Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.1 Solvent effects on chemical reactivity 759
The clathrate cage model states that the structure of water is strengthened around a hydrophobic
solute, thus causing a large unfavorable entropic effect. The surrounding water
molecules adopt only a few orientations (low entropy) to avoid “wasting” hydrogen bonds,
with all water configurations fully H-bonded (low energy). There is experimental evidence
of structure strengthening, such as NMR and FT-IR studies, 91 NMR relaxation, 92 dielectric
relaxation, 93 and HPLC. 94 A very common conclusion is that the small solubility of
nonpolar solutes in water is due to this structuring process.
In the cavity-based model the hard core of water molecules is more important to the
hydrophobic effect than H-bonding of water. The process of solvation is dissected into two
components, the formation of a cavity in the water to accommodate the solute and the interaction
of the solute with the water molecules. The creation of a cavity reduces the volume of
the translational motion of the solvent particles. This causes an unfavorable entropic effect.
The total entropy of cavity formation at constant pressure 54
( )
ΔScav, P = ρα p ∂ΔGcav / ∂ρ −ΔG
T
T cav /
[13.1.17]
where
ΔScav,P cavity formation entropy at constant pressure
ρ liquid number density
ΔGcav free energy of cavity formation
is the result of the opposing nature of the (positive) liquid expansibility term and the (negative)
chemical potential summand. Along these lines the large and negative entropy of cavity
formation in water is traced to two particular properties of water: the small molecular
size (σ = 2.87 Å) and the low expansibility (αp = 0.26x10 -3 K -1 ), with the latter having the
greater impact. It is interesting to note that in both aspects water is extraordinary. Water’s
low expansibility reflects the fact that chemical bonds cannot be stretched by temperature.
There is also a recent perturbation approach showing that it is more costly to accommodate
a cavity of molecular size in water than in hexane as example. 95 Considering the high fractional
free volume for water (Table 13.1.2), it is concluded that the holes in water are distributed
in smaller packets. 96 Compared to a H-bonding network, a hard-sphere liquid finds
more ways to configure its free volume in order to make a cavity.
In the cavity-based model, large perturbations in water structure are not required to explain
hydrophobic behavior. This conclusion arose out of the surprising success of the
scaled particle theory (SPT), 39 which is a hard-sphere fluid theory, to account for the free energy
of hydrophobic transfers. Since the theory only uses the molecular size, density, and
pressure of water as inputs and does not explicitly include any special features of H-bonding
of water, the structure of water is arguably not directly implicated in the hydration thermodynamics.
(However, the effect of H-bonds of water is implicitly taken into account through
the size and density of water.) The proponents of this hypothesis argue that the entropic and
enthalpic contributions arising from the structuring of water molecules largely compensate
each other. In fact, there is thermodynamic evidence of enthalpy-entropy compensation of
solvent reorganization. 97-100 Furthermore, recent simulations 101,102 and neutron scattering
data 103-105 suggest that solvent structuring might be of much lower extent than previously believed.
Also a recent MD study report 106 stated that the structure of water is preserved, rather
than enhanced, around hydrophobic groups.
Finally, the contribution of solute-water correlations to the hydrophobic effect may be
displayed, for example, in the framework of the equation