Orientational order of rod- and disk-like solutes in the nematic liquid crystal 5CB

Proton and deuteron nuclear magnetic resonance has been used to investigate orientational order in binary liquid crystalline mixtures. Results obtained from small rigid molecules (2-butyne, 2,4-hexadiyne, propyne, 1,2-propadiene, acetylene, 1,3,5-trichlorobenzene and 1,3,5-tribromobenzene) dissolved in the nematic phase of selectively deuterated 4-n-pentyl-4'-cyanobiphenyl (5CB-d^αβ₄), indicate that the solute and liquid-crystal molecules have different order parameters. Such observations are consistent with the predictions of mean-field theory, which, under certain assumptions, indicates that for a given mixture a plot of one order parameter, S⁽¹⁾, against that of the other parameter, S⁽²⁾, should yield a universal curve independent of temperature and composition. Fitting the experimental NMR results to these theoretical curves provides a virtual nematic-isotropic transition temperature, T^*_NI, for the pure solute.