Confinement-Suppressed Phase Transition and Dynamic Self-Assembly of Ionic Superdiscs in Ordered Nanochannels: Implications for Nanoscale Applications

Ionic liquid crystals are ionic liquids that exhibit liquid crystalline mesomorphism together with ionic conductivity. As is known, confined liquid crystal mesophases can show anomalous dynamics and phase behavior. Investigations considering the factors controlling the macroscopic properties of ILCs in confinement are rare in the literature. This study reports the molecular mobility and the phase transition behavior of a guanidinium-based columnar ILC confined in the nanopores of self-ordered anodic aluminum oxide membranes of various pore diameters (25-180 nm) using broadband dielectric spectroscopy (BDS), calorimetry, and X-ray scattering. It is aimed at revealing in which way the pore size as well as the pore surface wettability (hydrophobic or hydrophilic) alter the molecular dynamics and phase transition behavior of this system. These properties are crucial for applications. The DSC investigations reveal the following: (i) the phase transition temperature for the transition from the plastic crystalline to the crystalline-liquid state has nonmonotonic dependence versus the inverse pore diameter; and (ii) the transition from the liquid crystalline to the isotropic phase is suppressed for all nanoconfined samples. This transition suppressed in the thermal signal was made evident by BDS and X-ray scattering. It is discussed as a continuous phase transition taking place in the pores instead of a discontinuous first-order transition, as observed for the bulk. BDS investigations show different relaxation processes for bulk and nanoconfined ILC. The molecular origins of various relaxation processes are discussed and suggested. It is further shown that the self-assembly of this ILC is dynamic in nature, which might apply for other ILCs too. The obtained results will have implications for nanoscale applications of ionic liquid crystals.