Molecular Frustration, Symmetry Breaking, and an Escape to Infinity
Mahesh K. Mahanthappa
University of Minnesota
Lyotropic liquid crystal (LLC) mesophases emerge from the self-assembly of minimally hydrated small-molecule amphiphiles. The exquisite, periodic nanodomain structures of LLCs suggest myriad applications as selective ion transporting membranes, templates for mesoporous inorganic materials synthesis, and therapeutic delivery vehicles. This talk will focus on efforts to understand the molecular factors that drive phase selection in LLCs of simple, spherical micelles. Based on hard sphere colloidal crystals, spherical micelles are intuitively anticipated to form high symmetry face-centered cubic (FCC) and hexagonally close-packed (HCP) structures. However, we recently discovered that ionic surfactant micelles also spontaneously form low symmetry, tetrahedrally close-packed Frank-Kasper σ and A15 phases, and related C14 and C15 Laves phases, with gigantic unit cells that mimic those of well-known intermetallic compounds. These low symmetry LLCs arise from a frustrated non-covalent force balance that minimizes local variations in amphiphile solvation, while maximizing global micelle cohesion within the particle ensemble. We discuss how chemical features of the amphiphiles and the path-dependent processing of their aqueous dispersions drive LLC sphere packing symmetry selection. This work suggests that engineered molecular frustration offers a route to high fidelity, periodic pattern formation at length scales that far exceed those of their constituent molecular and supramolecular building blocks.