Size and Shape Effects and the Orientation in Nematic Liquid Crystals of Solutes Having Planar Symmetry

Experiments performed in our laboratory have suggested that the orientational order of solutes dissolved in nematic liquid crystals results from two mechanisms: a) the interaction between the average electric field gradient experienced by the solute and its molecular quadropole moment; b) the interaction with a potential which depends on the size and shape of a molecule. We model the size and shape potential by replacing the molecule by a cavity having a simple geometrical shape and assuming that the potential experienced by each surface element of the cavity depends only on its orientation with respect to the director. We use the lengths of the projections of the solutes onto axes parallel and perpendicular to the director to approximate the dimensions of the cavity. We use two different potentials based on these assumptions and fit the parameters describing the strength of the interactions with the different surfaces using the published experimental values of the order parameters of 27 molecules having C(2v) and higher symmetry. All solutes were dissolved in a special mixture where the electric field gradient mechanism is expected to be negligible. We measure the order parameters of 13 additional molecules with C(s) symmetry, also dissolved in the same special mixture. Then, without any adjustable parameters, we use the size and shape potentials to predict the order matrices of the solutes with C(s) symmetry. These order matrices contain three independent elements for each solute and thus these measurements provide a stringent test for the models. We compare both potentials and discuss the consistency of the parameters which were obtained from the different fits.