Hierarchical self-assembly in colloidal and molecular length scales in thermotropic zwitterionic liquid crystals

Abstract

Hierarchical self-assemblies suggest routes for emerging structural complexities of soft matter towards multifunctional behaviors, however, involving challenges to design the balanced competing attractive and repulsive interactions required for the different length scales. Herein, self-assembly of zwitterionic amphiphiles were explored in the solvent-free state, where especially bis-n-tetradecylphosphobetaine shows temperature-driven reversible transition from low-temperature molecular-scale self-assemblies to high-temperature hierarchical self-assembly, combining colloidal and molecular order. The colloidal order results from layered molecular packing to 2D-nanoplatelets, which form well-defined oblique self-assemblies in several nanometer periodicity, allowing viscous fluidity upon shearing. The colloidal-scale structure emerges due to packing frustration between the tightly packed zwitterionic moieties and alkyl chains, and can be relieved by plasticizing with ionic liquids for purely smectic-A liquid crystallinity. Thus, even molecules with seemingly simple chemical structure can lead to structural hierarchy and tunable complexity by balancing the competing long-range electrostatics and short-range nanosegregations.