Competition between a lamellar and a microemulsion phase in an ionic surfactant system

An experimental study of a microemulsion system consisting of equal volumes of brine (water plus salt) and oil (cyclohexane), sodium dodecyl sulfate (SDS) as surfactant, and a mixture of hexanol and pentanol as cosurfactant is presented. Increasing the hexanol fraction in the cosurfactant mixture causes a competition between the (disordered) microemulsion phase and a lamellar phase at vanishing spontaneous curvature, culminating in four-phase brinelamellar phase-microemulsion phase41 coexistence. A SAXS study of the lamellar phases is presented. Although a rather high salt concentration was used (0.2 M NaCl), several indications for the presence of electrostatic repulsion between the lamellae are found. When the dispersion size F of the middle-phase microemulsion exceeds the value of approximately 70 nm, the lamellar phase becomes stable in the presence of excess oil and water. A similar dispersion size is estimated for a nonionic surfactant system where the lamellar phase is close to the three-phase body (Kahlweit, M.; Strey, R.; Firman, P. J. Phys. Chem. 1986, 90, 67 1). The experimentally observed phase behavior is compared with a phenomenological theory (Andelman, D.; Cates, M. E.; ROUX, D.; Safran, S. A. J. Chem. Phys. 1987,87, 7229). It appears that the theory qualitatively correctly describes the competition between the microemulsion and the lamellar phases. However, quantitatively there is a difference between theory and experiment which is attributed to the effect of a negative Gaussian bending elastic modulus