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

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'-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.