Molecular orientation in adsorption of pyridine and pyrazine at water/mercury and water/air interfaces: Electrocapillary and reflectance studies

Abstract

The importance of orientation effects in adsorption at the water/mercury and water/air interfaces is demonstrated in relation to the electrosorption of pyridine and pyrazine. Pyrazine, being a chemically and electrically “symmetrical” molecule, provides an ideal basis for examination of electrochemical theories of adsorption of polar molecules in the double-layer at charged interfaces. It is shown to exhibit a relatively symmetrical change of free energy of adsorption with field about the potential of zero charge; this effect enables other solvent-solvent interaction effects to be evaluated. Evaluation of isosteric entropies and heats of adsorption, at constant charge, shows that the adsorption behavior of pyrazine is substantially different from that of pyridine, probably due to bifunctional II-bonding in the interphase. Important compensation effects between the heat and entropy of adsorption at various coverages and surface charges (at Hg) indicate the role of water-structure (H-bonding) effects in the interphase. Electric modulated reflectance studies at Hg show that the adsorption and desorption of pyridine can be followed optically. With isoquinoline, a surface reorientation transition can be detected before desorption.