Refractive index measurements in uniaxial and biaxial lyotropic nematic phases

The calamitic (NC) and discotic (ND) 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 (NB) 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 NC-NB-ND phase transitions in a lyotropic mixture of potassium laurate, decanol, potassium sulfate and water. Biaxial negative (NB−) and biaxial positive (NB+) nematic phases are characterized from refractive index data and confirmed via conoscopic images. A change from a NB− to a NB+ configuration is also determined and discussed.