Thermotropic and lyotropic liquid crystalline phases of Guerbet branched-chain -D-glucosides

Abstract

The effect of chain branching on glycolipid thermotropic and lyotropic phases was investigated for a series of synthetic β-D-glucosides derived from Guerbet alcohols, whose total hydrocarbon chain length ranged from C₈ to C₂₄. The compounds, which can be viewed as isosteric mimics for glycoglycerolipids, were synthesised in high purity and their liquid crystalline phases were studied using optical polarising microscopy (OPM), and small-angle X-ray diffraction. When dry, the shortest compound (total C₈) exhibits a monotropic L_α phase while longer ones (C₁₆ and C₂₀) adopt inverse hexagonal H_II phases. The C₂₄ compound forms an ordered lamellar phase at room temperature, but exhibits a metastable H_II phase upon cooling. Curiously the intermediate chain length homologue (C₁₂) adopts an isotropic inverse micellar (L₂) phase in the dry state over the range of temperatures studied. Upon hydration, the C₈ compound dissolves, and the C₁₂ compound forms a fluid lamellar L_α phase. The C₁₆ Guerbet glucoside (i.e. β-Glc-C₁₀C₆) exhibits an inverse bicontinuous cubic phase of space group Ia3d in excess water, never previously observed in branched-chain lipids, and very seldom observed in excess water. The C₂₀ compound remains in the H_II phase upon hydrating, with the lattice parameter swelling substantially.

Keywords:

• branched-chain glycolipid
• inverse bicontinuous cubic phase
• inverse micellar phase
• lyotropic phase behaviour
• self-assembly

Acknowledgements

This work was supported by Malaysian grants UM/MOHE HIR and UM.C/625/1/HIR/MOHE/05, and by EPSRC (UK) Platform grant EP/G00465X. We thank Diamond Light Source (UK) and the Australian Synchrotron for synchrotron beamtime.