Mineral hydroxides: From homogeneous to heterogeneous modelling

The electrical double layer of mineral hydroxides is analysed. A brief description of the solution side of the double layer based on the Gouy-Chapman and the Gouy-Chapman-Stern model is given together with some arguments against the triple layer model. For the description of the primary surface charge the protonation of the surface oxygen groups is discussed. Based on calculated proton affinity constants (K(H)s) the conclusion is drawn that ordinary surface oxygens can undergo only one protonation step in the normal pH range. To describe the protonation of homogeneous amphoteric oxides the existing one-pK(H) model is therefore to be preferred over the classical two-pK(H) model. With specific adsorption of ions other than protons competitive adsorption and adsorption on independent sites is discussed. The moderate heterogeneity of crystalline oxides can be described with the MUSIC model. For strongly heterogeneous surfaces a description based on random heterogeneity is suggested. Mono- and multicomponent Freundlich type equations can be used to describe the adsorption of both protons and specifically adsorbing ions in this case. Copyright (C) 1996 Elsevier Science Ltd