Influence of permanent molecular dipoles on surface anchoring of nematic liquid crystals

We consider the ordering of molecular dipoles at a nematic surface and the influence of this ordering on the equilibrium director orientation and the anchoring energy. Both phenomenological and molecular approaches are used. We show that a thin, spontaneously polarized layer will appear even at a structureless nematic surface without any charged impurities or gradients of the order parameter. The ordering of molecular dipoles in this layer is determined by strong dipole-quadrupole interactions, modified by the presence of the surface. Surface polarization gives a significant contribution to the anchoring energy of nematic liquid crystals composed of strongly polar molecules. The estimates indicate that in such systems the dipolar contribution is very large and could even be predominant, for example, at a free surface of some nematic liquid crystals. We show also that in the case of large longitudinal dipoles the equilibrium director orientation at a free surface is homeotropic. In the case of large transverse dipoles the alignment is planar. The principal results of this study are consistent with experimental observations on ferroelectric layers at nematic interfaces and with data on the orientation of typical polar nematics at a free surface.