Quantized layer growth at liquid-crystal surfaces

We report x-ray reflectivity measurements on the free surface of dodecylcyanobiphenyl (12CB) at the isotropic to smectic-A phase transition. At about 10?deC above TIA, smectic-A-like ordering develops at the surface while the bulk phase remains isotropic. The angular dependence of the specular reflectivity is consistent with a sinusoidal density modulation, starting at the surface and terminating abruptly, after an integral number of bilayers. As the transition is approached the number of layers increases in quantized steps from zero to five before the bulk undergoes a first-order transition to the smectic-A phase.