INVESTIGATION OF THE INTERACTIONS IN EMULSIONS STABILIZED BY A POLYMERIC SURFACTANT AND ITS MIXTURES WITH AN ANIONIC SURFACTANT

The interaction of a nonionic polymeric surfactant with an anionic sur factant at the oil-water interface has been studied by its effects on the droplet size, stability and rheology of emulsions. Oil-in-water (o /w) emulsions were prepared using isoparaffinic oil and mixtures of a nonionic polymeric surfactant with an anionic surfactant. The macromol ecular surfactant was a graft copolymer with a backbone of polymethyl methacrylate and grafted polyethylene oxide (a graft copolymer with PE O chains of MW = 750). The anionic surfactant was sodium dodecyl sulfa te (SDS). The stabilization of the emulsion droplets was found to be d ifferent when using one or the other surfactant. The mechanism of stab ilization of emulsion droplets by the macromolecular surfactant is of the steric type while the stabilization by anionic surfactant is of th e electrostatic repulsion type. Emulsions stabilized with mixtures pre sent both types of stabilization. Other effects on the preparation and stabilization of emulsions were found to be dependent on properties a ssociated with the surfactant molecular weight such as the Marangoni e ffect and Gibbs elasticity. The initial droplet size of the emulsions showed a synergistic effect of the surfactant combination, showing a m inimum for the mixtures compared to the pure components. Emulsion stab ility also shows a synergistic interaction of both surfactants. Rheolo gical measurements allow for the estimation of the interparticle inter action when measured as a function of volume fraction. Most of the eff ects observed can be attributed to the differences in interfacial tens ion and droplet radius produced by both surfactants and their mixtures . The elastic moduli are well explained on the basis of droplet deform ation. Ionic versus steric stabilization produce little difference in the observed rheology, the only important differences observed concern ed the extent of the linear viscoelasticity region.