Image credit: Horia Varlan
Superabsorbent polymers: Experiments with sodium polyacrylate
This page describes experiments performed at the Physics@work 2011 exhibition.
Superabsorbent polymers (SAP) are polymers that can absorb and retain a large amount of water. They are used for example in diapers or in dehumidifiers but are also sold in winter as fake snow. Sodium polyacrylate, that has the form of a dry powder, can absorb between 100 and 300 times its mass of water.
On this page
What you need:
Most of the experiments bellow indicate quantities to mix. But these are highly dependent on the microscopic properties of the sodium polyacrylate that is used (especially the fraction of crosslinks and the molecular mass). You might need to adjust the quantities to observe the results shown bellow.
You can also do the following additional experiments that can help you to understand what allows the SAP to absorbs so much water:
In the following explanations, ions are represented as spheres and a polymer as a worm:
Dissolution of a salt in water
To understand how a SAP can absorbs water, you need to remember first how a salt dissolves in water. Table salt is a crystal made of an equal amount of positive ions of sodium and negative ions of chloride. Remember that two charges of the same sign repeal and two charges of opposite sign attract.
The equation of dissolution of salt is the following:
In the cristalline state, the atoms are arranged in a dense lattice in which each positive charge is strongly linked to negative charges by attractive electrostatic forces. Water can break these links and allow the atoms to separate as ions that swim in the surrounding water.
What is sodium polacrylate?
Sodium polyacrylate is a polymer. A polymer is a very long and big molecule that is a linear chain made of a repetition of a pattern called monomer. The chemical formula of a polymer is described by the monomer and how the monomers are linked together:
In this representation, the monomer is the part inside the parenthesis, which is repeated n times. n is typically greater than 1000 and not known exactly. Because a polymer is a linear chain, we can represent it schematically as a worm.
Dissolution of sodium polyacrylate
Solid sodium polyacrylate (dry powder) contains sodium atoms linked to oxygen atoms. This link is broken when dissolving in water like in the case of the dissolution of table salt:
In the powder state, the polymer is a folded in a dense coil. Each sodium atom is linked to an oxygen atom of the linear chain and there is no net attraction or repulsion at large scales between different parts of the polymer. In water, the sodium ions swim in the water and the linear chain becomes negatively charged. In this case, different parts of the chain repeal and the dense coil tend to unfold.
The unfolding of the polymer because of electrostatic repulsion is the key point that allows to absorb a large quantity of water, but there is one more detail needed to understand the “fake snow” state. Independent unfolded polymer chains dissolved in water would lead to a liquid state in which each chain can swim freely. But each “snow flake” looks solid and is indeed a gel that traps water. This is possible by building links between different polymer chains. These links (blue points in the figure bellow) are present in the powder state and remain in the dissolved state. All these links build a polymeric mesh. When adding water, the volume of the mesh expands because the chains unfold and water is trapped inside the network.
Last update: 2012-01-02