Molecular Order in a Chromonic Liquid Crystal: A Molecular Simulation Study of the Anionic Azo Dye Sunset Yellow

Abstract

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.

Thermodynamics of the self-assembly of non-ionic chromonic molecules using atomistic simulations. The case of TP6EO2M in aqueous solution

“…We have performed simulations to compare the results of two force fields: GAFF and OPLS. The GAFF force field with modified electrostatics was successfully used previously by Chami and Wilson to study Sunset Yellow (SSY) 23 , and this force field was therefore initially chosen to study our target TP6EO2M molecules. GAFF is compatible with the TIP3P water model, and this was used for the simulations here.…”

“…23 SSY molecules quickly form small clusters, which organize into two tetramers after first 22 ns, followed by the merging of the tetramers into a single stack after 200 ns of the simulation time. (E) Number of molecules in clusters plotted against simulation time, each point represents an average over 10 ns (blue, triangles -GAFF; red, circles -OPLS).…”

“…3,4,23,39,40 In the simulations, the EO groups cause the molecules to lie at a slightly larger distance apart than would be preferred for the pure aromatic stacking. Two literature values have been reported for TP6EO2M.…”

“…5,23,39 For Sunset Yellow (SSY) and disodium cromoglycate (DSCG) the stacking free energy is about -7 RT 5, 23 , for benzopurpurin 4B (Direct Red 2) it was found to be -10.2 RT 39 and -12 RT for the dye Blue 27 5 . A collection of free energy values for several chromonic dyes is given by Dickinson et al in their chromonics review.…”

“…23 Here, a combination of long atomistic simulations and DFT calculations provide a detailed explanation for NMR shifts in solution; and the study is able to reproduce the molecular separation within stacks, the experimental association free energy and provide the first atomistic simulation of a chromonic nematic phase in water.…”