(435e) Reentrant Surfactant Driven Isotropic-Nematic Transitions in Thin Films

The control of the orientation of nematogens at surfaces and interfaces is of primary importance in the development of liquid crystalline devices as well as many applications in biological systems. One of the ways that the interfacial orientation is controlled is through using surfactant and lipid molecules [1]. Experimental observations have shown that the anchoring ability of surfactants is a non-monotonic function of their surface density [2]. The interplay between the conformational entropy of the chain molecules, the rotational entropy of the nematogens, the intermolecular and molecule-surface interactions, and the packing at the interfaces results in an inhomogeneous molecular distribution close to the interface with complex effective interactions that have large implications for the phase behavior of the nematogen and surfactant molecules.

We present theoretical predictions for the phase behavior of nematogen thin films in the presence of surfactants. The results shed light on the interplay between the conformational entropy of the surfactants, the penetration of the nematogen into the surfactant region, the propagation of the interfacial orientation to the phase behavior of the nematogenic film and the effects of nematogen orientation on the phase transitions of the surfactants themselves. We show that at low surface density of surfactants the nematogens prefer to orient parallel to the surface due to excluded volume interactions. At intermediate surface densities the surfactant induces a homotropic (perpendicular) orientation, where the nematogens penetrate slightly into the surfactant layer. When the surfactant surface is large enough so that the surfactants are highly stretched (since the surfactant chains are self avoiding), the orientation becomes parallel again since the coating is almost equivalent to a hard wall. These orientational effects have important consequences in the isotropic-nematic transition, where we predict a reentrant isotropic-nematic transition that depends on film thickness and surfactant surface density. We also present results on the effect of the presence of nematogens on the phase behavior of the surfactant molecules when there is an additional interaction included that favors the chains aligning perpendicular to the surface [3]. This interaction gives rise to a gel/liquid crystalline phase transition of the surfactant that is shifted due to the presence and concentration of the nematogens.

References

[1] J. M. Brake, M. K. Daschner, Y.-Y. Luk, and N. L. Abbott, Science 302, 2094 (2003).

[2] U. Kühnau, A. Petrov, G. Klose, and H. Schmiedel, Phys. Rev. E 59, 578 (1999).

[3] R. Elliott, K. Katsov, M. Schick, and I. Szleifer, J. Chem. Phys. 122, 044904 (2005).