Geoscience Reference
In-Depth Information
modification of its structure and properties, due to reorientation of surfactant
molecules. There is a disadvantage, energetic or entropic, in indefinite expansion,
which sets a limit to the solubility.
Cosolvents and salts mix completely with water to form homogeneous solu-
tions, but with different effects. Cosolvents decrease the polarity of water and
reduce the ability of an aqueous system to ''squeeze out'' nonpolar solutes,
resulting in an increase in the solubility of nonelectrolytes. On the other hand, salts
decrease the solubility on nonelectrolytes by increasing the polarity of water,
thereby increasing the ability of the aqueous system to ''squeeze out'' the nonpolar
solutes.
6.5 Salting-Out Effect
Effects of electrolytes on the solubility of organic compounds in aqueous solutions
were established empirically more than 100 years ago by Setschenow (
1889
). He
found that the presence of dissolved inorganic salts in an aqueous solution
decreases the aqueous solubility of nonpolar and weak polar organic compounds.
This effect, known as the salting-out effect, is expressed by the empirical Set-
schenow formula
log
ð
c
=
c
0
Þ
¼log
ð
S
0
=
S
Þ
¼k
s
C
s
;
ð
6
:
8
Þ
where c and c
0
are the activity coefficients of the organic solute in salt solution and
in water, respectively; S
0
and S are solubilities of the solute in water and in salt
solution, respectively; k
s
(L/mol) is the salting-out constant; and C
s
(mol/L) is the
molar concentration of the salt solution. In general, the dissolution process requires
overcoming water-water interactions that allow formation and occupation of a
cavity (Turner
2003
; Schwarzenbach et al.
2003
). In the presence of dissolved salt,
solutions become ''salted out'' or ''squeezed out'' due to higher organization and
compressibility of the water molecules when bound up in hydration spheres
(Millero
1996
). In some cases, the presence of salts leads to increases in solubility
of organic compounds; this behavior is known as the salting-in effect, and the
value of the Setschenow constant in this case becomes negative. Several examples
of ''salting in'' and ''salting out'' are given in Table
6.3
.
The salting-out constant expresses the potential of a salt or a mixture of salts to
change the solubility of a given nonpolar compound. On contact between organic
and aqueous phases, some of the organic molecules dissolve in the water, while
some water molecules enter the organic phase. The values of the activity coeffi-
cients or relative solubilities of nonelectrolytes in electrolyte solutions have been
examined over the years, mainly for saline concentrations close to seawater
salinity (Randall and Failey
1927
; Xie et al.
1997
; Millero
2000
;Droretal.
2000a
,
2000b
). Because it is virtually impossible to quantify (experimentally) the con-
tribution of individual ions, salting-out constants are available only for combined
