Solubility

Of the solvent properties necessary for extraction, solubility of the solute into the solvent is of fundamental importance. The general understanding of 'like dissolves like' is handy in the preliminary choice of extraction solvents. Solvent classification schemes are often helpful, especially if the selectivity of solvents is of interest. The Snyder selectivity triangle results in eight classifications of solvents according to proton donor, proton acceptor, and dipole interaction properties. Another solvent classification scheme is as follows.

• Class 1 solvents: capable of forming three-dimensional networks of strong hydrogen bonds.

• Class 2 solvents: have active hydrogen atoms and donor atoms, but do not form three-dimensional networks.

• Class 3 solvents: contain donor atoms, but not active hydrogen atoms.

• Class 4 solvents: contain active hydrogen atoms, but not donor atoms.

• Class 5 solvents: do not have hydrogen-bonding capability or donor atoms.

Comparisons of solubility values can give approximations for the partitioning of a solute between two solvents. Although experimentally generated solubility data is preferred, relative solubility scales can be used for estimation purposes. However, it is important to remember that no scale has been developed that completely accounts for all of the intermolecular interactions influencing solubility.

The most common relative solubility scale is the Hildebrand solubility parameter scale. The Hildebrand solubility parameter, 5, is a measure of the cohesion (interaction) energy of the solvent-solute mixture and is defined by 5 = (AEv/V)1/2, where Ev is the heat (energy) of vaporization necessary for volume V. Thus, the ratio AEv/V is the cohesive energy density. The 'total' Hildebrand solubility parameter (5t) is related to the hydrogen-bonding ability (5h), the dispersion coefficient (5d), and the polarity (5p) by 52 = 5h + 52 + 5p. Consequently, there is a strong correlation between the Hildebrand solubility parameter value and the polarity. For extraction purposes, it is preferable to use solvents that have 5 values similar to those of the solutes of interest. Several references provide detailed development of the Hildebrand solubility parameter, and similar scales, and these values are tabulated for several solvents.

When supercritical fluids are used in place of liquids, modified versions of the Hildebrand solubility parameter are used in which 5 = 1.25Py2(p/pliq) or 5 = 5liq(p/pliq), where Pc is the critical pressure, p is the density, and 5liq and pliq are the Hildebrand value and density at liquid conditions. This modification for supercritical fluids, while only approximate, provides for reference to liquid values of polarity and other 'chemical' properties, and for the relationship between supercritical density and solubility.

Solar Panel Basics

Solar Panel Basics

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