Stability of n-alkanes and n-perfluoroalkanes against horizontal displacement on a graphite surface.

Abstract

The stability of adsorbed molecules on surfaces is fundamental and important for various applications, such as coating, lubrication, friction, and self-assembled structure formation. In this study, we investigated the structures and interaction energies (Eint) of propane, n-pentane, n-heptane, perfluoropropane, n-perfluoropentane, and n-perfluoroheptane adsorbed on the surface of C96H24 (a model surface of graphite). The changes in Eint (ΔEint = Eint - Eint(0)) associated with the horizontal displacement from the stable position were calculated using dispersion-corrected density functional theory (DFT; B3LYP-D3), where Eint(0) is the Eint at the stable position. The maximum value of ΔEint (ΔEint(max)) associated with the horizontal displacement increased as the chain length increased. The ΔEint(max) for the three n-alkanes were 1.10, 1.82, and 2.35 kcal mol-1, respectively. The values for n-perfluoroalkanes were 0.57, 0.83, and 1.04 kcal mol-1, respectively. The ΔEint(max) values for the n-alkanes were significantly larger than those for the corresponding n-perfluoroalkanes. The Eint(max) value per carbon atom of the n-alkanes (ca. 0.30 kcal mol-1) is approximately 2.5 times as large as that of n-perfluoroalkanes (ca. 0.12 kcal mol-1). The ΔEint associated with the horizontal displacement of propane and perfluoropropane on circumcoronene (C54H18) obtained by the B3LYP-D3 calculations are close to those obtained by the second order Møller-Plesset (MP2) and dispersion-corrected double hybrid DFT calculations, suggesting the sufficient accuracy of the ΔEint obtained by the B3LYP-D3. Thus, our quantitative analysis revealed the higher stability of n-alkanes against horizontal displacement on a graphite surface than that of n-perfluoroalkanes.