Effect of Nonionic Surfactants on the Kinetics of Disproportion of Copper Sulfide Nanoparticles in the Aqueous Sols

The effect of nonionic surfactants on the disproportion kinetics of copper sulfide nanoparticles (aqueous sols) from the golden brown, copper-rich phase (Q-Cu₂S) to the green covellite phase (Q-CuS) has been presented. The transformation follows a first-order rate law. There is a substantial decrease in the rate of transformation in the presence of nonionic surfactants, viz. TX-100, NP-9.5, and NP-40. An attempt has been made to explain the phenomenon based on a surface state catalyzed transformation that is sensitive to the adsorption. The surface coverage (θ), estimated from the dependence of the rate constant on surfactant concentration, fitted the Langmuir adsorption isotherm. From these, the equilibrium constants and standard Gibbs free energy change of adsorption have been estimated.