Sorption of molecules

Bulk diffusional whiskerlike hydrate, which forms both in the volume of gas and in the bulk of liquid water through sorption of molecules on the growing crystal surface and by tunnel sorption of molecules at the base of the crystal... 

Al content (4). These results all exemplify how framework charge density affects heats of sorption of molecules possessing permanent electric moments. 

The principle of molecular shape-selective catalytic reaction was established by Weisz and associates in 1960 ( 70), and molecular engineering aspects have been reviewed recently by Chen and Weisz (12). The selective sorption of molecules differing only slightly in their critical dimensions has been applied in the development of industrial separation processes to remove straight chain hydrocarbons from a mixed hydrocarbon stream. In shape-selective catalysis, on the other hand, molecules of proper dimensions are continuously entering and leaving the intracrystalline cavities of the molecular sieve, thus allowing specific selectivi-ties to occur. Reactant selectivity (Scheme 7A) occurs when I of 2... 

We would like to emphasize that the global properties of molecular sieves, often described in terms of hydrophobicity and hydrophilicity [240] or fiamework polarity [241] will markedly influence the chemical preference for the sorption of molecules. This may lead to quite different relative concentrations of reactants in the zeolite pores (well demonstrated and discussed for the case of TS-1 in this review [163]) and in the intra ciystalline void space. The consequence of which is unexpected solvent-type effects. While these possibilities are hardly utilized up to now, one could think of systematically applying this approach for organic synthesis steps in which, for example, a high concentration of reactants is needed around the active site, but for which one wishes to dilute the products in the intra crystalline void to prevent further reaction. In such a case a solvent of different polarity than the reacting substrate should be chosen and the choice of the zeolite should be made in accordance to the polarity of the reactant(s). 

During the last two decades, the number of molecular modeling studies on zeolites published annually has grown enormously. As a result of advances in both the development of computational methodologies and new computer architectures, modeling has become an important tool in zeolite research. It can provide useful insight into the structure and reactivity of zeolites and into the sorption of molecules in zeolites. This section gives a brief introduction to computational approaches, as well as a short overview on applications in zeolite research. 

The method proposed by Parrinello and Rahman is of special interest in studies of crystalline solids and zeolites in particular. The method allows a variation of both volume and shape of simulation box and therefore is suitable for studying phase transitions induced by sorption of molecules, isomorphic substitution, or change of temperature. In this method, the Lagrangian of a system with kinetic energy T and potential energy U... 

Figure 9. Dependence of sorption of molecules M on concentration (pM, = -logM,) and dissociation constants of competing molecules. Curves were calculated from Eq. (18) using Kd = 10 mol/1, [M] = 10 mol/1.

Recent work on the role of solvated electrons in intra-DOM reduction processes has demonstrated the importance of trapped e in reactions with species adsorbed on the DOM matrix [98-100]. Modeling of DOM mediated photoreactions indicated the importance of sorption of molecules to DOM for reaction to occur [98, 99]. This is consistent with the lifetime of e" precluding escape from the aqueous DOM matrix into bulk solution. Since many important reactions with environmental implications involve binding or adsorption to DOM - see, for example, [3,101,102] - the role of matrix effects and the caged electron could be very significant. Some workers have suggested that since e remains primarily trapped within the DOM matrix, Oj must be formed by direct electron transfer from the excited triplet state of DOM to O2 [14]. However, it is equally if not more plausible that Oj may be produced by the reduction of Oj by radicals or radical ions produced by intramolecular electron transfer reactions from irradiated DOM [25]. The participation of radicals in the production of carbonyl sulfide and carbon monoxide from irradiated DOM in South Florida coastal waters was recently demonstrated by Zika and co-workers [81-83] and potential pathways for the formation of free radicals from irradiated DOM were discussed. Clearly, the relative contribution of e q and associated transients to the photochemistry of DOM has not been unequivocally resolved in the literature. 

Barrier properties of materials indicate their resistance to diffusion and sorption of molecules. A good barrier polymer has low values of both diffusion (D) and solubUity (S) coefficients. Since the permeabUity coefficient P is a derived function of D and S, a high-barrier polymer has low values of P. The diffusion coefficient is a measure of how fast a penetrant will move within the polymer in a particular direction, while the solubility coefficient gives the amount of the penetrant taken (or sorbed) per unit mass of polymer from a contacting phase per unit of sorbate concentration. Both diffusion and solubility can be applied to the reverse process of sorption, that is, the migration of compounds from the polymer to a surrounding media. Several factors influence the effective value of diffusion and solubility coefficients in polymers. 

The computer simulation of clay minerals or other complex soil phases is often limited by the large unit cells and supercells required to properly represent the bulk or surface structure of the sorbing material. Furthermore, multicomponent substitutions in the structure, especially for clay minerals, lead to complex disordered systems that are not amenable to molecular simulation based on a simple unit cell to represent the system. Recently, Cygan and Kirkpatrick (2002) determined the sorp-tion sites of several complex clay phases based on the forcefield method of Pat-(abiraman et al. (1985). This approach allows for the evaluation of the fundamental atomistic properties that control the sorption of molecules, metals, and anions onto the internal or external surface of clays. It is possible to evaluate the influence of crystallography, cleavage surface, composition, isomorphic substitution, and sur-l ace protonation state on the relative sorption energy. 

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