Sorbate localization

Drying of the subsurface solid phase can cause an increase in the rate of desorption. If penetration of a sorbate toward inner surfaces does not reach its equilibrium by the time drying commences, a fraction of the sorbate may remain localized at more accessible outer surfaces in an amount greater than that corresponding to the equilibrium level. Under these conditions, the drying of the system may increase the rate of desorption during successive rewetting. 

D limiting zeolitic diffusivity at zero sorbate concentration Dp macropore diffusivity (based on pore sectional area) rrt ratio of bed void space to zeolite crystal volume = e/(l — e ) q local sorbate concentration in a zeolite crystal... 

As the two sorbates methane and krypton on 5A appeared to have different mechanistic behaviour, further theoretical study appeared warranted. Two hypothetical gases P and Q whose properties are tabulated in Table 1 were used for comparison with the behaviour of methane and krypton. Hypothetical gas P was designed to have a Henry constant equal to methane, but to be a localized sorbate having entropy of sorption values decreasing incrementally as for krypton. Conversely, hypothetical gas Q had a Henry constant equal to that of krypton, but entropy of sorption values non-localized and decreasing incrementally as for methane. 

In Figure 7d the sorbate concentration of the higher Henry constant localized species totally dominates, indicating the reinforcement of the two factors. 

The packed bed breakthrough method for investigation of mass transfer phenomena in sorbent systems can in many instances offer certain advantages not found in other experimental methods. The method is especially useful when the adsorption isotherms for the principal sorbate exhibit favorable curvature (convex toward loading axis). In such a case, there is the potential for a portion of the sorption front to approach a stable wave form (shape of the front invariant with time). Given the existence of a stable or "steady-state" mass transfer zone (MTZ) and a detailed knowledge of the equilibrium loading characteristics within that zone, one can extract local values of the effective mass transfer resistance at any concentration in the zone. 

The adsorbates and adsorbents in this study were chosen to reflect a range of different types of molecular interactions as well as to observe whether the retention volume data trends could be generalized. In addition, solutes were picked which would rapidly equilibrate with the chosen adsorbents (no hysteresis) and whose distribution coefficients could be measured conveniently over as wide a pressure range as possible. As shown in Table I, the adsorbents corresponded to two distinctly different chemical types as classified by the criterion of Kiselev (40). The alumina represented an adsorbent capable of specific interactions with sorbates having localized peripheral... 

Localization of template molecules, sorbates, or other introduced molecular, semiconductor or metal clusters. 

Structural studies have been relatively few in number these are also typically performed at low temperature when sorbates are reasonably well localized. For examples, aromatic hydrocarbons in zeolites X, L, Y and good agreement with docking simulations is found. At higher temperatures the effective description of disordered sorbate distributions so as to reproduce the measure difiraction data remains, in general, a challenge [75]. 

Physical adsorption (physisorption) occurs when an adsorptive comes into contact with a solid surface (the sorbent) [1]. These interactions are unspecific and similar to the forces that lead to the non-ideal behavior of a gas (condensation, van der Waals interactions). They include all interactive and repulsive forces (e.g., London dispersion forces and short range intermolecular repulsion) that cannot be ascribed to localized bonding. In analogy to the attractive forces in real gases, physical adsorption may be understood as an increase of concentration at the gas-solid or gas-liquid interface imder the influence of integrated van der Waals forces. Various specific interactions (e.g., dipole-induced interactions) exist when either the sorbate or the sorbent are polar, but these interactions are usually also summarized under physisorption unless a directed chemical bond is formed. 

Reviews (9, 63, 64) of the reactions between hydroxylated mineral surfaces and aqueous solutions brought out the richness of variety found in surface phenomena involving natural particles. Isolated surface complexes, the principal topic of this chapter, are expected when reaction times are short and the adsorbate content is low [Figure 6, inspired by Schindler and Stumm 63)]. Thus, surface complexes occupy a reasonably well-defined domain in the tableau of reaction time scale versus sorbate concentration. Localized clusters of adsorbate (47, 48, 65, 66) that contain two or more adsorbate ions bonded together can form if the amount sorbed is increased by accretion or bv the direct adsorption of polymeric species (multinuclear surface complexes). Surface clusters can erase the hyperfine structure in the ESR spectrum of an immobilized adsorbate (33, 67) or produce new second-neighbor peaks from ions like the absorber in its EXAFS spectrum (47, 66). 

Figure 3. Representations of various local sorption phenomena and the effects of typical sorbent and sorbate properties.

This isotherm model was designed by Fowler and Guggenheim [12] to correct for the first-order deviations from the Langmuir isotherm. It assumes ideal adsorption on a set of localized sites on a homogeneous surface, with weak interactions between molecules adsorbed on neighboring sites. It assumes also that the interaction energy between two sorbate molecules is small enough that the random... 

Factors 1 and 2 are universally found in sorption systems, and the approximate additivity of dispersion energies for all atom pair interactions ensures the sorption, at low temperatures, of large molecules, even n-paraffins. When a sorbent is composed of positive and negative ions (as in a zeolite), there exist local electrostatic fields, F, that polarize the sorbate of polarizability a. Thus = — olF, where the negative sign implies exothermal reaction. Such effects could be visualized in terms of distortion of electron clouds in certain p- or 7r-electron systems. 

Thus, the magnitude of physisorption forces is most strongly affected by the size, polarity, polarizability, and quadrupolarity of sorbate atoms or molecules, as well as the electric field strength and the local field gradient of the solid surface. 

Optical processing systems are proposed based on relative intensity changes of second harmonic generation in sorbate complexes of p-nitroaniline and 2-methyl-p-nitroaniline in molecular sieve hosts [98]. Optical data storage would be one of a variety of potential electro-optical applications. In addition, molecular optical effects by third harmonic generation are suggested for optical data storage based on the possible formation of bistabilities for the local fields [65,99]. 

One can visualize, then, the two distinct models of the sorbate (1) The sorbate molecules occupy certain sites where their potential energy is a minimum, the distribution of the sites in space being determined by the lattice of the sorbent. The sorbate particles would then be localized in a lattice of sites and the statistics of particles in a lattice would apply under the condition that part of the lattice can be empty. (2) If the potential energy of the occluded molecules is independent of their position in the accessible volume of the host crystal, then the sorbate particles perform an irregular movement, characterized by a kinetic energy of IcTj per particle per degree of translational freedom. The statistics of a gas would apply to this case of a mobile sorbate. 

In order to discriminate between localized and mobile guest molecules, one can consider also the measured entropy of the sorbate and compare this quantity with the values obtained from theory for both models. For an energetically homogeneous sorbent, the calculation of the entropy of the sorbate is quite straightforward in principle. 

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