Nonamphiphilic assembly in water: Polymorphic nature, thread structure, and thermodynamic incompatibility

Self-assembly of large quantities of entirely water-soluble molecules is entropically challenging. In this work, we describe the design and synthesis of water-soluble aromatic (dichromonyl) molecules that can form nonamphiphilic assemblies and the so-called chromonic liquid crystal phase in water. We discover a new molecule, 5′DSCG-diviol, that exhibits a large birefringent phase, and we show that the formation of this unique class of nonamphiphilic lyotropic liquid crystal shares enormous similarity to the polymorphism observed for crystal formation. Small-angle neutron scattering (SANS) revealed a concentration-independent rod-shaped assembly at concentrations below and above the formation of liquid crystal phase. Adding a small percentage of monoanionic aromatic molecules to the liquid crystal resulted in the elimination of the liquid crystal phase, but addition of dianionic aromatic molecules retained the liquid crystal phase. Together, these results suggest a new assembly structure for nonamphiphilic molecules in water, which is comprised of long threads of small molecules connected by salt bridges stacked over aromatic groups, with the molecular threads heavily hydrated with solvent water. Furthermore, mixing molecules with different structures can result in new liquid crystalline materials, or in segregation of the molecules into different solvation volumes, each of which contains only one type of molecule. The unusual thermodynamic incompatibility of entirely water-soluble molecules also supports the model of molecular threads, in which two polymer-like assemblies do not mix.