Comparison of cationic excesses in the presence of perchlorate and chloride anions

Cationic surface excesses of Li+, Na+, Mg2+, and Al3+ are obtained from ele ctrocapillary curves corresponding to aqueous perchlorate solutions. They a re compared with earlier reported data on chlorides, obtained under similar conditions. The influence of ionic sizes on the ion-free layer thickness i s discussed in the framework of Gouy-Chapman and the generalized UDCA theor y. Differences between the; surface excesses of a given cation in the prese nce of perchlorate and chloride find a plausible explanation in terms of th e anion distance of closest approach to the electrode.