An effective potential for adsorption of polar molecules on graphite

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.