The helical twisting power of chiral dopants in lyotropic chromonic liquid crystals

Chiral dopants with different levels of lipophilicity were synthesised, and their helical twisting power (HTP) and thermal phase behaviour in mixtures with lyotropic chromonic liquid crystal (LCLC) were investigated. The experimental approach involved measurement of the positions of disclination lines in Θ-cells, where one surface is linearly rubbed, and the other is circularly rubbed. This technique allowed the use of minimal dopant concentrations. Experimental conditions were selected such that nematic temperature range of LCLC remained constant. We found that the twisting power of the used chiral dopants varies over nearly an order of magnitude between the molecules with positive and negative lipophilicity. All chiral molecules of hydrophobic nature investigated showed much higher effectiveness in twisting LCLC than hydrophilic ones, and in addition, the hydrophilic molecules had a much smaller effect on the transition temperatures of LCLC. The dependences were interpreted as a variation of the partition between the concentration of dopant dispersed in the host phase, between the columns and that adhering to the columnar stacks – with hydrophobic molecules better able to adhere, thus creating sources of chirality for the whole system.