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

The temperature coefficient is the enthalpy of phase transfer, e.g., pure solute to
solution for solubility or from solution to solution for partition coefficients. The
enthalpies must be consistent around the cycle air-water-octanol such that their sum
is zero. This provides another consistency check. It should be noted that the enthalpy
change refers to the solubility or partition coefficient variation when expressed in
mole fractions, not mol/m 3 concentrations. This is particularly important for partitioning
to air, where a temperature increase causes a density decrease, thus C or S
will fall while x remains constant. For details of the merits of applying the “three
solubility” approach, the reader is referred to Cole and Mackay (2000). We discuss
these partition coefficients individually in more detail in the following sections.
5.4.2 Air-Water Partitioning
The nature of air-water partition coefficients or Henry’s law constants has been
reviewed by Mackay and Shiu (1981), and estimation methods have been described
by Mackay et al. (2000) and Baum (1997), and only a brief summary is given here.
Several group contribution and bond contribution methods have been developed,
and estimation methods are available from websites such as the EPIWIN programs
of the Syracuse Research Corporation site at www.syrres.com. As was discussed
above, the simplest method of estimating Henry’s law constants of organic solutes
is as a ratio of vapor pressure to water solubility. It must be recognized that this
contains the inherent assumption that water is not very soluble in the organic
material, because the vapor pressure that is used is that of the pure substance
(normally the pure liquid) whereas, in the case of solubility of a liquid such as
benzene in water, the solubility is not actually that of pure benzene but is inevitably
of benzene saturated with water. When the solubility of water in a liquid exceeds a
few percent, this assumption may break down, and it is unwise to use this relationship.
If a solute is miscible with water (e.g., ethanol), it is preferable to determine
the Henry’s law constant directly; that is, by measuring air and water concentrations
at equilibrium. This can be done by various techniques, e.g., the EPICS method
described by Gossett (1987) or a continuous stripping technique described by
Mackay et al. (1979). A desirable strategy is to measure vapor pressure P S , solubility
C S , and H or K AW and perform an internal consistency check that H is indeed P S /C S
or close to it. K AW is, of course, Z A/Z W.
Care must be taken when calculating Henry’s law constants to ensure that the
vapor pressures and solubilities apply to the same temperature and to the same phase.
In some cases, reported vapor pressures are estimated by extrapolation from higher
temperatures. They may be of a liquid or subcooled liquid, whereas the solubility
is that of a solid. As was discussed earlier, subcooled conditions are not experimentally
accessible but prove to be useful for theoretical purposes.
Henry’s law constants vary over many orders of magnitude, tending to be high
for substances such as the alkanes (which have high vapor pressures, low boiling
points, and low solubilities) and very low for substances such as alcohols (which
have a high solubility in water and a low vapor pressure). There is a common
misconception that substances that are “involatile,” such as DDT, will have a low
Henry’s law constant. This is not necessarily the case, because these substances also
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