Anion-specific effects on the interaction forces between Al2O3 surfaces and dispersibility of Al2O3 colloids in electrolyte solutions

The interaction between an Al2O3 colloid and c-sapphire surface (0 0 0 1) was investigated in an aqueous solution containing citrate ion, Cl−, NO3− or Br− using the colloidal probe method. The force curve measured in electrolyte solutions at pH = 4 showed a good agreement with the force curve predicted by the Derjaguin–Landau–Vervey–Overbeek theory at a long separation distance. The repulsive forces were observed at a shorter separation distance. The order of the strength of the repulsive force was citrate ion > Cl− > NO3− > Br−, corresponding to the Hofmeister series. All the force curves measured in aqueous solutions at pH = 10 were the same. This is caused by the cation (K+) selectively affecting surface interaction because the Al2O3 surface charged to negative at this pH. A particle size distribution measurement of the Al2O3 colloids in the 0.001–0.1 mol/L Cl− and Br− aqueous solution was carried out. The agglomerates of the Al2O3 colloids were observed in 0.1 mol/L Br− solutions. However, Al2O3 colloids were well dispersed in the other aqueous solutions, which are reflected by the force curve results. Jump-in was observed only in the force curve measured in the 0.1 mol/L Br− solutions and not in any other solutions.