Sodium Polyacrylate and Water Absorption
Date: Summer 2009
Why does sodium polyacrylate absorbs water? If you could,
how can you make it?
Sodium polyacrylate does absorb a large amount of water. This shows itself in a
dramatic increase in viscosity when a small amount of the polyacrylate is "dissolved"
in water. However, making the polymer is beyond what you could do in an unsupervised
laboratory. That is a totally different question.
Sodium polyacrylate is a polymer (a long chain molecule) with ionic groups (negative
charges with accompanying positively charged sodium ions) all along its length. It is
these ionic groups that allow it to absorb a huge amount of water. The water molecules
cluster around the negative and positive charges and become attached to the polymer chain.
The reason they do this is the same reason why salt (or most other ionic compounds)
dissolves in water: the water molecules are polar - they have a negative and a positive
end and the negative end of the water molecule will be attracted to a positive ion and
the positive end of the water will be attracted to a negative ion.
Sodium polyacrylate is formed by polymerizing acrylic acid to make polyacrylate and then
treating the polymer with sodium hydroxide to change the neutral polymer into the ionic
polymer sodium polyacrylate.
Sodium polyacrylate is a polymer that has many chemical structures that naturally attract
water molecules. These structures are called "hydrogen bonding" - they are attractive
sites for both the oxygen and the hydrogen part of water.
More than that, the hydrogen bonds can be found in pores or holes within the polymer
structure such that whole water droplets can be held inside the polymer.
Sodium polyacrylate is difficult to prepare outside the lab since it involves many
chemical reaction steps. However, it is the main adsorbent material for diapers. It
is reasonably safe to handle, as long as you are careful not to inhale the fibers
when you break open the diaper lining.
Greg (Roberto Gregorius)
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Update: June 2012