Molecular surface and order parameters in liquid crystals

The surface model for solute ordering in nematics, which is based on the decomposition of the orientational potential according to the contributions of surface elements, leads to a simple procedure for the calculation of both orientational properties and the cholesteric order induced by a chiral compound. However, a realistic representation of the molecular surface accessible to the solvent is required. The rolling sphere algorithm, applied to the ensemble of atomic van der Waals spheres of a molecule, provides a natural determination of such a surface, since it smoothes away the small scale details. The surface ordering model implemented with the rolling sphere smoothing of the surface is described and applied to several molecular systems. It is shown that the orientational order parameters are substantially independent of the rolling sphere radius identified with the average curvature of the solvent molecular surface. On the contrary, a sensible dependence on such a parameter emerges for the chirality order parameter, this behaviour pointing out the role of the shape of solvent molecules in the chirality recognition of solutes. A fair agreement is obtained in the comparison with experimental data.