Structure of Normal-Alkanes Adsorbed on Hexagonal-Boron Nitride

We report the structures of submonolayer films of the homologous series of normal-alkanes hexane (C₆H₁₄) to hexadecane (C₁₆H₃₄) adsorbed on the surface of hexagonal-boron nitride (h-BN), as determined by X-ray powder diffraction. These n-alkanes are demonstrated to form solid monolayers and exhibit structures that are similar to those seen on graphite but with some important differences. Each n-alkane studied shows a fully commensurate structure, even at submonolayer coverage. Hexane and octane adopt structures in which the molecules are arranged in a “herringbone” pattern (2 × m√3a_bn for C_nH_n+2, where a_bn is the unit cell parameter of the underlying h-BN surface, n = 6 or 8 and m = (2n – 2)/2). Heptane, nonane, and undecane–hexadecane adopt structures in which the molecules within adjacent lamellae are arranged with their principal axis parallel to each other (√3 × ma_bn for C_nH_n+2, where n = 7, 9, 11–16 and m = n + 2). Decane is a transitional case with evidence of both structure types, depending on coverage and temperature. This transitional case occurs for shorter molecules on h-BN than on graphite, indicating a difference in the relative balance of intermolecular and molecule–surface interactions that has significant implications for the phase behavior of adsorbates on these two surfaces.