The influence of hydrophobic tail volume on thermotropic self-assembly of mannosides: Structural, dielectric, and rheological behaviours

Many aspects govern the nature of the resulting phase of a self-assembly of glycolipid, including its detailed stereochemical structure, solvent type, and state condition. Glycolipid has attracted considerable attention due to
its extensive lyotropic applications in surfactant industry and material science. However, its application as thermotropic liquid crystal is unknown and rarely investigated. Herein, the thermotropic properties of a series of
glycolipids, namely Guerbet branched chain α-D-mannosides (C8 to C24 total carbons) were studied by X-ray scattering, dielectric spectroscopy, and rheology. The shortest chain αManC6C2 exhibited lamellar phase over the entire temperature range whereas both αManC8C4 and αManC10C6 only at elevated temperatures since these have larger hydrophobic volumes. Interestingly, at the room temperature, both anhydrous αManC8C4 and αManC10C6 showed formation of rippled structures. Prior to transforming into the fluid lamellar phase, these complex structures possess greater viscosity than the former. The longer chain mannosides (αManC12C8 and αManC14C10) adopted an inverse bicontinuous Ia3d cubic and inverse hexagonal phases, respectively. The temperature-dependent evolution of dielectric relaxation times, τ(T) of primary relaxation within the lamellar, hexagonal, and isotropic phases is explored. Distortion-sensitive tests, enabled by derivative-based analysis,
evaluate the suitability of τ(T) parametrisation using the Vogel-Fulcher-Tammann (VFT) and critical-like equations. According to the dielectric and rheological analyses, as the temperature increases, both ε‖ and ε⊥ increased, while the viscosity decreased. The findings suggest that higher temperatures are accountable for
higher molecular mobility and fluidisation of the phase structure. These fundamental investigations are important to the bottom-up approach development of regulated and specially designed nanoscale material (e.g., a cryoprotective agent)