1. Why does Mentos make Cola “explode“?
Introduction
The Phenomenon
Different Explanations
The Composition of Cola
CO2 in Cola
Henry‘s Law
Two States of Equilibrium
CO2 in Solution
The Bubbles – Young-Laplace
The Bubbles – Nucleation
The Mentos Effect
Coca Cola vs. Diet Coke vs. Cheap Cola
Philipp Schüdel- Why does Mentos make Cola “explode”?

2. Philipp Schüdel- Why does Mentos make Cola “explode”?
Introduction
Philipp Schüdel- Why does Mentos make Cola “explode”?

3. Philipp Schüdel- Why does Mentos make Cola “explode”?
The Phenomenon
White Mentos, thrown into a bottle of Cola, results in a violent eruption of the soda-drink
It is found, that this works best with Diet Coke but the principle works with any drink containing CO2
Why does that happen?
Why does it work best with Diet Coke?
Philipp Schüdel- Why does Mentos make Cola “explode”?

4. Philipp Schüdel- Why does Mentos make Cola “explode”?
Different Explanations
A chemical reaction? According to a certain “professor“ from Brasil, some of the aromas in Mentos and the Acesulfam K found in Diet Coke react to give an explosive substance called Ta9V4.
Catalysis of the desorption of CO2 by heterogenous nucleation
Philipp Schüdel- Why does Mentos make Cola “explode”?

5. Philipp Schüdel- Why does Mentos make Cola “explode”?
The Composition of Cola
Main ingredients of normal Coca Cola: water, sugar, CO2 (called carbonic acid), phosphorus acid, food dyes, other
Main ingredients of Diet Coke: water, CO2 (called carbonic acid), phosphorus acid, citric acid, acesulfame K, aspartame, food dyes, other
=> Most probable reason for violent eruption: CO2
Philipp Schüdel- Why does Mentos make Cola “explode”?

6. Philipp Schüdel- Why does Mentos make Cola “explode”?
CO2 in Cola
CO2 is dissolved in the Cola Mixture by applying a pressure of approximately 4 atmospheres
Questions: - What does dissolved CO2 look like? What happens when the pressure is reset to 1 atm? (opening of the bottle)
In approximation, Cola can be described as a solution of CO2 in water. For such a system, Henry‘s law can be applied.
Philipp Schüdel- Why does Mentos make Cola “explode”?

7. Philipp Schüdel- Why does Mentos make Cola “explode”?
Henry‘s Law
The chemical potential of CO2 in water is
It has to be in equilibrium with its gas phase
The equation can be transformed in a way that the term on the left is a constant.
That means that the term on the right side has to be a constant, too
For an ideal solution, we obtain Henry‘s law
Philipp Schüdel- Why does Mentos make Cola “explode”?

8. Philipp Schüdel- Why does Mentos make Cola “explode”?
The Phenomenon
Another interesting application of Henry‘s law: The so called “Martini Effect“:
Divers can suffer from a sort of drunkness in deep waters. This is due to the higher solubility of N2 in blood at higher pressure (Henry)
An empirical law connects the depth of the diver to the amount of Martinis he would have to drink in order to get the same effect. Every ten meters corresponds to one Martini where the feeling of being drunk starts at 30 meters.
Because a Martini is not a very common unit, a recalculation has been done to form the “Beer Effect“ according to which every 17 meters is like drinking half a litre of beer.
Philipp Schüdel- Why does Mentos make Cola “explode”?

9. Philipp Schüdel- Why does Mentos make Cola “explode”?
Two States of Equilibrium
Henry‘s law can also be written this way: c(CO2)= KH*p(CO2)
with KH= atm*l*mol-1 at 25°C
Sealed bottle of Cola:
p(CO2)= 4 atm => c(CO2)= mol*l-1
Opened bottle:
p(CO2)= 2.96*10-4 atm => c(CO2)= 0,010 mmol*l-1
How does Mentos help in getting from one state of equilibrium to the other?
What does CO2 dissolved in water look like?
Philipp Schüdel- Why does Mentos make Cola “explode”?

10. Philipp Schüdel- Why does Mentos make Cola “explode”?
CO2 in solution
Contrary to common belief, CO2 in water is not carbonic acid H2CO3 or hydrogencarbonate HCO3-. Only about 1% of the dissolved CO2 reacts to give these species. In Cola, this reaction is even further reduced due to the low pH (phosphoric acid).
According to some scientists, CO2 in solution corresponds to small bubbles of a size of around 100 nm being dispersed in water. That would mean that it‘s not an actual solution but a metastable two phase system.
The best explanation is given by the CO2 forming a hydrationkomplex with 12 water molecules
Philipp Schüdel- Why does Mentos make Cola “explode”?

11. Philipp Schüdel- Why does Mentos make Cola “explode”?
The Bubbles - Young-Laplace
How does the system get to equilibrium?
Bubbles formed in the solution are only stable above a certain diameter due to the surface tension and the pressure difference between the inside and the outside of a bubble.
The free energy of the surface is G= 4pr2g where g is the surface tension
A change of the radius of the bubble results in a change of the free energy: DG= 8pgrdr
This is compensated by the pressure-volume-work
Dp*dV= Dp*4pr2dr
In equilibrium, those two terms are the same and we obtain the Young-Laplace-Equation:
Dp= 2g/r
One can see that the difference in pressure and therefore the instability of the bubble is smaller for small surface tensions and big bubble diameters.
Philipp Schüdel- Why does Mentos make Cola “explode”?

12. Philipp Schüdel- Why does Mentos make Cola “explode”?
The Bubbles - Nucleation
All units in this diagram are arbituary
Philipp Schüdel- Why does Mentos make Cola “explode”?

13. Philipp Schüdel- Why does Mentos make Cola “explode”?
The Bubbles - Nucleation
In a homogeneous solution, the bubbles can only surpass the critical diameter through pressure fluctuations. This is called homogeneous nucleation.
Another example of homogenous nucleation are hand warming pads, where an overcritical liquid is crystallized by inducing nucleation through a charge emitted by a small metal plate.
The process of nucleation can also be catalyzed, which is called heterogeneous nucleation. A good example is rice which is put into Weizen. The CO2 in the beer can form bubbles on the rice-surface which grow and make the rice “dance“.
Philipp Schüdel- Why does Mentos make Cola “explode”?

14. Philipp Schüdel- Why does Mentos make Cola “explode”?
The Bubbles - Nucleation
The formation of raindrops is also a nucleation process. Rain can only occur if small particles like dust, pollen etc. are present which is why it never rains in the Antarctic.
Furthermore, in regions with high particle concentration, like big cities, there are too many nucleation sites. Because of that, the raindrops can never grow enough in order to be able to fall.
Philipp Schüdel- Why does Mentos make Cola “explode”?

15. Philipp Schüdel- Why does Mentos make Cola “explode”?
The Mentos Effect
How do Mentos affect the CO2 in solution?
Atomic Force Microscopy (AFM) pictures have shown that Mentos have a very rough surface.
On the cavities of this surface, CO2-molecules can accumulate to form bubbles which surpass the critical size.
Philipp Schüdel- Why does Mentos make Cola “explode”?

16. Philipp Schüdel- Why does Mentos make Cola “explode”?
Coca Cola vs. Diet Coke vs. Cheap Cola
Why does the Mentos-Cola experiment work best with Diet Coke? => Higher CO2 concentration?
According to the Young-Laplace-equation ( Dp= 2g/r ) a higher surface tension means that for a given bubble size, the pressure difference between the inside and the outside of the bubble is bigger than for a lower surface tension. That means that the bubbles in a system with high surface tension must be bigger in oder to be stable.
Surface tension measurements of normal Coca Cola, Diet Coke and cheap Cola showed that there is almost no difference.
The reason for a higher CO2 concentration could also be a higher pressure applied in the production process, but an actual measurement of the amount of dissolved CO2 showed no difference between the sodas.
Philipp Schüdel- Why does Mentos make Cola “explode”?

17. Philipp Schüdel- Why does Mentos make Cola “explode”?
Coca Cola vs. Diet Coke vs. Cheap Cola
In conclusion, the real reason why the experiment works best with Diet Coke has yet to be found. The problem seems to be a bit more complicated than expected and there is no valid explanation found so far.
Philipp Schüdel- Why does Mentos make Cola “explode”?

18. Thank you for your attention and have a nice day!
Philipp Schüdel- Why does Mentos make Cola “explode”?