Comparative adsorption study of 1-butanol and 1-pentanol on mercury and gallium from aqueous 0.5 M Na2SO4 at 32°C

Adsorption of 1-butanol and 1-pentanol on dropping mercury and gallium electrodes from aqueous 0.5 M Na2SO4 at 32° C was determined on the basis of capacitive charge measurements carried out by a computerized chronocoulometric apparatus. Adsorption measurements on Ga were carried out both in neutral solutions and in acidic solutions of pH ≈ 3.5; in the latter medium, which avoids the formation of a submonolayer of Ga oxides induced by alcohol adsorption, the capacitive charge was corrected for the appreciable faradaic contribution due to hydrogen evolution by a special procedure. Adsorptivities of both alcohols are lower on Ga than on Hg, especially at less negative charge densities. However, even at charge densities as negative as -12 σ C cm-2, differences in the adsorptivity of the same alcohol on the two metals remain appreciable. Since at these negative charge densities both anionic specific adsorption and metal surface oxidation can be ruled out, the above differences in adsorptivity strongly suggest a certain coupling between the orientation of adsorbed water molecules and electron spillover, whose extent is expected to be greater on Ga than on Hg at the same negative charge density.