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Influences of Nanoscale Structural Features on Transport through Self-Assembled Monolayers

Piyush Srivastava, Walter G. Chapman, and Paul E. Laibinis. Rice University, Ms-362, 6100 Main Street, Houston, TX 77005

Self-assembled monolayers (SAM) on metallic surfaces offer a convenient approach for fabricating molecularly tailored interfaces with well-defined compositions, structures, and thicknesses. This work investigates the molecular scale effects governing the ability of SAMs to act as barrier films towards oxygen transport which has important implications in the field of corrosion inhibition. Our motivation for this work comes from experimental observations on these coatings using techniques such as cyclic voltammetry and electrochemical impedance spectroscopy that have shown that the coating resistance of n-alkanethiol (CnSH) SAMs on copper increase with chain length (n) for n≥16, whereas the coating resistances are negligible for n≤12. To gain insight into the molecular-level structural features responsible for these trends we have performed molecular dynamics (MD) simulations of oxygen transport across SAMs of different chain lengths on gold and copper using the z-constraint algorithm'. Our MD simulations reveal that by increasing the thickness of these coatings by only a few angstroms, the free energy barrier towards oxygen transport increases by as much as ~20 kJ/mole. MD simulations also show that the free volume alignment within these films have a huge impact on the values of diffusivities of oxygen in them. The resistances, as calculated by MD simulations, offered by these films towards oxygen transport were found to be a function of the crystallinity of the middle region of the SAMs. The lack of crystallinity of SAMs for n≤12 and the increasing crystallinity of the middle region of SAMs for n≥16 appear to be responsible for the experimentally observed trends.