GUEST-HOST-MIXTURES - A CORRELATION BETWEEN THE DYES ORDER-PARAMETER WITH THERMODYNAMIC AND STRUCTURAL QUANTITIES

A series of dyes containing a rigid aromatic core of the stilbene/azomethine type with approximate structural cylindrical symmetry as well as anthraquinone dyes with more spherical shape were investigated concerning their influence on thermodynamic properties of the liquid crystalline material CB5 (4'-Pentyl-4-cyano-biphenyl). Quantities like the transition temperatures, the biphasic range and the density discontinuity at the nematic-isotropic transition were estimated via precise density measurements at different solute concentrations. The derived results like the slopes of the phase transition boundary lines and the resulting transition entropy were used to evaluate a semiempirical model which describes the change of the optical order parameter with the solute's mole fraction and the solute's structure. Experimentally the optical order parameter was estimated using the main transition in the visible range, which is parallel to the dye's long axis. The comparison of the experimentally obtained values with the data derived from our model is quite acceptable.