Used commonly in diapers, sodium polyacrylate is a polymer that is recognizably found in supermarket "growing" animals, which also contain polyvinyl acetate (to maintain shape and color). Sodium polyacrylate is unique for its ability to absorb as much as 800 times its own mass in water. Sodium polyacrylate is a polymer form of -CH2-CH(COONa)- and is extremely hydrophyllic (water loving).
The mechanism by which sodium polyacrylate absorbes water deserves some mention. When dry, the polymer is coiled tightly and forms a selectively permeable pseudo-membrane. The chains of the polymer in this state are lined with carboxyl groups. When hydrated, the carboxyl groups dissociate, forming carboxylate (COO-1) ions. The ions repel each other, expanding the chain, which expands the overall coil shape of the polymer. As the coil expands, water equalizes the osmotic potential between the inside of the chain (rich in sodium) and the outside (low in sodium) until the relative concentration of sodium inside the membrane is negligible.
Since sodium polyacrylate's behavoir relies on osmotic balance, its absorbency is limited by the purity of the water it is exposed to. In fact, sodium polyacrylate toys (gro-saurs, etc...) will shrink when exposed to salt. As urine contains salt, diapers that use sodium polyacrylate are less effective with more concentrated urine.

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