Chemical Dynamics Simulations of Thermal Desorption of Protonated Dialanine from a Perfluorinated Self-Assembled Monolayer Surface

Classical chemical dynamics simulation results are presented for the thermal desorption kinetics and energetics of protonated dialanine ions (ala₂-H⁺) physisorbed on/in a perfluorinated self-assembled monolayer (F-SAM) surface. Previously developed analytic potentials were used for the F-SAM and the ala₂-H⁺/F-SAM intermolecular interaction, and the AMBER valence force field was used for ala₂-H⁺. The activation energy, E_a = 13.2 kcal/mol, determined from the simulations is consistent with previous simulations of the ala₂-H⁺/F-SAM binding energy. The A-factor, 7.8 × 10¹¹ s^-1, is about an order of magnitude lower than those representative of small molecule desorption from metal and semiconductor surfaces. This finding is consistent with the decreased entropies of ala₂-H⁺ and the F-SAM upon desorption. Using the Arrhenius parameters for ala₂-H⁺ desorption from the F-SAM, the lifetime of ala₂-H⁺ adsorbed on the F-SAM at 300 K is 5 × 10^-3 s. Larger peptide ions are expected to have longer adsorption lifetimes.