A new interpretation of the optical interference figures observed in a lyotropic biaxial nematic phase for a ternary mixture of DTMABr, 1-DDeOH and H2O

The lyotropic ternary mixture of dodecyltrimethylammonium bromide (DTMABr), 1-dodecanol (DDeOH) and water (H2O) was studied by optical conoscopy and Abbe refractometer techniques. The present mixture showed a large biaxial phase domain (N-B) located between two different uniaxial ones in the partial phase diagram, one optically positive (N-D) and the other negative (N-C). The N-B phase exhibits two optic signs as a function of the temperature, each of which was equal to the neighboring uniaxial phases. The temperature dependence of the refractive indices was measured along the N-C - N-B - N-D phase transitions. In addition, we are reporting for the first time a new interpretation of the optical interference figures observed along the N-B phase. (C) 2022 Elsevier B.V. All rights reserved.