Abstract
Isosteric entropies and enthalpies of adsorption of pyrazine, pyridine andn-amyl alcohol at the mercury electrode have been measured at various constant charges. The “TΔS” component of the free energy of adsorption is relatively large for pyrazine and pyridine but is compensated by a comparable “ΔH” component that makes the free energy of adsorption relatively less sensitive to changes of surface charge and coverage. The variations of ΔS are discussed in terms of field-dependent orientation and displacement of water molecules in the Hg-water interphase. The entropies of adsorption for pyrazine and pyridine are found to vary linearly with the respective enthalpies of adsorption over a wide range of values of surface charge and coverage. The compensation temperatures, Tc, in the relation ΔH= α + TcΔS are in the range 292–317 °K, indicating a large degree of compensation at ambient temperatures (298°K). The compensation effects observed provide further evidence for the very important role of water orientation and displacement in electrochemical adsorption of organic substances at electrodes. Similar compensation effects are found for many physical and biochemical processes in the water solvent and reflect the important role of H′-bonding and structural effects in that medium.
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