Phase Equilibria in Liquid Crystalline Systems Part II. Theory and Interpretation of Experimental Results

Characteristic temperatures T* have been evaluated for the oligomeric p-oxybenzoates, C6H5CO(-OC6H4CO)n-2-OC6H5 from the transition temperatures TNI presented in the preceding paper. For n = 3, 4 and 5 they are, respectively, 343, 316 and 306 K. These parameters that measure the intensities of the orientation-dependent intermolecular interactions are correlated with the optical anisotropy Δα and the average intermolecular cohesive energy expressed by the characteristic pressure p*. The characteristic temperature T* decreases slightly with chain length. The lattice theory of nematic fluids is extended to include mixtures of rodlike molecules having different characteristic temperatures T* in order to treat binary and ternary mixtures of the p-oxybenzoate oligomers. The wide biphasic gaps predicted for the several binary systems and the concomitant pronounced partitioning of the components between the nematic and isotropic phases are not confirmed by experiments. Compositions of the coexisting phases in ternary systems were found to be identical within limits of experimental determinations. These marked departures from theory are explicable on the premise that facile rotational diffusion of the nematogens here considered, which are of comparatively low chain length, leads to rapid generation of ordered domains of large dimensions. Subsequent diffusional transport over the required distances consequently is very slow. The transition temperatures observed in binary systems are in excellent agreement with calculations carried out in the “single component” approximation using mean values of the axial ratio and of T* for the mixture, these being taken to be the same in the two phases at equal concentrations.