Behavior of hexane on graphite at near-monolayer densities: Molecular dynamics study

Abstract

We present the results of molecular-dynamics studies of hexane physisorbed onto graphite for eight coverages in the range 0.875ഛ ഛ 1.05 ͑in units of monolayers͒. At low temperatures, the adsorbate molecules form a uniaxially incommensurate herringbone solid. At high coverages, the solid consists of adsorbate molecules that are primarily rolled on their side perpendicular to the surface of the substrate. As the coverage is decreased, the amount of molecular rolling diminishes until = 0.933, where it disappears ͑molecules become primarily parallel to the surface͒. If the density is decreased enough, vacancies appear. As the temperature is increased, we observe a three-phase regime for Ͼ 0.933 ͑with an orientationally ordered nematic mesophase͒; for lower coverages, the system melts directly to the disordered ͑and isotropic͒ liquid phase. The solid-nematic transition temperature is very sensitive to coverage, whereas the melting temperature is quite insensitive to it, except for at low coverages where increased in-plane space and, ultimately, vacancies soften the solid phase and lower the melting temperature. Our results signal the importance of molecular rolling and tilting ͑which result from the competition between molecule-molecule and molecule-substrate interactions͒ for the formation of the intermediate phase, while the insensitivity of the system's melting temperature to changing density is understood in terms of in-plane space occupation through rolling. Comparisons to experimental results are discussed.

Melting of Hexane Monolayers Adsorbed on Graphite: The Role of Domains and Defect Formation

“…The pair correlation function (not shown) also confirms that the system undergoes a sharp melting transition. It is noteworthy that the behavior of all the order parameters in Figure 1 illustrate that the system melts directly from a herringbone to the fluid, in contrast to all previous computer simulations where a nematic mesophase is observed [7,8,[10][11][12]. Figure 2 shows typical configurations of the layer at various temperatures.…”

“…The initial low-temperature configuration has an important departure from the previous ones [6][7][8][9][10][11][12]. There are N = 104 hexane molecules in a herringbone arrangement atop a six-layer 68.16 Å x 68.88 Å graphite structure.…”

“…Even with a considerable body of computational work [4][5][6][7][8][9][10][11][12][13], there remain poorly understood elements of this interesting system. In particular, the detailed mechanism of the melting transition and the effect that molecular stiffness has on it has not been elucidated.…”

Molecular dynamics simulation of the dynamical behaviors of an aromatic carboxylic acid molecule with different conformations on a Au (111) surface

“…[2][3][4] In recent studies, the twodimensional molecular monolayer was successfully fabricated by physical adsorption of these molecules on a solid surface. 2,3,[5][6][7][8][9][10][11][12][13][14][15][16] For example, the different geometric formations of the molecular monolayer can be controlled by the chirality of the adsorbed molecules, 5,11 the lattice arrangement of the substrate surface, 6 the placement of functional groups, 7 the length of side chain, 2 backbone length, 3 permeated additive metal, 3,8 and different adsorbed molecules. 13,14 However, according to the morphologies obtained in the studies above, the experimental equipment has difficulties in directly observing the stationary morphology of adsorbed molecules at the atomic level of resolution.…”

“…In order to compensate for the limitations of the empirical approach, molecular dynamics ͑MD͒ simulation is a useful method to observe the detailed behaviors of adsorbed molecules on solid substrates at the atomic scale. 4,[15][16][17] Therefore, in this study, MD simulation is employed to investigate the behavior of a single tricarboxylic acid derivative of 1,3,5-tris͑carboxymethoxy͒ benzene ͓TCMB, C 6 H 3 ͑OCH 2 COOH͒ 3 ͔ on the Au fcc͑111͒ substrate.…”