The surface transport of adsorbed molecules

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Abstract
A detailed mechanistic model is developed to describe the process by which adsorbed molecules migrate over the surface of an adsorbent. The migration is assumed to occur as a result of a random hopping of partially desorbed species; explicit expressions are derived for the hopping rate and the distance traversed in a single jump for the case of a gas adsorbed on a homogeneous surface. In the degree of detail developed herein experimental determinations are still required (to evaluate an otherwise unknown ratio of partition functions, and the activation free energy) but the pressure or surface concentration dependency of the transport rate is given explicity.The predictions are tested by comparison with three sets of experimental data for hydrocarbons adsorbed on porous glass; these systems were chosen as they may be shown to meet the assumption of an adsorbate migrating over an energetically homogeneous surface under the conditions studied. Reasonable values are found for the parameters and excellent agreement between the observed and predicting trends with pressure is noted in all cases.

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