Learning Aims:
  • To develop experimental series including control of variables
  • To explain the absorption of water by using structures and intermolecular forces (“functional holes”)
  • To present findings in the forms of texts and graphs
  • Baby nappy or superabsorber bought in the store
  • Sodium chloride solution ( = 0,01 mol/l, 0,1 mol/l, 1 mol/l)
  • Distilled water
  • 4 beakers (250 ml)
  • 4 paper tea filter-each with 2 paperclips to close it
  • Crucible tongs
  • Graduated cylinder
  • Weighing scale
  • Plastic bag
Suggestions for use:

To start the investigation, the teacher could demonstrate how much water a small amount of SAP can take up. This could even be done in a fun way by pretending to spill a cup of coffee which does not come out of its cup when turned over...

Referring to upper secondary level students, the investigation of properties and factors affecting them could be developed by the students themselves. In case this is not possible, the following activities can be suggested:

  • Examine the layered assembly of a baby nappy and find out which layer contains the superabsorbing polymer.
  • Pull apart the fibrous web with the pellets of superabsorber in a plastic bag. Blow into the bag and separate the pellets from the fibrous web by shaking the (closed) bag vigorously.
  • Add 200 ml of different concentrations of sodium chloride solution to each of three beakers and add 200 ml of distilled water to a fourth beaker.
  • Fill each of the 4 tea filters with 2 g of the superabsorber, close with the paper clips and hang one in each beaker.
  • Remove the filter after 45 minutes and allow excess liquid to drip off over the beakers for about 5 minutes (might have to use a funnel).
  • Weigh the expanded tea filters and record the volume of the liquid remaining in the beakers.
  • Repeat steps 3-6 with the aim to find out how the pH affects the retaining properties.
Possible questions:
  • How could you explain the absorption of water with a simple model?
  • How can you explain that the water does not get out again?
  • How can you explain the influence of pH and concentration of ions on the structure-property relation?
  • Compare the properties of SAP to other absorbing substances you could find at home. How could you explain the different properties through the different structures?
  • Looking at nappies, how do the properties of urine differ from pure water? How could that influence the properties of SAP?

If the facilities and regulations allow for it, the following synthesis could be carried out:

Safety Instructions

Wear protective gloves and goggles. Work in the fume hood.

  • Distilled water
  • Acrylic acid
  • N,N’-methylenebisacrylamid solution (MBA, w= 1%) (Xn)
  • Ascorbic acid solution (w= 1,9%)
  • H2O2 solution (w=0,6%) (C)
  • Sodium hydroxide solution (c=0,5 mol/l) (C)
  • Ethanol (T, F+)
  • Beakers (250 ml)
  • Syringes (1ml, 2ml, 5ml)
  • Temperature sensor
  • Spatula
  • Glass stirrers
  • 2 pointed tweezers
  • Spoon
  • Crystallizing dish
  • Graduated cylinders (50ml, 100ml)

(a) In a beaker, add the following solutions in the given order (use the syringes for measuring the liquids):

  • 2.7 ml distilled water
  • 2.0 ml acrylic acid
  • 0.2 ml MBA-solution (w=1%)
  • 1.4 ml ascorbic acid solution (w=1.9%)
  • 0.7 ml H2O2 solution (w=0.6%)

Mix the contents of the beaker by gently swirling it. Let the beaker sit at room temperature. Record the temperature change during the reaction. The next part (part b) can follow right after cooling or the following day.

b) Transfer the product obtained to an evaporating dish. Pour 40 ml of sodium hydroxide solution c=0.5 mol/l) over the product.

  • With the help of the tweezers, separate the polymer into little pieces (be sure to wear protective gloves and goggles).
  • After complete absorption of the liquid by the gel (about 30 minutes), add 100 ml of ethanol (do so in the fume hood).
  • The content of the evaporating dish needs to be swirled around carefully from time to time (do not stir with a utensil because the gel is very sticky).
  • After about 10 minutes, remove the remaining liquid (make sure to dispose of this as organic solvent) and add 60 ml of ethanol.
  • Repeat the last step twice (after waiting 10 minutes each time). Depending on the consistency of the polymer pieces, they might have to be separated from each other or the container with the help of the tweezers.
  • After the last removal of the liquid, leave the product sit under the fume hood overnight (or put into the oven for 1 hour at 80°C).