Multicomponent Equilibrium Thermodynamics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 75

are only of the order of 1 kJ/mol. The directional character of the H-bond in water
allows it to form a three-dimensional structure wherein the tetrahedral structure of
the ice lattice is propagated in all three dimensions.
Although the actual structure of liquid water is still under debate, it is generally
accepted that the tetrahedral geometry of the ice lattice is maintained in liquid water
to a large extent shown by infrared (IR) and Raman spectroscopic investigations. An
accepted model for water is called the ST2 model (Stillinger and Rahman, 1974).
In this model each H atom in the tetrahedron covalently bonded to an O carry a net
charge of + 0.24 e ( e is the electron unit) each, while the two H atoms on the opposite
side of the O atom that participate in H-bonds carry a net compensating charge of
− 0.24 e each. The H-O-H bond angle is 109 ◦ . Molecular computer simulations of

the ST2 model confirm that the tetrahedral coordination of the O atom with other H
atoms is the cause for the many unusual properties of water. In liquid water although
the ice-like structure is retained to a large extent, it is disordered and somewhat open
and labile. The interesting fact is that the number of nearest neighbors in the lat-
tice in liquid water increases to five on the average, whereas the average number of
H-bonds per molecule decreases to 3.5. The mean lifetime of a H-bond in liquid water
is estimated to be 10 − 11 s. Generally, it can be said that only a tetrahedral structure
in liquid water can give rise to this open three-dimensional structure, and it is this
property more than even the H-bonds themselves that imparts strange properties to a
low molecular weight compound such as water.

3.4.3.2 Hydrophobic Hydration of Nonpolar Solutes

The introduction of a solute changes the intermolecular forces in water. The solute-
water interactions are called hydration forces . Most of our current understanding of
hydration phenomena is based on indirect experimental evidence. An understanding
of hydration (or more generally solvation) is by starting with a thermodynamic cycle,
which incorporates all of the so-called standard thermodynamic transfer functions.
It is useful and instructive to compare the thermodynamic functions for transfer of
a solute molecule (say i) at a specified standard state from solvent A to solvent B.
Solvent B is water while solventA can be any other liquid or even air. Both solventsA
and B can be either pure or mixtures. Let us consider a solute (methane) that is trans-
ferred from different organic solvents (A) to solvent water (B/W).The values of molar
free energy, enthalpy, and entropy have been obtained and tabulated (Franks, 1983).
These values are shown in Table 3.9. The free energy change is positive in all the
cases. It is less positive for a more nonpolar solvent (e.g., methanol versus cyclohex-
ane). However, the enthalpy change in all the cases is negative, indicating exothermic
transfer of the molecule from solvent to water, that is, the enthalpic contribution to the
transfer of methane to water is highly favorable. The entropic contribution is, how-
ever, positive in all these cases. Thus, we arrive at the remarkable conclusion that the
dissolution of a nonpolar solute such as methane in water is entropicallyunfavorable .
Methane is not the only solute for which this behavior has been observed. Most non-
polar compounds or slightly polar compounds of environmental significance show
this feature. A majority of polar compounds that have only one polar group such as
alcohols, amines, ketones, and ethers also show this behavior.