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

A new model is introduced to describe the ordering of solute molecules in nematic liquid crystals where the average electric field gradient experienced by the solute is zero. For such cases the average orientation of the solute correlates with its size and shape. We assume a mean-field potential that depends on the length of the projection of the solute onto the axis parallel to the director and the circumference of the projection onto the plane perpendicular to the director. The model is used to fit the experimental values of the order parameters of a variety of rigid molecules having different symmetries. Very good fits for the order parameters of 1CB and the quadrupolar coupling of 5CB were obtained using the same set of parameters used to fit the rigid solutes. This shows that the contribution to orientational order from size and shape effects can be calculated from a potential with solute-independent parameters.