STUDY OF ANION ADSORPTION AT THE GOLD-AQUEOUS SOLUTION INTERFACE BY ATOMIC-FORCE MICROSCOPY

The forces between a gold coated colloidal silica sphere and a pure gold plate have been measured in aqueous solution as a function of electrolyte concentration using an atomic force microscope (AFM). Forces in the presence of gold(III) chloride (HAuCl4), sodium chloride, and trisodium citrate were recorded as a function of concentration. Each of these anion species is present during the formation of colloidal gold by the reduction of gold(III) chloride with trisodium citrate. In pure water the force between the gold surfaces was exclusively attractive. In sodium chloride or trisodium citrate solution a repulsive interaction was observed which is attributed to the adsorption of these anions at the gold/water interface. The observed interaction force in gold(III) chloride solution was always attractive, the surface potential never exceeding 20 mV. Data taken in aqueous solutions of citrate and chloride ions together suggested that the citrate ions were preferentially adsorbed to the surface of the gold. Addition of gold(III) chloride to the AFM liquid cell after the pre-adsorption of citrate anions caused the force of interaction to change from a repulsive force to an attractive one initially as the gold(III) chloride was reduced to gold by the citrate anions. After a few minutes the repulsive interaction between the surfaces reappeared as the gold(III) chloride was exhausted. Careful analysis of the short range (<10 nm) forces present for all of the above systems suggests that short range repulsive forces are important in determining the stability of gold colloids. Finally, the surface potentials obtained from the AFM data are compared to literature values obtained by electrophoresis for gold colloids prepared by citrate reduction and the role of each of the anions in determining gold colloid stability is assessed.