Solvent isotopic effect on the phase transition of lyotropic chromonic liquid crystals: Heavy water makes mesogens less charged

TL;DR

This study investigates how replacing water with heavy water (D₂O) affects the phase transition and microstructure of lyotropic chromonic liquid crystals, revealing that D₂O reduces aggregate charge and stabilizes the nematic phase.

Contribution

It demonstrates the isotopic effect of D₂O on lyotropic chromonic liquid crystals, showing changes in phase transition temperatures and aggregate charge not previously characterized.

Findings

D₂O increases nematic-to-isotropic transition temperature.

D₂O leads to longer inter-aggregate correlation lengths.

Aggregates in D₂O are less charged with more counter-ions.

Abstract

The interplay among solute and solvent molecules in lyotropic mesophases governs their physicochemical properties, such as phase behaviors and viscoelasticity. In our model system, a lyotropic chromonic liquid crystal (LCLC) made by disodium cromoglycate (DSCG), charged plank-like molecules self-assemble to form elongated aggregates via non-covalent attractions in water (H$_{2}$O). The aggregates align to exhibit liquid crystalline phases: nematic and columnar phases. Here, we report the isotopic effect on the phase behavior of the LCLC when D$_2$O is substituted for H$_2$O. D$_2$O-DSCG exhibits higher nematic-to-isotropic phase transition temperatures than H$_2$O-DSCG. X-ray scattering reveals considerably longer inter-aggregate correlation lengths in D$_2$O-LCLCs. In contrast, the other microstructural properties, such as inter-aggregate distances and intra-aggregate correlation lengths, remain almost the same. Our $^{23}$Na FT-NMR measurement reveals that D$_2$O-DSCG aggregates are less charged with more counter-ions, Na$^+$, bound to them than H$_2$O-DSCG aggregates. Weaker electrostatic repulsion between aggregates may stabilize the nematic phase, and this solvent isotopic effect may generally apply to diverse aqueous lyotropic mesophases with electrostatic interactions.