Effects of intramolecular dipolar coupling on the isotropic-nematic phase transition of a hard spherocylinder fluid.

The thermodynamics of a simple model, containing the minimum set of features required to provide liquid crystal-like phase behavior and the dipolar coupling observable in the NMR spectrum of orientationally ordered fluids, are presented within the framework of Onsager theory. The model comprises a fluid of hard spherocylinders with a pair of embedded freely rotating magnetic dipoles. The behavior of the isotropic-nematic phase transition is explored as a function magnetic field strength and of the relative orientation between the nematic director and the external magnetic field. When the field and director are aligned the phase diagram is similar to those predicted for a hard rod fluid in flow fields, electric fields, and magnetic fields, with the field promoting orientational order in the fluid and the isotropic-nematic phase transition being replaced by a paranematic-nematic phase transition. In contrast, when the field and director are perpendicular, the field destabilizes the nematic phase and the phase transition is shifted to higher densities. The variation of the mean magnetic moment and the dipolar coupling are examined as a function of the orientational structure of the fluid. The model is used to support the hypothesis that dipolar couplings observed in the spectra of human leg muscle originate from nematic-like liquid crystal phases in relatively small metabolite molecules. The fitted theoretical predictions of the dependence of the dipolar coupling on the orientation of the field with respect to the nematic director are shown to provide a good description of the experimental data.