Solvent restricted-access

This restricted access of solvent water is probably important for the close alignment of the substrate with respect to the catalytic groups which is required for effective catalysis. In addition, it was shown by Warshel - ... 

Figure 2.29. Schematic representation of a sorbent particle for restricted-access media chromatography. This medium allows proteins and macromolecules to be excluded and elute in the solvent front, while small analyte molecules enter the pores and are retained. (Reprinted with permission from Ref. 100. Copyright 2000 Elsevier Science.)...

There are several processes that can lead to the delocalization of an adsorbate molecule that prefers to adsorb with localization. One such process is illustrated in Fig. 2c, for the in-between molecule ii discussed above. This behavior of the molecule ii is referred to as restricted-access delocalization of the solvent molecule C (or M). Figure 2c is shown as an overhead view in Fig. 2d. Delocalized molecules C (shown as dashed circles) arise either from steric hindrance by surrounding localized molecules iib) or from the inability of C to center itself over a site as in iia and Fig. 2c. 

There are two reasons to suspect that restricted-access delocalization is not responsible for the rise in cb at small 0b in Fig. 10. First, an apparent function can be derived from these latter data and is plotted in Fig. 11. A single curve (solid line) fits the data points for different B-solvents, as expected for restricted-access delocalization. However, this solid curve in Fig. 11 differs dramatically from the dashed curve of Fig. 11, which is the plot of 7cversus 0b for localizing solvents (C) on alumina and silica [Eq. (40)]. Whereas the latter curve suggests localized adsorption of the B-solvent for 0b as large as 0.75 (% p = 0.5), the corresponding value of 0b for the localized monolayer on an amino-phase surface is only 0.18. It is reasonable to assume that localized solvent molecules might occupy as much as 75% of the adsorbent surface, but unreasonable to conclude that a localized monolayer exceed 18% of a monolayer. Furthermore,... 

A second reason to doubt the occurrence of restricted-access delocalization on amino-phase columns is that this increase in eu (Fig. 11) is observed for both localizing and nonlocalizing solvents ethyl acetate and tetrahydrofuran (localizing) and CH2CI2 and CHCHs (nonlocalizing). This contrasts with theory, which predicts that delocalization effects are only associated with localizing compounds. 

The data of Fig. 11 thus indicate that restricted-access delocalization does not exist for amino-phase packings. Furthermore, the importance of the variation of eo with 6n, shown in Fig. 11, is insignificant, so far as calculations of e" are concerned. We will therefore assume that cb is constant for a given solvent B and amino-phase packings, in the calculation of ° for multicomponent mobile phases (Appendix). 

Case 3 involves the use of mobile-phase mixtures B/C, where B and C are polar, but only C can localize. The solvent B can also be a blend of a nonpolar solvent A with a pure solvent B (A/B), in which case we deal with ternary-solvent mobile phases A/B/C. The effect of restricted-access delocalization on values of e in mixtures B/C is illustrated in Fig. 12 (solid circles) for mobile phases composed of acetone (C) and benzene (B), with silica as adsorbent. The open circles are comparative data for acetone/hexane (A/C) mixtures. The same tendency ofec to increase for smaller Oc is found for acetone in mixtures B/C as is found for mixtures A/C. This is further confirmed by the data of Fig. 13, where %tc for several mobile phases B/C (B is benzene, C varying) are plotted versus dc- The precision of these experimental values of is much reduced in the case of mobile phases B/C in comparison to those of type A/C, as can be seen in... 

The vertical lines through each point of Fig. 13 correspond to the experimental uncertainty in for an error of 0.005 unit in the associated e° value (a reasonable estimate of the precision of experimental ° values). In the case of the less polar C-solvents of Fig. 13a, all data points appear to fit the dashed curve [Eq. (40)] with acceptable precision. The fit for the more polar C-soIvents of Fig. 13b is poorer, and may reflect mobile-phase interactions as discussed in Section II,C. However, the overall trend in cb versus 0b is clearly that predicted by restricted-access delocalization. 

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). 

Solid-phase extraction (SPE) is an alternative to LLE. In SPE the analytes are partioned between a solid and a liquid [57, 58], Generally, interfering compounds are rinsed off the solid adsorbent and the analytes are then desorbed with an eluting solvent [58], A range (e.g., normal-phase, reversed-phase, ion exchange, restricted access) of sorbents and formats are available for SPE and the SPE systems are easy to automate [59, 60], In order to accomplish the isolation of the products from the fermentation matrix, both SPE and LLE were evaluated for use in papers I and II. 

Hogendoom, E. A., Huls, R., Dijkman, E., and Hoogerbmgge, R., Microwave assisted solvent extraction and coupled-column reversed-phase liquid chromatography with UV detection. Use of an anal)4ical restricted-access-medium column for the efficient multi-residue analysis of acidic pesticides in soils, J. Chromatogr. A, 938, 23-33, 2001. 

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