Surface forces and deformation at the oil-water interface probed using AFM force measurement

The immobilization of droplets of a hydrocarbon liquid (n-decane) on a flat solid surface has allowed the forces of interaction between a silica colloidal particle and the hydrocarbon-water interface to be measured using the atomic force microscope. Results are presented which demonstrate that nonpolar surfaces prepared in this way acquire a significantly negative diffuse layer potential in electrolyte solutions, as indicated by force distance relationships which obey DLVO theory at large separations. At smaller separations, deviations from DLVO theory are observed, followed by an attractive force which causes the instantaneous engulfment of the silica particle by the droplet. The presence of trace quantities of the anionic surfactant sodium dodecyl sulfate (SDS) radically alters the forces between the two surfaces, creating a significant barrier to engulfment, and an apparent "softening" of the fluid interface.