Optical and thermodynamic studies on binary mixtures composed of two-isothiocyanato liquid crystals

Abstract

Phase diagram and optical properties of the nematic mixtures containing two-isothiocyanatobenzen liquid crystal (7CHBT and 9CHBT) were studied by refractometry and DSC measurements. The thermodynamic and optical properties of the eutectic mixture (E79) composed of 7CHBT and 9CHBT were identified and studied. The phase diagram of the eutectic system was compared with the Schroder–van Laar calculation. The excess thermodynamic properties of the eutectic mixture were calculated and interpreted in terms of intermolecular interactions. The modified four-parameter model was used to fit the temperature dependence of the nematogen parameters on the refractive index. In addition, the temperature gradient of the ordinary refractive index (dno/dT) of the nematogens was analyzed in a convenient framework. The order parameters and polarizabilities of the nematic compounds, which are important parameters, were calculated through different methods based on birefringence measurements. Finally, the eutectic mixture and one of the constituent component (7CHBT) display crossover temperatures (TCO), which is very important liquid crystal parameter for specific applications such as optical devices and nonlinear optics.