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

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 (L α ) 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 (L 2 ), reverse anisotropic nematic (N 2 ), reverse micellar cubic (I 2 ), reverse hexagonal (H 2 ), 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 H 2 and L α phases. This area varies with the mole ratio [H 2 O]/[Lec] = W 0 at lower W 0 values, but saturates at an area of 90 Å 2 / molecule for W 0 ≳ 15. The phase diagram is discussed in relation to the known formation of giant wormlike reverse micelles in the liquid L 2 phase. Of particular interest here is the transition from liquid (L 2 ) to nematic (N 2 ) as the wormlike aggregate concentration is increased.