Salt effects on transition temperatures and orientational order of lyotropic chromonic liquid crystals.

Lyotropic chromonic liquid crystals (LCLCs) differ from conventional liquid crystals since the constituent units are self-assembled aggregates rather than individual molecules. The aggregates are stabilized by π-π interaction of poly-aromatic cores and ionic interactions between side groups. Hence, the aggregation behavior is sensitive to ionic impurities which interfere with the electrostatic interactions. In this study, we used monovalent salts to investigate salt effects on LCLC self-assembly by measuring transition temperatures and order parameters. We found that the size of cation plays a crucial role on the stability of nematic phase but the size of anion makes no difference. A mechanism is proposed based on the screening of electrostatic repulsion between adjacent molecules and aggregates. The order parameters of LCLCs at transition temperature are reduced by the addition of salts, which cannot be explained by Onsager model. The roles of temperature, LCLC concentration, and salts are separated when plotting the order parameters as a function of reduced temperature. The order parameters dependence on reduced temperature is independent of LCLC concentration, salt concentration, or cation size.