SURFACTANT MICROSTRUCTURES (MICROEMULSIONS, MICELLES)

Studies of microemulsions, vesicles and mixed micelles formed by ionic surfactants or surfactant mixtures are reported. The chief experimental tools were small-angle x-ray scattering (SAXS) and quasielastic light scattering (QLS). A modern SAXS facility incorporating a position-sensitive detector is described.

Many equilibrium microemulsion phases can solubilize hydrocarbon and water in all proportions in a continuous progression of states without any visibly abrupt transition. When the volume fraction of oil (phi)(,o) in the microemulsion is nearly zero or unity, the microemulsion structure almost certainly consists of either micelles swollen with oil (at small (phi)(,o)) or inverted micelles swollen with water (at (phi)(,o) near 1). Contending pictures of the mid-range progression of microstructures are that swollen micelles pack together as their sizes and numbers increase, or that swollen micelles fuse into chaotic and dynamic bicontinuous structures of low viscosity. A model that calls for totally random bicontinuous structures for volume fractions of oil or water between 0.16 and 0.84 was used to predict the SAXS intensity curve and the QLS autocorrelation function for microemulsions. These predictions compare well with the SAXS measurements of microemulsions formed with the pure alkyl aryl surfactant sodium 4-(1'-heptylnonly)benzenesulfonate and with the QLS measurements of others. However, there is a limited composition range over which the SAXS measurements on microemulsions made with a commercial mixture of alkyl aryl sulfonates (Witco TRS 10-80) are better correlated with a model of microemulsion as disjoint interacting swollen micelles.

Vesicles form upon sonication of aqueous dispersions of SHBS in either water or NaCl brine. The vesicles are less than 500(ANGSTROM) in diameter and apparently consist of single SHBS bilayer shells surrounding an aqueous core. They are thermodynamically unstable and eventually revert to the bulk smectic phase. Reversion is hastened by flow through a porous medium.

QLS studies of mixed micelles present in solutions of SHBS and sodium dodecylsulfate (SDS) show that the micelles have prolate shapes in NaCl brines and are larger than pure SDS micelles.