Ionic content dependence of viscoelasticity of lyotropic chromonic liquid crystal sunset yellow

TL;DR

This study investigates how ionic content, including salts and pH, affects the viscoelastic properties of lyotropic chromonic liquid crystals, revealing significant and versatile influences on elastic constants and viscosity.

Contribution

It provides the first detailed analysis of ionic effects on the viscoelasticity of LCLCs, highlighting ion-induced changes in aggregate properties unlike traditional liquid crystals.

Findings

NaCl decreases $K_3$ and $K_2$, increases $ au_1$

Elevated pH decreases all viscoelastic parameters

Ionic effects alter aggregate length and flexibility

Abstract

A lyotropic chromonic liquid crystal (LCLC) is an orientationally ordered system made by self-assembled aggregates of charged organic molecules in water, bound by weak non-covalent attractive forces and stabilized by electrostatic repulsions. We determine how the ionic content of the LCLC, namely the presence of mono- and divalent salts and pH enhancing agent, alter the viscoelastic properties of the LCLC. Aqueous solutions of the dye Sunset Yellow with a uniaxial nematic order are used as an example. By applying a magnetic field to impose orientational deformations, we measure the splay $K_1$, twist $K_2$ and bend $K_3$ elastic constants and rotation viscosity $\gamma_1$ as a function of concentration of additives. The data indicate that the viscoelastic parameters are influenced by ionic content in dramatic and versatile ways. For example, the monovalent salt NaCl decreases $K_3$ and $K_2$ and increases $\gamma_1$, while an elevated pH decreases all the parameters. We attribute these features to the ion-induced changes in length and flexibility of building units of LCLC, the chromonic aggregates, a property not found in conventional thermotropic and lyotropic liquid crystals formed by covalently bound units of fixed length.