Entropy and solvent structure effects in electrochemical adsorption at mercury.
Creators: name: Gordon, L. G. M.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 1969
Subjects: subject: Chemistry, Inorganic.

description: Interphase structure studies in the electrochemical adsorption of neutral organic substances at the mercury electrode have been carried out by electrocapillary measurements. New techniques have been developed which increase the accuracy and greatly facilitate the required measurements. Use has been made of a Langmuir type of adsorption equation to evaluate the standard free energy of adsorption of pyridine from an aqueous 0.03M NaClO 4 solution over a range of temperatures. This study enabled the entropies and heats of the electrochemical adsorption to be investigated as functions of electrode charge density and coverage. The adsorption behaviour of acetophenone and its pinacol, 2,3-diphenyl-butane-2,3-diol, were examined in order to provide information required in the interpretation of the kinetic reaction orders for the electrochemical reduction of acetophenone at mercury. The pinacol, the main product of the reduction in acid solution, was found to be more adsorbable than the acetophenone, but the kinetic investigations indicated that the presence of pinacol did not affect the rate of reaction. The order of the surface reaction was found to be dependent on the coverage. An important solvent effect was observed in the comparison of the surface pressure-surface coverage relations obtained for several systems (acetophenone, pinacol and naphthalene in methanolic and aqueous methanolic solutions) in which the same supporting electrolyte was used. The adsorption characteristics of the same solute from various solvents were found to be different, but were similar for different solutes adsorbed from the same solvent. This behaviour, in conjunction with other effects reported, led to a critical assessment of the value of considering solute-solute interactions in the description of the observed, "non-ideal" Langmuir-type behaviour. The orientation of dipolar neutral adsorbates as a function of charge and surface concentration (and its correlation with the molecular area of the adsorbed species) has been investigated and previous interpretations have been modified. A comparison between the results for the adsorption of pyridine from aqueous NaClO4 and KCl solutions has been made in terms of the analyses based on the Langmuir and Volmer isotherms. A new method for evaluating the true free energy, entropy and heat of adsorption of neutral molecules at a mercury electrode in the region of low surface concentration where adsorption is considered "ideal", was investigated. In this study, use was made of the Volmer equation of state. The adsorption of D- and L-mandelic acid in the presence of L-lactic acid was investigated in order to evaluate possible stereospecific effects in adsorption. However, the differences which were observed in the corresponding surface pressures can be interpreted in terms of intra- and intermolecular esterification processes in the ad-layer, a situation which renders identification of true stereospecific effects somewhat ambiguous. A review of a number of critical aspects of adsorption phenomena is presented in which the emphasis is placed on structural effects in the adsorbed phase.