Kids, which will dissolve more quickly, a Tic-Tac™ mint or a mouthful of cotton candy?  The answer may seem easy but why does it happen?  Exploring this can also help you to understand why, in the world of nanoscience, nanoparticles are so unique. A nanometer is a billionth of a meter, or 60,000 times smaller than a human hair. To help you realize how and why properties change dramatically at the nanometer scale, we’ll look at how an increase in surface area increases the reactivity of particles. 

Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

In March 2007, the ChemShorts article on Alka-Seltzer™ tablets established that if a tablet is divided into smaller pieces the surface area increases. In this experiment you will dissolve a sugar tablet (or cube or mint) and an equal mass of sugar crystals (or cotton candy) to see if there is a difference in how fast they dissolve.  Cotton candy can represent carbon nanotubes and the tic-tac can represent a chunk of graphite for this experiment.

You need small snack-size (1 oz, 28 gm) bags of fluffy cotton candy and a small package of Tic-Tac™ mints for this demonstration. There is roughly the same amount of sugar in one tic-tac mint (about 0.5 gm) as in about 1 tablespoon of cotton candy.  Devise a controlled way to measure the rate of dissolution for these two forms of sugar.  As a quick test you can try it out in your mouth, but think of better ways to control the variables (how to dissolve the sugar, weigh the sugar, what liquid to use, how to measure the time, etc.).

For large materials that you can see with your eyes (larger than micrometers), the percentage of atoms at the surface is tiny compared to the total number of atoms in the material. If the pieces are continually cut, the surface area will increase but the total volume does not change.

This is significant in nanoscience where nanoparticles acquire new chemical or physical properties different from bulk materials. Some properties of nanoparticles are due to the surface area of the particle. Small nanoparticles have a larger percentage of atoms on the surface. Small particles have a high surface to volume ratio.

Finer sugar grains have a vastly larger surface area than larger chunks. The larger the exposed surface, the faster the dissolution (or reaction rate) because the liquid has greater access to the sugar. 

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Kathleen Carrado Gregar, PhD, Argonne National Labs 
[email protected]
May 2011

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References:
University of Houston College of Education, http://atlantis.coe.uh.edu/texasipc/units/solution/sugar.pdf  and http://www.materialsworldmodules.org/modules/intro_to_nanoscale.shtml