FORCES BETWEEN EMULSION DROPLETS - ROLE OF SURFACE-CHARGES AND EXCESSSURFACTANT

We report direct measurements of the force distance profiles between e mulsion droplets in surfactant solutions of various nature. We probe t he evolution of double layer repulsive forces in mixture of ionic and non ionic surfactants. In the absence of any micelles, the force inten sity is shown to be significantly enhanced by the presence of non ioni c surfactant. Then, by varying the ratio of ionic over non ionic surfa ctant well above the critical micellar concentration, we investigate t he behavior of forces in the presence of micelles of various charge de nsities. The double layer repulsive force-distance profile is not affe cted by the presence of neutral non ionic micelles. However, as soon a s they get partially charged, a deviation from the classical electrost atic profile is observed. In that case, we establish that the repulsiv e force can be described by the sum of two contributions: one is the r epulsive double layer repulsive force while the other is attractive an d arises from the depletion of charged micelles. Because micelles and droplets are identically charged, they repel each other and this repul sion enhances the depletion force. This effect is accounted by conside ring an effective larger droplet diameter. We present the empirical re lation between this extra thickness and the Debye length. The simple a ddition of the two contributions is shown to successfully describe the force-distance profiles measured in mixed ionic-non ionic as well as in purely ionic micellar solutions.