Effect of ionic additives on elasticity of lyotropic chromonic liquid crystal*

Using a magnetic Frederiks transition technique, we determine how the splay $K_{1}$ and bend $K_{3}$ elastic constants of lyotropic chromonic liquid crystal Sunset Yellow (SSY) depend on concentration of ionic additives, sodium chloride (NaCl) and magnesium sulfate (MgSO$_{4}$). Both salts increase the ratio $K_{1}/K_{3}$, by mainly increasing $K_{1}$ (MgSO$_{4}$) or mainly decreasing $K_{3}$ (NaCl). The effects are attributed to the screening of electrostatic repulsions of chromonic molecules, which is expected to increase the contour length (thus increasing $K_{1}$) and to decrease the persistence length (thus decreasing $K_{3}$) of the chromonic aggregates in which the molecules are stacked face-to-face. As in salt-free SSY, the ratio $K_{1}/K_{3}$ increases when the temperature decreases.