THE DIRECT MEASUREMENT OF THE FORCES OF INTERACTION BETWEEN A COLLOIDPARTICLE AND AN OIL DROPLET

An atomic force microscope was used to measure the forces between a si lica sphere (diameter 10 mu m) and a n-decane droplet (diameter 0.3 mm ) in water. Force-distance profiles showed a weak attraction and adhes ion due to van der Waals forces. When the anionic surfactant sodium do decyl sulfate was added, there was electrostatic repulsion at all sepa rations due to the adsorption of the anionic surfactant at the oil-wat er interface, and no adhesion of the sphere to the oil droplet was obs erved upon retraction. Fitting the repulsive curves to the nonlinear P oisson-Boltzmann equation for heterogeneous surfaces yielded surface p otentials on the oil surface which were consistently lower than those values found from electrokinetic experiments on oil drops under simila r conditions. The measured Debye lengths were also found to be signifi cantly different from those calculated from the surfactant and electro lyte conditions employed. Possible reasons for the discrepancies are o utlined. (C) 1996 Academic Press, Inc.