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Q & A: de-ionized water and emulsions

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Most recent answer: 02/21/2015
Q:
There is a popular shampoo and conditioner made with deionized water. I noticed there is no emulsifier or solubilizer listed in their ingredient list. Does deionized water allow you to combine oils with water without separation? How is this possible?
- Rebecca (age 44)
Laramie, WY, US
A:

That's an insightful observation and question. I'm not sure if your idea fully accounts for what you observe but it does make sense. Emulsions stay reasonably stable for a long time because electrostatic repulsion between the little droplets (which all have the same sign of charge) keeps them from coalescing. The charge on the lipids can arise because they're a bit acidic, giving off H+ ions and remaining negatively charged. The more other ions that are present in the solution, the more their arrangements screen the electrostatic repulsion and, typically, the less stable the emulsion will be. When you want to make an emulsion unstable and have all the drops coalesce into a single lump, one way to do it is by adding the right types of salt ions. Tofu, for example, is typically made by adding calcium or magnesium salts to a soy emulsion to induce coagulation. Those divalent cations (Ca+2, Mg+2) are better than monovalent ones like those of sodium or potassium (Na+, K+) at screening the repulsion. So maybe with very few divalent cations and a minimal number (just enough to balance the charge on the emulsion) of monovalent ions, the emulsion is stable enough not to need special emulsifiers. A chemist would give a more confident answer.

Mike W.

p.s. On second thought, this is a great problem for an experiment. Take some of these emulsion-free products and add a few drops of the liquid from packaged tofu. Do they coagulate? How many drops does it take per tablespoon of product? Then you can compare with some shampoos and conditioners that do list emulsifiers as ingredients. If there's a clear difference, then it sounds like your idea may well be right. If not, then perhaps the products made with distilled water are just using emulsifiers described by chemical name rather than with the word "emulsifiers".


(published on 02/21/2015)

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