Introduction to Soil Chemistry

34 soil basics ii
Soils with anion exchange capacity can be expected to exchange anions in
the same way. However, in many soil anions are present as oxyanions, which
often react with soil components and thus do not and cannot act as exchangeable
anions. Phosphate anions are excellent examples of this type of interaction
[8].
2.2.4. Hydrogen Bonding
Hydrogen bonding is typified by the attraction of a partially positive hydrogen,
attached to a partially negative oxygen, which is attracted to a partially
negative oxygen on another molecule. A common example is the hydrogen
bonding in water, where the hydrogen of one water molecule is attracted to
the oxygen in another water molecule. Whenever hydrogen is bonded to a significantly
more electronegative element, it will have a partial positive charge.
It can then be attracted to lone pairs of electrons on other elements in other
molecules and thus produce an interaction that, although not classically a
hydrogen bond, is very similar to hydrogen bonding. Nitrogen and phosphorus
atoms would all fall into the category of electronegative atoms having lone
pairs of electrons.
Although hydrogen bonding is considered to be a much weaker interaction
than covalent or ionic bonding, it is nevertheless a relatively strong interaction.
When molecules have multiple sites for hydrogen bonding, there can be
significant strength in the association; for instance, paper is held together by
hydrogen bonding [9,10].
2.2.5. Polar–Polar Interactions
Whereas hydrogen bonding can be considered as a type of polar–polar interactions,
I define polar-polar interactions as the intermolecular attraction of
polar groups, which does not involve hydrogen. An example would the attraction
between two propanone (acetone) molecules, where a partially positive
carbon of the carbonyl group in one molecule is attracted to the partially negative
carbonyl oxygen of another molecule. This will be a weaker interaction
than hydrogen bonding but a stronger interaction than Van der Waals.
2.2.6. Van der Waals Interactions
Van der Waals attractions are described as the development of instantaneous
polar regions in one molecule that induce the development of polar regions
in another molecule and result in an attraction between the molecules. These
are considered to be instantaneous or short-lived polar regions that are in a
constant state of flux. The most common example is the attraction between
hydrocarbon molecules where this is the only discernable attracting force
between molecules.