An inverse micellar Fd3m cubic phase formed by hydrated phosphatidylcholine/fatty alcohol mixtures

Abstract

In this paper we report the observation of an inverse micellar cubic phase, of space group Fd3m, in fully hydrated diacyl phosphatidylcholine/fatty alcohol (PC/FAlc) symmetric mixtures, with each lipid component having the same chainlength. In the PC/FAlc systems described here, the Fd3m cubic phase is formed directly from the lamellar L β phase upon heating. All of the mixtures which form the Fd3m phase have an alcohol-rich composition, with the mol fractions of alcohol being in the region of 0.75–0.85. The Fd3m cubic phase occurs for PC/FAlc mixtures with chainlengths of C14, C16, C18 and C20. However, for the C14 mixture, the inverse hexagonal (H II) phase coexists with the Fd3m cubic phase over an extensive temperature range, and only the H II phase is observed above the chain melting transition temperature for the C12 PC/FAlc mixture. Thus decreasing chain length tends to favour the formation of the H II phase over the Fd3m cubic phase. In the pseudo-binary phase diagrams of PC/FAlc mixtures in excess water, the Fd3m cubic phases appear between the H II phase and the inverse micellar solution phase (L 2). This is consistent with the average mean curvature of the polar/non-polar interface in the Fd3m cubic phase being more negative than that in the H II phase. The lattice parameters of these Fd3m phases show little temperature dependence. The lattice parameter increases steeply with hydrocarbon chain length at constant temperature (90°C) for chainlengths above C16, which implies that the limiting hydration of the Fd3m phase increases for these longer chainlengths.