The direct measurement of the forces of interaction between a colloid particle and an oil droplet

Abstract

An atomic force microscope was used to measure the forces between a silica sphere (diameter 10 μm) and a n-decane droplet diameter 0.3 mm) in water. Force-distance profiles showed weak attraction and adhesion due to van der Waals forces. When the anionic surfactant sodium dodecyl sulfate was added, there was electrostatic repulsion at all separations due to the adsorption of the anionic surfactant at the oil-water interface, and no adhesion of the sphere to the oil droplet was observed upon retraction. Fitting the repulsive curves to the nonlinear Poisson-Boltzmann equation for heterogeneous surfaces yielded surface potentials on nthe oil surface which were consistently lower than those valu..