Polydimethylsiloxane

One of the most widely used silicon-based organic polymers is polydimethylsiloxane or PDMS. The formula for this polymer is shown on the right. Because it is optically clear and considered to be inert, non-toxic and non-flammable, it is used in medical devices including contact lenses, and in personal care products such as shampoo in which it makes hair shiny and slippery.

Northwestern University scientists have also found that compressing polymers such as PDMS releases free radicals which can be used to power chemical reactions in water.

To demonstrate this, the scientists took a Nike Air LeBron, similar to that shown on the right, and filled the air pockets in the polymer sole of the shoe with a solution of a compound that lights up in the presence of free radicals. After a person walked in the shoe for 30 minutes or more, enough radicals were created to generate a blue glow visible to the naked eye.

The scientists demonstrated that they can squeeze a polymer, such as what might be found in a shoe, tire or plastic bag, and get a mechanical-to-chemical energy conversion of up to 30%, about the same as the energy efficiency of a car engine. You could recharge a battery from the energy produced by walking or by driving a car!
Just imagine how much energy could be released by compacting the millions of plastic bags in rubbish tips around the world!

Reference:
H. Tarik Baytekin, Bilge Baytekin, Bartosz A. Grzybowski. Mechanoradicals Created in “Polymeric Sponges” Drive Reactions in Aqueous Media. Angewandte Chemie, 2012; DOI: 10.1002/ange.201108110

Further Reading:
Polymers and Polymerization
Covalent Bonding


Suggested Study Questions:

1. Polydimethylsiloxane can be prepared from dimethylchlorosilane, Si(CH₃)₂Cl₂, and water. Draw a possible structural formula for dimethylchlorosilane.
2. When dimethylchlorosilane reacts with water to form polydimethylsiloxane, HCl is formed as a by-product of the reaction. Write a chemical equation to represent this polymerization reaction.
3. Draw a structural formula for polydimethylsiloxane showing 3 repeating monomer units.
4. Polydimethylsiloxane can also be produced using a monomer in which acetate groups, COO, replace the chlorines in dimethylchlorosilane. Draw a possible structural formula for this monomer.
5. What is the expected by-product of the reaction between the monomers in question 4 and water to produce polydimethylsiloxane?
6. The food additive E900 is mixture of polydimethylsiloxane and silicon dioxide. It is used as an antifoaming and anticaking agent in many processed foods including chicken nuggets and french fries. What properties of polydimethylsiloxane might make it suitable for use in fried food?
7. Write the chemical formula for silicon dioxide.
8. Compare the structural formula of silicon dioxide and polydimethylsiloxane. How would you expect the three dimensional structure of silicon dioxide to differ from the three dimensional structure of polydimethylsiloxane?
9. Silicon dioxide is a hard substance while polydimethylsiloxane feels slippery. Explain this difference based on your understanding of the three dimensional structure of each solid.
   

AUS-e-NEWS March 2012

The March 2012 issue of AUS-e-NEWS takes a look at recent research into the presence of toxic aldehydes in fried food.

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Magnetic Detergent

Scientists have long been searching for a way to control surfactants once they are in solution to increase their ability to dissolve oils in water and then remove them from a system. Surfactants sensitive to light, carbon dioxide or changes in pH, temperature or pressure have all been investigated. Bristol University scientists have produced the world's first magnetic surfactant by dissolving iron in inert detergents composed of chloride and bromide ions. The addition of the iron creates magnetic centres within the detergent particles.

To test the properties of their magnetic detergent, the scientists introduced a magnet to a test tube containing their new detergent lying beneath a less dense organic solution. When the magnet was introduced the iron-rich detergent overcame both gravity and surface tension between the water and oil, to levitate through the organic solvent and reach the source of the magnetic energy, demonstrating its magnetic properties.

Commercial development of magnetic detergents could be a big bonus for the environment. In the future oil spills could be cleaned up using magnetic detergents, and the oildetergent mixture removed using magnets.

Reference
Paul Brown, Alexey Bushmelev, Craig P. Butts, Jing Cheng, Julian Eastoe, Isabelle Grillo, Richard K. Heenan, Annette M. Schmidt. Magnetic Control over Liquid Surface Properties with Responsive Surfactants. Angewandte Chemie International Edition, 2012; DOI: 10.1002/anie.201108010

Further Reading
Soaps and Saponification
Detergents
Transition Metals
Ligands and Complex Ions

Suggested Study Questions

1. What other elements, besides iron, could be used to produce magnetic detergents?
   
2. Use the concept of the micelle to explain how iron can create magnetic centres within these new detergent mixtures.
3. For the experiment described in the second paragraph,
   
   • write a suitable aim
   • write a suitable method (procedure)
   • draw the results of the experiment as described
   • write a suitable conclusion
4. You have been asked to determine the effectiveness of this new magnetic detergent at different pH levels.
   • Write a suitable hypothesis.
     
   • If you were to undertake an experiment to test this hypothesis, what variables would need to be controlled?
     

5. You have been asked to determine the effectiveness of this new magnetic detergent at different pH levels.

   • write a suitable aim
   • write a suitable method (procedure)

6. Surfactants sensitive to light, carbon dioxide or changes in pH, temperature or pressure have all been investigated. Why do you think that a magnetic surfactant might be a better alternative for cleaning up oil spills in natural waterways?

7. What disadvantages can you see in using magnetic surfactants in cleaning up oil spills?
   

   
   
   
   

4 Water Experiments to Download

The International Year of Chemistry may be over, but you can still contribute to the Global Experiment until the end of March.

There are 4 experiments to conduct using water:

1. Acidity
2. Salinity
3. Build a water filtration unit using household materials
4. Using solar energy to purify water

You can download the details of all 4 experiments at

http://water.chemistry2011.org/web/iyc/experiments

You will be able to conduct the 4 experiments in class and submit your results to the website until 31^st March.

Even if you can't upload your results to the website, the 4 experiments above make a terrific teaching and learning resource for the future!

2012 Chemistry Calendar

The quintessential 2012 chemistry calendar is now available for download from ausetute.com.au

Now there's no excuse for forgetting to celebrate the birthday of some celebrated chemists !