Uniaxial-to-biaxial cholesteric and nematic phase transitions on a lyotropic alcohol-free mixture

Interferometric measurements of the chirality-induced biaxiality in the vicinity of the uniaxial-to-biaxial cholesteric transition of the lyotropic system potassium laurate, decylammonium chloride, ${\mathrm{H}}_2$O, and brucine sulfate are reported. The mean-field approach to describe the cholesteric-to-cholesteric transition allows an estimate of the ratio between coefficients of the Landau expansion. The values of the bare correlation length, ${\xi }_0$∼1000 Å, calculated are larger than in lyotropic systems with alcohol. The chirality-induced biaxiality measurements in the limit of zero elastic field (${\mathit{q}}^2$=0) and the birefringence measurements in the nematic phase confirm critical properties of the uniaxial-to-biaxial nematic transition. The critical exponents for the order parameter (β) and the susceptibility (γ) are in good agreement with the values calculated for the XY model.