Title: Specificity of hydration of individual ions in relation to their adsorption at electrodes
Abstract: In the aqueous medium, H-bonding structural equilibria play a major role in the hydration of ions and in their adsorption behaviour at electrodes. Not only energy but entropy factors in adsorption are consequently important and lead to specificity in ion hydration interactions and ion adsorption. Specificity of ion-water interactions is reflected in the way such properties as the partial molar entropies, heat capacities and compressibilities of ions in aqueous solutions depend on ionic radii. Also, ion specificity arises in such spectroscopic properties as the 1H nmr chemical shift and the changes in overtone stretching and bending infrared band frequencies in water caused by the presence of ions. These properties, derived for individual ions, will be examined in relation to specificity of adsorption of ions at electrodes, especially Hg and Pt. At the former metal, hydration effects in ion adsorption can be investigated in studies of the adsorption of alkylammonium ions in the series Rn N+4−n where 1 n 4. Here the hydration of the N+ centre is controlled by steric effects associated with varying coordination of the N-atom by R groups (R = Me or Et). When n = 4, at the tetralakylammonium type of ion, hydrophobic adsorption effects arise and the unusual dependence of standard free entropy of adsorption of the cation on coverage can be accounted for in terms of hydration co-sphere sharing effects in the two-dimensional interphase at the Hg electrode. ΔH/ΔS compensation effects in adsorption at Hg from aqueous medium indicate the importance of water structure effects in the interphase. Generally, it is found that the dependence of thermodynamic and spectroscopic properties on ionic size or charge density for cations is quite different from that for anions. This effect is connected with the different orientation of OH bonds and lone-pair electrons in water molecules at anions in comparison with cations. At the former, better H-bonding in hydration shells with the remainder of the solvent can arise for reasons connected with H2O dipole orientation. In the case of Pt, interesting information on the state of adsorbed ions such as Cl−, Br−−, I−, CF3SO−3 and PF−6 can be derived from their competitive adsorption behaviour vis à vis chemisorbed H and OH or O species. In particular, information on lateral interactions and the electrosorption valence can be derived.
Publication Year: 1980
Publication Date: 1980-01-01
Language: en
Type: article
Indexed In: ['crossref']
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