Page 77 - Partition & Adsorption of Organic Contaminants in Environmental Systems

68 CONTAMINANT PARTITION AND BIOCONCENTRATION

consider the set of substituents as a whole rather than to treat them as a sum
of independent components, although the latter approach offers a quick rough
estimate of the K ow value of a solute with disubstituted or multisubstituted
substituents with respect to that of the parent solute (in this case, benzene).
The preceding analysis of the relationship between p X(oct-water) and D X
with benzene as a reference also applies reasonably well to many systems with
other compounds as reference standards. For instance, when aniline (logK ow
= 0.90) is used as the reference, the values of p X and D X are 0.98 and 0.98 for
X = Cl (meta); 0.39 and 0.42 for X = —CH 3 (ortho); and 0.50 and 0.46 for
X = —CH 3 (meta). If toluene (logK ow = 2.69) is used as the reference, p X and
D X are 0.43 and 0.48 for X = —CH 3 (ortho) and 0.51 and 0.49 for —CH 3 (meta).
These results agree with the earlier findings that the group contribution to K ow
derives essentially from the variation of solute incompatibility with water,
although p X may vary from one reference standard to another. Because of the
sensitivity of apolar heptane (or another highly nonpolar solvent) to the polar-
ity of the solute, a close relationship between p X(hep-water) and D X exists only
for nonpolar substituents (e.g., alkyl and halogen groups) with benzene and
other nonpolar compounds as the parent (reference) solutes.
As we have seen with the octanol–water and heptane–water systems, p X for
a substituent would be numerically close to the respective D X when the parent
solute and its derivatives exhibit comparable solubilities in the (water-
saturated) solvent and when the amount of the solvent dissolved in water is
not large enough to affect significantly the solute solubility in water. In those
solvent–water mixtures where the solvent–water mutual solubility is consid-
erable, the resulting p X values might thus deviate more significantly from the
respective D X values calculated from solute solubilities (or activity coefficients)
in water, although they might be correlated in some fashion with the corre-
sponding p X(oct-water) values for a series of substituted solutes. This is
because the high solvent–water mutual saturation can affect unequally the
solubilities of parent and substituted solutes in both water- and solvent-rich
phases [i.e., the log(g w/g* w) and logg* o terms in Eq. (5.1) can differ significantly
between the solutes].

5.6 LIPID–WATER SYSTEMS

5.6.1 Solubility of Solutes in Lipids

Knowledge of the partition behavior of compounds in lipid–water mixtures
forms a crucial link to the potential for bioconcentration of contaminants into
aquatic biotic species, such as fish, which constitutes an important part of our
biological resources. The lipid–water mixture is also a system of special inter-
est from the standpoint of solution theory because the molecular weights of
most biological lipids are considerably greater than those of ordinary solutes
and solvents but are substantially smaller than those of typical polymers. The