Proton binding at clay surfaces in aqueous media

摘要

Proton binding at a phyllosilcate clay surface takes place at oxygen-containing groups. As in the case of oxygen-containing molecules, the groups behave as monoprotic entities and undergo only one protonation-deprotonation step in a pH range 3-11. The protonation behavior depends on the intrinsic affinity of surface groups and long range electrostatics. The intrinsic affinities of siloxane groups and gibssite surface groups located at the basal surfaces (001 planes) of 2:1 or 1:1 clays are very low. They might be protonated only under very acidic condition. On the contrary, the intrinsic affinity of surface groups at the broken edges is sufficient to undergo protonation-deprotonation reactions under weakly acid, neutral or weakly basic conditions. The negative charges originated by isomorphic substitutions within the clay structure produce an electric field that affect the electrical potential at both, the basal and edge surfaces. When these surfaces have no net electrical charge, the electrical potential is negative due to the presence of the negative charges at the edges. This potential increases the affinity of basal surface for cations and also the affinity of surface groups at the edges for protons. Evidences for this can be obtained from proton adsorption curves performed at different supporting electrolyte concentration.