Physical Chemistry of Foods

3.2 SOLVATION

As mentioned above, net attractive interactions occur between all pairs of
molecules (excluding ions for the moment), except at very small distances. In
a pure liquid, all these interactions are on average the same, and it makes no
difference whether the one or the other molecule is close to a third one. In a
solvent (1) with a solute (2), however, some different interactions occur. In
the simplest case, where we consider spherical molecules of the same size, the
solvent–solute interaction is governed by

Unet¼U 12 

1

2

U 11

1

2

U 22 ð 3 : 2 Þ

where U 12 stands for the attractive energy between solvent and solute
molecules etc.; hence allU’s are negative. IfUnet<0,solvationoccurs, i.e.,
the solute molecules are preferentially surrounded by solvent molecules
rather than by other solute molecules, as illustrated in Figure 3.3a. In such a
good solvent, two solute molecules are on average farther away from each
other than they would be if there were no net attractionðUnet¼ 0 Þ. This
results then in arepulsiveforce between the solute molecules, that may be
felt over a range of at most a few solvent molecules. It generally implies that
the activity coefficient of the solute is smaller than unity (see Section 2.2.1);
hence the solute is well soluble.
Solvation should not be interpreted in terms of permanent binding of
solute by solvent: a solvent molecule is merely longer near a solute molecule
than it would be ifUnet¼0, for instance 10
^9 s instead of 10
^12 s. It may be

FIGURE3.3 Solvation. (a) Solute molecules (hatched) are preferentially sur-

rounded by solvent molecules. (b) Solvent molecules tend to stay away from solute

molecules (negative solvation).