Effect of surfactant alkyl chain length on uniaxial to biaxial cholesteric phase transitions in lyotropic mixtures

In the present work, some ionic surfactant-based lyotropic mixtures, exhibiting cholesteric phases, were prepared to investigate the effect of surfactant alkyl chain length on (a) uniaxial-to-biaxial phase transitions and (b) stabilization of different cholesteric phases. Potassium alkanoates, KCx, (potassium undecanoate, KC11, potassium dodecanoate, KC12, and potassium tridecanoate, KC13) were chosen as the surfactant molecules. The lyotropic mixtures, which were composed of KCx/potassium sulfate (K2SO4)/decanol (DeOH)/water/brucine, where brucine is a chiral dopant molecule, were studied in the frame of the present work. Partial phase diagrams were constructed to show how the surfactant alkyl chain affects the phase topologies in the phase diagrams. It was observed that while the longer the surfactant alkyl chain favors the formation of the larger discotic cholesteric, ChD, phase domain, the cholesteric biaxial, ChB, and calamitic discotic, ChC, regions expense. The phase transitions from the uniaxial cholesteric phases to biaxial cholesteric phase were determined from polarising optical microscopy measurements and it was shown that the ChD-to-ChB and ChB-to-ChC phase transitions shifted to lower temperatures.