Handbook of Solvents - George Wypych - ChemTech - Ventech!

758 Roland Schmid
Table 13.1.5. Solution thermodynamics
of methane in water and
carbon tetrachloride at 25°C.
[Data from T. Lazaridis and
M. E. Paulaitis, J. Phys. Chem., 96,
3847 (1992) and ref. 108]
Water CCl4 ΔS*, J/mol K - 64.4 - 7.1
300ΔS*, kJ/mol - 19.3 - 2.1
ΔH°, kJ/mol - 10.9 - 1.2
ΔG*, kJ/mol + 8.4 + 0.9
ΔCp, J/mol K 217.5 0 to 42
The hydrophobic effect
In some respects the hydrophobic effect may be
considered as the converse of the like-dissolves-like
rule. The term hydrophobic effect refers to the unusual
behavior of water towards nonpolar solutes.
Unlike simple organic solvents, the insertion of
nonpolar solutes into water is (1) strongly unfavorable
though slightly favored by enthalpy, but (2)
strongly opposed by a large, negative change in entropy
at room temperature, and (3) accompanied by
a large positive heat capacity. An example is given
in Table 13.1.5 for the thermodynamic properties of
methane dissolved in water and in carbon tetrachloride.
In dealing with the entropy (and free energy) of
hydration, a brief remark on the choice of standard
states is in order. The standard molar entropy of dis-
solution, Δ solvS° pertains to the transfer from a 1 atm gas state to a 1 mol L -1 solution and
hence includes compression of the gas phase from 1 mol contained in 24.61 L (at 300 K) to 1
mol present in 1 L. Since theoretical calculations disregard volume contributions, it is
proper to exclude the entropy of compression equal to -Rln24.61 = -26.63 J K -1 mol -1 , and
instead to deal with Δ solvS*. 82 Thus,
and
o
Δ S* = Δ S + 26. 63JK
mol
solv solv
−1 −1
[13.1.15]
*
o
Δ G = Δ H − 300Δ
S *
[13.1.16]
solv 300 solv
solv
Hydrophobicity forms the basis for many important chemical phenomena including
the cleaning action of soaps and detergents, the influence of surfactants on surface tension,
the immiscibility of nonpolar substances in water, 83 the formation of biological membranes
and micelles, 84,85 the folding of biological macromolecules in water, 86 clathrate hydrate formation,
87 and the binding of a drug to its receptor. 88 Of these, particularly intriguing is the
stabilization of protein structure due to the hydrophobicity of nonpolar groups. Hydrophobic
interactions are considerably involved in self-assembly, leading to the aggregation of
nonpolar solutes, or equivalently, to the tendency of nonpolar oligomers to adopt chain conformations
in water relative to a nonpolar solvent. 89
Ever-increasing theoretical work within the last years is being lifting the veil of secrecy
about the molecular details of the hydrophobic effect, a subject of vigorous debate.
Specifically, the scientific community would eagerly like to decide whether the loss in entropy
stems from the water-water or the water-solute correlations. There are two concepts.
The older one is the clathrate cage model reaching back to the “iceberg” hypothesis of Frank
and Evans, 90 and the other, newer one, is the cavity-based model. It should be stressed here
that the vast literature on the topic is virtually impossible to survey comprehensively. In the
following we will cite only a few papers (and references therein) that paved the way to the
present state of the art.