DETERMINATION OF MICROEMULSION PHASE CONTINUITY AND DROP SIZE BY ULTRA-CENTRIFUGATION

Microemulsions containing two synthetic petroleum sulfonate surfactants, an alcohol cosurfactant, an oil, and NaCl brine were studied with the analytical ultracentrifuge. Drop diameters calculated from measured sedimentation rates for both oil-continuous and water-continuous microemulsions were in the range of 50 to 400 Å and varied with salinity and surfactant chain length in a manner consistent with solubilization data. The surfactant phase was found to have somewhat larger drops and to be oil continuous in some portions of its region of existence and water continuous in others. This result confirms a previous proposal that this phase forms as a result of phase separation in a microemulsion or micellar solution. At still other compositions the surfactant phase showed behavior characteristic of both oil-continuous and water-continuous systems. The observed variation in drop size with salinity in all the microemulsion phases is explained in terms of the screening effect of the brine on interaction between surfactant ions. © 1979.