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

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 noncovalent 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 $p\mathrm{H}$ 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 $p\mathrm{H}$ 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.

Figure 1

Molecular structure (a), schematic of nematic LCLC phase (b), and phase diagram (c) of SSY aqueous solution with ionic additives. (a) The prevailing NH hydroazone tautomer form is shown. (b) Red dots on aggregates surface represent sulfonate groups, while isolated circles represent disassociated ${\mathrm{Na}}^{+}$ ions. (c) Dashed line marks the nematic to nematic-isotropic phase transition temperature $T_{N\to N+I}=316.5\mathrm{K}$ of the original SSY LCLC.

Figure 2

Temperature and ionic-content dependence of (a) splay constant $K_1$, (b) bend constant $K_3$, and (c) twist constant $K_2$ of SSY LCLC, $c_{\mathrm{SSY}}=0.98\mathrm{mol}/\mathrm{kg}$ with ionic additives. Insets illustrate the corresponding director deformation, where (a) splay creates vacancies that require free ends to fill in, (b) bend can be accommodated by bending the aggregates, and (c) twist can be realized by stacking “pseudolayers” of uniformly aligned aggregates within the layers; director rotates only when moving across layers.

Figure 3

(a)$K_3$ as a function of $\Delta T=T-T_{N\to N+I}$ for different ionic contents; insets show schematically that enhanced Debye screening increases the flexibility of aggregates. (b) For values measured at different $\Delta T,K_3$ show linear relations with ${\lambda }_D^2$, which decreases as $c_{\mathrm{NaCl}}$ increases.

Figure 4

Temperature and ionic-content dependence of rotation viscosity ${\gamma }_1$ of SSY LCLC, $c_{\mathrm{SSY}}=0.98\mathrm{mol}/\mathrm{kg}$ with ionic additives. Insets show the linear relation, ${\gamma }_1\propto K_1^2$ for all measurements.

Figure 5

Temperature and ionic-content dependencies of volume fraction $\varphi$ of SSY.

Figure 6

Measurement of rotational viscosity ${\gamma }_1$ from the relaxation of magnetic field induced twist. (a) Intensity increase in response to abruptly applied magnetic fields $B>B_{\mathrm{th}}$; $B_{\mathrm{th}}$ is the threshold field of twist Frederiks transition. (b) Intensity decay in response to abruptly reduced magnetic field. (c) Numerical simulation shows that intensity increases quadratically with the midplane twisting angle ${\phi }_0$. (d) Linear fit of $1/\tau$ vs $B^2$, where the rotation viscosity ${\gamma }_1$ can be extracted from the slope.