Phosphatidylcholine (PC) forms only lamellar mesophases in water, but it can be driven to self-assemble spontaneously into nonlamellar lyotropic liquid crystalline (LLC) mesophases by the addition of a third apolar component (oil) [2]. We report the formation of a reverse micellar cubic mesophase of symmetry Fm3m (Q225) in ternary mixtures of soy bean PC, water, and an organic solvent, including cyclohexane, (R)-(+)-limonene, and isooctane, at room temperature [1]. The mesophase structure consists of a compact packing of remarkably large reverse micelles in a face-centered cubic (fcc) lattice, a type of micellar packing not yet reported for reverse micellar mesophases.
The mesophase spacegroup was identified by Small Angle X-Ray Scattering (SAXS) based on spacing ratios and peak intensities. The variations of structural parameters point out to a classical hard-sphere phase diagram, showing an order-disorder transition Fm3m-L2 with an extended coexistence region. Form factor fitting in the pure L2 phase and in the Fm3m-L2 coexistence region yields quantitative estimations of the micellar low polydispersity (σ/Rc below 0.2), and PC interface rigidity 2κ+κ’ of 1.6-2.0 kBT.
The Fm3m structure is compared with the non-compact Fd3m structure found in the PC/water/α-tocopherol system. The compact Fm3m structure results mainly from (i) the release of lipid tail frustration and (ii) hard-sphere interactions between remarkably monodisperse micelles. The oil fills the geometric voids in the cell and modifies interface bending properties by penetrating the PC tails.
Acknowledgements: This study was funded by NESTEC Ltd.
[1] Martiel, I.; Sagalowicz, L.; Mezzenga, R., Langmuir 2013, 29(51), 15805-12.
[2] Martiel, I.; Sagalowicz, L.; Mezzenga, R., Advances in Colloids and Interface Science (March 2014, DOI:10.1016/j.cis.2014.03.005).
isabelle.martiel@hest.ethz.ch
