Size and shape effects on the orientation of rigid molecules in nematic liquid crystals

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Abstract
Order parameters, as measured from N.M.R. experiments, are reported for a series of small rigid molecules dissolved in the liquid crystals N-(p-ethoxybenzylidene)-p′-n-butylaniline, Merck ZLI 1132, and a 55 wt per cent 1132 mixture of the two. Previous studies have shown that in this mixture the external electric field gradient due to the liquid crystal environment is zero at the site of the deuterium nucleus in D2. The order parameters are discussed in terms of a simple model for the interaction between the solute molecules and their liquid crystal environment. It is shown that the ordering of the solutes in the liquid crystal solvents can be largely explained in terms of: (i) the interaction between the molecular quadrupole moment of the solute and the average electric field gradient due to the solvent and (ii) the short range interactions between the solvent and solute which depend on the size and shape of the solute.