Handbook of Solvents - George Wypych - ChemTech - Ventech!

760 Roland Schmid
ΔG = ΔG + ΔG
[13.1.18]
sol cav att
where:
ΔGsol free energy of dissolution
ΔGcav free energy of cavity formation
ΔGatt free energy of attractive interactions
This equation has been used by de Souza and Ben-Amotz 107 to calculate values of ΔGatt from the difference between experimental solubilities of rare gases, corresponding to ΔGsol, and ΔGcav assessed from eqn. [13.1.10], i.e., using a hard-sphere fluid (HF) model. The values
of ΔGatt so obtained have been found to correlate with the solute polarizabilities suggesting
a dispersive mechanism for attractive solvation. It is interesting to note that, in water, the
solubility of the noble gases increases with increasing size, in contrast to the aliphatic hydrocarbons
whose solubility decreases with size. This differential behavior is straightforwardly
explained in terms of the high polarizability of the heavy noble gases having a large
number of weakly bound electrons, which strengthens the vdW interactions with water. It
can be shown that for noble gases, on increasing their size, the vdW interactions increase
more rapidly than the work of cavity creation, enhancing solubility. On the contrary, for the
hydrocarbons, on increasing the size, the vdW interactions increase less rapidly than the
work of cavity creation, lowering the solubility. 108
We have seen that there is evidence of either model, the clathrate cage model and the
cavity-based model. Hence the importance of water structure enhancement in the hydrophobic
effect is equivocal. The reason for this may be twofold. First, theoretical models have
many adjustable parameters, so their physical bases are not always clear. Second, the free
energy alone masks the underlying physics in the absence of a temperature dependence
study, because of, amongst other things, the entropy-enthalpy compensation noted above.
In place of the free energy, other thermodynamic derivatives are more revealing. Of these,
the study of heat capacity changes arguably provides a better insight into the role of changes
in water structure upon hydration than analysis of entropy or enthalpy changes alone. Note
that heat capacity is the most complex of the four principal thermodynamic parameters describing
solvation (ΔG, ΔH, ΔS, ΔCp), with the following connections,
ΔC
p
2
∂ΔH
∂ΔS
2 ∂ ΔG
= = T =−T
2
∂T
∂T
∂T
[13.1.19]
It should be stressed that the negative entropy of hydration is virtually not the main
characteristic feature of hydrophobicity, since the hydration of any solute, polar, nonpolar,
or ionic, is accompanied by a decrease in entropy. 109 The qualitative similarity in hydration
entropy behavior of polar and nonpolar groups contrasts sharply with the opposite sign of
the heat capacity change in polar and nonpolar group hydration. Nonpolar solutes have a
large positive heat capacity of hydration, while polar groups have a smaller, negative one.
Thus, the large heat capacity increase might be what truly distinguishes the hydrophobic effect
from other solvation effects. 110
Recently, this behavioral difference of nonpolar and polar solutes could be reproduced
by heat capacity calculations using a combination of Monte Carlo simulations and the random
network model (RNM) of water. 110-112 It was found that the hydrogen bonds between
the water molecules in the first hydration shell of a nonpolar solute are shorter and less bent
(i.e., are more ice-like) compared to those in pure water. The opposite effect occurs around