Page 68 - Partition & Adsorption of Organic Contaminants in Environmental Systems

HEPTANE–WATER SYSTEMS 59

from the unique molecular structure of n-octanol, which possesses a nonpolar
chain of moderate size and a hydrophilic OH group; these impart to the mol-
ecule a unique polar-to-nonpolar balance with a weak-to-moderate polarity.
This unique molecular property empowers octanol to accommodate a wide
range of organic compounds with comparable solvencies through its hydro-
philic OH and/or its nonpolar alkyl chain. As a consequence, the g* o values of
a wide range of sparingly water-soluble solutes fall into a small range. We shall
see later that the partition effects of solutes in some other solvent–
water systems share some common features with those in the octanol–
water system, while other systems manifest important differences in solute
partition behavior as a result of large discrepancies in solvent composition and
polarity.

5.3 HEPTANE–WATER SYSTEMS

The n-heptane/water mixture offers an extreme but instructive system for
examining important differences in the partitioning of polar and nonpolar
compounds into a highly nonpolar organic phase. As with the octanol–water
system, the molecular-size differences between most solutes and heptane are
usually not too large to negate the use of Raoult’s law for treating solute par-
tition with heptane. Note here that the mutual solubility of heptane and water
is very small at room temperature, the solubility of heptane in water being
-5
-3
about 9.5 ¥ 10 M and that of water in heptane being 5.3 ¥ 10 M. Thus,
Eq. (3.11) can be simplified by treating the molar volumes of water-saturated
heptane and heptane-saturated water to be essentially the same as the molar
volumes of the respective pure solvents. A further approximation can be made
by assuming that the small amount of heptane in water has no significant effect
on the solubility of solutes in the water phase. With these simplifications,
Eq. (3.11) is reduced to

logK hw =-logS w - logV h - logg h (5.4)

where K hw is the heptane–water partition coefficient of the solute, V h the molar
volume of heptane (0.146L/mol), and g h the activity coefficient of the solute
in heptane. The ideal line for the heptane–water system is therefore
logK° hw =-logS w - logV h (5.5)

where logK° hw - logK hw = logg h applies for a solute at a particular logS w .
The K hw values for a series of organic solutes and the corresponding K ow
values at room temperature are given in Table 5.2 for comparison. If polar and
nonpolar solutes should exhibit comparable compatibilities with heptane as
with octanol, an equally good linear correlation would exist between logK hw
and logS w , as noted for the octanol–water system. However, as shown in