Nematic Order of a Binary System : Mbba-biphenyl

A binary system composed of nematogenic solvent (MBBA) and non-nematogenic solute (biphenyl) is investigated for its phase behavior and the dependence of the order parameter on temperature and composition over its entire nematic region. The nematic to isotropic transition temperature is predicted to be a linear function of solute concentration, whereby the dilation effect is shown to be moderated by a measure of orientational anisotropy of the solute molecules, in the context of Maier-Saupe mean field theory. Further, a correspondence relationship between temperature and concentration for the specification of the nematic order parameter is deduced. These predictions are made on the bases of athermal mixing of the two components in the dilute solution limit and of negligible thermal expansion of each component. The experimental phase diagram and justification of athermal mixing from the DSC thermal data are first offered. The order parameter is deduced from the anisotropic optical properties and the correspondence relationship is established by determinations of the order parameter as a function of composition at constant T and of temperature at constant X. Finally, the order surface is constructed from the correspondence relationship.