Temperature dependence of refractive indices near uniaxial–biaxial nematic phase transition

The uniaxial discotic (ND)(ND) and calamitic (NC)(NC) nematic phases are characterized by an order parameter which is a second-rank, symmetric, traceless tensor with two different eigenvalues and three in the biaxial nematic (NB)(NB) phase. The optical dielectric tensor may be chosen as the microscopic order parameter. 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 reentrant isotropic (IRE)–ND–NB–NC(IRE)–ND–NB–NC–isotropic (I)(I) phase transitions in a lyotropic mixture of potassium laurate, decanol and D2O. The dynamic of micellar configuration of the uniaxial and biaxial nematic phases is discussed as a consequence of our refractive index data.