Ion Pair Formation and Primary Charging Behavior of Titanium Oxide (Anatase and Rutile)

Abstract

The primary charging behavior of titanium oxide (anatase, rutile, and P25) and the ion pair formation of the electrolyte ions with the surface groups have been extensively studied. A large number of titration and electrokinetic data sets available in the literature have been successfully described, using the MUSIC (MultiSite Complexation) model with a Basic Stern double-layer option and applying the ion pair formation concept. The systematic analysis of the data, over a large number of different monovalent electrolytes and various ionic strength values, allowed the determination of a number of “best estimated” values for the ion pair formation constants. The values suggest that the interaction of the cations with the titania surface is stronger than that of the anions. This is in accordance with the observed shift of the IEP of titanium oxide to higher pH values, at high electrolyte concentrations. The binding of the cations follows the sequence Cs + < K + < Na + < Li + and that of the anions follows the sequence Cl - > NO 3 - > ClO 4 - > I - . Titanium oxide can be divided in two classes of materials, having a low ( C = 0.9 F m - 2 ) and a high ( C = 1.6 F m - 2 ) capacitance value, respectively. The low capacitance value corresponds with the low values found for well-crystallized gibbsite and goethite. On the basis of the low capacitance value and the absence of correlation with the dielectric properties of the solids, it is hypothesized that the first layer of physically adsorbed water has a unique relative dielectric constant ε of about 40 on well-crystallized oxides. The high capacitance may correspond to a situation with a distorted water layer, which has bulk water properties (ε = 78). No other significant differences between the interfacial charging parameters of anatase and rutile were found.