Chemistry Reference
In-Depth Information
Appendix B: Solubility of
Organic Molecules in Water
Using a
Surface Tension-
Cavity
Model System
The mechanisms of solubility in water of an inorganic salt, such as NaCl, is differ-
ent from that of an alkane (such as hexane) molecule. NaCl dissociates into Na and
Cl ions and interact with water through hydrogen bonds. Hexane molecules dissolve
(although at a very low solubility) in water when placed inside a water structure.
Since the water structure is stabilized mainly by hydrogen bonds, the hexane mol-
ecule will give rise to some rearrangements of these bonds but without breaking.
If a salt exhibits maximum solubility (called saturation solubility) of 10 mol/L in
water, then it corresponds to ca. 10 mol of salt:55 mol of water (ratio of 1:5.5). On the
other hand, an alkane may show a solubility of 0.0001 mol/L in water (0.0001 mol:55
mol), or a ratio of 1:550.000.
Many decades ago, this model was found to be able to predict the solubilities of
both simple and more complicated organic molecules. In the most simple case, the
solubility of heptane is lower than that of hexane due to the addition of one -CH
2
-
group. In the case of alkane molecules, a linear relation between the solubility and
the number of -CH
2
- groups is found (Birdi, 1997).
This model thus is based on the following assumptions when the alkane molecule
is placed in water:
Alkane(CCC) is placed in a cavity in the water(ww).
wwwwwwwwwwwwwwwwwwwwwwwwwwwwwww
wwwwwwwwwwwCCCCCCCCCwwwwwwwwwwww
wwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww
The energy needed to create a surface area of the cavity will be proportional to the
degree of solubility of the alkane. Thus, the solubility of any alkane molecule will
be given as
Free energy of solubility = Proportional to the cavity surface area
(surface tension of the cavity)
(B.1)
By analyzing the solubility data of a whole range of alkane molecules, the following
relation was found to fit the experimental data:
235
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