Condensation of Self-assembled Lyotropic Chromonic Liquid Crystal Sunset Yellow in Aqueous Solutions Crowded with Polyethylene glycol and Doped with Salt

TL;DR

This study investigates how polyethylene glycol and salt NaCl influence the phase behavior and aggregation of Sunset Yellow chromonic liquid crystals in water, revealing mechanisms of condensation and phase separation.

Contribution

It provides new insights into the effects of crowding agents and salt on chromonic liquid crystal phases and aggregate structures, combining multiple microscopy and scattering techniques.

Findings

PEG induces condensation into liquid-crystalline phases.

Salt NaCl modifies phase diagrams and aggregate properties.

Depletion effects and electrostatic screening explain phase behavior.

Abstract

We use optical and fluorescence microscopy, densitometry, cryo-transmission electron microscopy (cryo-TEM), spectroscopy, and synchrotron X-ray scattering, to study the phase behavior of the reversible self-assembled chromonic aggregates of an anionic dye Sunset Yellow (SSY) in aqueous solutions crowded with an electrically neutral polymer polyethylene glycol (PEG) and doped with the salt NaCl. PEG causes the isotropic SSY solutions to condense into a liquid-crystalline region with a high concentration of SSY aggregates, coexisting with a PEG-rich isotropic (I) region. PEG added to the homogeneous nematic (N) phase causes separation into the coexisting N and I domains; the SSY concentration in the N domains is higher than the original concentration of PEG-free N phase. Finally, addition of PEG to the highly concentrated homogeneous N phase causes separation into the coexisting columnar hexagonal (C) phase and I phase. This behavior can be qualitatively explained by the depletion (excluded volume) effects that act at two different levels: at the level of aggregate assembly from monomers and short aggregates and at the level of inter-aggregate packing. We also show a strong effect of a monovalent salt NaCl on phase diagrams that is different for high and low concentrations of SSY. Upon the addition of salt, dilute I solutions of SSY show appearance of the condensed N domains, but the highly concentrated C phase transforms into a coexisting I and N domains. We suggest that the salt-induced screening of electric charges at the surface of chromonic aggregates leads to two different effects: (a) increase of the scission energy and the contour length of aggregates, and (b) decrease of the persistence length of SSY aggregates.