Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.1 Solvent effects on chemical reactivity 761
polar solutes (the waters become less ice-like). The increase in H-bond length and angle has
been found to decrease the water heat capacity contribution, while decreases in length and
angle have been found to cause the opposite effect.
Note further that a large heat capacity implies that the enthalpy and entropy are strong
functions of temperature, and the free energy vs. temperature is a curved function, increasing
at low temperatures and decreasing at higher temperatures. Hence there will be a temperature
at which the solubility of nonpolar in water is a minimum. The low solubility of
nonpolar species in water at higher temperatures is caused by unfavorable enthalpic interactions,
not unfavorable entropy changes. Some light on these features has been shed by using
a “simple” statistical mechanical MB model of water in which the water molecules are represented
as Lennard-Jones disks with hydrogen bonding arms. 113 (the MB model is called
this because of the resemblance of each model water to the Mercedes-Benz logo.) As an important
result, the insertion of a nonpolar solute into cold water causes ordering and
strengthening of the H bonds in the first shell, but the reverse applies in hot water. This provides
a physical interpretation for the crossover temperatures T H and T S, where the enthalpy
and entropy of transfer equal zero. T H is the temperature at which H-bond reorganizations
are balanced by solute-solvent interactions. On the other hand, T S is the temperature at
which the relative H-bonding strengths and numbers of shell and bulk molecules reverse
roles.
Although the large positive free energy of mixing of hydrocarbons with water is dominated
by entropy at 25°C, it is dominated by enthalpy at higher temperatures (112°C from
Baldwin’s extrapolation for hydrocarbons, or 150°C from the measurements of Crovetto for
argon) 113 where the disaffinity of oil for water is maximal. Ironically so, where
hydrophobicity is strongest, entropy plays no role. For this reason, models and simulations
of solutes that focus on cold water, around or below 25°C, miss much of the thermodynamics
of the oil/water solvation process. Also, a clathrate-like solvation shell emerged from a
recent computer simulation study of the temperature dependence of the structural and dynamical
properties of dilute O 2 aqueous solutions. 114 In the first hydration shell around O 2,
water-water interactions are stronger and water diffusional and rotational dynamics slower
than in the bulk. This calls to one’s mind an older paper by Hildebrand 115 showing that at
25°C, methane’s diffusion coefficient in water is 40% less than it is in carbon tetrachloride
(D(H 2O) = 1.42x10 -5 cm 2 /s vs D(CCl 4) = 2.89x10 -5 cm 2 /s). Presumably the loose clathrate
water cages serve to inhibit free diffusion of the nonpolar solute. From these data it seems
that both the nonpolar solute and the aqueous solvent experience a decrease in entropy upon
dissolving in water. It should also be mentioned in this context that pressure increases the
solubility. The effect of pressure on the entropy was examined and it was found that increase
in the pressure causes a reduction of orientational correlations, in agreement with the
idea of pressure as a “structure breaker” in water. 116 Actually, frozen clathrate hydrates
trapped beneath oceans and arctic permafrost may contain more than 50% of the world’s organic
carbon reserves. 117,118 Likewise, the solubility of aromatics is increased at high pressure
and temperature, with π bond interactions involved. 119
Only at first glance, the two approaches, the clathrate cage model and the cavity-based
model, looked very different, the former based on the hydrogen bonding of water, and the
later on the hard core of water. But taken all results together it would appear that both are
just different perspectives on the same physics with different diagnostics reporting consequences
of the same shifted balance between H bonds and vdW interactions. Actually, in a