Solvating power, scale

Steady-State Solvatochromism. The majority of the reports on supercritical fluid solvation have used steady-state solvatochromic absorbance measurements (21-28). The original aim of these experiments was to determine the solvating power of supercritical fluids for chromatography and extraction (SFC and SFE) (26,28). To quantify solvent strength, researchers (21-28) adopted the Kamlet-Taft x solvent polarity scale (50-55). This scale best correlates solvatochromic effects on a- x and x- x electronic absorption transitions. 

The wide variety of possible solvent-solute interactions requires that any scale used to quantify solvent properties will be complex. Unfortunately, no universally accepted scale of solvating power has been devised. It does not seem reasonable to develop an entirely new scale for supercritical fluid solvents, especially since it is desirable to compare the solvent behavior of supercritical fluids with that of liquid solvents. 

Solvent polarity is very difficult to define, but essentially refers to the solvation power of a solvent. Quantitative determination of solvent polarity is equally difficult, and quantitative methods rely on physical properties such as dielectric constant, dipole moment, and refractive index. It is not possible to determine the solvent polarity by measuring an individual solvent property due to the complexity of solute-solvent interactions and for this reason empirical scales of solvent polarity, based on chemical properties, are most widely used. The principal properties used to estimate solvent polarity are summarized in Table 2 and the most important of these methods are embellished below. 

Steam distillation is the main commercial extraction procedure for the production of essential oils from almost any type of plant material. Solvent extraction is also used commercially and yields a resinoid, concrete or absolute according to the solvents and techniques used (see Chapter 4). Both steam distillation and solvent extraction are used on a laboratory scale to produce oils and extracts for analysis. Other methods of extraction, such as supercritical fluid extraction (SFE), which uses supercritical CO2 as the extraction solvent, are now being developed and used on both commercial and laboratory scales. The extracts produced by SFE may contain different materials from the steam-distilled oil because of the solvating power of C02 and the lower extraction temperature, which reduces thermal degradation. The C02 extract may therefore have an odour closer to that of the original material and may contain different fragrant compounds. The choice of extraction procedure depends on the nature and amount of material available, and the qualities desired in the extract. Solvent extraction is better suited to small sample amounts or volatile materi-... 

The fundamentals of miscibility (solvation power, E ) of various solvents from nonpolar, aprotic tetramethylsilane (TMS with 0 as defined) to polar water E = 1) are given by the solvent polarity scale in Figure 5 [5]. 

Research workers investigating the solvent effect selected model systems with some well measurable property (e.g., light absorption in the UV, visible or IR spectrum, heat of formation, an NMR, Mossbauer or NQR parameter, the redox potential, reaction rate, etc.) which changes appreciably due to the effect of the solvent. Hence, these experimentally measurable data, characteristic of the extent of the interaction between the solvent and the solute, may serve to categorize the solvating powers of solvents. Of course, solvent scales obtained in this way can be compared with one another only if the solvation process in the different model systems is governed by analogous factors. 

The scale frequently used to characterize the solvating power (the acceptor strength) of an acceptor solvent is the Kosower Z scale [Ko 58]. The procedure is based on the fact that the cation of the l-ethyl-4-carbomethoxypyridinium iodide ion pair used as the model system is not an electron pair acceptor, while the iodide anion is capable of hydrogen bonding. Consequently, the extent of ion pair formation between the l-ethyl-4-carbomethoxypyridinium cation and the iodide ion in solutions prepared with various solvents depends on the solvation of the iodide ion. 

Nevertheless, the solvatochrome effect has served as the basis of a number of scales characterizing the solvating powers of solvents. These include the Kosower Z scale and the Dimroth-Reichardt Ej scales, dealt with in Chapter 4. 

As clearly shown by the examples in Chapters 2 and 3, the solvating power of a solvent is the resultant of a combination of several specific and non-specific interactions. It is diflRcult to differentiate these from one another. This is the reason why so many different types of empirical solvent scales have been proposed for the characterization of the solvating power. 

The simplest and easiest-to-use tools for the interpretation of solvent effects are the phenomenological models, which rely on a very simple description of the solvent in terms of one (or more) empirical parameter, so that the solvation effects are interpreted in terms of the strength of the solvents on the basis on the values of such parameters. As a consequence, the different solvents are classilied in scales, on the basis of the value of the chosen parameters, and the experiments are interpreted consequently. One very popular approach tries to rationalise the solvent effects in terms of the solvent polarity, empirically defined as the overall solvation power, which can be estimated with the dielectric function or using other more refined and more case-specific approaches. 

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