NMR characterization of the aggregation state of the azo dye sunset yellow in the isotropic phase.

The azo dye sunset yellow is known to form lyotropic liquid crystals as a function of both temperature and sample composition. Numerous studies have been performed to investigate the aggregation processes in these liquid crystals; however, less attention has been paid to the nature of the aggregates in the isotropic phase. In this study we employ diffusion nuclear magnetic resonance methods to investigate the hydrodynamic properties of sunset yellow aggregates at a range of concentrations in isotropic solution. The results of these experiments are interpreted in terms of a simple thermodynamic model for aggregation and suggest that the aggregates are comprised of tens to hundreds of monomer units at the concentrations investigated. The results also demonstrate that the average number of molecules per aggregate is a factor of approximately 5 larger than previously reported.