INFLUENCE OF CHARGES ON STRUCTURE AND DYNAMICS OF AN O W MICROEMULSION - EFFECT OF ADMIXING IONIC SURFACTANTS/

The effect of charges on the properties of oil-in-water (O/W) microemu lsions has been studied. These investigations started from an aqueous system made up from a zwitterionic surfactant (alkyldimethylamine oxid e) and hydrocarbon. The O/W droplets in this system can be charged by the partial substitution of the nonionic surfactant by either a cation ic or an anionic surfactant. The properties of these charged microemul sion aggregates were studied by static and dynamic light scattering, v iscosimetry, and interfacial tension measurements. The charge density on the droplets can simply be fixed at a desired value by the composit ion of the surfactant mixture. The light scattering experiments showed that the aggregates remain constant in size over a large concentratio n regime (0.1-30 wt %) and in addition their size is only very little influenced by the admixture of ionic surfactant. The scattering behavi or of the cationically substituted microemulsions could adequately be described by a simple random phase approximation (RPA) model that cont ains a hard-sphere interaction with an additional DLVO-potential term that accounts for the electrostatic repulsion. Deviations toward lower scattering intensities were observed for the anionically substituted microemulsions; i.e., this system behaves asymmetrically with respect to cationic and anionic surfactant substitution. These deviations resu lt both from an increased effective hard-sphere radius (larger hydrati on shell) and from a more effective electrostatic interaction. This ef fect is not particular for the system studied, but similarly observed for a microemulsion based on another nonionic surfactant (Brij 96). Dy namic light scattering experiments showed an increase of the diffusion coefficient with rising ionic content of the system. This increase is entirely due to the decreasing structure factor and only little influ enced by the hydrodynamic factor, which is slowly decreasing with incr easing ionic content. The diffusion coefficient of the charged microem ulsions exhibits a maximum at similar to 0.03 volume fraction.