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

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 (H2O). 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 D2O is substituted for H2O. D2O-DSCG exhibits higher nematic-to-isotropic phase transition temperatures than H2O-DSCG. X-ray scattering reveals considerably longer inter-aggregate correlation lengths in D2O-LCLCs. In contrast, the other microstructural properties, such as inter-aggregate distances and intra-aggregate correlation lengths, remain almost the same. Our 23Na FT-NMR measurement reveals that D2O-DSCG aggregates are less charged with more counter-ions, Na+, bound to them than H2O-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.