New Methods for Calculating the Temperatures and the Enthalpy Changes of Virtual Transitions and Predicting Phase Diagrams-Applied to the Tolans

For a mesogen which does not exhibit an enantiotropic nematic phase when pure, two new methods are proposed for the determination of the temperatures and enthalpy changes of the nematic-isotropic liquid virtual transitions. These calculations, based on Schröder-van Laar and van Laar equations, use the value of the liquid appearance temperature (the determination of which is easy and accurate) for mixtures of the compound with another mesogen which exhibits the enantiotropic mesophase when pure. Both methods are applied to the study of the first ten compounds in the series: 4-methoxy-4′-n-alkyl-tolan. The results are then used to determine the eutectic, liquid appearance and nematic disappearance temperatures of mixtures of two tolans that do not exhibit an enantiotropic nematic phase when pure. In seven cases, an enantio-tropic nematic phase is predicted and observed; in one case, the phase diagram differs from that expected; for the remainder, there is a good agreement between calculated and experimental data.