Collisions of polar and nonpolar gases with hydrogen bonding and hydrocarbon self-assembled monolayers

Molecular beam scattering experiments are used to explore collisions of 60 kJ/mol Ne, CD4, ND3, and D2O with long-chain CH3-, NH2-, and OH-terminated self-assembled monolayers (SAMs) created via the chemisorption of alkanethiols on gold. Time-of-flight measurements for the scattered gases reveal the extent of energy exchange and the propensity for a gas to thermally accommodate with the surface during a collision. Of the four gases studied, Ne transfers the least amount of translational energy into the monolayers and D2O the most. Neon atoms recoil from the OH-SAM with an average of 14.4 kJ/mol of energy, while D 2O retains only 6.4 kJ/mol of its 60 kJ/mol incident energy when scattering from the same surface. Overall, the trend in final translational energies follows the order Ne \u3e CD4 \u3e ND3 \u3e D2O for scattering from all three SAMs. The observed trend in the energy exchange is correlated with the gas-surface attractive forces, as determined by ab initio calculations. The thermal accommodation efficiencies of the four gases follow the opposite trend. Thermalization for the Ne atoms is nearly negligible for all three monolayers, whereas D2O and ND 3 approach near complete accommodation on all of the monolayers studied. The overall energy exchange and thermal accommodation efficiencies also depend markedly on the terminal group of the SAM. For Ne scattering, the trend for the overall energy transfer follows: CH3- \u3e NH2- ≈ OH-SAMs. In contrast, the overall D2O energy transfer is greater when colliding with the OH-SAM than the nonpolar CH3-SAM. Together, the results show that the extent of energy transfer depends on a balance between the rigidity of the surface, as affected by intrasurface hydrogen bonding, and the strength of the gas-surface attractive forces, as determined by intermolecular interactions. © 2008 American Chemical Society