McKay, Donald. "Front matter" Multimedia Environmental Models ...

(1982), Baum (1997) and Leo (2000). Extensive databases are also available as
reviewed by Baum (1997). Octanol was selected because it has a similar carbon to
oxygen ratio as lipids, is readily available in pure form, and is only sparingly soluble
in water (4.5 mol/m 3 ). The solubility of water in octanol of 2300 mol/m 3 , however,
is quite large (Baum, 1997). The molar volumes of these phases are 18 ¥ 10 –6 m 3 /mol
and 120 ¥ 10 –6 m 3 /mol, a ratio of 0.15. It follows that K OW is 0.15 g W/g O.
K OW is a measure of hydrophobicity, i.e., the tendency of a chemical to “hate”
or partition out of water. As was discussed earlier, it can be viewed as a ratio of
solubilities in octanol and water but, in most cases of liquid chemicals, there is no
real solubility, because octanol and the liquid are miscible. The “solubility” of
organic chemicals in octanol tends to be fairly constant in the range 200 to 2000
mol/m 3 , thus variation in K OW between chemicals is primarily due to variation in
water solubility. It is therefore misleading to assert that K OW describes lipophilicity
or “love for lipids,” because most organic chemicals “love” lipids equally, but they
“hate” water quite differently. Viewed in terms of Z values, K OW is Z O/Z W. Z O is
(relatively) constant for organic chemicals; however, Z W varies greatly and is very
small (relatively) for hydrophobic substances.
Because K OW varies over such a large range, from approximately 0.1 to 10 7 , it
is common to express it as log K OW. It is a disastrous mistake to use log K OW in a
calculation when K OW should be used!
K OW is usually measured by equilibrating layers of water and octanol containing
the solute of interest at subsaturation conditions and analyzing both phases. If K OW
is high, the concentration in water is necessarily low, and even a small quantity of
emulsified octanol in the aqueous phase can significantly increase the apparent
concentration. A “slow stirring” method is usually adopted to avoid emulsion formation.
An alternative is to use a generator column in which water is flowed over
a packing containing octanol and the dissolved chemical.
5.4.4 Octanol-Air Partition Coefficients
This partition coefficient is invaluable for predicting the extent to which a
substance partitions from the atmosphere to organic media including soils, vegetation,
and aerosol particles. It can be estimated as K OW/K AW or measured directly,
usually by flowing air through a column containing a packing saturated with octanol
with the solute in solution. Values of K OA can be very large, i.e., up to 10 12 for
substances of very low volatility such as DDT, and values are especially high at low
temperatures. Harner et al. (2000) have reported data for this coefficient and cite
other data and measurement methods.
5.4.5 Solubility in Water
This property is of importance as a measure of the activity coefficient in aqueous
solution, which in turn affects air-water and octanol-water partitioning. It can be
regarded as a partition coefficient between the pure phase and water, but the ratio
of concentrations is not calculated. A comprehensive discussion is given in the text
by Yalkowsky and Banerjee (1992), and estimation methods are described by Mackay
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