Heterogeneous interface adsorption of colloidal particles†
The heterogeneous adsorption behaviors of charged colloidal particles to oil–water interfaces were quantitatively and statistically investigated. Using optical laser tweezers, the particles in a sessile water drop formed in an oil phase were laterally translated toward the slope of the oil–water interface and their attachment to the interface was attempted. The adsorption probability was found to logarithmically decrease as the ionic strength decreased and to depend on the holding time during which an optically trapped particle was held at the position closest to the interface. Non-unity of the adsorption probability at particular salt concentrations and the holding time dependence offer an important clue that the particle adsorption to the interface is not deterministic but stochastic. The stochastic adsorption process can be attributed to the surface heterogeneity of colloidal particles that consequently leads to changes in the electrostatic interactions between the particles and the interface. We also demonstrated that the salt dependence on the adsorption properties of the particles, as measured by optical laser tweezers, was consistent with their bulk behaviors with regard to the stability of particle-stabilized emulsions. Furthermore, we revealed the gravity-induced spontaneous adsorption of the particles to the interface under conditions of sufficiently strong ionic strength.