Adsorption of heptane-toluene binary mixtures on a hydrophobic polymer surface

Polymer coatings offer a means to modulate the adsorption of molecules onto solid surfaces by offering a surface functionality, charge, roughness, and hydrophobicity that is different from the underlying substrate. One application is to provide anti-fouling functions for metal surfaces. Understanding solvent-surface interactions is an essential component to gaining mechanistic insight into the adsorption process. In this work, we study the adsorption of toluene-heptane binary mixtures onto a perflurorinated polymer surface. We use a combination of IR absorption and Raman scattering spectroscopy to study the mixture in the bulk phase, and surface-specific visible-infrared sum-frequency generation to probe the surface layers. Through the use of homo- and heterospectral two-dimensional correlation spectroscopy, we conclude that the adsorption of the two solvents is reversible and that the surface structure is generally independent of the surface composition, with a small change in toluene orientation as the toluene content increases. We also find that the hydrophobic fluropolymer has very little preference for either solvent, similar to previous studies on hydrophilic surfaces.