Solvent-solute localization

Fig. 3. Solvent-solute localization and separation selectivity, (a) representation as in Fig. 2d of adsorption of mobile phase A/B (A, —nonpolar , —polar, nonlocalizing) (b) same for mobile phase A/C (C—polar, localizing) (c) adsorption of localizing solute X from mobile phase A/B (d) adsorption of nonlocalizing solute Y from mobile phase A/B (e) adsorption of X from mobile phase A/C (f) adsorption of Y from mobile phase A/C (g) completion of localized layer of molecules C at = 0.75 with maximum effect on solvent-solute localization selectivity.

As a result of solvent-solute localization, a change from a localizing mobile phase AlC to a nonlocaUzing mobile phase AIB can create large differences in solvent selectivity, and the a values of various solute pairs. The effect is limited to solutes which show some degree of localization, and is therefore more pronounced for more polar samples and the stronger mobile phases that are required for their optimum separation. 

Because solvent-solute localization leads to decreased retention of the solute, the term is negative. 

The above model [Eqs. (30a) and (31a)] can be generalized to the case of mobile phases containing more than two solvents (18). It is also found that the parameter of Eq. (31a) is constant for a particular mobile-phase combination (e.g., A/C or A/B/C), but some variation of Cj is possible between various localizing solvents C, D, etc. This latter effect, which is less important than the variation of a with m, is referred to as solvent-specific solvent-solute localization (.18), and is discussed in Section III,B,3. 

Equation (34) is a general expression which accounts for the three known contributions to solvent selectivity in LSC (i) solvent strength, (ii) solvent-solute localization (including solvent-specific localization), and (iii) solvent-solute hydrogen bonding. The second term ii) is generally the most important and most general contribution to solvent selectivity. 

Solvent selectivity effects in LSC are accounted for by terms (i), (ii), and (lil) of Eq. (34). We will first discuss solvent-strength selectivity term (i) and solvent-solute localization selectivity term (ii), leaving hydrogenbonding selectivity term (iii) to the following section. As already indicated, solvent-strength selectivity term (i) is of limited value in optimizing retention in LSC. This effect is directly based on the validity of Eq. (8),... 

The eflFect of solvent-solute localization and its effect on selectivity is given by Eq. (31a) ... 

The dependence of m on (- for a localizing solvent C in mobile phases A/C or A/B/C is given by Eq. (30a). According to our theory of how these solvent-solute-localization effects arise (i.e., competition of localizing solvent and solute molecules for the same site), we expect that/( e) will increase slowly with 6c until the maximum localized-coverage is approached (6c > 0.5). The function/(flc) should then increase sharply and level out for 6c > 0.75. Figure 16 shows actual datafor/(do) in the case of alumina (Fig. 16a) and silica (Fig. 16b). The same function/( c) is shown in each plot, and it is apparent that experimental data fall close to the solid curves (best-fit values of m° for each solvent C). The shape of the solid curve is as predicted, based on the analysis of Fig. 3. 

Fio, 15. Correlation of solvent selectivity effects with solvent-solute localization and Eq. (31a) (a) alumina, different solute pairs for each plot, 18 different polar solvents (b) silica, as in (a), except ternary- and quaternary-solvent mixtures used localizing agent , MTBE , ACN (O) chloroform or dichloromethane. The solute pairs are as follows (a) 1—1-naphthaldehyde and 1-cyanonaphthalene 2—1-nitronaphthalene and 1,2-dimeth-oxynaphthalene 3—1,5-dinitronaphthalene and 1-acetylnaphthalene (b) I—1-nitronaphthalene and 2-methoxynaphthalene 2—1,5-dinitronaphthalene and 1,2-dimethoxynaphtha-lene 3—methyl l-naphthoate and 2-naphthaldehyde. Reprinted from Snyder< f /. (/J. /it). 

Fig. 18. Correlation of solvent selectivity effects with solvent-solute localization and Eq. (31a. Data for selected diastereomeric solute pairs. Reprinted with permission from Snyder (39).

Solvent-specific localization is generally only one-half to one-third as Important as solvent-solute localization in affecting a values (18). However, its effects are nevertheless important in overall solvent-optimization strategies for LSC (see Section 1II,E). 

Less is known about the relative Importance of solvent-solute hydrogen bonding in affecting solvent selectivity, although several workers have postulated such effects (e.g., /. 42, 43). However, one must be cautious in accepting these various observations at face value, because solvent-solute localization effects will normally be large in systems that can ex-... 

Increase in As [(A, Xxw A,] due to localization of a solute group i Solvent-solute localization contribution to retention [Eq. (29)1 Solute parameters for solutes X and Y, increasing in value for solutes that are localized to a greater extent (Eq. (30) and Fig. 17] Dimensionless free energy of adsorption of solute X from mobile phase M [Eq. (3)]... 

L. R. Snyder, J. L. Glajch, and J. J. Kirkland, Theoretical basis for systematic optimization of mobile phase selectivity in liquid-solid chromatography solvent-solute localization effects, /. Chromatogr. 218 (1981), 299-326. 

Agitation of the Fluid. Agitation of the solvent increases local turbulence and the rate of transfer of material from the surface of the particles to the bulk of the solution. Agitation should prevent settling of the soHds, to enable most effective use of the interfacial area. 

Photochemistry can be used to demonstrate solvent effects in supercritical fluids. The analysis revealed trimodal fluorescence lifetime distributions near the critical temperature, which can be explained by the presence of solvent-solute and solute-solute clustering. This local aggregation causes an increase in nonradiative relaxations and, therefore, a decrease in the observed fluorescence lifetimes. Concentration and density gradients are responsible for these three unique lifetimes (trimodal) in the supercritical fluid, as contrasted with the single lifetime observed in a typical organic solvent. The... 

Influence of Solvent-solute Clustering on Reactions Due to Augmentation in Local Polarity... 

Persistent interactions are not limited to hydrogen bonds. We mention for example those appearing in solutions of molecules with a terminal C=0 or C=N group dissolved in liquids such as acetone or dimethylsulfoxide. These solvents prefer at short distances an antiparallel orientation which changes at greater distances to a head-to-tail preferred orientation. The local antiparallel orientation is somewhat reinforced by the interaction with the terminal solute group and it is detected by the PCM calculation of nuclear shielding and vibrational properties. Recent experimental correlation studies [25] have confirmed the orientational behaviour of these solvents found in an indirect way from continuum calculations. The physical effect found in this class of solvent-solute pairs seems to be due to dispersion forces. 

The key differences between the PCM and the Onsager s model are that the PCM makes use of molecular-shaped cavities (instead of spherical cavities) and that in the PCM the solvent-solute interaction is not simply reduced to the dipole term. In addition, the PCM is a quantum mechanical approach, i.e. the solute is described by means of its electronic wavefunction. Similarly to classical approaches, the basis of the PCM approach to the local field relies on the assumption that the effective field experienced by the molecule in the cavity can be seen as the sum of a reaction field term and a cavity field term. The reaction field is connected to the response (polarization) of the dielectric to the solute charge distribution, whereas the cavity field depends on the polarization of the dielectric induced by the applied field once the cavity has been created. In the PCM, cavity field effects are accounted for by introducing the concept of effective molecular response properties, which directly describe the response of the molecular solutes to the Maxwell field in the liquid, both static E and dynamic E, [8,47,48] (see also the contribution by Cammi and Mennucci). 

While the effect of solvent in local geometry can be reasonably captured by minimization protocols implemented in most continuum models, the effect of solvent in conformation and in intermolecular geometry requires more powerful methods, since the conformational space of the solute(s) needs to be sample to explore regions which might be quite different from those important in the gas phase. This means that solvation methods need to be... 

Solvent Effects. The values oi Pim obtained from dilute solution measurements differ somewhat from values obtained from the pure solute in the form of a gas. This effect is due to solvent-solute interactions in which polar solute molecules induce a local polarization in the nonpolar solvent. As a result p. as determined in solution is often smaller than p for the same substance in the form of a gas, although it may be larger in other cases. Usually the two values agree within about 10 percent. A discussion of solvent effects is given in Ref. 4. 

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