Refractive index measurements in uniaxial and biaxial lyotropic nematic phases

The calamitic (N-C) and discotic (N-D) uniaxial nematic phases are characterized by an order parameter which is a second-rank, symmetric, traceless tensor with two different eigenvalues and the other three in the biaxial nematic (N-B) phase. The optical characterization of these uniaxial and biaxial nematic phases requires the measurements of two and three refractive indices, respectively. These optical parameters were determined near the N-C-N-B-N-D phase transitions in a lyotropic mixture of potassium laurate, decanol, potassium sulfate and water. Biaxial negative (N-B(-)) and biaxial positive (N-B(+)) nematic phases are characterized from refractive index data and confirmed via conoscopic images. A change from a N-B(-) to a N-B(+) configuration is also determined and discussed. (C) 2015 Elsevier B.V. All rights reserved.