Semiconducting oxides: infrared and rate studies of the effects of surface blocking by surfactants in dissolution kinetics

Abstract

For nickel oxide annealed at different temperatures (after preparation from the hydroxide), surface blocking by adsorbed anionic surfactant reduces dissolution rates per unit surface area in acid solution. Cationic and non-ionic surfactants have little effect. Infrared studies indicate coulombic interaction of the anionic group with the positive surface charge, with little molecular structural modification. For NiO annealed at 700°C, there is a systematic decrease in rate with anionic surfactant concentration to a minimum rate at ≈ 20% of the rate without surfactants. The minimum rate is found close to monolayer coverage. For 1450°C annealed material, weaker attraction of adsorbed surfactant, due to lower surface charge on the less imperfect surfaces, produces less reduction of the rate and the minimum rate (≈ 35% of the rate without surfactant) occurs at concentrations corresponding to about six times monolayer coverage. Surface blocking by the anionic surfactant causes a reduction (to 10%) of the enhanced dissolution rate produced by Co³⁺ in solution; a cationic surfactant again has no significant blocking effect and oxidation by Co³⁺ gives a more than one hundred fold increase in rate.

Rate changes are discussed in relation to changes in the oxide structure caused by pretreatment and in relation to the structure of the adsorbed surfactant layer.