Solvation power

Gas-expanded liquids (GXLs) are emerging solvents for environmentally benign reactive separation (Eckert et al., op. cit.). GXLs, obtained by mixing supercritical CO2 with normal liquids, show intermediate properties between normal liquids and SCFs both in solvation power and in transport properties and these properties are highly tunable by simple pressure variations. Applications include chemical reactions with improved transport, catalyst recycling, and product separation. 

High solvating power, tuneable via mechanical compression... 

A single SFE/ESE instrument may perform (i) pressurised C02 (SFE), (ii) pressurised C02/modifier and (iii) pressurised modifier (i.e. ASE /ESE , organic solvent) extractions. The division between SFE and ASE /ESE blurs when high percentages of modifier are used. Each method has its own unique advantages and applications. ESE is a viable method to conduct matrix/analyte extraction provided a solvent with good solvating power for the analyte is selected. Sample clean-up is necessary for certain matrix/analyte combinations. In some circumstances studied [498], SFE may offer a better choice since recoveries are comparable but the clean-up step is not necessary. 

On-line SFE coupled to GC or SFC, according to the thermal stability of the analytes, are both very competitive with classical methods of analysis in terms of sensitivity and analysis time. Since all of the extracted analytes are transferred to the GC system, much higher method sensitivities can be obtained. Several modes of operation are possible utilising on-line SFE-GC, including quantitative extraction of all analytes from a sample matrix quantitative extraction and concentration of trace analytes selective extractions at various solvating powers to obtain specific fractions and periodic sampling (multiple-step extractions) of the effluent at various pressures for qualitative characterisation of the sample matrix. 

In the second place, it must still be recognized that the solvating power of a... 

The so-called G-H solvents were introduced by Palit et al.90 they usually consist of 1 1 mixtures of ethylene or propylene glycol and isopropyl or ra-butyl alcohol (with use of the same mixtures in the titrants) the glycol acts as a solvent for polar groups and the alcohol for the non-polar hydrocarbon groups, which explains the solvating power of their mixtures for both salts and soaps, whereas the alcohol chosen is beneficial for wider pH ranges (see Figs. 4.1 and 2). 

In the radical anions of the norbornane-linked naphthalenes [37] mentioned earlier (Gerson et al, 1990) no counterion effects were detected for [37a], which has a small spatial separation, but the esr/ENDOR spectra of [37b]- and [37c]- indicate that the electron-spin transfer between the naphthalene moieties is determined by the rate of synchronous counterion migration (Gerson et al., 1990). For tight ion pairs the electron is localized, while for loose ion-pair conditions, e.g. by using solvents of high cation-solvating power, the transfer becomes fast on the hyperfine timescale (k > 107 Hz). 

However, unlike most conventional solvents, many ionic liquids combine high solvating power for polar catalyst complexes (polarity) with weak coordination (nucleophilicity) [38], It is this combination that enables a biphasic reaction mode with these ionic liquids even for catalyst systems which are deactivated by water or polar organic solvents. 

With traditional solvents, the solvent power of a fluid phase is often related to its polarity. Compressed C02 has a fairly low dielectric constant under all conditions (e = 1.2-1.6), but this measure has increasingly been shown to be insufficiently accurate to define solvent effects in many cases [13], Based on this value however, there is a widespread (yet incorrect ) belief that scC02 behaves just like hexane . The Hildebrand solubility parameter (5) of C02 has been determined as a function of pressure, as demonstrated in Figure 8.3. It has been found that the solvent properties of a supercritical fluid depend most importantly on its bulk density, which depends in turn on the pressure and temperature. In general higher density of the SCF corresponds to stronger solvation power, whereas lower density results in a weaker solvent. 

---