Histogram of number of particles as an indicator for 2D phase transition in adsorption of gases on graphite

Grand canonical Monte Carlo simulation is used to study the adsorption of gases with strong and weak molecular interaction on graphite. We choose nitrogen adsorption at 77 K, ethylene at 104 K, methanol at 240 K and ammonia at 300 K as model examples. The adsorption mechanism of these species can be studied by analysing the radial distribution and the 'number of particles histogram' as a function of loading. At low pressures, at which the surface is barely covered with molecules, nitrogen and ethylene adsorb in a similar manner, while ammonia and methanol show a distinct difference because of the formation of clusters, resulted from the hydrogen bonding. Small clusters are observed for methanol and larger ones for ammonia, which is in agreement with the fact that hydrogen bonding is more significant in ammonia than in methanol. Analysis of the number of particles distribution can identify 2D phase transition as a sudden shift of the peak in the number histogram as exemplified with the adsorption of ethylene at 104 K and ammonia at 300 K. © Copyright 2011 Elsevier B.V., All rights reserved