PROPERTIES OF SURFACTANT MONOLAYERS IN RELATION TO MICROEMULSION PHASE-BEHAVIOR

The relationship between the properties of surfactant monolayers at oi l-water interfaces and the phase behaviour in bulk of mixtures of oil + water + surfactant is discussed. Such monolayer properties include t he spontaneous curvature, c(o), the interfacial, tension, gamma, the e lasticity K (or rigidity) associated with the mean curvature, and the elasticity KBAR associated with the Gaussian curvature. The model syst em chosen for investigation is the anionic surfactant AOT + aqueous Na Cl + n-alkane at 20-degrees-C. In such systems, inversion of microemul sion type from oil-in-water (o/w) to water-in-oil (w/o) is possible wi th increasing electrolyte concentration. The tension, gamma, passes th rough an ultralow minimum value at conditions corresponding to the for mation of three phases. Using small angle neutron scattering, we have determined the structure of surfactant-rich third phases (c(o) approxi mately 0) formed with the different alkanes. Lamellar phases consistin g of surfactant monolayers separated alternately by oil and water appe ar with short alkanes, whereas L3 and bicontinuous phases form in syst ems containing longer alkanes. The bending elasticity K has been measu red for planar monolayers at the oil-water interface by ellipsometry. K is independent of salt concentration but depends markedly on alkane chain length N, falling from approximately 1 k(B)T for N < 11 to appro ximately 0.1 k(B)T for N = 14. This is discussed in terms of the diffe ring extents of oil penetration into the surfactant chains. Higher rig idities favouring lamellar phases and lower rigidities favouring bicon tinuous microemulsions are in line with the theoretical predictions of de Gennes and Taupin. Estimates of the constant KBAR have been obtain ed in droplet microemulsions (w/o) from a knowledge of their size, K a nd gamma. The sign of the constant is in agreement with the geometry o f the phases formed in three phase systems. Finally, the ideas and con cepts developed in the oil-water systems described above are used to e xplain the wetting behaviour by alkanes of AOT monolayers at the air-w ater surface.