Basic Considerations for Sorbent Design

For van der Waals (dispersion) interactions, the polarizabilities of the sorbate molecule and the atoms on the sorbent surface are both important (see Eq. (9)). For electrostatic interactions, for a given sorbate molecule, the charges and van der Waals radii of the surface atoms are important. The roles of these parameters are discussed separately. 

Because the ionic radius determines the distance r, it has a strong effect on the electrostatic interactions. The ionic radii of selected cations are given in Table III. 

The potentials discussed previously are those between two molecules or atoms. The interactions between a molecule and a flat, solid surface are greater because the molecule interacts with all adj acent atoms on the surface, and these interactions are assumed pairwise additive. When a molecule is placed between two flat surfaces, i.e., in a slit-shaped pore, it interacts with both surfaces, and the potentials on the two surfaces overlap. The extent of the overlap depends on the pore size. For cylindrical and spherical pores, the potentials are still greater because more surface atoms interact with the  

The effects of the pore size and pore geometry are best illustrated by Table IV. Table IV lists the threshold pressure for adsorption in different  

As expected, the total interaction energies depend strongly on the van der Waals radii (of both sorbate and sorbent atoms) and the surface densities. This is true for both HK type models (Saito and Foley, 1991 Cheng and Yang, 1994) and more detailed statistical thermodynamics (or molecular simulation) approaches (such as Monte Carlo and density functional theory). Knowing the interaction potential, molecular simulation techniques enable the calculation of adsorption isotherms (see, for example, Razmus and Hall, (1991) and Cracknell etal. (1995)). 

---