Hildebrand solubility parameter supercritical fluids

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. 

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. 

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. 

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 ... 

Choosing a supercritical fluid on the basis of its Hildebrand solubility parameter is pointless when different fluids are compared rather, it seems more appropriate to rely on the dipole moments of the fluids. Thus, based on its dipole moment, CHCIF, should be more suitable than CO, and N2O for extracting polar analytes — the opposite conclusion is reached if their solubility parameters are compared, however. 

They were the calculation of the Hildebrand solubility parameter as a function of density using tabulated thermodynamic data for carbon dioxide and Raman spectroscopy of test solutes dissolved in supercritical carbon dioxide compared to liquid solvents to evaluate solvent-solute interactions. The results of these recent approaches indicated that while the maximum solvent power of carbon dioxide is similar to that of hexane, probably somewhat higher, there is some solvent-solute interaction not found with hexane as the solvent. The limiting solvent power of carbon dioxide is resolved by choosing the alternative of a supercritical fluid mixture as the mobile phase. The component added to the supercritical fluid to increase its solvent power and/or to alter the chromatograph column is referred to as the "modifier."... 

The works of Giddings and coworkers in the late 1960s (1968, 1969) are, perhaps, the most well-known and most referenced papers on the extension of Hildebrand solubility parameters to supercritical fluids. We excerpt from Giddings et al. and we italicize parts of their statements and phrases to accent their feelings and intentions. We concentrate on the applicability of their methods for calculating solubility parameters of dense gases their telling statements are often not heeded. 

---