added that entropic effects can be involved in the interactions if the solvent
has some structure (as in water), and one then speaks of contact entropy. In
such a case, one cannot express the solute–solvent interaction as a bond
energy, but one may—in principle—calculate an interaction free energy.
If Unet>0, we havenegative solvation or, in other words, apoor
solventfor the solute considered. Now the solute molecules are preferentially
near to each other, rather than near to solvent molecules: Figure 3.3b. This
generally implies that the solute has a high activity coefficient and poor
solubility.
Solvation repulsion may also act between segments of one polymer
molecule (Section 6.2.1) or between colloidal particles that have groups at
their surface that become solvated (Section 12.4). Negative solvation leads
to attraction between polymer segments or between particles.
Hydration. If the solvent is water, solvation is called hydration. It is
an intricate phenomenon, since water is such an intricate, not fully
understood liquid. Hydration nearly always involves considerable change
in entropy, since anything altering the fluctuating network of hydrogen
bonds alters entropy. Four kinds of solute molecules or groups may be
conveniently distinguished:
- Ions or ionic groups. Due to the ion–dipole interactions
mentioned, small ions tend to be strongly hydrated. Ions move,
by diffusion or in an electric field, as if they were accompanied by
a number of water molecules. Again, this does not imply that these
water molecules are permanently bound: they interchange with
other water molecules. Ion hydration is stronger for a smaller ion
and a higher valence; cations tend to be more strongly solvated
than anions of the same size and valence. The attraction between a
proton and a water molecule is so strong that hydronium ions
ðH 3 OþÞ occur in water, leaving very few free protons. The
hydronium ion is, in turn, hydrated.
It may further be noted that the formation of ion pairs (e.g.,
NaþþCl!NaCl, orCOOþHþ!COOH) requires de-
solvation (‘‘dehydration’’); especially if the ions or ionic groups
involved are small, the increase in free energy involved can be
appreciable. In other words, hydration then strongly promotes
dissociation of ionizable species. - Groups with a strong dipole moment also become hydrated. A
case in point is the peptide bonds in proteins. - Other somewhat polar groups, such as -OH groups, that can make
H bonds with water. Substances with several -OH groups, like
sugars, often are said to be hydrophilic. Nevertheless, hydration is