Phase Diagram and Phase Properties of the System Lecithin−Water−Cyclohexane

Abstract

The isothermal quasi-ternary-phase diagram of the lecithin−cyclohexane−water system was determined at 25 °C using a combination of polarizing microscopy, small-angle X-ray diffraction, and NMR techniques. The system contains four lyotropic liquid-crystalline phases and two isotropic liquid phases. Apart from the lamellar (Lα) phase, there are only reverse-type aggregates with a water interior, in addition to an essentially pure water phase, whose relative locations in the phase diagram follow the sequence (from the oil corner to the surfactant corner): reverse micellar solution (L2), reverse anisotropic nematic (N2), reverse micellar cubic (I2), reverse hexagonal (H2), and finally, the lamellar phase. The aggregates have a finite swelling with water, and coexistence with excess water is found at higher water contents. The area per lecithin molecule was determined in the H2 and Lα phases. This area varies with the mole ratio [H2O]/[Lec] = W0 at lower W0 values, but saturates at an area of 90 Å2/ molecule for W0 ≳ 15. The phase diagram is discussed in relation to the known formation of giant wormlike reverse micelles in the liquid L2 phase. Of particular interest here is the transition from liquid (L2) to nematic (N2) as the wormlike aggregate concentration is increased.