Oil-induced phase transition from lamellar to reverse hexagonal liquid crystals in the Aerosol OT system

Abstract

The effect of added n-decane or m-xylene on the lamellar liquid crystal (Lga) phase in a water/ purified Aerosol OT system was investigated by small-angle X-ray scattering (SAXS) at 25 °C. Upon addition of n-decane at constant water/surfactant weight ratio, the reverse micellar solution phase (water-in-oil microemulsion) separates beyond the phase boundary of the single Lga phase, whereas the Lα-hexagonal liquid crystal (H2) phase transition takes place by adding m-xylene. In the m-xylene system, the effective cross-sectional area per surfactant molecule at the interface, a s, calculated from SAXS data suddenly drops off at the Lα-H2 phase transition. In the poly(oxy-ethylene)-type nonionic surfactant system, however, a s monotonically increases at the same transition upon addition of n-decane. It is considered that the difference in the size of surfactant head group and the penetration behavior of oil affect the change in a s at the Lα-H2 transition.