Almost 30 years ago, when the study of chromonic liquid crystals was in its infancy, Alfred Saupe and L.J. Yu investigated disodium cromoglycate using deuteron magnetic resonance spectroscopy. Among their findings were that the quadrupolar splitting of the D₂O spectrum increased with decreasing temperature and that the addition of salt to the solution increased the temperature stability of the nematic phase. A good deal of research on chromonic liquid crystals has been reported since that time, and therefore a lot more is known. However, it is also interesting that some of the questions from that time remain unanswered.

Chromonic liquid crystals form when certain molecules are dissolved in a solvent, usually water, and the molecules spontaneously assemble into anisotropic structures. If the density of these structures is high enough and the temperature is low enough, they organize into a liquid crystal phase with orientational and sometimes with positional order also. Chromonic liquid crystals have been studied for more than half a century, yet theoretical, computational, and experimental investigations in the last decade have revealed many more details about them. The molecular structures that form chromonic liquid crystals are quite varied, and as a result the assemblies that these molecules form vary significantly also. Recent research has begun to shed light on these assembly processes, revealing that these too can be quite different from one system to another.

Trans–cis isomerization was investigated in a room temperature liquid crystal mixture of two azoxybenzene compounds. Experiments were performed on isolated molecules in dilute solutions and on the liquid crystal phase composed of the pure compounds. The absorption spectra of the trans and cis isomers were found to be similar to those of azobenzene compounds, as were the birefringence and order parameter of the nematic liquid crystal phase. The photo‐optic properties were also similar in that irradiation by ultraviolet light caused the conversion from trans to cis isomers, while short wavelength visible light incident on these compounds resulted in the conversion from cis to trans isomers. The activation energy for thermal relaxation from the cis to trans isomer in the liquid crystal phase was determined to be (66±7) kJ/mole, which is less than for azobenzene in solution. While a photostationary state in a dilute solution with approximately equal numbers of trans and cis isomers was achieved, the nematic–isotropic transition of the mixture of the pure compounds decreased from 70°C to room temperature with a cis concentration of only about 12%. One unusual finding was that the photostationary concentration of trans and cis isomers due to irradiation with light of a specific visible wavelength depended on the starting concentrations of the two isomers, indicating that there may be a molecular conformation that is not photo‐responsive and relaxes only thermally.

Past and recent x-ray and absorption data on chromonic liquid crystals are analyzed in a consistent manner to reveal both the cross-sectional areas and stacking free energy changes for a number of systems. Two tentative conclusions result from this study. First, the locations of the charged groups on the dye molecule determine the number of molecules in the cross-section of the aggregate. Second, among systems with the same aggregate structure, the higher the stacking free energy change as measured in dilute solutions, the lower the concentration at which a room temperature liquid crystal phase forms at higher concentrations.

Significance: Some soft matter systems behave similarly and some differently. Understanding this is one of the significant challenges in the field. For example, while the splay and bend elastic constants for thermotropic and lyotropic nematic liquid crystals are similar, the twist elastic constant is nearly an order of magnitude smaller in lyotropic nematics. Unfortunately, complete elastic data on lyotropic nematics are restricted to one polymer and two chromonic systems, omitting the most common and highly studied lyotropic surfactant systems. This article reports that a surfactant system of disk-like micelles also possesses an extremely small twist elastic constant, demonstrating that this property is general for lyotropic liquid crystals. The reason likely stems from the higher flexibility of molecular assemblies compared to molecules.