Competing Ordering Processes at Liquid Crystal Surfaces Laden with Semifluorinated Alkane Molecules

Ellipsometric measurements elucidate the interplay between the surface order at the isotropic liquid crystal/air interface and a structural phase transition in a Gibbs film on the same interface. Gibbs films formed by the semifluorinated alkane C18H37--C12F25 exhibit a sharp transition from a dilute state at higher temperatures to a dense state at lower temperatures. The transition temperature can be tuned by controlling the C18H37--C12F25 concentration in the bulk liquid crystal phase. When the transition takes place in the temperature range in which a molecular thin smectic or nematic film exists at the isotropic liquid crystal/air interface, the smectic surface order is destroyed while the nematic surface order is affected by a change of the orientation of the liquid crystal molecules. The ellipsometric data indicate that both behaviors result from a change of the anchoring condition of the liquid crystal molecules in contact with the Gibbs film.