Effect of Hofmeister anions on the existence of the biaxial nematic phase in lyotropic mixtures of dodecyltrimethylammonium bromide/sodium salt/1-dodecanol/water

Lyotropic mixtures including different sodium salts of Hofmeister anions were studied in order to investigate the effect of these anions on the existence of biaxial nematic phase and on the uniaxial to biaxial phase transitions. For this purpose, these sodium salts were added singly into different mixtures of dodecyltrimethylammonium bromide (DDTMABr)/1-dodecanol/water, keeping the relative molar concentration of all the constituents constant. The uniaxial to biaxial phase transitions were determined from the temperature dependence of the birefringences by laser conoscopy. Micellar shape anisometry and average micellar volume were evaluated from small-angle X-ray scattering measurements. The results indicated that the Hofmeister anions were bounded to the head groups of DDTMABr molecules at the micelles' surfaces, which significantly affect the different orientational fluctuations responsible for the formation of different nematic phases, biaxial phase domains and uniaxial to biaxial phase transition temperatures.