Forces between emulsion droplets stabilized with Tween 20 and proteins

We measured forces between tiny emulsion droplets by using the magnetic chaining technique. Force versus distance profiles were obtained for emulsions stabilized by Tween 20, Tween 20/BSA (bovine serum albumin) mixtures, and beta-casein. For emulsions stabilized with Tween 20 alone, we observed classical DLVO behavior. At very large Tween 20 concentration, the force profile is clearly deviating from the double-layer repulsion. An interpretation of the data is proposed combining classical DLVO with steric-like additional effective repulsion due to the micellar condensation on the droplet surface. The systems that contain both Tween 20 and BSA qualitatively show the same kind of behavior: the purely electrostatic repulsion is progressively transformed into a steric one in the presence of BSA. The emulsions stabilized by beta-casein obey a different type of behavior: in that case the depletion attraction is found to be of great importance. The data are interpreted assuming additivity of standard DLVO forces and depletion attraction. The threshold flocculation force, as a function of the ionic strength, was measured for these systems.