Effects of pressure and temperature on the self-assembled fully hydrated nanostructures of monoolein-oil systems.

Synchrotron small-angle X-ray scattering (SAXS) was applied for studying the effects of hydrostatic pressure and temperature on the structural behavior of fully hydrated tetradecane (TC)-loaded monoolein (MO) systems. Our main attention focused on investigating the impact of isobaric and isothermal changes on the stability of the inverted type discontinuous Fd3m cubic phase as compared to the inverted type hexagonal (H(2)) liquid crystalline phase. The present results show that compressing the TC-loaded Fd3m phase under isothermal conditions induces a significant increase of its lattice parameter: it approximately increases by 1 A per 75 bar. Further, the Fd3m phase is more pressure-sensitive as compared to the Pn3m and the H(2) phases. At ambient temperatures, we observed the following structural transitions as pressure increases: Fd3m --> H(2) --> Pn3m. Our findings under isobaric conditions reveal more complicated structural transitions. At high pressures, we recorded the interesting temperature-induced structural transition of (Pn3m + L(alpha)) --> (Pn3m + L(alpha) + H(2)) --> (L(alpha) + H(2)) --> H(2) --> Fd3m --> traces of Fd3m coexisting with L(2). At high pressures and low temperatures, the TC molecules partially crystallize as indicated by the appearance of an additional diffraction peak at q = 3.46 nm(-1). This crystallite disappears at high temperatures and also as the system gets decompressed. The appearance of the Pn3m and the L(alpha) phases during compressing the fully hydrated MO/TC samples at high pressures and low temperatures is generally related to a growing hydrocarbon chain condensation, which leads to membrane leaflets with less negative interfacial curvatures (decreasing the spontaneous curvatures |H(0)|). Both the effects of pressure and temperature are discussed in detail for all nonlamellar phases on the basis of molecular shape and packing concepts.