Supercritical fluids solubility parameter

Solubility Parameters of the Most Common Fluids for Supercritical Fluid Extraction and Chromatography Solubility Parameters of Supercritical Fluids Solubility Parameters of Liquid Solvents Instability of Modifiers Used with Supercritical Fluids... 

Following this, elastomers can be swollen by some high-pressure gases (especially CO2) as the densities of these gases approach liquid-like levels, at appropriate temperatures they become supercritical fluids which possess a solubility parameter magnitudes that, however, are highly dependent on temperature and pressure... 

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. 

A gradient that runs with 30-80% methanol or acetonitrile is not uncommon. This amount of modifier is generally not needed in supercritical fluid chromatography to affect the same separation. Typical modifier composition in SFC is 1.0-10% and would achieve higher Hildebrand Solubility Parameter adjustment overall than the broader gradients found in LC. 

Important solvent properties of SC-CO2 (e.g., dielectric constant, solubility parameter, viscosity, density) can be altered via manipulation of temperature and pressure. This unique property of a supercritical fluid could be exploited to control the behavior (e.g., kinetics and selectivity) of some chemical processes. 

The evaluation of the sublimation pressure is a problem since most of the compounds to be extracted with the supercritical fluids exhibit sublimation pressures of the order of 10 14 bar, and as a consequence these data cannot be determined experimentally. The sublimation pressure is thus usually estimated by empirical correlations, which are often developed only for hydrocarbon compounds. In the correlation of solubility data this problem can be solved empirically by considering the pure component parameters as fitting-parameters. Better results are obviously obtained [61], but the physical significance of the numerical values of the parameters obtained is doubtful. For example, different pure component properties can be obtained for the same solute using solubility data for different binary mixtures. 

The advantage of this extraction method is that the parameters pressure, temperature and solvent to feed ratio can be varied in each extraction step. By this way a very accurate fractionation of the different compounds included in the feed can be achieved. The solubility of the compounds in the supercritical fluid, depending on pressure and temperature, can be changed in each extraction step. The highly soluble substances are extracted in the first step at low fluid density. Increasing the density in the following extraction steps leads to the removal of the less soluble substances. Further, the flow rate of the supercritical fluid can be adjusted in each extraction step, either constant flow for each step or different flow rates, depending on the separation to be achieved. 

Polarity of Supercritical Fluids. In order to successfully perform any chromatographic separation, the analytes must be sufficiently soluble in the mobile phase. Efforts to ensure solubility have often been based on matching the polarity of the sample components and the mobile phase. For pure fluids, Giddings has reported a polarity classification based on the solubility parameter (18). In contrast to essentially constant values for incompressible fluids (liquids), the solubility parameter (mobile phase strength) of a supercritical fluid varies with its density. 

SOLUBILITY PARAMETERS OF THE MOST COMMON FLUIDS FOR SUPERCRITICAL FLUID EXTRACTION AND CHROMATOGRAPHY... 

The following table provides the solubility parameters, 8, for the most common fluids and modifiers used in supercritical fluid extraction and chromatography. The data presented in the first table are for carrier or solvent supercritical fluids at a reduced temperature, T of 1.02 and a reduced pressure, P of 2. These values were calculated with the equation of Lee and Kesler.1 2 The data presented in the second table are for liquid solvents that are potential modifiers.3... 

In this table, we provide solubility parameters for some liquid solvents that can be used as modifiers in supercritical fluid extraction and chromatography. The solubility parameters (in MPa1/2) were obtained from reference 3, and those in cal1/2cm 3/2 were obtained by application of Equation 4.1 for consistency. It should be noted that other tabulations exist in which these values are slightly different, since they were calculated from different measured data or models. Therefore, the reader is cautioned that these numbers are for trend analysis and separation design only. For other applications of cohesive parameter calculations, it may be more advisable to consult a specific compilation. This table should be used along with the table on modifier decomposition, since many of these liquids show chemical instability, especially in contact with active surfaces. 

The E s of the nonpolar solvents, CF3CI and C2H4, become equal to tnat of n-hexane at a pressure in the range of 1-2 kilobar. Notice that the Hildebrand solubility parameters of these three solvents are roughly equivalent at this condition of constant E. The same result is also observed for the polarizabilities/ volume of these solvents. Again, the molar densities of these supercritical fluids are considerably higher than that of n-hexane at this equivalence point in solvent strength, since the polarizabilities/molecule are lower. 

Table 2 shows critical parameters of the fluids most used for SFE. When it comes to choosing a supercritical fluid, the critical pressure and the critical temperature are two important parameters. The critical pressure determines, from a first approximation, the importance of the solvent power of the fluid. Ethane, for example, which has a lower critical pressure than carbon dioxide, will not dissolve a moderately polar soluble in the same way as carbon dioxide. Similarly, fluids with a higher critical pressure are more able to dissolve polar compounds. The critical temperature has practical implications. Indeed, one should always consider the influence of the extraction temperature on the stability of the component to extract. 

Thermodynamic Properties The variation in solvent strength of a supercritical fluid from gaslike to liquidlike values may be described qualitatively in terms of the density, p, or the solubility parameter, 5 (square root of the cohesive enei density). It is shown for gaseous, liquid, and SCF CO2 as a function of pressure in Fig. 22-17 according to the rigorous thermodynamic definition ... 

The Hildebrand solubility parameter, 6, is a semi-quantitative entity related to the thermodynamic properties of dense gases (supercritical fluids) and solutions.t The solubility parameter in calories per cubic centimeter is calculated from the equation ... 

From Eqs. 1-3, the following relationship can be derived to relate the Hildebrand solubility parameter with the density of supercritical fluid. 

Equation 4 was used to calculate the Hildebrand solubility parameters for various supercritical fluids. Here p is the density of the supercritical fluid which is related to the pressure and temperature as described earlier. 

People have taken other approaches to understanding the solubilizing characteristics of a SCF. One common method is solubility parameters. The solubility parameter, 6, is the square root of the internal pressure, or cohesive energy density of a liquid. This concept has been modified for a supercritical fluid to be the internal energy of the SCF relative to the isothermally expanded ideal gas state as shown in Eq. 3 by S. R. Allada.1 1... 

The solubility parameter model has difficulty with temperature effects and also fails to predict solubilities in several instances, such as with silicones. However it is a good starting point for estimating the solubility characteristics of a SCF as a function of temperature and pressure. The most likely temperatures and pressures under which a material is soluble in a supercritical fluid are where the solubility parameters are within a value of unity of each other. See Fig. 1, taken from Fig. 2 of Ref 5 by Allada, for a graph of 6 versus T and P for CO2. This effect allows one to selectively remove a particular component from a material by tuning the 5 of the SCF using T and P. 

The solubility of a solute in a supercritical fluid can be quantitatively estimated using Giddings theory, which relies on differences between the Hildebrand solubility parameters for the SF and solute concerned. Solubility in a supercritical fluid can be understood by examining the Gibbs-Helmholtz equation ... 

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