ChemCafé » Funny chemistry

Mentos-Diet Coke Geyser: Heterogeneous Catalysis in Popular Culture

admin — Mon, 03 Aug 2009 19:55:34 +0000

The Internet is seething with videos of “chemical geysers“, the most famous -and spectacular- being the one one obtains by adding mint Mentos candies to diet coke. A host of moreless reliable explanations can be found, but they are seldom based on good old science. The main explanation to the phenomenon -which by the way tells why the ‘experiment’ works with any sparkling beverage- relies on a very chemical concept known as heterogenous catalysis. Usually chemists use this phenomenon to promote reactions like hydrogenations by adding a porous catalyst such as palladium on carbon to their reaction medium. What happens (in the case of a ‘chemical fountain’) is that the pores in the mentos candy facilitate nucleation of carbon dioxide molecules which are dissolved in the beverage, which results in a fast formation of bubbles, resulting in the ‘geyser effect’ – nucleation is also what you observe when you see bubbles forming in a cup of Champagne. Of course, the faster the bubbles form, the faster they will escape from the bottle, ultimately creating this foaming mixture that can be projected meters away!

A detailed study conducted at Appalachian State University in Boone, North Carolina, by Dr. Tonya S. Coffey, showed that, in addition to the candy roughness and porosity (they even made AFM study on Mentos candies !!), a second factor deeply influences the success or failure of a coke fountain: the surface tension of the liquid directly influences the bubbles formation rate, and a liquid containing an artificial sweetener (such as aspartam in diet coke) has a lower surface tension than a liquid containing a natural sugar. Furthermore, the gum arabic contained in mentos candies also brings a contribution to lowering the surface tension of the mixture.

It was also observed than, contrary to what is frequently postulated, no acid-base reaction is involved (actually, there is no base in a mentos candy, and the pH of the beverage does not change during the course of the ‘reaction’), the caffeine present in coke has hardly any influence on the result, contrary to the speed at which mentos candies sink into the liquid, which turns out to be of great importance!

In summary, the chemical geyser is due to the presence of a rough, porous ‘material’, in addition to chemicals that decrease the surface tension of the liquid, which contains dissolved carbon dioxide: these are the three basic ingredients of this funny recipe.

For more details:
T. S. Coffey, Am. J. Phys 2008, 76, 6, 551-557. DOI: 10.1119/1.2888546

Thanks Sophie for having suggested the topic of this post – btw, ideas always welcome!

Nerdy Science Commercials for the (Chem)Coffee Break

admin — Mon, 11 May 2009 21:30:18 +0000

If you had appreciated the PCR song (still can’t believe the guy kisses the thermo-cycler…) or the famous epMotion video, it’s likely you’ll also like these commercials for SciFinder… They really make it look like an exceptional instrument, allowing you to succeed easily in research, but also making you so much happier in the out-of-the-lab life !! (Actually, SciFinder IS of course an incredible tool, I can simply not imagine how people were working without it…)

Enjoy…

www.youtube.com/watch?v=4_WoAR9GdCQ

www.youtube.com/watch?v=AEUXJ7ydpKI

Actually, writing this post I just found out Bio-Rad made a sequel to the PCR song… it’s definitely worth a few more minutes… the only risk is that you may be humming the song till the end of the day… or even later !! Now, before forwarding the links to your friends and relatives to show them how scientists can be funny and sexy, remember that probably more than 95% of the world population doesn’t have a clue on what it’s talking about, and therefore would not find this funny at all…

www.youtube.com/watch?v=CQEaX3MiDow

Well next time, I’ll come back with more scientific stuff such as molecules as nanoreactors with chemistry inside!

Pacman complexes – Chemists imagination at work

admin — Sun, 03 May 2009 18:57:23 +0000

Doing my weekly lit overview, I found on the Chem Comm Advance articles page a review, which title immediately attracted my eye. During my studies, I learnt how chemists can be smart when having to coin new names for molecules and techniques – think for example about the DEAD reagent, the INADEQUATE NMR technique, or the scorpionate ligands – but there I found a category I had never heard of: the PACMAN complexes… It reminded me the computer game I used to play on an old Commodore 128 many years ago – and yes! It was actually it! With some imagination, the molecules of interest can be associated to the famous yellow, pizza-missing-a-slice, figure. The authors probably don’t indicate how many pac-dots these molecules can eat, but many more informations can be found here!