The Adsorption on Ionic Surfaces

There have been numerous theoretical and experimental investigations on the adsorption of argon, oxygen, and nitrogen on potassium chloride (126-128) and in this connection we may refer to a survey in Brunauer s book on physical adsorption (129). There seems to be a general agreement that the most favorable positions for the adsorbed atoms or molecules will be found just above the center of a lattice cell. The electrostatic polarization is minimum at such spots, but the nonpolar van der Waals forces are at their maximum and dominate (130). Drain 

We saw in Sec. V,5 that molecules with peripheral dipoles, such as OH, Nil, and, COOH groups are attracted strongly by the electrostatic field of the surface. These dipole forces form the most important contributions toward the adsorption energy when such molecules are adsorbed on ionic surfaces, nonpolar van der Waals forces and electrostatic polarization giving smaller contributions (133). 

Healy et al. (134) studied experimentally the heats of adsorption of many polar and nonpolar gases on polar and nonpolar surfaces by means of their heats of immersion. It was found that the heat of immersion of rutile on a series of straight-chain compounds was a linear function of the dipole moment of the wetting liquid. In a later article (135)- this work was extended and it is shown that nearly the entire heat effect on immersion of the clean solid surface is due to adsorption of molecules in the first layer. From the slope of the line, giving the values found for the net heat of adsorption as a function of the dipole moments, the average field strength, F, of rutile can be found by means of Eq. (22). The experimental value found by these investigators is 

A further analysis of the various contributions toward the adsorption energies (135) has revealed that the adsorption energy of alcohol on rutile consists mainly of the contribution of the attraction of the dipole the nonpolar van der Waals forces contribute less than 40% of this part and electrostatic polarization less than 10%. The adsorption energies of hydrocarbons on rutile are mostly due to the van der Waals forces, and half the amount of the van der Waals contribution (one third of the total) originates from the electrostatic polarization. 

The conversion of the surface layers of many oxides into surface hydroxide layers is the result of the chemisorption of water on these oxides. On heating, H20 desorbs and the OH groups are converted into 0 ions again. 

---