Page 71 - Partition & Adsorption of Organic Contaminants in Environmental Systems

62 CONTAMINANT PARTITION AND BIOCONCENTRATION

Because of the noted large variation in the response of nonpolar versus polar
solutes to a nonpolar medium, the heptane–water system differs in one impor-
tant respect from the octanol–water system: namely, that the solute water sol-
ubility (S w) has a predominant effect on K hw only for relatively nonpolar solutes.
This point is corroborated further in the later section by the effects of some
polar and nonpolar substituents in benzene on the partition coefficients of
substituted benzenes in heptane–water and octanol–water systems.

5.4 BUTANOL–WATER SYSTEMS

The n-butanol/water mixture represents an opposite extreme to the
heptane–water system, in which the solvent phase is remarkably more polar
than apolar heptane or weakly polar octanol. Note that butanol is the lowest-
molecular-weight alcohol, whose polarity stays just below the level for it to be
partially miscible with water; methanol, ethanol, and propanol are completely
miscible with water. The solubility of butanol in water is 1.1M and the solu-
bility of water in butanol is 9.4M at room temperature. This relatively high
mutual solubility affects not only the molar volumes of the two solvent phases
(water and butanol) but also, more critically, the solubility behavior of the
solutes in the two equilibrium phases.
Although the experimental butanol–water partition coefficients (K bw) are
quite limited in number, the magnitude of K bw relative to K ow may be under-
stood in terms of the solvent polarity and solvent–water mutual solubility.
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Dissolved water in octanol at 4.5 ¥ 10 M is known to reduce the solubility of
DDT and hexachlorobenzene (two of the most water-insoluble compounds)
in water-saturated octanol relative to that in pure octanol by about 20%
(Chiou et al., 1982b). The related effect for more water-soluble solutes is
expected to be less. Since the dissolved water in butanol at 9.4M is about 2000
times greater than in octanol and since butanol is much more polar a solvent
than octanol, the solubility of nonpolar solutes in water-saturated butanol
should be significantly lower than that in water-saturated octanol. Similarly,
the greater (but not large) amount of butanol than octanol in water should
enhance to a greater extent the solubility of nonpolar solutes in water than
that exhibited by the dissolved octanol. Thus, for nonpolar solutes, K bw < K ow
is expected. On the other hand, for relatively polar or water-soluble solutes,
one may expect K bw > K ow to occur, although the difference may not be very
substantial, mainly because the water-saturated butanol may act as a better
partition phase than water-saturated octanol for solute partitioning; for such
solutes, the solubility enhancement in water by either dissolved butanol or
dissolved octanol would not be significant. Overall, the relatively high
butanol–water mutual solubility would result in a large compression of the
range of K bw relative to that of K ow for solutes that span a wide range of water
solubility; the effect should be most noticeable for highly water-insoluble
solutes because their water solubilities are sensitive to a dissolved organic
solvent.