Solvent-selectivity triangle, Snyder

The Snyder-solvent-selectivity triangle concept can be combined with a mixture-design statistical technique to define the optimum mobile-phase composition for a particular separation A feature of this mixture-design... 

Solvents used here for a general liquid-liquid extraction method were selected from Snyders solvent selectivity triangle. As extraction liquids have to be composed of mixtures of three solvents which may enter into maximum interaction with the analyte, three solvents had to be selected that represent a wide variety of selective interactions. In addition, the solvents should be sufficiently polar to ensure quantitative extraction. Besides selectivity and polarity requirements, the solvents should also meet a few other criteria, mainly for practical reasons they should not be miscible with water, have low boiling points (for relatively fast evaporation procedures) and have densities sufficiently different from the density of water, for pure solvents as well as for selected binary or ternary mixtures of solvents. 

The separation and identification of natural dyes from wool fibers using reverse-phase high-performance liquid chromotog-raphy (HPLC) were performed on a C-18 column. Two isocratic four-solvent systems were developed on the basis of the Snyder solvent-selectivity triangle concept (1) 10% acetonitrile, 4% alcohol, and 2% tetrahydrofuran in 0.01 M acetic acid and (2)7% acetonitrile, 8% alcohol, and 5% tetrahydrofuran in 0.01 M acetic acid. Samples were also eluted in 30% acetonitrile. Spot tests and thin-layer chromatography were performed on all samples to confirm HPLC results. The systems also were found to be potentially useful in the identification of early synthetic dyes. A system of sample preparation that minimizes the reaction of samples was discussed. The application of this HPLC separation technique to samples from 20th century Caucasian rugs and American samples unearthed from the foundation of Mission San Jose was examined. 

The technique used to develop the four-solvent systems was based on procedures elucidated by Lehrer (6), Rohrschneider (7), and Glajch (8). After trials with individual solvents chosen from the comers of the Snyder solvent-selectivity triangle—a system of classification of solvents by the degree to which they function as proton donors, proton acceptors, or dipole interactors—an ideal solvent system was calculated. Ethanol, acetonitrile, and tetrahydrofuran were the reverse-phase solvents used, and water was the carrier solvent. Once the ideal solvent strength of one solvent-water combination was empirically determined, that of the other combinations could be estimated by use of the following equation (9) ... 

Fig. 18 Snyder solvent selectivity triangle for solvents of Table 3.

In Figure 4.9 Snyder s solvent selectivity triangle is presented. The solvents of Table 4.2 are marked in the plot with triangular coordinates for the eight groups. 

Figure 5-2. Snyder s selectivity triangle for solvents. (Reprinted from reference 9, with permission.)...

The literature of QSRR with LSS is dominated by a specific SSD, the I ER solute parameters V, E, S, A, and B, as defined in Equation 15.2. An extraordinary amount of attention has been paid to predict retention (24,25) and to establish phase selectivity in MEKC using LSER (5, 7, 26-31). Attempts to classify and to contrast micellar phases with basis on the LSER coefficients have been pursued by many researchers (5,26,27,29). Interesting approaches comprise the classification of micellar phases by the combined use of LSER parameters and retention indexes (32), the clustering of micellar systems by principal component analysis (26), the use of LSER parameters to compose vectors for characterization of lipophilicity scales (33), and, more recently, the establishment of micellar selectivity triangles (34,35) in analogy to the solvent selectivity triangle introduced by Snyder to classify solvents and ultimately mobile phases in liquid chromatography. 

Solvent strength and selectivity parameters based on Snyder s selectivity triangle. (Si is an empirical solvent strength parameter for reversed-phase chromatography)... 

Figure 7 Snyder s solvent selectivity triangle. Solvent are classified in eight groups (I-VIII), according to the proton acceptor (Xe), proton donor (Xj), and strong dipole (X,i) contribution.

A chemometric approach where the /ty-values of forty-seven flavonoids in seven TLC systems were studied using principal component and cluster analyses, has made it possible to choose the minimum number of chromatographic systems needed to perform the best separation (20). Another method (the PRISMA model) based on Snyder s solvent selectivity triangle has been described to aid mobile phase optimization (21). This model is reported to give good separation of flavonol glycosides from Betula spp. (1). When tested in our laboratory no improvements were obtained in comparison with established systems (22) such as the solvent ethyl acetate-formic acid-acetic acid-water (100 11 11 27) on silica support, which can be used for separation of a wide range of flavonoids. 

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... 

The solvent triangle classification method of Snyder Is the most cosDBon approach to solvent characterization used by chromatographers (510,517). The solvent polarity index, P, and solvent selectivity factors, X), which characterize the relative importemce of orientation and proton donor/acceptor interactions to the total polarity, were based on Rohrscbneider s compilation of experimental gas-liquid distribution constants for a number of test solutes in 75 common, volatile solvents. Snyder chose the solutes nitromethane, ethanol and dloxane as probes for a solvent s capacity for orientation, proton acceptor and proton donor capacity, respectively. The influence of solute molecular size, solute/solvent dispersion interactions, and solute/solvent induction interactions as a result of solvent polarizability were subtracted from the experimental distribution constants first multiplying the experimental distribution constant by the solvent molar volume and thm referencing this quantity to the value calculated for a hypothetical n-alkane with a molar volume identical to the test solute. Each value was then corrected empirically to give a value of zero for the polar distribution constant of the test solutes for saturated hydrocarbon solvents. These residual, values were supposed to arise from inductive and... 

There are several systems which can be used to select the solvents of the mobile phase. The number of selected solvents and the solvents which are selected not only depend on the chromatographic problem but also on the method which will be used to optimize the system. With response surface methodology it is appropriate to use a minimum number of solvents. For reasons stated below this minimum number of solvents was four. The second question is, which solvents will be selected, is more difficult to answer when a small number of solvents is used because the consequences of a wrong selection are large. Several approaches are possible to select the solvents. The most simple method is comparison with common solvent systems for the solutes under investigation. A more general approach is to use the selectivity triangle of Snyder [4] in the selection of the solvents. 

Snyder, Glajch and Kirkland [570) introduced two new parameters to describe the selectivity effects in the optimization triangle for LSC. If methylene chloride (MC), acetonitrile (ACN) and methyl t-butyl ether (MtBE) are used as the preferred modifiers in n-hexane, then an empirical solvent selectivity parameter (m) can be defined which is low for methylene chloride and can be made equal for the other two binary solvents. The latter is achieved by adding the appropriate amount of methylene chloride to the hexane-ACN binary. Addition of MC is required at any rate, because hexane and ACN are not miscible in all proportions. By definition we can assume m to equal zero for the hexane-MC binary mixture and m to equal one for the two other binaries. 

Organic solvent selection in LLE can be guided by using the Snyder triangle (Figure 9.2). In this triangle, solvents are classified according to their properties ... 

Figure 2.3 Solvent classification according to Snyder. The insert (figure 2.3a) shows how the location of a solvent in the triangle is related to the values for the selectivity parameters xe (eqn.2.15), xd (eqn.2.16) and x (eqn.2.17). For identification of classes see table 2.9. Figure taken from ref. [215]. Reprinted with permission.

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