Chromonic liquid crystals formed by CI Acid Red 266 and related structures

Chromonic liquid crystals are currently receiving increased attention because they have applications in a wide range of products. In this study, we have compared the chromonic mesophase behaviour of four azo dyes with similar chemical structures. Our objective is to determine if there is an obvious link between mesophase formation and dye chemical structure. Orange G does not form mesophases over the concentration range examined (saturated solution > ~20–30 wt%). The other three compounds all form nematic (N) and hexagonal (H) mesophases, but over very different concentration ranges. X-ray diffraction shows that the ordered Edicol Sunset Yellow (ESY) aggregates present in the mesophases have a single molecule cross section, while those of CI Acid Red have a cross section equivalent to six to eight molecules, probably organised in a ‘water-filled pipes’ structure. NMR quadrupole splittings of ²H₂O demonstrate that water binding to the aggregates is similar to that found for surfactant lyotropic mesophases. The sodium (²³Na) quadrupole splittings for Orange II and CI Acid Red are similar to the values found for surfactant hexagonal phases, suggesting that most sodium ions are ‘bound’ to the aggregates. This is unlike the behaviour of ESY where only one of the two sodium ions is bound.