these series are approximately as follows:
SO^24 >HPO 42 >acetate>citrate>Cl>NO 3 >I>SCN
NHþ 4 >Kþ>Naþ>Liþ>Mg^2 þ>Ca^2 þ>guanidiniumþThe anions on the left-hand side are strongly hydrated, cause a
‘‘hydrophilic’’ water structure around them, and increase the surface
tension of water; they tend to stay away from apolar surfaces, which can be
interpreted as negative solvation of apolar molecules or groups.
Consequently, they enhance hydrophobic bonding and decrease the
solubility of apolar substances (‘‘salting out’’). The anions on the right-
hand side are less hydrated and are not so greatly different from apolar
solutes in their effect on water, making the local water structure more
‘‘hydrophobic.’’ They tend to increase the solubility of apolar substances
(‘‘salting in’’) and have a fairly small effect on surface tension. The
explanation of the series for cations is somewhat less clear, and the observed
order is not always the same. The effects of anions and cations are roughly
additive, although the anions seem to be dominant. LiCl and NaCl are
mostly about neutral with respect to solubility of apolar substances. A
solution ofðNH 4 Þ 2 SO 4 is strongly salting out.
It should be noted that these arespecific effects of ions that are
independent of their valence. This is different from the general effects of ions
as discussed in Section 2.3, which primarily depend on total ionic strength—
i.e., on ion concentration and valence—and which become manifest at far
smaller salt concentration.
Note The reader should realize that the term hydration also tends
to be used fairly indiscriminately for a number of other phenomena,
like the taking up of moisture by a dry material. This is further
discussed in Chapter 8.3.3 RECAPITULATION
Forces acting between molecules or groups not only determine chemical
reactivity but also affect several other phenomena, such as formation of
condensed phases and aggregation of colloidal particles. Several kinds of
forces occur, greatly differing in strength, effective range, effective direction,
and additivity. Strength of Coulombic type bonds is inversely proportional
to dielectric constant. Bond strength is measured in terms of energy, but in
some cases entropy is involved and free energy should be used.
Attractive interaction between solvent and solute molecules causes
solvation, as a result of which solute molecules repel each other; this