Effect of symmetric and asymmetric head group growth of ionic surface active molecule on lyotropic uniaxial-to-biaxial nematic phase transitions

Lyotropic quaternary mixtures of some tetradecylalkylammonium bromides (tetradecyldimethylammonium bromide, TDMABr; tetradecyltrimethyl-ammonium bromide, TTMABr; tetradecyldimethylethylammonium bromide, TDMEABr; tetradecyldiethylmethylammonium bromide, TDEMABr; tetradecyl-triethylammonium bromide, TTEABr; tetradecyltripropylammonium bromide, TTPABr; tetradecyltributylammonium bromide, TTBABr), were prepared to examine the effect of the symmetric and asymmetric head-group growth of the surfactants on the stabilization of different lyotropic nematic phases and uniaxial-to-biaxial phase transitions. The lyotropic mixtures were prepared by the dissolution of the tetradecylalkylammonium bromides (TAABr) in the mixture of NaBr/DeOH/water. The textures of the lyotropic mesophases were characterized by polarizing optical microscope. The uniaxial-to-biaxial nematic phase transitions were determined from the temperature dependence of the birefringences of the nematic phases via laser conoscopy. The results indicated that the head-group size of the surfactant molecules influences both the amphiphilic molecular aggregate topology and uniaxial-to-biaxial nematic phase transitions, and, in addition, the minimum area per surfactant head group is a key parameter on stabilizing the lyotropic biaxial nematic phase.