Physical adsorption of simple linear molecules on graphite

Abstract

<p>Some properties of the small linear molecules N₂, CO and C₂H₂, physically absorbed on a graphite surface, are investigated in experiments and computer simulations. Acetylene, physically absorbed on graphite, is known to have two solid phases, but no clear indications of liquid-gas and liquid-solid coexistence has been observed. Vapor pressure isotherms of C₂H₂ on exfoliated graphite are measured revealing the two solid phases and also both coexistence phases. At high temperature Henry's law constants of C₂H₂ and CO on graphite are measure and used to refine the parameters of potential models. For CO, it is found that a 10-4 model is easily fit to the data, but in the case of C₂H₂ no realistic parameters in a 10-4 model resulted in an acceptable fit. A qualitative explanation, invoking an image force, is offered but quantitative attempts fail as the large quadrupole moment of C₂H₂ causes the model for the image interaction to break down. Monte Carlo calculations of the two solid structures are presented which disagree with the proposals in the literature but are in agreement with diffraction data. X-ray and LEED studies of monolayer densities of CO and N₂ absorbed on graphite have revealed the low temperature structure to be a 1x√3 herringbone and to change to an incommensurate herringbone structures at slightly greater densities. Orientational order-disorder transitions occur at both densities but the character of the transition is different in N₂ than in CO. Classical Monte Carlo calculations of the integral heat absorption vs. coverage are presented for several potential models are compared to experimental measurements. The best of these models are used in Monte Carlo simulations of the orientational order-disorder transition in N₂ and CO. The result for N₂ compare well with experiment but in the case of CO the agreement is not a good as the potential model seems to correspond to a molecule which is too small</p>