Multicomponent Equilibrium Thermodynamics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 79

10 − 21 J whereas in water it increases to

− ×

space, the interaction energy is 2.5

10 − 21 J. It should be emphasized that what we classify as hydrophobic inter-
action is not a true bond between solute species in water, although earlier thoughts
on this led to the formulation of terms like hydrophobic bonds that are mislead-
ing. The type of interactions between two solutes in water occurs via overlapping
hydration layers and is of much longer range than any other types of bonds. Ther-
modynamic parameters for hydrophobic interactions are not extensive because of the
inherent low solubilities of hydrophobic compounds. Tucker and Christian (1979)
determined that the interactions between benzene molecules in water to form a dimer
is about

− ×

14

8.5 kJ/mol
between two methane molecules in water.A satisfactory theory of hydrophobic inter-
action for sparingly soluble organics in water is still lacking and progress is being
made in this field. The same hydrophobic interactions play a major role in the
formation of associated structures from surfactants in water, biological membrane
structures, and conformations of proteins in biological fluids. For these cases ther-
modynamic parameters are well established and hydrophobic interactions are pretty
well understood.

− 8.4 kJ/mol, whereas Ben Naim (1980) calculated a value of −

3.4.3.4 Hydrophilic Interactions for Solutes in Water

Noninteracting solutes in water experience repulsive forces among one another. Polar
compounds, in particular, exhibit this behavior. They are mostly hygroscopic and
tend to incorporate water when left exposed in humid environments. Many of the
strongly hydrated ions, and some zwitter ions (those that have both anionic and
cationic characteristics), are strongly hydrophilic. Some nonpolar compounds also
show this behavior if they contain electronegative atoms capable of interacting with
the H-bonds in water. In contrast to the structuring of water imposed by a hydropho-
bic solute, the hydrophilic compounds tend to disorder the water molecules around
them. As an example, urea dissolved in water tends to make the water environment
so different that it can unfold a hydrophobic protein molecule in water. In summary,
for polar molecules in water, the contributions to free energies are determined pri-
marily by solvation effects with water giving a much more favorable conformation
than most other organic solvents.
Electrolytes in water provide multiple species (ions) that interact differently with
water. Aqueous electrolytes behave nonideally even at very low concentrations.
Fortunately, the limiting behavior (infinitely dilute solution) for electrolytes is well
understood and is called the Debye-Huckel theory . The final result from this theory
was presented in Section 3.2.4, where the effects of ionic strength on the mean activ-
ity coefficient of ions and on the activity coefficient of neutral solutes in water were
discussed.
Using the Debye-Huckel theory, the definition of mean activity coefficient is
(Bockris and Reddy, 1970)

) e 2

N A (z z

+ −

ln γ ± =− · κ , (3.64)

2 ε RT