Neutron scattering from mixtures of isotopically labelled molecules. A new method for determining the orientational distribution function in liquid crystals

Neutron scattering from a mixture of normal hydrogeneous and perdeuteriated 4-n-pentyloxybenzylidene-4′-n-heptyl quiline (50.7) has been shown to contain a component which depends only on the structure and orientational distribution of the molecules. It has been used to determine the orientational order parameters Pn for n = 2, 4 or 6 in the smectic and nematic phases of 50.7. These values were much less than predicted by the Maier-Saupe molecular field theory, but in qualitative agreement with a model based on biaxial intermolecular interactions. Comparison with the P2 values obtained by analysing the smearing into arcs of intermolecular interfence features, suggested that the correlation of orientation of neighbouring molecules is not very high so that the method tends to overestimate the order parameters. Other possible applications of the technique of neutron scattering from isotopically labelled mixtures are discussed.