Self-Assembly of Lyotropic Chromonic Liquid Crystal Sunset Yellow and Effects of Ionic Additives

Abstract

Lyotropic chromonic liquid crystals (LCLCs) are formed by molecules with ionic groups at the periphery that associate into stacks through noncovalent self-assembly while in water. The very existence of the nematic (N) phase in the typical LCLC, the dye Sunset Yellow (SSY) is a puzzle, as the correlation length associated with the stacking, as measured in the X-ray experiments, is too short to explain the orientational order by the Onsager model. We propose that the aggregates can be more complex than simple rods and contain "stacking faults" such as junctions with a shift of neighboring molecules, 3-fold junctions, etc. We study how ionic additives, such as salts of different valency and pH-altering agents, alter the N phase of SSY purified by recrystallization. The additives induce two general trends: (a) stabilization of the N phase, caused by the mono and divalent salts (such as NaCl), and evidenced by the increase of the N-to-I transition temperature and the correlation length; (b) suppression of the N phase manifested in the decrease of the N-to-I transition temperature and in separation of the N phase into a more densely packed N phase or the columnar (C) phase, coexisting with a less condensed I phase. The scenario (b) can be triggered by simply increasing pH (adding NaOH). The effects produced by tetravalent spermine fall mostly into the category (b), but the detail depends on whether this additive is in its salt form or a free base form. The base form causes changes through changes in pH and possible excluded volume effects whereas the salt form might disrupt the structure of SSY aggregates.

Using chiral tactoids as optical probes to study the aggregation behavior of chromonics

“…Twisted structures have been reported previously in disodium cromoglycate (DSCG) tactoids (4), in Sunset Yellow FCF (SSY) droplets (29), and for LCLCs confined to cylinders (30,31). Another interesting aspect of LCLCs is that the length distribution of the constituent units is polydisperse and is determined both by temperature and concentration (18,28,32). A consequence of the polydisperse size distribution of LCLCs is a broad biphasic regime where the nematic and isotropic phases coexist (32).…”

“…The model assumes that the aggregation process is isodesmic. A commonly accepted value of aggregation energy (E ) is about 7.25 kB T , as estimated from several experiments (18,20,32,42) and molecular dynamic simulations (43). The relative volume fraction XN of aggregates containing N molecules is XN = N (X1 exp(E /kB T )) N exp(−E /kB T ), where…”

“…Another interesting aspect of LCLCs is that the length distribution of the constituent units is polydisperse and is determined both by temperature and concentration (18,28,32). A consequence of the polydisperse size distribution of LCLCs is a broad biphasic regime where the nematic and isotropic phases coexist (32). However, there are several fundamental questions pertaining to LCLCs including their aggregation behavior, elasticity, and anchoring that need to be rigorously addressed.…”

Chiral symmetry breaking and surface faceting in chromonic liquid crystal droplets with giant elastic anisotropy

“…SSY was purchased from Sigma-Aldrich at a purity of 90% (wt/wt); it was then further purified using a published precipitation method (22,26,46 height and 2 mm in width, VitroCom) was filled with this emulsion solution. While in the capillary, water in the droplet undergoes a slow evaporation through the oil phase leading to an increase of SSY concentration in the droplet and an eventual phase transition from the nematic to the columnar phase.…”

Aligning lyotropic liquid crystals with unconventional organic layers

“…Despite Cromolyn being more studied, Sunset Yellow (SSY) was preferred, because SSY presents a wider range of concentration where the nematic phase appears: 27% < N < 35% w/w at RT, (compared to Cromolyn: 13% < N < 17% w/w at RT) [9], as well as a wider temperature range where the nematic phase is present [10]. Besides, SSY viscosity is lower than that of Cromolyn in the working concentration ranges, making it more convenient to manipulate.…”