosen so that the cation in the reacting electrolyte neutralizes precisely the exposed functional group charge associated with isomorphic substitutions and dissociated hydroxyls and so that the anion neutralizes only the exposed protonated functional groups, then q+ and q. will have optimal magnitude for the chosen pH value and CTjn will be truly an intrinsic surface charge density. On the other hand, if the cation in the probe electrolyte is not able to displace all of the native adsorbed cations in, e.g., inner-sphere surface complexes, or if the anion cannot displace all of the native anions bound to protonated functional groups, or if either ion does not form only neutral surface complexes in the soil clay, then Ojn will differ from its optimal value. 

The field X is taken as that due to a surface charge density i e., X = 47T M/e, where e is the effective dielectric constant of the interphase. This theory provides a good basis for quantitative interpretation of organic molecule adsorption but when the organic molecule is n times as large as the adsorbed solvent, the form of the isotherm will be... 

The properties of the diffuse double layer depend directly on the surface charge density and not on the potential. In order to correct for this effect quantitatively, one needs to convert the dependence of frequency on potential A/( ), observed experimentally, to its dependence on charge density, A/( q). Having the analogous dependence A/o( q) for the supporting electrolyte, it is possible to evaluate the real response of the EQCM to specific adsorption, 5f q) = A/( ) — A ( ), and use this response for interpretation of the data obtained. This approach was taken in [74, 108,111] for several systems as seen in Figs. 9 and 10. For all cases studied, the surface excess was known from independent electrochemical experiments. 

Interpretation of the EQCM data related to adsorption requires knowledge of other electrochemical properties, such as the surface excess and the surface charge density, in addition to the data on the EQCM response as a function of surface charge density in the same system in the absence of the adsorbate. 

A qualitative theoretical explanation of this model based on ionic surface charge densities or continuum electrostatics is possible, but quantitative interpretations are difficult, even more so when molecular ions like SCN" are studied, which cannot be approximated by spheres with an isotropic surface charge density. 

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