Wetting transition of a binary liquid mixture at a solid boundary

We present the observation of a first-order wetting transition of the binary liquid mixture 2-propanol and perfluoromethylcyclohexane at a liquid-solid interface, where one component (2-propanol) of the gravity-separated mixture is selectively adsorbed on the surface of a silicon substrate. The system undergoes a wetting transition from a thin adsorbed film to a macroscopic wetting layer at T w = 50.6 °C with pronounced premonitory growth of the adsorbed film. The microscopic density profile of the prewetting film is derived from fits to temperature-dependent X-ray reflectivity measurements. Our experimental data clearly indicate a film structure dominated by strong density gradients, which is in contrast to the frequently used "homogeneous slab" models, but confirms density functional calculations. Time-resolved measurements during equilibration reveal new details of the adsorption film growth process, in particular the existence of separate density and thickness growth regimes.