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 (E_int) 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 E_int (ΔE_int = E_int − E_int(0)) associated with the horizontal displacement from the stable position were calculated using dispersion-corrected density functional theory (DFT; B3LYP-D3), where E_int(0) is the E_int at the stable position. The maximum value of ΔE_int (ΔE_int(max)) associated with the horizontal displacement increased as the chain length increased. The ΔE_int(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 ΔE_int(max) values for the n-alkanes were significantly larger than those for the corresponding n-perfluoroalkanes. The E_int(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 ΔE_int 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 ΔE_int 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.