Geoscience Reference
In-Depth Information
log S
m
¼ log S
w
þ
r
s
f
s
with
r
s
¼
ð
A log K
ow
þ
B
Þ;
ð
6
:
7
Þ
where f
s
is the volume fraction of cosolvent in binary mixed solvent, A and B are
empirical constants dependent on cosolvent properties, and K
ow
is the octanol-
water partition coefficient of the solute. The solute mixture of interest could
comprise, for example, two hydrophobic organic chemicals or one hydrophobic
and one ionizable organic chemical.
Crystalline salts of many organic acids and bases often have a maximum sol-
ubility in a mixture of water and water-miscible solvents. The ionic part of such a
molecule requires a strongly polar solvent, such as water, to initiate dissociation. A
mixture of water-miscible solvents hydrates and dissociates the ionic fraction of
pollutants at a higher concentration than would either solvent alone. Therefore,
from a practical point of view, the deliberate use of a water-soluble solvent as a
cosolvent in the formulation of toxic organic chemicals can lead to an increased
solubility of hydrophobic organic contaminants in the aqueous phase and, con-
sequently,
to
a
potential
increase
in
their
transport
from
land
surface
to
groundwater.
When industrial effluents or other waste materials, constituting a mixture of
contaminants, are disposed of on the land surface, situations arise in which
completely water-miscible organic solvents (CMOS) change the solvation prop-
erties of the aqueous phase. A similar situation may be encountered when, during
remediation procedures, a mixture of water and water-miscible solvents is used to
''wash'' a contaminated soil (Li et al.
1996
). A large number of organic solvents
with various molecular properties (e.g., methanol, propanol, ethanol, acetone,
dioxane, acetonitrile, dimethylformamide, glycerol) are included in the CMOS
group. Each has a different effect on the activity coefficient and thus the solubility
and partitioning behavior of an organic contaminant in a water/CMOS mixture.
Table
6.2
presents data showing the effect of various CMOS on the activity
coefficient or mole fraction solubility of naphthalene, for two different solvent/
water ratios. To examine the cosolvent effect, Schwarzenbach et al. (
2003
) com-
pare the Hildebrand solubility parameter (defined as the square root of the ratio of
the enthalpy of vaporization and the molar volume of the liquid), which is a
measure of the cohesive forces of the molecule in pure solvent.
Schwarzenbach et al. (
2003
) note that, qualitatively, the more ''waterlike''
solvents (e.g., glycerol, ethylene glycol, methanol) have a much smaller impact on
the activity coefficient of an organic solute than organic solvents for which
hydrogen bonding is important but not the overall dominating factor. The CMOS
are relatively small molecules with strong H-acceptor or H-donor properties. In an
aqueous mixture, CMOS are able to break the hydrogen bonds between the water
molecules and thus form a new H-bond solvent. In this case, the properties of the
water-cosolvent solution change as a function of the nature and the relative
