Adsorption sequence of the alkali cations at the tungsten trioxide-water interface

The stability of pyrogenic tungsten trioxide hydrosols is studied in the presence of various monovalent cations between pH 2 and 12. The hydrosol critical coagulation concentrations (c*) are determined by extrapolation of the experimental log W-log c curves up to log W = 0. The c* of K (+), Rb (+) and Cs (+) slightly increase with the pH of the solution while the c* of Li (+) presents a pronounced maximum at pH 4.5. A less marked maximum is also observed at the same pH in the presence of Na (+). The corresponding ionic adsorption sequence at the water-WO3 interface deduced from these experimental data is Cs (+) > Rb (+) > K (+) > Na (+) > Li (+) in the pH domain under investigation. This overall behavior is successfully explained by the model proposed by Gierst who has generalized the Gurney concept describing the ion-ion interactions in solution to the ion-surface interactions. The results reported in this work are also compared with those obtained previously for other oxides; they confirm the validity and the generality of the conclusions already drawn concerning the ionic adsorption sequences at the oxide-water interface. (C) 1999 Elsevier Science B.V. All rights reserved.