X-ray reflectivity measurements and Landau theory of smectic wetting in liquid crystal-benzyl alcohol mixtures

Smectic layering has been observed at the free surface of decylcyanobiphenyl (10CB) and dodecylcyanobiphenyl (12CB) and mixtures of 10CB and 12CB with benzyl alcohol (BA). The effect of BA is to suppress the bulk isotropic-smectic transition temperature TIA and surface layer ''transition'' temperatures Tj, as well as sharpening these surface transitions by reducing the temperature range ΔT over which layers grow. The observed sharpening appears to saturate for concentrations x0.118. A Landau theory for the growth of a single layer j has been developed, in which the free energy contains a term coupling the concentration x to the local smectic order parameter ψj such that Fj∼xψj2. Applying this theory to pure 12CB and eight mixtures of 12CB-BA, we find that the j=1->j=2 transition is continuous in pure 12CB, but that the addition of small amounts of impurity drives the transition first order.