The structure and dynamics of a Langmuir–Blodgett film of stearic acid, physisorbed on graphite, has been examined by a combination of energy minimization and molecular dynamics. The model predicts that the molecules of this system are normal to the surface at head group areas below 21 Å2, but tilted away from the normal at head group areas above 21 Å2, and that this change occurs over a very small range in head group area. The system exhibited cooperative reorientations involving the average tilt and precessional angles. Gauche bond defects tended to appear near the ends of the molecules, and did not open up any significant free volume within the layer.
Atomic-beam diffraction and molecular-dynamics calculations have been combined to study the structures of overlayers of the dipolar molecules CH3F and CH3CI physisorbed on bare and xenon-plated graphite crystals. On the former substrate, CH3F adopts a commensurate triangular lattice with one molecule per unit cell, whereas on the latter the unit cell doubles. On both substrates CH3CI is incommensurate with antiferroelectrically ordered molecules arranged in a uniaxially distorted structure.
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.
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