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Soft Matter and Biological Physics
Eur. Phys. J. E 9, 443-446 (2002)
DOI: 10.1140/epje/i2002-10101-6

How do smectic liquid crystals of different molecular length mix in thin films?

H.R. Keymeulen1, W.H. de Jeu1, J.T. Slattery2 and M. Veum2

1  FOM-Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands
2  University of Wisconsin, Department of Physics and Astronomy, Stevens Point, WI54481, USA

dejeu@amolf.nl

(Received 13 August 2002 / Published online: 4 February 2003)

Abstract
We present a model for the structure of binary mixtures of smectic compounds in freely suspended films of 2-7 layers. The compounds are the hexyl (6AB) and dodecyl (10AB) homologues of p, $p^\prime$-dialkylazoxybenzene that differ by about 40% in molecular length. X-ray reflectivity indicates that no demixing occurs between 6AB and 10AB molecules, while also there is no indication found of increased roughness at the film surfaces. However, the surface layers are somewhat expanded compared to the interior layers. This can be explained by backfolding of the dodecyl end chains of 10AB molecules at the surface via two gauche kinks, which ensures dense packing. This model is supported by surface tension measurements that indicate an increased amount of alkyl groups at the surfaces.

PACS
61.10.Kw - X-ray reflectometry (surfaces, interfaces, films).
61.30.Cz - Molecular and microscopic models and theories of liquid crystal structure.
61.30.Hn - Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitions.

© EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2002