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Superabsorbent polymers: Experiments with sodium polyacrylate

Introduction

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.

Sodium polyacrylate powder

Sodium polyacrylate as a dry powder


On this page


What you need

What you need:

  • Sodium polyacrylate. It is a white powder that you can find sold as “fake snow” or “instant snow” in many shops during the winter. You can usually also buy some at any time in the year on Amazon or eBay.
  • Water
  • Table salt
  • Glasses and plates
  • Water
  • Precise weighing scale (precision < 1 g) and measuring cup if you want to do measurements of how much the polymer can absorb

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.


Experiments

  1. In a glass containing a teaspoon of SAP, add half a glass of water. The SAP absorbs all the water and seems to be still dry. This is the “fake snow” state of the polymer.

    Polymer and water Snow

    A small amount of SAP mixed with a lot of water gives a solid gel (fake snow).

  2. Add slowly more water by stirring at the same time. You can measure the volume after which the polymer cannot absorb water anymore (when the mix becomes wet, or when water is flowing out of the polymer when you incline the glass). This is the “mud” state of the polymer.

    Snow state Mud state

    Snow state and mud state of the SAP

  3. Add to the previous SAP, which is saturated in water, 1/2 spoon of table salt and stir. By dissolving the salt, the mix becomes liquid with some small flakes of polymer. The SAP absorbs less water when it contains salt.

  4. You can measure the amounts that the SAP can absorbs for different liquids: tap water, distilled water, water with vinegar, water with lemon juice...

You can also do the following additional experiments that can help you to understand what allows the SAP to absorbs so much water:

  1. Make some SAP in the “mud” state with water. Let it dry in a plate until you obtain a powder again (it can take more than 24 hours). Add water to that powder. The polymer can absorbs again a large amount of liquid. This is a reversible phenomenon.

  2. Repeat the previous experiment with salted “mud”: Dissolve one spoon of salt in a glass of water. In another glass, put 1 g of SAP. Add slowly the salted solution until you reach a “mud” state. This happens for a small volume, as the SAP absorbs less a salted solution. Let this solution dry. Add water and measure how much the SAP can absorb. The polymer can not absorb a lot of water. Why?


Explanations

In the following explanations, ions are represented as spheres and a polymer as a worm:

Notations

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.

Forces between charges

The equation of dissolution of salt is the following:

Equation of dissolution of sodium chlorure

Dissolution of sodium chlorure

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:

Sodium polyacrylate

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:

Equation of dissolution of sodium polyacrylate

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.

Dissolution of a SAP

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.

Crosslinked polymer


References

Last update: 2012-01-02
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