An Effective Potential for Adsorption of Polar Molecules on Graphite

Abstract

Each carbon atom in the graphite crystal has a quadrupole moment due to the symmetry of the crystal. We show that these graphite quadrupoles, along with the polarizability of graphite, have a substantial effect on adsorption of strongly polar molecules. We present an approximate method for accounting for the change in the solid–fluid potential energy due to polar interactions with graphite. The potential function is integrated over the graphite surface using a truncated Fourier series, so that the resulting potential is analogous to the Steele 10-4-3 potential. The interactions included in this potential include dipole-induced dipole, dipole–quadrupole, and quadrupole–quadrupole interactions. Hence, the potential can be used for fluid molecules with dipole and/or quadrupole moments. Fluid–fluid multipole interactions can be computed with any model; but point multipoles must be used in the solid–fluid potential. The multipole solid–fluid potential is most accurate for nearly spherical molecules.

Keywords:

• Adsorption
• Polar molecules
• Graphite
• Graphite quadrupole moment

Acknowledgements

This work was supported by NSF under Grant No. EEC 0085480. Calculations were performed at the Center for Molecular and Materials Simulations at the University of Pittsburgh. We thank L. W. Bruch, M. W. Cole, R. F. Cracknell, and K. D. Jordan for helpful comments.