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

Abstract

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 ${\mathit{T}}_{\mathit{I}\mathit{A}}$ and surface layer ``transition'' temperatures ${\mathit{T}}_{\mathit{j}}$, as well as sharpening these surface transitions by reducing the temperature range \ensuremath{\Delta}T over which layers grow. The observed sharpening appears to saturate for concentrations x\ensuremath{\ge}0.118. 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 ${\mathrm{\ensuremath{\psi}}}_{\mathit{j}}$ such that ${\mathit{F}}_{\mathit{j}}$\ensuremath{\sim}x${\mathrm{\ensuremath{\psi}}}_{\mathit{j}}^{2}$. 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.