No 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 -> NO3 -> ClO4 -> 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 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 ( ) 78). No other significant differences between the interfacial charging parameters of anatase and rutile were found.

In this paper, we present a novel methodology, called the potentiometric mass titration (PMT) technique, for determining the point of zero charge (pzc) of mineral hydr(oxides) immersed in electrolytic solutions. Following PMT, the pzc is identified as the common intersection point (CIP) of the potentiometric curve of the blank solution with the corresponding curves of the impregnating suspensions containing different amounts of the immersed mineral (hydr)oxides. Full experimental results related to the determination of pzc using the PMT technique and four traditional techniques (potentiometric titrations, mass titrations, immersion, and microelectrophoresis (for determining the isoelectric point, equal to pzc in cases where no specific adsorption takes place)) are presented for four oxides, namely, MgO, γ-Al 2 O 3 , TiO 2 , and SiO 2 . The comparison of the pzc values determined by PMT, with the corresponding ones determined using the traditional methodologies, strongly suggested that the PMT technique can be used to determine the pzc of oxides. A simulation procedure of the PMT technique has been developed and applied to model oxides with properly selected acid-base characteristics and to various combinations of models related to the charging mechanism of the oxide surface (1 site/1 pK, 1 site/2 pK, multisite models) and to the description of the interfacial region (diffuse double layer, constant capacitance, basic Stern models). The intensive application of this simulation procedure offered a quantitative interpretation of the methodology. Specifically, it was demonstrated that (a) the application of the "quick scan" version of the PMT technique, realized by recording the titration curve of the blank solution (pH vs V added acid ) and the corresponding curve of a suspension of a given amount of the immersed oxide, indeed results in the determination of the pzc, provided that this is greater than a value of about 4; (b) the application of the "typical" version of PMT, realized by recording the titration curves of three different suspensions (pH vs V consumed acid ) containing different masses of the immersed oxide, provides the pzc value of this oxide over the whole pH range; and (c) the CIP that is determined, using PMT, corresponds to the pzc irrespective of the charging mechanism of the oxide surface and the structure of the double layer developed between the oxide surface and the solution. However, in the case where the basic Stern model is used to describe the interfacial region, the pzc value determined by PMT deviates slightly from the true value when the value of the affinity constants of the ion pairs formed between the positive counterions and the surface is different than the corresponding value of the negative counterions.