Solvents classification schemes

A classification scheme for solvents needs, therefore, to reflect to some extent the uses for which the solvents are put. Many classification schemes have been proposed, and a single major property, that may form the basis for the usefulness of solvents for certain applications, can often be employed in order to classify solvents. On the other hand, a few selected properties may advantageously be used to form the basis for the classification. Various solvent classification schemes have been presented (Reichardt, 1988) and a common solvent classification scheme is ... 

Fig. 5 Solvent classification scheme according to Snyder (Ref. 9). (Reproduced with permission from Preston Publications.)...

TABLE 7.5 A Solvent Classification Scheme for Nonaqueous Solvents... 

The data plots of Fig. 15b (silica) are differentiated for the use of methyl- -butyl ether (MTBE) or acetonitrile (ACN) as localizing solvent C in the mobile phase. It is seen that for some solute pairs (Fig. 15a and c) the open squares (MTBE) fall on a different curve than the closed squares (ACN). This implies that the constant in Eq. (31a) is solvent-specific, rather than being constant for all solvents (as first-order theory would predict). A similar behavior is observed for alumina as well. Figure 15a plots data for 18 different polar solvents B or C, and some scatter of these plots of log a versus m is observed here, as in Fig. 15b for silica. The variation of Q with the localizing solvent C used for the mobile phase has been shown (18) to correlate with the relative basicity of the solvent, or its placement in the solvent classification scheme of Refs. 40 and 41. Thus, for relatively less basic solvents (groups VI or VII in Refs. 40 and 4/),... 

The Snyder solvent classification scheme (29, 30) that has been successfully applied to LC systems (31-37) for LC separation optimization appears to be a general framework that can be used for guidance in the selection of the modifier. Some preliminary experiments have been done to determine the applicability of this approach to SFC systems as a prelude to a more in-depth, statistically-designed modifier selection study. These first experiments are the topic of this paper the basis of the proposed modifier selection scheme will now be described in detail. 

Within the last several years HPLC separations have been optimized in terms of the most appropriate mobile phase composition for a particular set of solutes by exploring the whole plane of solvent selectivities using this solvent classification scheme with a minimal number of measurements in statistically-designed experiments. For reversed phase HPLC systems, the selectivity triangle is often defined by methanol, acetonitrile, and tetrahydrofuran with water as the diluent (37). 

The purpose of these first experiments in the modifier survey work was to determine the applicability of the Snyder solvent classification scheme to the choice of modifier to see if an in-depth study would be warranted. The experimental conditions are described in the final section of this paper. 

The main problem with solvent classification schemes based on the solvatochromic parameters is that it considers only the polar interactions of the solvents and not their cohesive energy [578,582]. The transfer of solute from one solvent to another occurs with (approximate) cancellation of dispersion interactions, but the energy required for cavity formation in the two solvents is not necessarily self-canceling, and when one of these solvents is water, cancellation of the cavity term is unlikely. Solvent classification schemes needs to consider the cohesive energy of the solvent as well as its capacity... 

The rather simple solvent classification schemes yield complex fractions of botanochemicals. Their detailed composition depends not only on the species but also on maturity of the plant and the method of extraction (1 5, ). The polar fraction isolated by acetone extraction and readily soluble in 87.5% aqueous ethanol, termed "polyphenol" by Buchanan and coworkers (11,12), no doubt consists of phenolics and a wide variety of other substances. For plants of high tannin content, (e.g., Rhus g/aubra) the polyphenol fraction might well be called tannin (24)... 

A classification by chemical type is given ia Table 1. It does not attempt to be either rigorous or complete. Clearly, some materials could appear ia more than one of these classifications, eg, polyethylene waxes [9002-88 ] can be classified ia both synthetic waxes and polyolefins, and fiuorosihcones ia sihcones and fiuoropolymers. The broad classes of release materials available are given ia the chemical class column, the principal types ia the chemical subdivision column, and one or two important selections ia the specific examples column. Many commercial products are difficult to place ia any classification scheme. Some are of proprietary composition and many are mixtures. For example, metallic soaps are often used ia combination with hydrocarbon waxes to produce finely dispersed suspensions. Many products also contain formulating aids such as solvents, emulsifiers, and biocides. 

The grouping of solvents into classes with common characteristics can be useful in focusing attention on features that may play a role in experimental solvent effects. Reichardt s - review of classification schemes is thorough. 

However, not withstanding the above objections, further discussion of the Snyder solvent triangle classification method is justified by its common use in many solvent optimization schemes in liquid chromatography. The polarity index, P, is given by the sum of the logarithms of the polar distribution constants for ethanol, dioxane and nltromethane and the selectivity parameters, X, as the ratio of the polar distribution constant for solute i to... 

Nnmerons solvents are used in solvent extraction. They can be divided in the context of solvent extraction into different classes as follows (but see Refs. [1] and [2] for some other classification schemes) ... 

In their thoughtful 1983 review, Nielsen and coworkers noted that particles of diesel soot or wood smoke can absorb significant amounts of water. Thus, they suggested that the most plausible mechanism(s) for nitration (and possibly other electrophilic reactions) of particle-associated PAHs in ambient air may involve reactions both in a liquid film and on solid surfaces and that fundamental laboratory studies of the rates, products, and mechanisms of PAHs in polar solvents would be atmospherically relevant for reactions in the liquid films. Based on this, they proposed a classification scheme for the reactivities of key PAHs in electrophilic reactions, which was subsequently described in detail (Nielsen, 1984). 

The grouping of solvents into classes with common characteristics can be useful in focusing attention on features that may play a role in experimental solvent effects. Reichardt s review of classification schemes is thorough (Reichardt, 1988). It is remarkable that solvent classification correlates strongly with the chemist s intuition. The new direction of the science demands that new properties be incorporated into mundane practices. These will include safety properties and environmental properties, as well as chemical properties. 

Some other classification schemes are provided in a work by Kolthoff (Kolthoff, 1974). It is according to the polarity and is described by the relative permittivity (dielectric constant) e, the dipole moment p (in 10 ° C.m), and the hydrogen-bond donation ability Another suggested classification (Parker, 1969) stresses the acidity and basicity (relative to water) of the solvents. A third one (Chastrette, 1979), stresses the hydrogen-bonding and electron-pair donation abilities, the polarity, and the extent of self-association. A fourth is a chemical constitution scheme (Riddick et al., 1986). The differences among these schemes are mainly semantic ones and are of no real consequence. Marcus presents these clearly (Marcus, 1998). 

Unfortunately, a single classification scheme suited to all areas of nonaqueous solvent study has not yet been devised the criteria that are useful in one area often are not appropriate for another.38... 

This classification has been broadened39,40 by replacing the Brpnsted acid (proton donor) with a Lewis acid (an electron acceptor) and the Brpnsted base with a Lewis base (an electron donor). (A Brpnsted acid is a Lewis acid but not necessarily vice versa.) Solvent-proton interactions are therefore included as one subdivision of this classification, but many solvation reactions of cations with solvents also will be included as reactions of Lewis acid-base systems. This approach still does not solve the problem of fitting specific solvation interactions into the classification scheme. For example, acetonitrile behaves as a good Lewis base toward silver ion, but a poor one toward hydronium ion. The broader scheme also does not specifically take into account hydrogenbonding effects in hydroxylic and other solvents, which affect both the dielectric... 

There also is difficulty in accommodating fused-salt systems in a classification scheme designed primarily for organic solvents, because the dielectric constants are not comparable and the measures of solvent polarity appropriate to organic solvents are not generally useful in fused salts. Therefore, fused-salt systems are not discussed, nor are they included in the general classification scheme. 

Nonaqueous Solvents. Many organic compounds are not soluble in water, and the investigator who desires to study their electrochemistry must resort to organic solvents. The solvents most often used are the so-called dipolar aptotic solvents that belong to Class 5a in the classification scheme of Table 7.5. These are solvents with moderately large dielectric constants and low proton availability. This aptotic character tends to simplify the electrochemical reactions often the primary product is a stable radical cation or anion that is produced by removal or addition of an electron. 

It is remarkable that this overall solvent classification, obtained entirely using statistical methods (PCA), correlates strongly with the chemist s intuition Some of the solvent elasses of Parker s scheme cf. Fig. 3-4) are reproduced in Fig. 3-6. 

In light of the variety of behaviour exhibited by solvates, Byrn (1982) has suggested a classification scheme for crystal solvates based on that behaviour, rather than on stability. He proposed that the solvates for which the solvent can be removed from the crystal and added back to the crystal reversibly without greatly changing the X-ray powder diffraction pattern (Section 4.4) would be considered pseudopoly-morphic solvates. Those which undergo a change in structure, as evidenced by a different powder diffraction pattern, would be described as polymorphic solvates. The appellation does not seem to have been adopted by many other workers. 

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