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Saturday, August 20, 2011

How Superabsorbents Work

Dry Polyacrylamide
Have you ever wondered how a diaper can absorb so much water?  Or you may have seen disposable rags claiming to soak up hundreds of times their mass in liquid.  This absorbance capability is all thanks to a family of polymers known as superabsorbents.
Current uses for superabsorbents are in diapers, medical bandages, electrical circuit components, and potting soil additives.  The ability to soak up 300 times their weight in water makes superabsorbents very successful at keeping things dry or storing water.
Polyacrylamide after absorbing 300
 times its mass in water
Polyacrylamide is the king of superabsorbents, transforming from a tiny white pellet to a large colorless blob when immersed in water.  This pellet can quickly absorb over 300 times its weight in water.  Polyacrylamide is also easily regenerated with several days of exposure to dry air.
            The structure of polyacrylamide contains repeating crosslinked chains containing amide groups.  The amide groups, made up of a Nitrogen atom bonded to two Hydrogen atoms, has Hydrogen bonding forces which strongly attract water.  Water diffuses into the polymer network and is held within by strong attraction to the Amide groups.
In a sense, the polymer acts as a semi-permeable membrane.  When a concentrated solution of salt, alcohol, or acid is exposed to the polymer, very little water will diffuse in since there are more attractive forces outside the polymer than inside.  When a sugar solution is exposed to the polymer, the network will freely absorb both water and sugar molecules.  This is because dissolved sugar is small enough to penetrate the polymer network, and has little attractive force on water.
By observing the mechanism of absorbance, it is easy to see how these polymers are capable of holding so much water.  The uses for superabsorbents are constantly increasing with new knowledge of absorbing mechanisms.  Due to its affinity for water and exclusion of large or hydrophilic molecules, this technology could be applied to separations of water from mixtures.  Future uses currently under investigation even involve concentrating dissolved solids using superabsorbents.