We report the results of molecular dynamics simulations of a complete monolayer of hexane physisorbed onto the basal plane of graphite. At low temperatures the system forms a herringbone solid. With increasing temperature, a solid-to-nematic liquid-crystal transition takes place at T 1 = 138± 2 K followed by another transition at T 2 = 176± 3 K into an isotropic fluid. We characterize the different phases by calculating various order parameters, coordinate distributions, energetics, spreading pressure, and correlation functions, most of which are in reasonable agreement with available experimental evidence. In addition, we perform simulations where the Lennard-Jones interaction strength, corrugation potential strength, and dihedral rigidity are varied in order to better characterize the nature of the two transitions. We find that both phase transitions are facilitated by a "footprint reduction" of the molecules via tilting and to a lesser degree via creation of gauche defects in the molecules.

We discuss molecular dynamics (MD) computer simulations of a tetracosane (C24H50) monolayer physisorbed onto the basal plane of graphite. The adlayer molecules are simulated with explicit hydrogens, and the graphite substrate is represented as an all-atom structure having six graphene layers. The tetracosane dynamics modeled in the fully atomistic manner agree well with experiment. The low-temperature ordered solid organizes into a rectangularly centered structure that is not commensurate with underlying graphite. Above T=200 K, as the molecules start to lose their translational and orientational order via gauche defect formation a weak smectic mesophase (observed experimentally but never reproduced in united atom (UA) simulations) appears. The phase behavior of the adsorbed layer is critically sensitive to the way the electrostatic interactions are included in the model. If the electrostatic charges are set to zero (as for a UA force field), then the melting temperature increases by approximately 70 K with respect to the experimental value. When the nonbonded 1-4 interaction is not scaled, the melting temperature decreases by approximately 90 K. If the scaling factor is set to 0.5, then melting occurs at T=350 K, in very good agreement with experimental data.

The melting transition of N2 on graphite is examined using a Monte Carlo procedure with several different boundary conditions. On the basis of a simple interpretation of the experimental environment, we are able to explain and accurately reproduce the large change in melting temperature between partial and monolayer coverages.

We report the results of constant-temperature, constant-density, molecular-dynamics simulations of the melting transition of Kr atoms physisorbed onto small Lennard-Jones spheres. Adlayer depletion takes place when free boundary conditions are used but utilization of a soft reflecting shell results in a broad melting transition at around Tϭ65 K without any appreciable hysteresis. We introduce structural and bondorientational parameters and utilize thermodynamic parameters to monitor the behavior of the system. The spherical geometry causes compression of the neighbor distances with respect to those for the corresponding planar case, causing enhanced interaction of the neighbor shells via interstitials/vacancies, the nature of which is further elucidated using the bond-orientational information. Quantities related to the radial degree of freedom change slowly during melting, and bond-orientational order of the Kr lattice persists well into the fluid.