The Polarization of an Adsorbed Molecule by a Conducting Adsorbent

The same conclusion may be obtained from the study of the behavior of many gases adsorbed on charcoal. We shall discuss the mobility of adsorbed molecules in Sec. VII, but we may mention here one of the results of such studies. Many gases, such as A, N2, 02, CO, CH4, etc., when adsorbed on charcoal, behave as two-dimensional nonideal gases (44)- This behavior can be described by a two-dimensional van der Waals equation, from which a two-dimensional van der Waals constant o2 (comparable with the normal three-dimensional van der Waals a) may be derived. The two-dimensional van der Waals constants can also be calculated from the three-dimensional values of a (45). The experimental results show that the actual a2 constants for gases adsorbed on charcoal or on mercury are always far lower than the theoretical ones and are very often even negative (45). The adsorbed molecules tend to repel each other instead of showing a mutual attraction. This behavior also points to a polarization of the adsorbed molecules by the field of the charcoal or of the mercury (47). 

We may assume this polarization to be caused by an electric double layer formed by an electron distribution over the surface of the conducting adsorbent and corresponding positive charges in the metal. The dipole moment induced in the adsorbed molecules by the field of this double layer may be calculated from the difference between the theoretical value of a2 and the actual value which is found. This difference forms the a2 contribution caused by the dipole and is given by the expression 

Some figures obtained in this way for gases adsorbed on charcoal are given in Table I. 

It is clear that this effect leads to a rather important contribution to the adsorption energy, if not to the most important, in those cases of adsorption of gases on conducting surfaces where no chemical interaction results. 

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