Surfactant driven surface anchoring transitions in liquid crystal thin films

Abstract

The surface anchoring transitions of nematogens confined in thin films in the presence of surfactants at the surfaces are studied theoretically. The theoretical approach is derived from a free energy functional that includes a local Onsager type interaction as well as a microscopic description of the conformations of the surfactant molecules. The theory predicts that the effective interactions between the surfactants and the nematogens are a non-monotonic function of the surfactant area per molecule. As a result surface anchoring transitions occur from planar to homeotropic and back to planar as the surface coverage of the surfactant is increased. The location of the transitions depends on the ability of the nematogens to penetrate into the surfactant layer. The findings presented here provide a molecular picture of the role that packing plays in the interactions between flexible and rigid anisotropic molecules and how they can be used to control the phases of nematogenic thin films in devices. Furthermore, we calculate the potential of mean force on a nematogen molecule due to the presence of surfactants, confinement, and the other nematogen molecules present. The potential of mean force gives us valuable insight into the interplay of competing intermolecular forces at the surfactant-nematogen interface. The effective interactions between the surface and the nematogens can be used as one of the building blocks of field mesoscopic theories, enabling the systematic introduction of molecular information into continuous coarse-grained approaches.