Net adsorption energies

The same sample of AlP04-5 was used to obtain the results shown in Figure 12.13. Here, the isotherms of methane, argon, nitrogen and carbon monoxide and the corresponding net adsorption energies were determined at 77 K after the adsorbent had teen outgassed by CRTA to 353 K. 

McGuire and Suffet [728] proposed the calculated net adsorption energy concept which is based on the solubility parameter of the adsorbate. They justified their approach by noting that the interactions involved in the adsorption of nonpolar and polar compounds onto a nonpolar [activated carbon] surface are, for the most part, governed entirely by the dispersion forces. Their results are summarized in Fig. 31. Even on a log-log plot, the r correlation coefficient is only 0.7. The authors cautioned against extrapolating such a correlation to predict the adsorption capacities of other neutral organic compounds. Clearly, incorporation of model parameters that quantify the chemistry of the carbon surface is necessary. 

FIG. 31 Relationship between adsorption capacity for a range of organic compounds on activated carbon and the net adsorption energy. (Adapted from Ref. 728.)... 

The net adsorption energy A varies with the surface activity of the adsorbent, and we shall next examine the theoretical nature of this relationship. ... 

Equation 47a describes the difference between the adsorption energies of solute and equivalent amount of solvent (one solute molecule can replace one or more solvent molecules on the sorbent surface, depending on the stoichiometry of a given process). Thus A can be regarded as the net adsorption energy of the solute. With a simplifying assumption as to the monocomponent mobile phase we can further write (7) ... 

If we consider the case of a gas in adsorption equilibrium with a surface, there must be no net free energy change on transporting a small amount from one region to the other. Therefore, since the potential represents the work done by the adsorption forces when adsorbate is brought up to a distance x from the surface, there must be a compensating compressional increase in the free energy of the adsorbate. Thus... 

The experimental thickness measurements may also be compared with theoretical results based on profiles generated by the S.F., Scheutjens Fleer, theory (11). For this calculation we use a value for xs °f 1 (net adsorption free energy), for x of 0.45 (experimental value of the Flory-Huggins parameter) and a polymer solution concentration of 200 ppm. Although the value for xs seems rather arbitrary it has been shown (10) that 6jj is insensitive to this parameter. 

In adsorption of a species onto a surface, a bond is formed. When the binding is very weak, for example, through van der Waals interactions, the molecule is said to be physically adsorbed (physisorbed). Such species are typically bound by a few (< 10) kcal/mol. In other cases a chemical bond forms between the adsorbate and the surface, meaning the molecule is chemically adsorbed (chemisorbed). Thus chemisorbed molecules are typically bound to the surface by tens of kcal/mol. In either case, the net internal energy change in an adsorption process A Eads is always negative ... 

Originally Ex - EL was known as the net heat of adsorption (Lamb and Coolidge, 1920). It is now recommended that the more general term net molar energy of adsorption should be adopted (cf. Chapter 2). 

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