PHYSICAL-CHEMICAL INTERPRETATION OF PRIMARY CHARGING BEHAVIOR OF METAL (HYDR)OXIDES

The primary charging behaviour of metal (hydr)oxides is of great practical and theoretical importance. It has been shown in the literature that it is possible to describe this charging behaviour with widely different models. This ability to describe the experimental observations has not contributed to a consensus with respect to the physical interpretation. The most popular models for interpretation are, in essence, all a combination of a description for adsorption of protons ("site-binding model") with a double layer model. The classical one- and two-pK site-binding models assume that the surface can be treated as chemically homogeneous. Recently, a multisite complexation model has been formulated (MUSIC model) that allows for a priori estimation of the proton affinity constants for various types of reactive groups present on metal (hydr)oxides. Large differences between experimental capacitance (d-sigma-0/dpH) as well as model constants for the Stern layer capacitance (0.2-4 F m-2) can be found for different metal (hydr)oxides, and are discussed. A physical model for the compact part of the double layer on metal (hydr)oxides is derived and compared with that for AgI. From the model it follows that for well crystallized non-porous metal (hydr)oxides the Stern layer capacitance is expected to be smaller than or equal to around 1.7 F m-2. Analysis of primary charging curves of non-porous colloids like gibbsite (Al(OH)3), rutile (TiO2) and goethite (FeOOH) in combination with the MUSIC model approach leads to a Stern layer capacitance of 1.2 +/- 0.4 F m-2, in agreement with the proposed double layer structure. Salt dependency of the charging curves as well as the value of the capacitance support the assumption of ion-pair formation. For silica a higher capacitance is derived which can easily be interpreted in the light of the surface structure since the reactive groups are not confined to a layer of densely packed reactive surface groups. Goethites with a specific surface area of less than 50 m2 g-1, prepared by rapid neutralization of an iron salt, charge significantly better. This is interpreted as being due to the presence of other crystal faces and/or porosity.