Pore Size and Geometry

The potentials discnssed above are those between two molecnles/atoms. The interactions between a molecule and a flat solid surface are greater because the molecule interacts with all adjacent 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 

Pore Size (A) P/Po for Slit-Shaped P/Po for Cylindrical Shape P/Po for Spherical Shape 

As expected, the total interaction energies depend strongly on the van der Waals radii (of both sorbate and sorbent atoms) and the surface atom 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). By knowing the interaction potential, molecnlar simnlation techniques enable the calculation of adsorption isotherms (see for example, Razmus and Hall, 1991 CrackneU et al., 1995 Barton et al. 1999). 

C dispersion constant average number of sorbate molecules per cage in zeolite E interaction energy 

Q heat of adsorption linear quadrupole moment r distance between centers of pair pore radius r, ionic radius 

---

This chapter discusses the fundamental principles for designing nanoporous adsorbents and recent progress in new sorbent materials. For sorbent design, detail discussion is given on both fundamental interaction forces and the effects of pore size and geometry on adsorption. A summary discussion is made on recent progress on the following types of materials as sorbents activated carbon, activated alumina, silica gel, MCM-41, zeolites, n -complexation sorbents, carbon nano tubes, heteropoly compounds, and pillared clays. 2001 Academic Press. 

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 pore sizes and geometries for N2 on carbon. The calculation was based on... 

The possibility of obtaining different pore sizes and geometries allows studying the specific role of the pore diameter and interconnection in confinement effects. However, the main problem in the use of MCM materials in radiolysis is the poor definition of the silica-based walls. The presence of micropores and a high content in non-condensed silica (silanols groups) has been evidenced in some cases. 

It has been known that condensation pressure depends on the adsorbate, temperature, pore size, and geometry of sorbent. As increasing temperature, the capillary condensation pressures is increased and adsorption capacity is decreased. This fact implies that adsorption and desorption can be easily achieved by only little adjustment of pressure and temperature. Therefore the effective removal of VOC can be done by pressure swing adsorption (PSA) or thermal swing adsorption (TSA) processes. 

Friedel Crafts type alkylations of benzene by alkenes involve the initial formation of a lattice associated carbenium ion, formed by protonation of the sorbed olefin. The chemisorbed alkene is covalently bound to the zeolite in the form of an alkoxy group and the carbenium ion formed exists only in the transition state. As would be expected fixjm conventional Friedel Crafts alkylation, the reaction rate over acidic molecular sieves also increases with the degree of substitution of the aromatic ring (tetramethyl > trimethyl > dimethyl > methyl > unsubstituted benzene). The spatial restrictions induced by the pore size and geometry frequently inhibit the formation of large multisubstituted products (see also the section on shape selectivity). 

If the active reverts to a solid while confined in the pores (e.g., after cooling), its physical properties are likely to be determined by the nanostructure - the freezing and melting points of pore-confined solids, for example, have been shown to be dependent on pore size and geometry (Chamaya et al. 2011 Jackson and McKenna 1990), and, depending on pore size, the active may remain amorphous or recrystallize. 

Scaffold porosity can be controlled by varying the particle concentration, while pore sizes depend on the size of the particles added to the polymer solution (De Nardo et al, 2012 Janik and Marzec, 2015). If the particle concentration is insufficient, isolated pores will be generated as the polymer surrounds each particle. Hariraksapitak et al. reported an increase in porosity with higher concentrations of particles (25x to 40x) due to the generation of more voids. However, pore sizes remained in the range of 200 00 gm as a result of the particle size utilized during scaffold synthesis, which indicates that particle size and shape are directly related to the pore size and geometry... 

The pore size and geometry of a chromatographic packing are of paramount importance for efficient chromatography. In order to minimise the band broadening by limiting restricted diffusion it is necessary for the pore size and shape to be suffi-... 

---