Page 84 - Partition & Adsorption of Organic Contaminants in Environmental Systems

LIPID–WATER SYSTEMS 75

triolein. By Eq. (5.15), since /VV * t is neither constant nor approaching zero, no
single ideal line relating logK tw versus logS w V or logK tw versus logS w can be
established to describe the solute incompatibility with triolein (c/2.303). The
magnitude of c/2.303 can only be determined individually for each solute by
reference to Eq. (5.15), in which log(g w/g* w) is neglected for approximation.
Calculated c/2.303 values are generally quite small yet positive (<0.25) for
benzene derivatives with relatively large logS w; respective values for larger,
less-soluble solutes, such as 2,4¢-PCB (0.32), 2,4,4¢-PCB (0.81), hex-
achlorobenzene (0.45), and DDT (1.1), are somewhat greater. True c/2.303
values for latter nonpolar solutes are probably smaller because of the neglect
of log(g w/g* w), which may be significantly greater than zero because of the
sensitivity of their very small water solubilities to a small amount of triolein
dissolved in water. However, since the sum of c/2.303 and log(g w/g* w) is never-
theless small compared to -logS w for all solutes and since the variation of
V should also be relatively small, logS w is evidently the principal determinant
of logK tw. In this sense, the triolein–water system is quite similar to the
octanol–water system as far as the solute partition is concerned. Therefore, the
K tw values exhibit similar magnitudes as the corresponding K ow values for all
solutes.
The noted small differences between K tw and K ow values appear to be
related to the solute size. The logK tw values for all simple nonpolar benzene
derivatives tend to be slightly greater than logK ow values, by 0.1 to 0.2. By
contrast, logK tw values of larger nonpolar solutes (hexachlorobenzene, some
PCBs, and DDT) are either about the same as or smaller than the respective
logK ow values. The first result manifests that triolein is somewhat less
polar than octanol, so that the solution of low-polarity solutes with water-
saturated triolein is closer to being ideal or athermal than that with water-
saturated octanol. The second result is not well understood. In addition to the
effect of c on K tw, the transition in order between K tw and K ow could be related
to the solute water-solubility enhancement, in which the log(g w/g* w) term is
much greater with dissolved triolein in water (at low mg/L levels) than with
dissolved octanol in water (585mg/L). This could potentially happen because
triolein is eight times larger in size and is somewhat less polar than octanol
(which would make triolein a more effective solubility enhancer per unit
weight), although the dissolved triolein mass in water is far less than the dis-
solved octanol mass. The problem remains to be resolved. The influence of
the polarity and molecular size of a dissolved organic matter on solute water
solubility is studied in more detail in Chapter 7, where a series of natural
macromolecules and synthetic organic materials are employed as the dissolved
organic matter.
Because logS w is the major factor for both logK tw and logK ow, logK tw can
be estimated in terms of logS w (or logS w ) and logK ow. Plots of logK tw versus
V
logS w V and logK tw versus logK ow are given in Figures 5.5 and 5.6, respectively.
The results show that although logK tw is closely related to logS w V and to
logK ow, the correlations show a noticeable curvilinearity when logK tw or