Sodium polyacrylate is a white powder with a characteristic odor. But when we add water to this powder, instead of forming an aqueous solution, it instantaneously solidifies and forms more amount of powder like substance, also called "hot ice". Why and how this phenomenon occur?
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asked Jul 19, 2015 at 5:44
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$\begingroup$ Strictly speaking, the polyacrylate isn't solidifying as it starts as a solid and ends as a solid. The solid starting material is absorbing vast quantities of water and expanding as a result. $\endgroup$– matt_blackCommented May 9, 2018 at 12:28
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2 Answers
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Sodium polyacrylate is an example of a super-absorbing polymer. It is a cross-linked (network) polymer that contains sodium atoms. It absorbs water by a process called osmosis. When the (sodium-containing) polymer is placed in contact with water, there is a tendency for the sodium to distribute equally between the network and the water.
That means, some of the sodium atoms want to leave the network and move to the water. When these sodium atoms leave, they are replaced with water molecules. Water swells the polymer network to try to keep the sodium concentration balanced between the polymer and the water.
The cross-links that connect the chains together prevent them from dissolving/breaking apart in the water. The resulting product appears very similar to fake snow, as you mentioned, a solidified powder like substance.
Sodium polyacrylate can absorb it’s own weight 300x over making it an amazing chemical to aid in water cleanups.
The question was good.
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The answer would be clear when we know the mechanism why sodium polyacrylate absorbs water:
Mechanism
It consists of a set of polymeric chains that are parallel to each other and regularly linked to each other by cross-linking agents, thus forming a network.
When water comes into contact with one of these chains, it is drawn into the molecule by osmosis. Water rapidly migrates into the interior of the polymer network where it is stored.
Tip: Metal ion can deeply reduce or destroy the water absorbing capacity of sodium polyacrylate.
