Ordering of apolar and polar solutes in nematic solvents

The quadrupolar splittings of deuteriated para- and ortho-dichlorobenzene (1,4-DCB and 1,2-DCB, respectively) are measured by nuclear magnetic resonance (NMR) in the nematic solvents hexyl- and pentyloxy-substituted diphenyl diacetylene (DPDA-C6 and DPDA-OC5, respectively). Measurements are taken for all four combinations of the nominally apolar (1,4-DCB) and polar (1,2-DCB) solutes in the apolar (DPDA-C6) and polar (DPDA-OC5) solvents, and throughout the entire nematic temperature range of the solutions. The temperature dependence of the second-rank orientational order parameters of the solutes are obtained from these measurements and the respective order parameters of the mesogenic cores of solvent molecules are obtained independently from carbon-13 NMR measurements. The order parameter profiles of the two solutes are found to be very different but show little variation from one solvent to the other. The results are analyzed and interpreted in terms of the underlying molecular interactions using atomistic solvent-solute potentials. The influence of electrostatic interactions on solute ordering is directly evaluated by computing the order parameters with and without the electrostatic component of the atomistic potential. It is observed to be small. It is also found that the important interactions in these solvent-solute systems are operative over short intermolecular distances for which the representation of the partial charge distributions in terms of overall molecular dipole and quadrupole moments is not valid. (C) 2003 American Institute of Physics.