Effects of citrate adsorption on the interactions between zirconia surfaces

Abstract

The adsorption of the trivalent citrate anion on zirconia has been studied as a function of additive concentration and solution pH. The effects of citrate adsorption on the surface properties of the zirconia were monitored by using adsorption isotherms, FTIR spectroscopy and electrokinetic measurements. Microscopic information about the adsorbed species and its surface conformation were obtained with an atomic force microscope (AFM). The force–distance profiles were obtained at AFM scan rates equivalent to those expected for Brownian collision rates between colloidal particles. For a given concentration of added adsorbate, the adsorbed amount of citrate was seen to increase with decreasing solution pH. This was attributed to increased Coulombic repulsions between the citrate and the surface at higher pH. The effective pH_iep after adsorption was seen, from the electrokinetic data, to move towards lower pH values as the adsorbed amount increased. A limiting value of pH 3–3.2 was observed at an added citrate concentration of 10^–3 mol dm^–3. AFM force–distance measurements demonstrated the presence of an electrosteric barrier to flocculation in the presence of adsorbed citrate. A short-range steric barrier of ca. 10 Å per surface was seen under conditions where citrate was adsorbed, this barrier was in addition to any long-range electrostatic component of the overall force profile. Best fits to these long-range electrostatic parts of the data were used to calculate a diffuse-layer interaction potential. Comparison of these potentials with the electrokinetic data showed good agreement.