A molecular dynamics study of the low temperature structure and dynamics of ethane monolayers physisorbed on the graphite basal plane

Abstract

The three known low-temperature structures of ethane monolayers physisorbed on the graphite basal plane (hereafter called S1, S2, and S3) have been examined by molecular dynamics calculations with intermolecular and molecule–surface potentials based on atom–atom interactions. The structure of the low density S1 phase is shown to be strongly influenced by the corrugation of the graphite surface. The intermediate density S2 phase is incommensurate with a slightly different structure than found experimentally. By contrast, the structure of the high density S3 phase, in which all of the molecules stand erect, appears to be independent of surface corrugation. The dynamical behavior of the three surface phases has been probed by examining power spectra calculated from the hydrogen atom trajectories. The resulting spectra for the S1 and S3 phases agree reasonably well with experimental inelastic incoherent neutron scattering data but not as well in the case of the S2 phase.

Monolayers of Linear Molecules Adsorbed on the Graphite Basal Plane: Structures and Intermolecular Interactions

“… a Gas−solid energy parameters for these molecules have been estimated using Lorentz−Berthelot combining rules together with ε/ k = 28°, σ = 3.40 Å for the CC interactions. b The simulations of ethane and ethylene in graphite have been performed using eight-site and six-site models, respectively; see Klein et al. ,,, for details. c This quadrupole moment is parallel to the C−C axis; for estimates of all three moments, see Cheng and Klein …”

“…Ethane and Ethylene on Graphite. Both ethane − and ethylene 104-115 have been the subject of intensive study, in part because of the large number of monolayer phases observed in these systems. It is possible to treat ethane and ethylene as quasi-linear on graphite only at temperatures high enough that rotation around the long axes of these molecules is fully excited.…”

Molecular Dynamics Simulation of a Binary Hydrocarbon Mixture near an Adsorbing Wall: Benzene/n-Heptane on Graphite

“…There are numerous simulation studies, where graphite has been used to test modeling methodologies and interaction potentials in the context of physisorption by comparing simulation and experiment. These investigations include physisorbed noble gases, , small nonpolar − and polar molecules, and rather large and more complex adsorbates. − …”

Molecular Dynamics Simulations of the Melting of a Hexane Monolayer: Isotropic versus Anisotropic Force Fields

“…In recent years there has been considerable interest in the study of the structural and thermodynamic properties of adsorbed monolayers by computer simulation. − When used in combination with diffraction and calorimetric measurements, molecular dynamics (MD) has proved to be a particularly powerful technique. Monolayers of rare gases, especially krypton, on graphite have been investigated extensively , and these simulations have been extended to complicated adsorbates, such as small linear , polar, and tetrahedral molecules. , Recently, long-chain molecules, and alkanes in particular, have been the subject of considerable theoretical and experimental research, − due in part to the widespread application of scanning tunneling microscopy and other proximal probe techniques to study these adsorbates. The structural and orientational ordering of these alkane molecules physisorbed from solution onto various substrates is a topic of considerable interest. , There is a delicate balance between the energy associated with the flexibility of the chains, the molecule−surface energy, and the intermolecular interactions within the adsorbates which gives rise to a rich phase diagram and the appearance of new structures.…”

“…For such dense systems, the packing is sensitive to the parameters and functional forms of the potential and similar effects might be expected for the melting of adsorbed monolayers at full coverage. A limited number of studies of phase transitions for adsorbed flexible molecules, have appeared in the literature so far. ,− ,,− The effect of the isotropy of the nonbonded potential on the melting behavior of these layers is still unknown. An excellent system for such a study is n -hexane adsorbed on graphite, which has been extensively investigated.…”