Page 67 - Partition & Adsorption of Organic Contaminants in Environmental Systems

58 CONTAMINANT PARTITION AND BIOCONCENTRATION
(l)
rather than their supercooled-liquid logS w of -4.48 and -5.24mol/L, in the
logK ow –logS w plot, the resulting data points would shift toward the left by
0.66 and 0.94 unit, respectively, which would then separate these data points
from the rest and reduce the overall logK ow –logS w correlation. Another vivid
example of the melting-point effect on S w , and thus on the logK ow –logS w
correlation, is illustrated by the S w and K ow data of m-dichlorobenzene
and p-dichlorobenzene (Table 5.1). At room temperature, the S w of the
para isomer, which is a solid, is about half that of the meta isomer, which is a
liquid, but the supercooled-liquid para isomer and the liquid meta isomer
(l)
exhibit about the same S w and hence about the same K ow , as expected. The
same is true for phenanthrene and anthracene at room temperature in that
the two solid isomers differ significantly in melting point (101 and 216°C,
(s)
respectively) and in water solubility (logS w =-5.14 and -6.60, respectively)
(l)
but they display comparable supercooled-liquid S w and thus K ow , as shown in
Table 5.1.
The observed logK ow –logS w correlation [Eq. (5.3)] together with the
octanol–water ideal line [Eq. (5.2)] provides an effective means to account for
the change in solute solubility in the octanol phase with increasing solute log
K ow or decreasing solute logS w . By treating octanol as a lipidlike substance,
as substantiated later, one can see how the lipophilicity of a group of solutes
varies with S w . The lipophilicity of a solute should in a strict sense be related
to the inverse of g* o . The logg* o values of most solutes, except highly insoluble
ones such as DDT and HCB, are simply equal to the vertical distances between
the ideal line and the experimental line in Figure 5.1. As noted, this vertical
distance increases with decreasing S w . This implies that in a homologous series
of solutes, the higher-molecular-weight, less water-soluble compounds (i.e.,
the ones with larger K ow values) are not more lipophilic than the more
water-soluble compounds. According to the g* o data, the solute affinity with
octanol decreases with increasing K ow (or decreasing S w ), indicating that
there is actually an increase in solute–octanol incompatibility as the solute
molecular-weight increases. In essence, the higher K ow values, or lipid–water
partition coefficients, for the latter solutes result from their much lower S w
values rather than from their enhanced solubilities in octanol (or a lipid). To
avoid the confusion of the term lipophilicity or lipophilic being used to refer
to a compound, one must keep in mind that it only implies that the compound
has a high lipid–water partition coefficient (i.e., its solubility in lipids is sig-
nificantly higher than that in water). Thus, although all compounds with
low S w values tend to be lipophilic, their solubilities in lipids usually bear no
direct relation to the order of their solvent–water or lipid–water partition
coefficients.
The correlation presented in Eq. (5.3) has also been found to give a rea-
sonable account of the partition coefficients for many other classes of organic
compounds, including moderately soluble alcohols, ketones, and ethers and
sparingly soluble esters, alkyl halides, alkanes, and alkenes (Chiou et al.,
1982b). This wide correlation for solutes of many classes presumably results