Proton binding at clay surfaces in water

Abstract

This article discusses the points that have more impact in the protonation–deprotonation behavior of a phyllosilicate clay surface in aqueous media. Some concepts that are currently used to explain the behavior of the oxide–water interface are now applied to the clay–water interface, and a rather new interpretation of the role of structural charges on the affinity of edge groups for protons is also given. Proton binding at a phyllosilicate 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 normal pH range. The protonation behavior depends on the intrinsic affinity of surface groups and long-range electrostatics. The MUSIC model confirms that the proton affinity of siloxane and gibbsite-like groups located at the basal surfaces (001 planes) of 2:1 or 1:1 clays is very low. On the contrary, the proton affinity of surface groups at the broken edges is adequate to undergo protonation–deprotonation reactions in a normal pH range. The reactivity of edge groups is also affected by the presence of structural charges within the clay layer. These charges produce an electric field that modifies not only the reactivity of the basal surfaces but also that of the edge surfaces. When the edge surface has a zero net charge, the electrical potential is negative due to the presence of the mentioned negative charges. Evidences for this are obtained from proton adsorption curves performed at different supporting electrolyte concentration.