Surface forces and deformation at the oil-water interface probed using AFMforce 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 collo idal particle and the hydrocarbon-water interface to be measured using the atomic force microscope. Results are presented which demonstrate that nonpo lar surfaces prepared in this way acquire a significantly negative diffuse layer potential in electrolyte solutions, as indicated by force distance re lationships which obey DLVO theory at large separations. At smaller separat ions, deviations from DLVO theory are observed, followed by an attractive f orce which causes the instantaneous engulfment of the silica particle by th e droplet. The presence of trace quantities of the anionic surfactant sodiu m dodecyl sulfate (SDS) radically alters the forces between the two surface s, creating a significant barrier to engulfment, and an apparent "softening " of the fluid interface.