Molecular Origin of UV-Induced Irreversible Phase Changes in a Chromonic Liquid Crystal

TL;DR

This study reveals that UV-induced chemical degradation of disodium cromoglycate alters its molecular self-assembly, leading to phase diagram shifts in chromonic liquid crystals, enabling light-controlled soft matter self-assembly.

Contribution

It identifies the molecular mechanism behind UV-induced phase changes in chromonic liquid crystals, linking photodegradation products to disrupted molecular self-assembly.

Findings

UV irradiation causes specific chemical degradation of DSCG.

Degradation products disrupt DSCG self-assembly.

Phase diagram shifts correlate with chemical changes.

Abstract

Aqueous solutions of disodium cromoglycate (DSCG), a representative model system for chromonic liquid crystals, exhibit temperature- and concentration-dependent phase behaviors spanning isotropic, nematic, and columnar phases, as well as their coexistence regions. Nastishin et al. (2018) reported that UV irradiation can alter the phase diagram, transforming a nematic phase into a nematic-isotropic biphasic state due to weakened molecular attractions, accompanied by a slow post-irradiation relaxation. Here, we revisit this phenomenon and elucidate the molecular origin of this phase diagram shift: the UV-induced photodegradation of DSCG into specific photodegradation products, which we identify using liquid chromatography-mass spectrometry. Through an integrated approach combining in situ X-ray scattering and polarized optical microscopy, we demonstrate that these degradation products disrupt the self-assembly of DSCG aggregates, thereby expanding the isotropic and biphasic regions in the phase diagram. These findings demonstrate that chromonic assemblies and their phase behaviors are highly sensitive to minor chemical alterations, providing a potential route toward light-controlled self assembly of soft matter.