Classification of solute-solvent interactions

The dielectric constant and refractive index parameters and different functions of them that describe the reactive field of solvent [45] are insufficient to characterize the solute-solvent interactions. For this reason, some empirical scales of solvent polarity based on either kinetic or spectroscopic measurements have been introduced [46,47]. The solvatochromic classification of solvents is based on spectroscopic measurements. The solvatochromic parameters refer to the properties of a molecule when its nearest neighbors are identical with itself, and they are average values for a number of select solutes and somewhat independent of solute identity. 

An example of an application is shown in Fig. 30.10. This concerns the classification of 42 solvents based on three solvatochromic parameters (parameters that describe the interaction of the solvents with solutes) [13]. Different methods were applied, among which was the average linkage method, the result of which is shown in the figure. According to the method applied, several clusterings can be found. For instance, the first cluster to split off from the majority of solvents consists of solvents 36, 37, 38, 39, 40, 41, 42 (t-butanol, isopropanol, n-butanol. 

Classification of Solvents in Terms of Specific Solute-Solvent Interactions Parker divided solvents into two groups according to their specific interactions with anions and cations, namely dipolar aprotic solvents and protic solvents (Parker, 1969). The distinction lies principally in the dipolarity of the solvent molecules and their ability to form hydrogen bonds. It appears appropriate to add to these two groups a third one, namely, the apolar aprotic solvents. 

Solvents can be classified as EPD or EPA according to their chemical constitution and reaction partners [65]. However, not all solvents come under this classification since e.g. aliphatic hydrocarbons possess neither EPD nor EPA properties. An EPD solvent preferably solvates electron-pair acceptor molecules or ions. The reverse is true for EPA solvents. In this respect, most solute/solvent interactions can be classified as generalized Lewis acid/base reactions. A dipolar solvent molecule will always have an electron-rich or basic site, and an electron-poor or acidic site. Gutmann introduced so-called donor numbers, DN, and acceptor numbers, AN, as quantitative measures of the donor and acceptor strengths [65] cf. Section 2.2.6 and Tables 2-3 and 2-4. Due to their coordinating ability, electron-pair donor and acceptor solvents are, in general, good ionizers cf. Section 2.6. 

Classification of Solvents in Terms of Specific Solute/Solvent Interactions... 

Solvent classification In acid-base reactions the solvent plays an active or specific role in two ways it may react generally with ions and molecules (solvation), and as indicated above, it has acidic and basic properties that are of active concern. Broadly, solute-solvent interactions are studied by electrical and spectral methods. ... 

Solvents may also be classified according to their acid-base properties and in terms of specific solute-solvent interactions. These various classification methods are summarized in Figure 2. The listed classifications facilitate the selection of the appropriate solvent to dissolve a compound, i.e., a solvent of low polarity dissolves covalent compounds of low polarity whereas a highly polar solvent dissolves ionic compounds. 

The symbols quoted there, e.g. MeOH, THF, DME etc., are used in this text. No classification is universally applicable. Overlapping of the solvent classes is inevitable and some specific solute-solvent interactions evade classification. Specific interactions, however, are often sought in connexion with technological problems and have led to a arch for appropriate solvent mixtures which are gaining importance in many fields of applied research. In spite of all its limitations, the classification of solvents is useful for rationalizing the choice of appropriate solvents and solvent mixtures for particular investigations. 

Despite such limitations as the overlapping of solvent classes or possible interactions evading the unambiguous classification of a solvent, such classifications are useful for understanding the properties of electrolyte solutions and for rationalizing the choice of appropriate solvents and solvent mixtures for particular investigations. 

Fundamental principles governing the use of solvents such as chermcal stractine, molecular design, basic physical and chemical properties, as well as classification of inter-molecular solute/solvent interactions, modeling of solvent effects, and solvent influence... 

The solvatochromic classification of solvents takes into consideration only the polar interactions of the solvents and not their cohesion. The transfer of a solute from one solvent to another occurs with the cancellation of dispersion interactions [38]. 

Fig. 30.10. Hierarchical agglomerative classification of solvents according to solvent-solute and solvent-solvent interactions [13].

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... 

Kg is the experimental distribution coefficient and K g the corrected value. This correction is required, because any measure for the interactions that occur in certain solvents should be more related to the ratio of mole fractions than to the ratio of concentrations of the solute in the liquid phase and in the gas phase. We may assume the molar volume of the gas phase to be constant and hence irrelevant if our purpose is a classification of solvents. However, the molar volumes of solvents vary a great deal. The Kg values for n-octane in various hydrocarbon solvents vary up to a factor of 3.9 between cyclohexane and squalane [216]. The Kg values vary by a more realistic factor of 1.5 [214]. 

A knowledge of the physico-chemical principles of solvent effects is required for proper bench-work. Therefore, a description of the intermolecular interactions between dissolved molecules and solvent is presented first, followed by a classification of solvents derived therefrom. Then follows a detailed description of the influence of solvents on chemical equilibria, reaction rates, and spectral properties of solutes. Finally, empirical parameters of solvent polarity are given, and in an appendix guidelines to the everyday choice of solvents are given in a series of Tables and Figures. 

In capillary electrophoresis (CE), several criteria can be applied to classify solvents [e.g., for practical purposes based on the solution ability for analytes, on ultraviolet (UV) absorbance (for suitability to the UV detector), toxicity, etc.]. Another parameter could be the viscosity of the solvent, a property that influences the mobilities of analytes and that of the electro-osmotic flow (EOF) and restricts handling of the background electrolyte (BGE). For more fundamental reasons, the dielectric constant (the relative permittivity) is a well-recognized parameter for classification. It was initially considered to interpret the change of ionization constants of acids and bases according to Born s approach. This approach has lost importance in this respect because it is based on too simple assumptions limited to electrostatic interactions. Indeed, a more appropriate concept reflects solvation effects, the ability for H-bonding, or the acido-base property of the solvent. 

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