HEAT OF ADSORPTION
The sources of adsorption heat in neutral molecules without strong dipole moments are the van der Waals dispersion forces, the same forces which operate in liquids to give rise to the heat of vaporization. These attractive forces operate at very short range, and are relatively independent of the compositions of the neighboring pair. They are the result of an attraction between the rapidly moving nucleus-electron dipole in the orbital of one molecule and the dipole it induces in a nearby orbital of the neighboring molecule. An adsorbed molecule will generally have somewhat less than half of its orbitals in close proximity to the surface, the rest facing in the wrong direction. In a liquid, a molecule is surrounded by neighbors on all sides. Thus, an adsorbed molecule should be stabilized somewhat less than half of its heat of vaporization. But the surface on which it is adsorbed is similarly stabilized, and thus a crude estimate of the heat of adsorption for the system molecule+surface would be the heat of vaporization, or perhaps a little less.