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204 CHAPTER 2
to be dealt with. The first is salting out—the decrease of solubility that the ions cause.
This is easily understood because of course the ions remove quite a lot of the waters
from availability to the incoming solute by taking them off into temporary inactivity
in the hydration shell so that the organic molecule has less water (per liter of solution)
to dissolve in and its solubility is thus decreased. Salting in is a little bit more difficult
to understand, especially anomalous salting in, which occurs when the equations for
salting out indicate that there should be a decrease in the solubility of a nonelectrolyte
upon the addition of ions but in fact there is an increase. It turns out that this is caused
by dispersion force interactions by which the ions (large ions such as those of the
series are involved here) attract the organic molecules to themselves and push the
water out, thus giving more water for the organic molecules to dissolve in and an
increased solubility. Such phenomena provide some basis for an interpretation of
hydrophobic effects in hydration.
Electrostriction is the study of the effects of squeezing of ions and molecules by
the electrical forces that are exerted upon them by the ions we have been dealing with
(Section 2.22). It is only recently that modelers have begun to take into account the
shapes formed by these compressed bodies. In fact, they do become lenslike in shape
(not spheres) and when this is taken into account, agreement between theory and
experiment is improved.
Hydrophobic effects are on a list of special phenomena. They are closely tied to
salting in because one of the reasons for hydrophobic effects (water pushing-out
effects, one could say) is that the ions of the solute tend to attract each other or other
nonelectrolytes present and push the water between them out. Structure breaking in a
solution, some part of which rejects water in the rearrangements formed, also gives
hydrophobic effects.
Polyelectrolytes occur in ion-exchange membranes and thus their study has great
material value. They have a central importance in biology and the study of their
electrochemistry, as ions, their natural interactions in solution, etc., are important although
we are only able to give a short description of them in a chapter of restricted length.
Finally, the question of the structure of biological water is one of far-reaching
importance. Some workers in the last few decades have suggested that water in
biological systems is special but our answer is that this special structure is so readily
explicable that no mystery exists. Biological cells are sized on the micron scale and
contain much solid material. The surface-to-volume ratio inside such cells is very
large. Most of the waters in cells are in fact surface waters. In this sense, biological
water is special but only because it has lost the netted-up properties of bulk water and
adopted the individual two-dimensional structure of water at all surfaces.
APPENDIX 2.1. THE BORN EQUATION
In the preceding text, particularly in Section 2.15.11, use is made of Born’s
equation, a famous classical equation first deduced in 1920. This equation is generally