TERNARY PHASE-DIAGRAM OF MIXTURES OF PALMITOYL-OLEOYL-PHOSPHATIDYLCHOLINE, TETRAOXYETHYLENE DODECYL ETHER, AND HEAVY-WATER AS SEEN BY P-31 AND H-2 NMR

The isothermal phase diagram at 25 degrees C of ternary mixtures of 1- palmitoyl-2-oleoyl-phosphatidylcholine tetraoxyethylene dodecyl ether, and (H2O)-H-2 has been determined by P-31 and H-2 NMR. The liquid cry stalline lamellar phase (L(alpha)) occupies the largest area in the tr iangle. It extends from the lipid-water to the surfactant-water axis. Water separation (L(alpha) + W) occurs at high water concentrations. A surfactant-rich isotropic phase (L(2)), an inverted hexagonal (H-II), and a gel phase (L(beta)) are observed at low water content depending on the lipid and surfactant concentrations. Most of the phase transit ions occur via two-phase regions. A small three-phase region is detect ed in the water-poor and surfactant-rich region. A second three-phase region should exist at low water and surfactant concentrations accordi ng to the phase rule. Simple model considerations allow for a qualitat ive understanding of some of the phase transitions observed. The L(alp ha)/L(alpha) + W boundary at low surfactant content is calculated by a ssuming an ideal mixing behavior. The mesophases formed in the water-p oor region are discussed with respect to the molecular shape of the am phiphiles. Deuterium quadrupolar splitting measurements using partiall y deuterated surfactant are consistent with a relocation of the surfac tant relative to the lipid/water interface at the lamellar to hexagona l phase transition. (C) 1995 Academic Press, Inc.