On points of zero charge

Following on the pioneering efforts of Werner Stumm and his collaborators, environmental chemists have developed increasingly detailed molecular models of the adsorbent-aqueous solution interface in order to quantify the pH dependence of surface charge. This model development, irrespective of its level of sophistication, is subject to a few broad constraints, which may be termed generic properties of surface charge, that in turn reflect certain macroscopic conditions imposed by electroneutrality and thermodynamic stability. In this paper, rigorous proofs of these generic properties are given, and the results are used to establish general connections among conventional points of zero charge that all molecular models of the electrical double layer must respect. The nonunique relationship between a molecular model of the electrical double layer and the observed effects of ion adsorption on points of zero charge is emphasized.