Concentration-Induced Wetting Transition in Water-Tetrahydrofuran-Isobutane Systems

The pressure-dependent wetting of isobutane at the aqueous tetrahydrofuran (THF) solution-isobutane interface was studied by means of X-ray reflectivity measurements. Using pure water and mixtures at low THF concentrations, a completely wetting isobutane layer is adsorbed onto the substrate. The pressure-dependent layer thickness can be described by a simple adsorption isotherm. In contrast, the formation of thick layers with low electron density is observed at high THF concentrations. The film growth shows an unpredictable behavior. This finding can be explained by the formation of partially wetting isobutane droplets on the water/THF substrate caused by a decrease of the liquid's surface tension with increasing THF concentration.