Physical Adsorption on Charcoal and Metals

Physical adsorption of these gases on charcoal, therefore, must be regarded as being caused by the cooperative action of a polarization by the field of the charcoal nonpolar van der Waals forces and repulsion forces. 

The same picture holds for physical adsorption on metal surfaces. The polarization of the adsorbed molecules causes dipoles pointing with their positive ends away from the metal surface. The work function of the metal will be lowered by this effect, and it seems as if the increase of the normal nonselective photoelectric emission of metals by the adsorption of water molecules (122) or molecules of organic substances such as pyridine, propionic acid, and benzene (123) or alcohol, diethyl ether, and acetone (124) is caused by this effect. The explanation, which, many years ago, was given by the author (125), viz., polarization by positive hydrogen ions which should still be present, may seem to be unnecessary and obsolete. 

As already mentioned in Sec. V,8,b, water molecules may even cause dipoles by forming a coordinate bond with the metal surface, these dipoles working in the same sense as those formed by polarization in the molecules themselves. 

When chemical action enters into the picture, in other words when chemisorption can also take place, more forces come into action, as in the above-mentioned adsorption of water molecules. We shall discuss the chemisorption of hydrogen and of oxygen on charcoal and on metal surfaces in subsequent sections. 

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