Refractive index measurements in uniaxial and biaxial lyotropic nematic phases

Abstract 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.