Adsorption at metal oxide surfaces

Colloidal iron or aluminum oxides are an important component in many systems: two good examples are soils and the particles used in treatment of wastewater. Organic materials, especially products of organic decomposition such as humic or tannic acids as well as limiting nutrients such as phosphates, are often present in these systems. Thus, we would like to examine how these molecules interact with alumina or iron oxide surfaces, and particularly how they affect the chemistry of the surface; this research may also have important industrial applications in the processing of aluminum metal.

In our early work, we showed there was a close relationship between changes in colloid morphology and the coating of the colloid particles with various organic species in solution. Chemical force titration methods -- where we measure the strength of the adhesive interaction between tip and sample as a function of pH -- are suited to studying the interaction of phosphate species in solution with these colloids. Using AFM tips coated with a phosphate or benzoic acid-terminated alkanethiols, such as those shown below, we showed that phosphate interacts preferentially with Fe-OH sites on the surface; however, colloids coated with various organic acids, such as gallic and tannic acid, block phosphate adsorption. Using force titration methods on colloids has the added advantage that, unlike self-assembled surfaces, colloids are readily amenable to analysis by independent methods to determine the surface charge, such as zeta potentiometer. The zeta potential, solvation effects and the width of the electrical double layer are all important in determining the magnitude of force interaction between tip and sample.

Currently, research in this area is focused on examining the interaction of variously modified tips on these colloids. The long term goal in the project, is to study the interaction of organic acids with the colloids directly, by assembling species with the hydroxyl and/or carboxylate groups arranged in various meta or ortho positions. Since the binding strength of these species is thought to be dependent on the relative positions of these groups on the benzene ring, CFM should give insight into the mechanism of the organic groupsí interaction with the colloids.

Figure 4 Chemical force titration curve showing the tip-sample adhesive force as a function of pH between a Au-coated AFM tip terminated with an SAM of 4-(12-mercaptododecyloxy)benzoic acid and (a) a Au-coated mica substrate which has been terminated with an SAM of bis(11-thioundecyl)phosphate; (b) iron oxide colloids that were post-precipitated and (c) coprecipitated with K2HPO4. Note how the trace on the postprecipitated sample closely resembles that of the phosphate SAM on Au, demonstrating that under low loading conditions the K2HPO4 adsorbs in a bidentate fashion on the surface. The insets show the zeta potential of the post-precipitated and coprecipitated colloids as a function of pH in addition to an AFM image of the colloid.