Ask A Scientist Chemistry Archive

name         Aviva K.
status       educator
age          50s

Question -   Is there a connection between the size of a molecule and
solubility?
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Yes, but the connection is not simple. If a molecule is extremely large
(e.g. a polymer with a molecular weight of several thousand) the solubility
will be less that of a molecule with a low molecular weight (e.g. about
100). Also, some theories of solution take into account the size difference
of the solute and solvent molecules, but this is not a huge effect. In
another context, the solubility of CsI (cesium iodide) is much greater than
LiF (lithium fluoride) -- in the series of alkali halides -- but there are
other factors also operating in such cases so that it is difficult to
separate the effect of size from all the other factors that are also
changing.

Vince Calder
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Aviva,

Indeed, there is a relationship between molecular size and solubility. More
important is the "compatibility" between the solute (that which is to be
dissolved) and the solvent (that which does the dissolving).

For example: The solubility of alcohols in water decreases as the molecular
size of the alcohol molecule increases. It is the affinity (hydrogen bonding)
between the -OH part of the water (H-O-H) molecule and the -OH part of the
alcohol (R-OH) that enables them to intermix. When the R- (hydrocarbon tail)
part of the alcohol molecule gets larger and larger, the hydrogen bonding
effect between the respective solute/solvent -OHs becomes less able to drag
the "tail" into solution because the "tail" is compositionally very unlike
the water molecule itself.

Likewise, even though both motor oil and road tar are soluble in gasoline,
the oil dissolves more readily because the molecules are smaller.

There is more to this story that I can fit into this brief response to your
query. Check out a current chemistry textbook for further information.

Regards,
ProfHoff 488
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