Chromonic Liquid Crystal Formation by Edicol Sunset Yellow

Chromonic liquid crystalline phases of pinacyanol acetate: characterization and use as templates for the preparation of mesoporous silica nanofibers

We report on the self-aggregation of the cationic dye pinacyanol acetate and its use for the preparation of nanostructured silica via templated sol-gel reaction. The dye forms nematic and hexagonal chromonic liquid crystals at low concentrations in water (i.e., from 0.75 wt %); the type of counterion appears to play an important role in liquid crystal formation. From analysis of small X-ray scattering (SAXS) curves, it is inferred that dye aggregates have the morphology of hollow long tubes with one-molecule-thick walls; the diameter of the tubes does not to change much with concentration. The dye aggregates can be aligned by shear or by a magnetic field. The high-resolution (1)H NMR spectra show that aggregation takes place over a range of concentrations rather than having a sharp "critical" aggregation. Within the aggregates the conjugated moiety, including the three-carbon link, is in close proximity to the aromatic groups of stack neighbors. On the other hand, dye aggregates direct the formation of silica nanofibers synthesized via sol-gel reaction, mimicking the elongated structures found in aqueous media. The nanofibers show a hierarchical organization; i.e., they contain hexagonal arrays of 3 nm cylindrical mesopores left after calcination of the templating molecules, and the pore walls are 2.7 nm thick. As the nanofibers form entangled networks, the obtained materials also show interparticle porosity. The present findings open new possibilities for the use of commercial cationic dyes in the synthesis of nanostructured materials.

Chiral symmetry breaking by spatial confinement in tactoidal droplets of lyotropic chromonic liquid crystals

In many colloidal systems, an orientationally ordered nematic (N) phase emerges from the isotropic (I) melt in the form of spindle-like birefringent tactoids. In cases studied so far, the tactoids always reveal a mirror-symmetric nonchiral structure, sometimes even when the building units are chiral. We report on chiral symmetry breaking in the nematic tactoids formed in molecularly nonchiral polymer-crowded aqueous solutions of low-molecular weight disodium cromoglycate. The parity is broken by twisted packing of self-assembled molecular aggregates within the tactoids as manifested by the observed optical activity. Fluorescent confocal microscopy reveals that the chiral N tactoids are located at the boundaries of cells. We explain the chirality induction as a replacement of energetically costly splay packing of the aggregates within the curved bipolar tactoidal shape with twisted packing. The effect represents a simple pathway of macroscopic chirality induction in an organic system with no molecular chirality, as the only requirements are orientational order and curved shape of confinement.

Molecular order in a chromonic liquid crystal: a molecular simulation study of the anionic azo dye sunset yellow

We have carried out a detailed atomic simulation study of molecular order within a chromonic liquid crystalline material (sunset yellow) in aqueous solution. Self-assembly occurs in dilute solutions to form stacked aggregates, which show a preference for head-to-tail stacking and antiparallel dipole order. This feature is independent of solution concentration and aggregate size. Stacks are found to be dynamic entities in which rotational transitions (flips) can occur between antiparallel and parallel configurations. At a concentration matching the nematic phase of sunset yellow, the simulations show chromonic columns with a loose hexagonal packing and an intercolumn distance of 2.36 nm. Partial condensation of sodium ions occurs around a chromonic stack, with two preferred binding sites identified for sodium ions, corresponding to strong binding with the oxygens of a sulfonate group and a bridging site between a pair of molecules in a stack. A value for the free energy of binding of a molecule to a stack of 7 k(B)T was obtained for stacks of three and eight molecules, with a slightly larger value (additional 2 kJ mol(-1)) obtained for the dimer binding energy, indicating that aggregation is approximately isodesmic.

Concentration, temperature, and pH dependence of sunset-yellow aggregates in aqueous solutions: an x-ray investigation

The dye sunset yellow (SY) forms columnar aggregates via pi-pi stacking in aqueous solutions. These aggregates develop orientational and translational order at elevated concentrations to exhibit the nematic (N) and columnar (C) mesophases. Positional-order correlation lengths of the aggregates in the directions parallel and perpendicular to the stacking direction were measured as functions of temperature, concentration, and ionic content of solutions with synchrotron x-ray scattering in magnetically aligned samples. Average length of aggregates (i.e., the number of SY molecules in an aggregate) grows monotonically with concentration while their effective transverse separation decreases. The scission energy, E , determined from the Arrhenius thermal evolution of the longitudinal correlation length, is found to be 4.3+/-0.3 kBT and 3.5+/-0.2 kBT , in the N and C phases, respectively. Temperature and concentration dependence of E suggests that chromonic aggregation is not an isodesmic process. The aggregate length decreases with decreasing pH when HCl is added to the solution.