AROMATICITY, ELECTRONIC-STRUCTURE AND MOLECULAR DIMENSION IN THE ADSORPTION OF ORGANIC-COMPOUNDS ON MERCURY .1. THE METAL ADSORBATE INTERACTION

The adsorbability of a wide series of aromatic derivatives (38) on mercury from aqueous solution at constant potential (E(pzc) or close to it) cannot be simply accounted for by the '' aromaticity '' of the compounds, at any rate modulated by substituent groups on the aromatic moiety, even if the adsorbates show a common planar disposition on the surface in the adsorbed state. The interfacial activity can be successfully related to the process of formation of a ''soft-soft'' couple (in the Pearson's concept) between mercury and the organic, in competition with the 'hard-soft'' couple between the metal and water. Desorption is then explained with a higher 'hard'' character of mercury at increasing negative potential. In this view, the feasibility of a partial charge transfer, as expressed by lambda, calculated for neutral organic adsorbates, corresponding to a positive DELTAN value (DELTAN, fractional number of electrons transferred from the adsorbate to the mercury) can be confirmed. A simple relation between adsorbability and ''bulkyness'' of the compounds, in relation to the dislodgement of water molecules from the surface occurring in the adsorption process is proposed and discussed.