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 cry o-transmission electron microscopy, the phase sequence in less solvent rich regions of the phase diagram is linked to the aggregate structur es appearing in dilute samples. The focus is set on the lamellar phase s and their relationship to neighboring phases of positive curvature. NMR line shape studies on H-2-labeled surfactants are employed to reve al changes in bilayer structure. The results demonstrate large differe nces in the way that the two surfactants disintegrate the lamellar str ucture in the two closely related systems. Lecithin in mixture with th e surfactant of the longer hydrocarbon tail (C(16)TAC) forms lamellar phase regions where the bilayers display curvature defects. A correspo nding defect formation is not observed in the lamellar phase from mixt ure 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 la mellar phase in altering the phase behavior is discussed. We also conc lude that a local segregation of the amphiphiles appears to be associa ted with curvature defects in the bilayer structure of C(16)TAC and le cithin.