The effect of molecular steric properties on the melting of quasi-two-dimensional solids is investigated by comparing results of molecular dynamics simulations of the melting of butane and hexane monolayers adsorbed on the basal-plane surface of graphite. These molecules differ only in their length, being members of the n-alkane series [CH3(CH2)n_2CH3] where n=4 for butane and n=6 for hexane. The simulations employ a skeletal model, which does not include the hydrogen atoms explicitly, to represent the intermolecular and molecule-substrate interactions. Nearest-neighbor intramolecular bonds are fixed in length, but the molecular flexibility is preserved by allowing the bend and dihedral torsion angles to vary. The simulations show a qualitatively different melting behavior for the butane and hexane monolayers consistent with neutron and x-ray scattering experiments. The melting of the low-temperature herringbone (HB) phase of the butane monolayer is abrupt and characterized by a simultaneous breakdown of translational order and the orientational order of the molecules about the surface normal. In contrast, the hexane monolayer exhibits polymorphism in that the solid HB phase transforms to a rectangular-centered structure with a short coherence length in coexistence with a fluid phase. A significant result of the simulations is that they demonstrate the importance of molecular flexibility on the nature of the melting transition. The formation of gauche molecules is essential for the melting process in the hexane monolayer but unimportant for butane. The effect of molecular length on the qualitative nature of the melting process is discussed for both monolayers. J. INTRODUCTIONFor some time we have been interested in the question of how the melting of quasi-two-dimensional (2D) physisorbed monolayers is influenced by the steric properties of the constituent molecules. 1 It seems plausible that the melting behavior of 2D solids should depend more strongly on molecular shape than in the bulk phase. For example, if one considers a rigid rod-shaped molecule adsorbed with its long axis parallel to a surface, the potential energy barrier to rotation about the surface normal could be quite large compared to the steric hindrance encountered in the bulk phase. This is because molecules in the bulk do not remain confined to or near a single plane. In general, orientational and translational disorder can be achieved more easily than in a monolayer due to exchange of molecules between neighboring layers.Our approach to studying the effect of molecular steric properties on monolayer melting has been to consider a series of isostructural rod-shaped molecules, the n-alkanes [CH3(CH2)n_2CH3], physisorbed on a graphite basalplane surface. We have investigated the dependence of their monolayer melting behavior on the length of the molecule (n) using both neutron scattering and x-ray diffraction techniques. crease in the monolayer melting point as the length of the molecule increases in the series ethane (n = 2 ), butane (n =4), and...

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The structure of deuterated n-butane [CD3(CD&)2CD3] adsorbed on the (0001) surfaces of an exfoliated graphite substrate has been investigated by elastic neutron diffraction. The aim of this study was to elucidate the effect of steric properties, particularly the rodlike shape of the molecule, on both the monolayer and multilayer film structure. Our principal findings are as follows: (1) The solid monolayer has a rectangular unit cell commensurate with the graphite lattice in one direction (2~3 X 00). Profile analysis

Elastic neutron diffraction has been used to study the structure of N2 films adsorbed on the (0001) surfaces of an exfoliated graphite substrate at coverages 0 between 1.0 and 1.67 layers and at temperatures below 11 K. For 0=1.0, the diffraction patterns can be fit by the in-plane rectangular V 3 X 3 herringbone structure denoted C (commensurate), previously inferred from low-energy electron diffraction (LEED) experiments. Analysis of the relative Bragg-peak intensities in the neutron diffraction pattern of the C phase extends the LEED results by yielding a value of /=45'+5 for the angle between the N -N bond and the short axis of the unit cell. Also, we find a substantially smaller Debye-Wailer factor for this phase than previously inferred from x-ray experiments. At coverages 8=1.13 and 1.27, the diffraction patterns are consistent with the uniaxial incommensurate (UI) phase seen by LEED. The patterns can be fit with the same molecular orientational parameters as for the C phase. We find the compression of the monolayer to be complete at 6=1.67 where the film density is -10% greater than for the C phase. Fits to the diffraction pattern at this coverage indicate a slight oblique distortion of the unit cell from hexagonal symmetry. For this nearly triangular incommensurate (TI) phase, we obtain orientational parameters in the ranges 30' & P &45' and 10 & P & 20 where l3 is the tilt angle of the N -N bond with respect to the surface.At coverages between 6=1.27 and 1.40, there is evidence of coexistence of the UI and TI phases.

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