Ground state energy and structure of physisorbed monolayers of linear molecules

Model calculations of the ground state configuration and energy of monolayers of CO2 and of N2 adsorbed on the basal plane surface of graphite are reported. The interaction models consist of atom–atom and site–site potentials and point quadrupole moments, as in models for the three-dimensional bulk solids, supplemented by effects of static and dynamic screening of multipole fields by surface charges on the graphite. There is a good quantitative agreement with estimated zero temperature heats of adsorption for N2/Gr and the calculated ground state configuration is the √3 R 30° registered monolayer solid. For CO2/Gr, the calculated low coverage isosteric heat differs by 15% from the experimental value, but the calculated lateral energy of the monolayer solid is consistent with available experimental information. Within the uncertainty caused by this discrepancy, the calculations are consistent with a suggestion that CO2/Gr is a nonwetting system at low temperatures. The ground state configuration of monolayer CO2/Gr is calculated to be an incommensurate solid of number density within 5% of the number density of the √3 R 30° solid.