Simulation studies of the fluid–solid monolayer transition in argon adsorbed on graphite at 77.5 K

Abstract

Simulations have been carried out, using the GEMC method, for Ar adsorbed on graphite at near-monolayer coverages. Two model potentials were used for the Ar substrate interaction: a uniform potential which treats the adsorbent as a continuum, and a periodic potential with barrier heights artificially raised by a factor of three compared to those which would be obtained with conventional 12–6 potentials. For the uniform potential, both 2D and 3D systems were examined and a 3D system was simulated for the periodic potential. Isosteric heats and isotherms for these three systems are compared with experiment and discussed in the light of distribution functions from the simulations.

Comparison with experiment and decomposition of the simulated heat curves into wall and intermolecular parts demonstrates that the characteristic shape of the experimental curves is essentially a three-dimensional phenomenon in which lateral periodicity is of minor importance only. The main mechanism appears to be adsorption into the second layer rather than buckling of the fluid in the first layer. On the other hand, the pressure at which the transition occurs is greatly affected by the substrate periodicity although this pressure is lower in all the simulations than that observed experimentally. Rather strong localisation effects are revealed by the lateral distributions for molecules close to the surface in the fluid phase on the periodic adsorbent. In addition to providing a molecular machanism for the phenomena associated with this transition, the investigation permits some general requirements to be stated, for a potential which would bring theory and experiment into closer agreement.