Wetting transitions of simple liquid films adsorbed on self-assembled monolayer substrates: an ellipsometric study

We report on an ellipsometric experimental study designed to explore the re levance of the wetting phase diagram predicted by liquid state physics of b asic models, to the wide class of simple organic liquid films that adsorb f rom saturated vapour onto planar substrates at room temperature. The wettin g properties are explored by measuring adsorption isotherms in the approach to saturation, in particular, for adsorption of n-hexane on a variety of s pecially constructed substrates (self-assembled monolayers) spanning a wide range of surface energy, and by carrying out the microscopic equivalent of contact angle experiments at saturation. We locate a wetting transition, w hich in our case is continuous, and then study its properties in detail. Th e general prediction of the wetting phase diagram, that wetting transitions should be ubiquitous in nature and readily located via control over the su bstrate field, is supported by our data, but the quantitative nature of the thick film adsorption regime is not in agreement with Lifshitz theory. Thi s conclusion supports the work of a variety of earlier related studies, but contrasts with recent results for adsorption onto the surface of water. In addition, the correlation length determined from our complete wetting adso rption isotherms is mesoscopic, suggesting that equilibrium statistical mec hanics of simple models of inhomogeneous fluids cannot explain the data.