Surface diffusion of n-alkanes: Mechanism and anomalous behavior

In this Letter, we investigate, using molecular dynamics simulations, the diffusion of n-octane and n-tetradecane on a graphite surface at 300 K under incomplete coverage condition. For both molecules, we observe that the lateral diffusion coefficient exhibits a Λ-shape anomaly with surface coverage i.e., the diffusion coefficient increases with the increase in surface coverage until a critical surface coverage, beyond which the diffusion coefficient decreases. Moreover, for low surface coverages, the longer n-tetradecane molecule moves faster compared to the shorter n-octane molecule and for high surface coverages, the n-octane molecule moves faster. We develop a new theory to understand the surface diffusion of n-alkanes and show that D ∼ frac(r_ee², N^p τ_R).