Adsorption of acetonitrile at the mercury/aqueous solution interface. Comparison of constant-potential and constant-charge analysis

Abstract

The adsorption of acetonitrile (ACN) on Hg from aqueous solutions of NaF has been studied by means of both electrocapillary and capacity curves. Full analysis of data has been carried out at constant charge as well as at constant potential. Both types of analysis have been found to give essentially the same picture of the adsorption process. Isotherms have been found to be congruent with respect to both of the electrical variables in the region more negative than the point of maximum in adsorption, whereas they are non-congruent with respect to both in the positive region. A Langmuir isotherm is obeyed in the former region, while in the latter the system is described by a Frumkin isotherm with positive interaction constant, corresponding to lateral repulsion, increasing as the charge or the potential becomes more positive. Reasons for this behaviour are discussed. The capacity of the inner layer has been also determined at constant amount adsorbed, and hypotheses have been made on the detailed structure of the inner layer in the presence of ACN.