COMPOUND COMPLEX-FORMATION IN PHOSPHOLIPID-MEMBRANES INDUCED BY A NONIONIC SURFACTANT OF THE OLIGO(ETHYLENE OXIDE) ALKYL ETHER TYPE - A COMPARATIVE DSC AND FTIR STUDY

The phase behavior of pseudobinary mixtures DMPC/C12E4 at high water e xcess was studied between 0 and 50 degrees C as a function of the surf actant mole fraction x using DSC and FTIR. The phase transitions of ea ch component could be followed separately by FTIR spectroscopy mixing acyl-chain-deuterated DMPC-d(54) and alkyl-chain-protonated C12E4. Cha nges of the corresponding methylene stretching band frequencies yield the solidus and liquidus lines in the temperature-composition phase di agram. Two eutectics appear at x approximate to 0.55 and x approximate to 0.85. In the concentration range in between these values, the exis tence of a gel compound complex of about 1:2 lipid:surfactant composit ion was established. A peritectic point was suggested at low surfactan t concentration between the rippled, gel, and liquid crystalline state s of the lipid-rich bilayers on the basis of the Gibbs phase rule. A m icellar transition (cloud point) precedes the formation of lamellar ag gregates as the surfactant rich mixtures are heated. The phase transfo rmations are discussed in terms of fluidity and hydration of the molec ules on the basis of spectral data obtained from selected infrared abs orption bands of the lipid and of the surfactant. The phase behavior o f C12E4/DMPC was compared with literature data of related systems with respect to the chain melting transition, membrane solubilization, and complex formation. (C) 1998 Academic Press.