DOI: 10.1021/la9023019 1177 Langmuir 2010, 26(2), 1177–1185 Published on Web 08/14/2009 pubs.acs.org/Langmuir © 2009 American Chemical Society Effects of Pressure and Temperature on the Self-Assembled Fully Hydrated Nanostructures of Monoolein-Oil Systems Anan Yaghmur,* ,†,§ Manfred Kriechbaum, † Heinz Amenitsch, † Milo s Steinhart, ‡ Peter Laggner, † and Michael Rappolt † † Institute of Biophysics and Nanosystems Research (IBN), Austrian Academy of Sciences, Graz, Austria, and ‡ Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Prague, Czech Republic. § Permanent address: Department of Pharmaceutics and Analytical Chemistry, Faculty of Pharmaceutical Sciences, University of Copenhagen, Universitetsparken 2, DK-2100 Copenhagen, Denmark. Received June 26, 2009. Revised Manuscript Received July 30, 2009 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 f H 2 f Pn3m. Our findings under isobaric conditions reveal more complicated structural transitions. At high pressures, we recorded the interesting temperature-induced structural transition of (Pn3m þ L R ) f (Pn3m þ L R þ H 2 ) f (L R þ H 2 ) f H 2 f Fd3m f 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 R 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. Introduction Lyotropic liquid crystalline phases originating from the self- assembly of biologically relevant lipids in water are hierarchical complex assemblies. Their nanostructures are tunable and strongly depend on hydration level, lipid molecular structure, and composition and depend of course on the experimental conditions. 1-7 Among the experimental parameters, several re- ports on modulating the nanostructure by varying pressure, 3,8-17 temperature, 1,2,5,7,18-21 and pH value 22 were published. It has also been demonstrated that salt concentration, 23,24 the presence of peptides, 25-27 and proteins 28-30 play a vital role in controlling the lipid-based nanostructures. The one-dimensional (1D) lamellar (L R ), the two-dimensional (2D) inverted hexagonal (H 2 ), and the three-dimensional (3D) reversed bicontinuous (V 2 ) or discontinuous *Corresponding author: Tel þ45 35 33 65 41, Fax þ45 35336030, e-mail aya@farma.ku.dk. (1) Kaasgaard, T.; Drummond, C. J. Phys. Chem. Chem. 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