It seems that nitrous oxide $(\ce{N2O})$ is frequently used to create whipped cream. But why can't just regular nitrogen gas $(\ce{N2})$ be used instead?
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2$\begingroup$ Probably because of the much higher boiling point, i.e. you need to apply less pressure to store it. But that's just a wild guess that sprung to mind. $\endgroup$– Martin - マーチン ♦Jun 20, 2017 at 8:09
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2$\begingroup$ Whipping cream is a surface tension issue so there might also be that oxides work better, @Martin. $\endgroup$– StianJun 20, 2017 at 8:48
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$\begingroup$ If you make the whipped cream yourself, air and a whisk works fine. It gets more complicated if you want to create a spray bottle. $\endgroup$– Karsten ♦Jan 1 at 0:13
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3 Answers
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There are two ways to efficiently make an aerosol product:
- Use a gas that liquifies under the pressure inside the can. For example, butane lighters. Nitrogen is one of the "fixed gases", meaning it's a gas under most conditions (but take a look at the temperatures and pressures needed for liquid nitrogen—it's not going to ever be found in consumer products).
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- Use a gas that is highly soluble in the liquid (carrier) and that will "substantially" vaporize when the higher pressure inside the can is reduced to atmospheric.
The US government restricts the pressures that can be used in aerosol cans (and requires 100% quality control testing—when's the last time you heard of an aerosol can exploding? [although it does happen]). If you cut up a can, you'll notice that it's pretty flimsy. The higher the pressure (and a gas that has been dissolved in a liquid doesn't exert much pressure), the more expensive it will be to build the container (aerosol can).
Thus, the fixed gases are almost never used, except in some medical products. Why? Because they just aren't soluble enough to help move the liquid (or, in other cases, solid) out of the can and also to disperse it into a very fine mist. The customer wants basically one thing when using an aerosol: uniform, consistent spray from first to last drop. Using a very soluble gas helps, and using one with a boiling point near room temperature also helps. But the laws of thermodynamics say that the temperature of the can will drop as you spray out its contents. This may dramatically interfere with the liquid-to-gas phase change, while solubility is less sensitive to temperature. The trick is to make a product (and I've made some) that sprays out consistently and also doesn't leave so much left behind in the can that the customer feels ripped off.
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According to Nitrous oxide, carbon dioxide, and their mixtures as propellants, Proc. Chem. Specialties Mtrs. Assoc., June 1950, page 45, William Strobach, $\ce{CO2}$ and $\ce{N2O}$ are both suitable because they each have some solubility in both the aqueous and oil phase of the cream emulsion. $85\%~\ce{N2O}/15\%~\ce{CO2}$ has the most preferable taste, with too much $\ce{CO2}$ being too acidic. Other references say that the $\ce{N2O}$ provides a sweet taste, which is partially offset by the sour taste of the $\ce{CO2}.$
Freon- C318 Propellent for Aerosol Food Products, F. W. Blodgett and R. C. Webster, in a 1958 issue of the same journal, reports that adding perfluorocyclobutane to $\ce{N2O}$ resulted in a firmer, longer lasting cream.
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1$\begingroup$ Ahh, so just plain nitrogen gas (N2) would be "too inert" for that purpose? $\endgroup$– ManRowJun 20, 2017 at 11:06
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7$\begingroup$ I don't think it would be soluble enough. It seems to be solubility that is desirable. $\endgroup$– DavePhDJun 20, 2017 at 11:09
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$\begingroup$ Well, what I meant was, is the nitrous more soluble possibly because it may somewhat react more with the cream? $\endgroup$– ManRowJun 20, 2017 at 11:12
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3$\begingroup$ @ManRow I don't think "react" is the right word, maybe "interact". There is no chemical reaction. $\endgroup$– DavePhDJun 20, 2017 at 11:13
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3$\begingroup$ Ahh, key factor I seem to have missed -- nitrous oxide is probably a lot more "polar" than N2! And that then probably explains the differences in their respective (even somewhat inert) chemistries as well! $\endgroup$– ManRowJun 20, 2017 at 12:30
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The critical temperature of nitrous oxide is 36 Celsius. That means under pressure and shaken well it is distributed thruout the cream as a dissolved or nanodroplet liquid. As the pressure is released microbubbles form thruout the cream forcing a foam emulsion out. Similar propellants are iso and neopentane that happen to be flammable, not a good idea about birthday candles or food; I think isopentane is used in shaving cream. Aerosol cans are again a chemical art form to maintain stability, homogeneity and a pleasing product. An alternative to N2O might be CO2 but that that forms dry ice crystals upon releasing the pressure. N2O has a lower FP and probably tastes better.
