THERMOTROPIC PHASE-BEHAVIOR OF THE PSEUDOBINARY MIXTURE DPPC C12E4 ATEXCESS WATER

The pseudobinary phase diagram of DPPC/C(12)E(4) at excess water was i nvestigated by DSC, H-2-, P-31-NMR and freeze fracture electron micros copy in a wide range of concentrations and temperatures. Compared with previously published papers on phase behaviour of DPPC/Triton X-100 a nd 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 an d 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 pe ritectic point with an incongruently melting lamellar gel compound com plex formation at a molar ratio of n(DPPC)/n(C12E4) = 3:4 (x = 0.57) a nd 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 p hase L, for the high-temperature phase in the whole mixing range. At l ow surfactant concentrations 0 < x < 0.15 the formation of the rippled phase P-beta' in a defined region of temperature can be observed by f reeze fracture electron microscopy.