Solvent site-competition

Moreover, the membrane could be mounted as an interface between the apolar substrate and the polar oxidant in a membrane reactor, avoiding the use of any solvent. Dilution of the reagents by solvent and competition between solvent and reagents on the active sites can thus be avoided. In the countercurrent membrane reactor, the substrate and the oxidant are circulated at each side of the membrane and alkanes can be oxidized with peroxides without solvents. Of course, the system carries all of the other advantages of membrane reactors continuous operation and easy separation. 

Less-polar solvent molecules B (CHCI3, CH2CI2, benzene, etc.) that do not localize nevertheless interact with adsorption sites. This is illustrated in Fig. Id for the binary mixtures A/B (A is nonpolar), and is contrasted in Fig. le for adsorption of a mobile phase A/C, where C is localizing. When nonlocalizing molecules of a polar mobile phase M are adjacent to localizing molecules of solute X (Fig. Ic) or solvent C (Fig, If), these noncova-lent interactions of M with surface sites can interfere with or displace corresponding interactions between the localized molecule and its site. This effect is referred to as site-competition delocalization. 

The requirement for site-competition delocalization of the solute would appear to be an adsorption site which allows both (a) the localization of a solute molecule X, and (b) the lateral interaction with the site by an adjacent mobile phase molecule M—as in Fig. Ic. Thus, the presence of this phenomenon for some LSC systems (e.g., silica) and not others (e.g., alumina) implies some fundamental difference in the relative accessibility of the adsorbent sites to both solvent and solute molecules. 

When a binary-solvent mobile phase B/C is used, where C is localizing and B is not, the value of °a for the localized solvent C [analogous to E°a for a localized solute X, as in Eq. (13)] will vary with cd, just as in the case of site-competition delocalization of the solute. This has been discussed in Ref. 16. and the resulting relationship is [cf. Eq. (13)] ... 

Note that Et can change with mobile-phase composition due to site-competition de-localization, but this effect does not arise from solvent-solute or solvent-solvent interactions. Its effect on retention was discussed in Sections II,B,2 and 3. 

Returning to Fig. 12, it is seen that the value of Cc for a mixture B/C is smaller than in a mixture A/C, as predicted by Eq. (15). This is the result of site-competition delocalization (superimposed onto restricted-access delocalization), the same phenomenon that leads to increase in the value of localizing solute molecules, as compared to the value calculated from the molecular dimensions of the solute. The function/,(C) of Eq. (15) is the same function as/,(X) in Eq. (14) for delocalization of solute molecules. A previous study (Fig. 3 of Ref. /6) has shown that plots of//(C) and /i(X) versus the adsorption energy Qi of the solute or solvent substituent A that is localized (Efca) give a single curve through points for both solvents (C) and solutes (X). This function/j(C) is tabulated in Table II and can be used to estimate values of ec for mobile phases B/C, when the experimental value offi C)IA is not known for the solvent C (see Table I). 

Site-competition delocalization with resulting decrease in el for stronger solvents B in mixtures B/C [Eq. (I5)j has been observed for silica as adsorbent (/6) and several solvents C this effect should also occur for... 

While the precision of the resulting values is poor, it appears (as predicted) that the value of ( is independent of the strength of the mobile phase exclusive of C (A/B). That is, site-competition delocalization of the C-solvent does not occur on alumina. 

Site-competition delocalization will be favored by surface sites that are accessible to lateral interaction by solvent molecules that are adjacent to a localized solute or solvent molecule. The relative accessibility of sites for this type of interaction is as follows alumina (least), Qg-silica, and silica or amino phase (most). This is in fact the order of increasing site-competition delocalization noted in Table III for delocalization of either solvent or solute molecules. 

See Refs. 14 and /5 for further examples, keeping mind that the complication of site-competition delocalization of localizing solvents was unknown when these articles were written. 

The retention of polar solutes is also affected by site-competition delocalization. A moderately polar non-localizing solvent molecule can interact laterally with sites upon which a solute molecule is localized. This added competition for the site by both the solute and solvent molecules weakens the net interaction of the solute with the surface. For solvents of increasing polarity a greater decrease in the retention factor with increasing polarity of the non-localizing solvent occurs than is predicted by the simple competition model. This effect can be quantitatively accounted for by assuming a larger value of As than is calculated from the molecular dimensions of the solute. 

It is proposed that a monolayer of solvent is adsorbed to the surface of the silica and that the solute is adsorbed onto the surface when the polarity is less than that of the solvent. No displacement or site competition is envisaged. When the polar modifier concentration is high, a bilayer is formed and the solute then competes with a molecule of the polar solvent in the second layer (Fig. 6.3). Therefore, the major difference between the two models is the level of impor-... 

Tertiary haloalkanes react by an S l mechanism because 3° carbocation intermediates are relatively stable and tertiary haloalkanes are protected against backside attack. In fact, 3° haloalkanes are never observed to react by an mechanism. In contrast, halomethanes and primary haloalkanes are never observed to react by an mechanism. They have little crowding around the reaction site and react ty an Sj 2 mechanism because methyl and primary carbocations are unstable. Secondary haloalkanes may react by either 8, 1 or 8, 2 mechanisms, depending on the nucleophile and solvent. The competition between electronic and steric factors and their effects on relative rates of nucleophilic substitution reactions of haloalkanes are summarized in Figure 9.3. 

Various functional forms for / have been proposed either as a result of empirical observation or in terms of specific models. A particularly important example of the latter is that known as the Langmuir adsorption equation [2]. By analogy with the derivation for gas adsorption (see Section XVII-3), the Langmuir model assumes the surface to consist of adsorption sites, each having an area a. All adsorbed species interact only with a site and not with each other, and adsorption is thus limited to a monolayer. Related lattice models reduce to the Langmuir model under these assumptions [3,4]. In the case of adsorption from solution, however, it seems more plausible to consider an alternative phrasing of the model. Adsorption is still limited to a monolayer, but this layer is now regarded as an ideal two-dimensional solution of equal-size solute and solvent molecules of area a. Thus lateral interactions, absent in the site picture, cancel out in the ideal solution however, in the first version is a properly of the solid lattice, while in the second it is a properly of the adsorbed species. Both models attribute differences in adsorption behavior entirely to differences in adsorbate-solid interactions. Both present adsorption as a competition between solute and solvent. 

The solvents themselves are adsorbed on the electrode surface, as is shown by the capacitance-potential graphs illustrated in Fig. 9 (Payne, 1967, 1970) potassium hexafluorophosphate, the electrolyte in each of the solvents, is thought to be adsorbed only very weakly. The solvents show somewhat differing curves and the peaks have been interpreted both in terms of competition between the solvent and anions for sites at the surface and also in terms of solvent reorientation. Ethers are adsorbed from the amide solvents most strongly at the potentials around the peaks and this has been postulated to be due to an increase in freedom for the solvent to rotate at these potentials (Dutkiewicz and Parsons, 1966). 

The conformation of a polymer in solution is the consequence of a competition between solute intra- and intermolecular forces, solvent intramolecular forces, and solute-solvent intermolecular forces. Addition of a good solvent to a dry polymer causes polymer swelling and disaggregation as solvent molecules adsorb to sites which had previously been occupied by polymer intra- and intermolecular interaction. As swelling proceeds, individual chains are brought into bulk solution until an equilibrium solubility is attained. 

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