Adsorption of H2O, CO2 and Xe on soft surfaces

The interactions of water, carbon dioxide, and Xe with octadecanethiol (C18H37SH, ODT) self-assembled monolayers (SAMs) were studied under ultrahigh vacuum conditions employing temperature-programmed desorption and optical diffraction measurements. The ODT layer was grown on a 1 nm thick gold film deposited over a Ru(001) single-crystal substrate. The gases used in this report differ in their lateral interactions while adsorbed on ODT-SAM being either repulsive (Xe) or attractive (H2O, CO2). The activation energies for desorption of the first layer from ODT are E-a = 3.6 +/- 0.9, 4.1 +/- 0.5, and 8.5 +/- 0.9 kcal/mol for Xe, CO2, and H2O, respectively. Sticking probabilities of the three gases on the soft ODT surface are S-0 = 0.7 +/- 0.1, 0.8 +/- 0.1, and 0.95 +/- 0.05 for xenon, CO2, and water, respectively, derived from the respective adsorption curves. Optical diffraction studies from multilayer coverage grating of Xe on ODT-SAM have demonstrated that sublimation is a thermodynamically more favorable process over diffusion and wetting. The significantly lower binding energy of the first layers of H2O and CO2 adsorbed on the soft surface of ODT compared to that on clean metals and oxides, reflects generally weak (CO2) and hydrophobic (H2O) interactions that are important for understanding the behavior of these molecules on interfaces that are found in biological systems.