Page 38 - Partition & Adsorption of Organic Contaminants in Environmental Systems

COHESIVE ENERGY DENSITY AND SOLUBILITY PARAMETER 29

A related parameter, d, which is the square root of CED, has been widely
used to account for the compatibility of a liquid (or a supercooled liquid) with
others. The d parameter, developed initially by Hildebrand (Hildebrand and
Scott, 1964), is termed the solubility parameter of a substance:

d= (CED ) 12 (2.41)

The d values at room temperature for some hydrocarbons, halogenated hydro-
carbons, and compounds with polar groups are presented in Table 2.2. As
noted in Table 2.2, the d values for aliphatic hydrocarbons (e.g., n-pentane to
n-octane) are comparable in their magnitudes but are significantly smaller
than for aromatic hydrocarbons (e.g., benzene, naphthalene, and phenan-
threne). This is because aromatic compounds with labile p electrons are more
polarizable, thus promoting the molecular attraction by London forces. Mean-
while, the d values for small polar liquids, such as alcohols, ketones, and
nitrogen-containing aromatics, are considerably higher than for aliphatic
liquids, because the polar or H-bonding force adds to the London force, the
net effect being more pronounced for small molecules than for large mole-
cules. It is also worth noting that aliphatic hydrocarbons substituted with halo-
gens (except F) show a significant increase in d. This may be reasoned on the
basis that the large halogen atoms (e.g., Cl and Br) of the substituted mole-
cules contain many labile outer-shell electrons, making the compounds more
easily polarizable as well as enabling them to form dipole moments if their
electron clouds become unevenly oriented.
Strictly speaking, the concept of solubility parameters as a criterion for
compatibilities of two components in a solution is followed strictly only when
the same molecular forces are operative for two components. It thus works
well either among nonpolar liquids or among those polar liquids with the same
or similar polar functional groups that respond with the same principal molec-
ular forces. Finally, for macromolecules or polymers, it is difficult to determine
CED or d directly from Eq. (2.40) or (2.41) because of their nondetectable
vapor pressures. In this case, the CED or d values are usually estimated from
their solution properties with suitable solvents. As many polymers or macro-
molecules often possess large polar and apolar domains, the estimated d values
depend strongly on the polarity of the solvent used (Barton, 1975). Therefore,
the d values reported for polymers may fall into a range rather than being dis-
crete values.