decreases the activity coefficient of the solute and increases its solubility. In
other cases, negative solvation occurs, leading to net attraction between
solute molecules. Hydration, i.e., solvation by water molecules, is an
intricate phenomenon, due to liquid water being strongly hydrogen-bonded.
Negative hydration of apolar groups leads to the formation of hydrophobic
bonds between these groups. Hydrophobic bonding strongly increases with
temperature, being weak or absent below 0 8 C.
Some solutes, if present in significant concentration, affect water
structure and thereby hydrophobic bonding. In this respect salts are
arranged in a lyotropic series of decreasing hydration and increasing
tendency to enhance solubility of apolar substances.
BIBLIOGRAPHY
We refer again to the textbooks on physical chemistry mentioned in Chapter 1. A
clear description is also given by
J. N. Israelachvilli. Intermolecular and Surface Forces, 2nded. Academic Press.
London, 1992.
A thorough and extensive discussion is found in Chapters 4 and 5 of
J. Lyklema. Fundamentals of Interface and Colloid Science, Vol. 1. Fundamentals.
Academic Press, London, 1991.
Theory and consequences of hydrophobic interactions are extensively and clearly
discussed in
C. Tanford. The Hydrophobic Effect, 2nded. John Wiley, New York, 1980.