The Free Surfaces of Liquid Crystals and Liquid Crystal/benzyl Alcohol Mixtures Studied by X-Ray Reflectivity.

The surface-normal structure of two liquid crystals, 10CB and 12CB, has been measured in the bulk isotropic phase by the technique of x-ray reflectivity. Reflectivity yields the square of the Fourier transform of the derivative of electron density along the normal direction, partialrho_{e}(z)/partial z. The temperature-dependence of this structure shows that smectic layers form at the free surface. An attempt is made below to interpret this temperature-dependence in terms of the general features of the bulk isotropic -smectic-A (I-A) transition of these materials as well as the phenomenology of multilayer adsorption. Several mixtures of 12CB and benzyl alcohol (BA) have also been measured, as well as one mixture of 10CB and BA. The apparent influence of BA is to cause the growth of individual layers to become more "first-order-like", in that pre- and post-transitional growth of the local smectic order parameter for the layer n=2 is suppressed in mixtures and that this order parameter grows to saturation over a smaller range of temperature than in the pure materials. A Landau theory has been applied to the growth of n=2 in 12CB/BA mixtures, and indicates that the growth of the layer is continuous for pure 12CB, but becomes discontinuous at the smallest concentration of BA measured. Diffuse x-ray scattering studies of pure 12CB near n=2 formation were also performed to search for the possibility of coexistence between regions of n=1 and n=2 surface layers. The diffuse scattering does not exhibit features which could be interpreted as coexistence and appears to agree with the predicted scattering for thermally-induced capillary waves.