Environmental Engineering Reference
In-Depth Information
20
18
Heptachlor
16
14
12
10
Benzene
8
6
0.00310
0.00315
0.00320
0.00325
0.00330
0.00335
0.00340
0.00345
0.00350
(1/T)/K
-1
FIGURE 3.9
A plot of ln
(
1
/x
i
)
versus 1
/T
for heptachlor and benzene.
How can we explain the large entropic contribution to solution of nonpolar com-
pounds in water? The first convincing explanation was given by Frank and Evans
(1945), and further expanded by Franks (1983) and Israelchvili (1992). The essential
tenets of the theory are summarized here. Most nonpolar compounds are incapable of
forming H-bonds with water. This means that in the presence of such a solute, water
molecules lose some of their H-bonds among themselves. The charges on the tetra-
hedral water structure will then have to be pointed away from the foreign molecule
so that at least some of the H-bonds can be re-established. If the solute molecule is
of very small size, this may be possible without loss of any H-bonds, since water has
an open flexible structure. If the molecule is large, thanks to the ability of the tetrahe-
drally coordinated water to rearrange themselves there is, in fact, more local ordering
among the water molecules in the hydration layer (Figure 3.9). Thus, the introduction
of a nonpolar or a polar molecule with a nonpolar residue will reduce the degrees of
freedom for the water molecules surrounding it. Spatial, orientational, and dynamic
degrees of freedom of water molecules are all reduced. As stated earlier, the average
number of H-bonds per molecule in water is about 3.5. In the presence of a nonpolar
solute this increases to four. If there is local ordering around the solute, we should
expect from thermodynamics that the local entropy of the solvent is reduced; this is
clearly
unfavorable
. Such an interaction is called
hydrophobic hydration
. It should
be remembered that in transferring to water, although the solute entropy increases,
the solvent entropy decrease is so much larger that it more than offsets the former;
hence the overall entropy of the process is negative. None of the present theories of
solute-solvent interactions can handle this phenomenon satisfactorily. In Table 3.11
the entropy contribution is about 60-70% of the overall free energy of transfer.
Table 3.11 shows the thermodynamic functions of solution of several gases from
the vapor phase into water. Once again, we see that the entropic contribution toward
the overall free energy of dissolution is large. The incompatibility of nonpolar and
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