The influence of sodium chloride on the self-association and chromonic mesophase formation of Edicol Sunset Yellow

Abstract

We have investigated the effect of an inorganic electrolyte (sodium chloride) on the aggregation behaviour and liquid crystals of Edicol Sunset Yellow. Edicol self-aggregates in aqueous solution to form single molecule stacks, which then become ordered to form nematic and hexagonal (columnar) mesophases at high concentrations. We have employed changes in the ¹H nuclear magnetic resonance (NMR) chemical shifts to monitor the aggregate formation in solution. A single spectrum is observed at all concentrations because the exchange between Edicol monomers in solution and those in stacks is fast on the NMR time scale. The results show that at low Edicol concentrations (<1 wt%) the concentration of aggregates is small, but at high concentrations (20 wt%) the fraction of monomers is tiny. At low Edicol concentrations, low levels of salinity appear to alter aggregate shape and size, resulting in a disaggregation/aggregation effect occurring over four orders of magnitude of added electrolyte. However, little alteration is seen in the fraction of aggregates. At high electrolyte levels, when the Debye length is comparable to the stack lengths (a few nanometres), the fraction of aggregates increases, presumably because of the reduced intra-stack electrostatic repulsion. Importantly, we have also shown that the isodesmic theory of aggregation (equal K) is too simple to describe accurately the aggregation process from the monomer to the pre-nematic phase concentrations. NMR quadrupole splittings indicate that there is no specific Na⁺ ion binding to the stacks. At the very highest concentrations of Edicol and sodium chloride the aggregates and mesophases are destabilised. The reason for this has yet to be elucidated.

Keywords:

• chromonic
• aggregation
• isodesmic
• Edicol Sunset Yellow
• salt
• sodium chloride
• NMR
• chemical shift
• quadrupole

Acknowledgements

We thank EPSRC and (the late) ICI for financial support. In addition, we are grateful to Professor A. Masters, Professor O. Lavrentovich, Dr L. Tortora, Dr O. Lozman, Dr C. Rodríguez-Abreu1, Dr M. Sintyureva and D.G. Edwards for valuable discussions. We also thank Professor M. Wilson and Dr F. Chami who provided details of unpublished structures from molecular simulations.