ADSORPTION OF MALONATE AND SUCCINATE AT THE HG AQUEOUS SOLUTION INTERPHASE/

The surface excesses of malonate and succinate are derived from differ ential capacity, potential of zero charge (pzc) and interfacial tensio n measurements. When the electrode is very negatively charged, the int erphase is described well by a simple electrostatic model assigning di fferent distances of closest approach to the electrode for Na+ (lambda = 1.6 angstrom), succinate (lambda = 1.6 angstrom) and malonate (lamb da - 4.4 angstrom). When the electrode is positively or not very negat ively charged, malonate and succinate are specifically adsorbed on Hg. In both cases the adsorption is described well by a virial isotherm, having similar adsorption parameters. The adsorption of succinate on H g when compared with earlier results in the literature on (210) and (3 11) gold faces follows the sequence Au(311) > Au(210) > Hg. Integral c apacities and electrosorption valency are also calculated and analyzed , suggesting that (a) at potentials close to or more negative than the pzc, specifically adsorbed malonate and succinate place their electri cal charges in the vicinity of the outer Helmholtz plane and probably do not make direct contact with the electrode, and (b) at potentials m ore positive than the pzc, succinate progressively replaces water mole cules in contact with the electrode and probably lies flat on the meta l surface, while malonate retains its carboxylate groups partially ori ented towards the solution.