INTERACTIONS BETWEEN WEAKLY CHARGED OIL-IN-WATER MICROEMULSION DROPLETS

This study concerns oil-in-water microemulsions stabilized by a nonion ic surfactant plus a few mole percent of an ionic surfactant at low dr oplet volume fraction (phi < 0.15). The interactions in these weakly c harged systems have been studied by using turbidity measurements. In t he absence of added ionic surfactant, the microemulsion droplets behav e as hard spheres at low temperatures close to the lower temperature p hase boundary. Increasing the temperature causes the turbidity to incr ease due to either droplet growth, clustering of droplets, or a combin ation of both effects. Droplets with added ionic surfactant show a dec reased turbidity due to electrostatic repulsion between the droplets. The turbidity of charged droplets is temperature independent up to the upper temperature phase boundary of the single-phase microemulsion re gion. Increasing the temperature above the upper temperature phase bou ndary causes a sharp increase in turbidity. At the low temperature pha se boundary, the variation of microemulsion turbidity with concentrati on of ionic surfactant (either anionic or cationic), droplet radius, a nd concentration of added electrolyte is semiquantitatively predicted by a simple theory with no adjustable parameters.