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This has gone far enough. In Problem 7.11 you’ll finish the calculation to get a direct derivation
of Equation 7.31. For a deeper derivation from thermodynamics, see Israelachvili, 1991,§12.7.
7.5.1′
- The discussion in Section 7.5.1 described the electric field around a water molecule as that due
to two positive point charges (the naked protons) offset from a diffuse negative cloud (the oxygen
atom). Such a distribution will have a permanent electric dipole moment, and indeed water is highly
polarizable. But we drew a distinction between the H-bond and ordinary dipole interactions. This
distinction can be described mathematically by saying that the charge distribution of the water
molecule has many higher multipole moments (beyond the dipole term). These higher moments
give the field both its great intensity and rapid falloff with distance.
Forcomparison, propanone (acetone, or nail-polish remover, CH 3 -CO-CH 3 )has an oxygen atom
bonded to its central carbon. The oxygen grabs more than its share of the carbon’s electron cloud,
leaving it positive, but not naked. Accordingly, propanone has a dipole moment, but not the strong
short-range fields responsible for H-bonds. And indeed, the boiling point of propanone, while higher
than a similar nonpolar molecule, is 44◦Clower than that of water. - The picture of the H-bond given in Section 7.5.1 was rooted in classical electrostatics, and so is
only part of the story. In fact, the H-bond is also partly covalent in character. Also, the formation
of an H-bond between the hydrogen of an OH group, for example, and another oxygen actually
stretchesthe covalent bond in the original OH group. Finally, the H-bond accepting sites, described
rather casually as the “back side” of the water molecule, are in fact more sharply defined than our
picture made it seem: The molecule strongly prefer for all four of its H-bonds to be directed in the
tetrahedral directions shown in Figure 7.12. - Another feature of the ice crystal structure (Figure 7.12a on page 241) is important, and general.
In every H-bond shown, two oxygens flank a hydrogen, and all lie on a straight line (that is, the H-
bond and its corresponding covalent bond are colinear). Quite generally, the H-bond isdirectional:
There is a significant loss of binding free energy if the hydrogen and its partners are not on a line.
This additional property of H-bonds makes them even more useful for giving binding specificity to
macromolecules. - The books listed at the end of the chapter give many more details about the remarkable properties
of liquid water.
7.5.2′
- The term “hydrophobic” can cause confusion, as it seems to imply that oil “fears” water.
Actually, oil and water molecules attract each other, by the usual generic (van der Waals) interaction
between any molecules; an oil-water mixture has lower energy than equivalent molecules of oil and
water floating separately in vacuum. But liquid water attractsitselfeven more than it attracts oil
(that is, its undisturbed H-bonding network is quite favorable), so it nevertheless tends to expel
nonpolar molecules. - In his pioneering work on the hydrophobic effect, W. Kauzman gave a more precise form
of the solubility argument of Section 7.5.2. Figure 7.14 on page 245 shows that at least some
nonpolar molecules’ solubilities decrease as we raise the temperature beyond room temperature.
Le Chˆatelier’s Principle (to be discussed later, in Section 8.2.2′on page 294) implies that for these
substances solvation releases energy, since raising the temperature forces solute out of solution.