Chemgineer
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Hygroscopic vs High Solubility
It is my understanding that hygroscopic materials usually high a high solubility in water. Why is it the case that some high solubility materials are
not hygroscopic?
[Edited on 29-5-2022 by Chemgineer]
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Amos
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I'm going to try to explain hygroscopy as a concept and hopefully not mess anything up. Solubility and hygroscopicity might have some correlation but
are not really related properties.
A compound or material has a given hydration energy, which is the amount of energy released by(positive hydration energy) or required to create a bond
or bonding with water molecules. Water molecules are always in the air passing by a material open to the air, interacting with the surface and
occasionally forming bonds. When a material has a very high bonding energy, a lot of energy is released upon the formation of one of these bonds or
water-material interactions, and since there isn't a lot of energy in the form of heat to drive the opposite interaction, the water molecule stays
bonded to or in the immediate vicinity of that material. This large release of energy might grow smaller as the material hydrates, such as with metal
salts like magnesium sulfate or copper chloride that generally stop hydrating after forming a particular hydrate, or it can continue to stay high
until the material is literally dissolving in attracted water (deliquescing). You can actually experience the hydration energy firsthand for a lot of
compounds(like anhydrous calcium chloride or sodium carbonate) by simply adding water to some in a beaker; usually they grow warm on account of the
energy released as they associate and bond with water molecules.
If a compound that has high solubility in water doesn't have a meaningfully high hydration energy or if that energy rapidly declines after a small
amount of water is absorbed, it won't be hygroscopic enough for you to notice a change. They're really just separate properties.
[Edited on 5-30-2022 by Amos]
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j_sum1
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I think Amos nailed it. It comes down to energy changes.
An ionic compound is soluble if it is energetically favourable for ions to leave a solid ionic lattice and form bonds with water in solution.
When the concentration in solution becomes sufficiently high then an equilibrium is reached and so this action is no longer energetically favourable.
This sets a limit to the solubility of a substance.
An ionic compound is hygroscopic if it is energetically favourable for water molecules in the air to adsorb onto the surface of the solid, enter the
lattice and form a hydrated salt.
These two processes are both quite different. One involves water in a liquid state. The other involves gaseous water air mixture. One involves the
formation of bonds between between ions and water in solution. The other involves the formation of bonds between ions and water in a hydrated salt,
One involves the breakdown of a crystal lattice. The other involves the reorganisation of the lattice to accommodate interstitial water molecules.
Therefore, the correlation between solubility and hygroscopicity is somewhat incidental.
I say "somewhat" because a practical correlation does exist. Both properties are increased for materials where there is a weaker ionic lattice
structure and where the bond between individual ions and the water molecule is relatively strong.
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chornedsnorkack
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One part of hygroscopicity is osmotic pressure. Solutions which have a high osmotic pressure have a low vapour pressure, low freezing point and are
strongly hygroscopic. Solutions which have a low osmotic pressure are weakly hygroscopic.
Osmotic pressure is directly dependent on solubility... but solubility in the specific terms of molal concentration as dissociated in solution.
This means that substances which have high solubility by mass may have low solubility in molal terms if they dissolve as high molecular mass solutes,
in which case they are not hygroscopic.
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