Disintegration of the lecithin lamellar phase by cationic surfactants

The phase behavior of egg lecithin mixed with two cationic surfactants, hexadecyl- and dodecyltrimethylammonium chloride (C(16)TAC and C(12)TAC, respectively), in brine (100 mM NaCl) has been studied. Combining results from NMR spectroscopy, small-angle X-ray diffraction, and cryo-transmission electron microscopy, the phase sequence in less solvent rich regions of the phase diagram is linked to the aggregate structures appearing in dilute samples. The focus is set on the lamellar phases and their relationship to neighboring phases of positive curvature. NMR line shape studies on H-2-labeled surfactants are employed to reveal changes in bilayer structure. The results demonstrate large differences in the way that the two surfactants disintegrate the lamellar structure in the two closely related systems. Lecithin in mixture with the surfactant of the longer hydrocarbon tail (C(16)TAC) forms lamellar phase regions where the bilayers display curvature defects. A corresponding defect formation is not observed in the lamellar phase from mixture of lecithin and the short cationic surfactant(C(12)TAC). Instead, broad coexistence regions of (defect free) lamellar phase and micellar or hexagonal phases are observed. The role played by the defective lamellar phase in altering the phase behavior is discussed. We also conclude that a local segregation of the amphiphiles appears to be associated with curvature defects in the bilayer structure of C(16)TAC and lecithin.