THERMOTROPIC PHASE-BEHAVIOR OF THE PSEUDOBINARY MIXTURE DPPC C12E4 AT EXCESS WATER

The pseudobinary phase diagram of DPPC/C(12)E(4) at excess water was investigated by DSC, H-2-, P-31-NMR and freeze fracture electron microscopy in a wide range of concentrations and temperatures. Compared with previously published papers on phase behaviour of DPPC/Triton X-100 and DMPC/Triton X-100 [13,41], we do not find any total solubilisation of the bilayer membrane within the range of molar fractions x = 0 to x = 0.95. The estimated phase diagram implies a strong non-ideal mixing behaviour in the gel phases with peritectic and eutectic behaviour and complete miscibility in the liquid crystalline L, phase. Two mixing gaps were observed in the gel phase between x = 0.2 and x 0.95. One peritectic point with an incongruently melting lamellar gel compound complex formation at a molar ratio of n(DPPC)/n(C12E4) = 3:4 (x = 0.57) and one eutectic point at x = 0.9 are a peculiar feature of this system. H-2-NMR measurements employing selectively deuterated surfactant and P-31-NMR experiments clearly indicate a lamellar liquid crystalline phase L, for the high-temperature phase in the whole mixing range. At low surfactant concentrations 0 < x < 0.15 the formation of the rippled phase P-beta' in a defined region of temperature can be observed by freeze fracture electron microscopy.