Forces Between Emulsion Droplets: Role of Surface Charges and Excess Surfactant

(Received 28 Novembre 1995, revised 11 April 1996, accepted 22 May 1996)

Abstract
We report direct measurements of the force distance profiles between emulsion droplets in surfactant solutions of various nature. We probe the evolution of double layer repulsive forces in mixture of ionic and non ionic surfactant. In the absence of any micelles, the force intensity is shown to be significantly enhanced by the presence of non ionic surfactant. Then, by varying the ratio of ionic over non ionic surfactant well above the critical micellar concentration, we investigate the behavior of forces in the presence of micelles of various charge densities. The double layer repulsive force-distance profile is not affected by the presence of neutral non ionic micelles. However, as soon as they get partially charged, a deviation from the classical electrostatic profile is observed. In that case, we establish that the repulsive force can be described by the sum of two contributions: one is the repulsive double layer repulsive force while the other is attractive and arises from the depletion of charged micelles. Because micelles and droplets are identically charged, the repel each other and this repulsion enhances the depletion force. This effect is accounted by considering an effective larger droplet diameter. We present the empirical relation between this extra thickness and the Debye length. The simple addition 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.