Dynamics of Ar atom collisions with a perfluorinated alkanethiol self-assembled monolayer (F-SAM) surface on gold were investigated by classical trajectory simulations and atomic beam scattering techniques. Both explicit-atom (EA) and united-atom (UA) models were used to represent the F-SAM surface; in the UA model, the CF3 and CF2 units are represented as single pseudoatoms. Additionally the nonbonded interactions in both models are different. The simulations show the three limiting mechanisms expected for collisions of rare gas atoms (or small molecules) with SAMs, that is, direct scattering, physisorption, and penetration. Surface penetration results in a translational energy distribution, P(Ef), that can be approximately fit to the Boltzmann for thermal desorption, suggesting that surface accommodation is attained to a large extent. Fluorination of the alkanethiol monolayer leads to less energy transfer in Ar collisions. This results from a denser and stiffer surface structure in comparison with that of the alkanethiol SAM, which introduces constraints for conformational changes which play a significant role in the energy-transfer process. The trajectory simulations predict P(Ef) distributions in quite good agreement with those observed in the experiments. The results obtained with the EA and UA models are in reasonably good agreement, although there are some differences.

We have developed a molecular dynamics scheme in order to understand the dynamics of water adsorption on calcite surfaces as a function of relative humidity. In contrast to previous studies where either a monolayer or bulk water was assumed to cover the surface, we observe the formation of two to three prominent layers of water depending on the relative humidity. Due to the fact that these simulations are at room temperature, the distribution of water molecules on the surface is inhomogeneous and nonuniform. Our simulation results agree well with recent grazing incidence X-ray diffraction studies. The free energy of adsorption of a single water molecule onto the bare calcite (101̄4) surface is predicted to be −10.6 kcal/mol at room temperature while the enthalpy for the same process is −21.3 kcal/mol. The time scale for the bare calcite surface to become in dynamic equilibrium with water vapor at 100% relative humidity is determined to be close to 6 ns, and the adsorption follows a BET (Brunauer−Emmet−Teller)-like isotherm, in that multilayers form. From our orientational distribution functions, we are able to determine the existence of three binding modes for water. The mobility of water adsorbed on the calcite surface is greater in directions parallel to the surface. Motion perpendicular to the suface is slower. The diffusivity of water significantly increases with increasing relative humidity.

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