Intermediate Phases in the System Egg Lecithin/CTAC/Brine. SAXS and NMR Studies

Abstract

The phase behavior and phase structures in the pseudo ternary system egg lecithin (EPC)/cetyltrimethylammonium chloride/brine (100 mM NaCl) have been investigated by 2H and 31P NMR spectroscopy and small-angle X-ray diffraction (SAXS). At solvent contents higher than 55 wt %, micellar, hexagonal, and lamellar phases are formed. At lower solvent content, an intermediate phase occurs consisting of rods with noncircular cross-sections forming a centered rectangular phase of space symmetry c2mm. This phase is connected to both the lamellar phase and the hexagonal phase by two-phase regions. There is a large region of the lamellar phase where the bilayers contain water-filled defects in the form of pores and/or slits. SAXS diffractograms from the defect lamellar phase exhibit interlamellar Bragg reflections as well as a broad reflection originating from the water-filled defects. The variation of the distances obtained as a function of water content and CTAC/EPC ratio is consistent with theoretical predictions on water-filled defects in lamellae. The rectangular phase is characterized by several sharp SAXS reflections which have been indexed to the space group c2mm. The NMR line shapes are of biaxial symmetry with an asymmetry parameter ranging from 0.6 to 1.0. From an analysis of the SAXS and NMR data, we have found evidence of molecular segregation in that lecithin is enriched in the flat regions of the noncircular cylinders.