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To save soap during this pandemic, I mixed foaming soap of two different brands and then the solution became cloudy (both originally transparent) . It seems foaming okay. But does this make the soap less efficient in washing away germs/virus? Does the foaming show that the mixture is still working as a regular soap?

I know that soap is both hydrophilic and hydrophobia which makes it capable of washing away bacterias. Is it possible for the ingredients of two soap make it non polar?

Thanks.


According to the answers, more details are added. The ingredients of the two foaming soaps are given below. According to the ingredients, can we tell if the effectiveness of soap is reduced by mixing them?

  1. About 1/3 is method foaming hand soap with ingredients--WATER (AQUA), SODIUM LAURYL SULFATE*, COCAMIDOPROPYL BETAINE*, GLYCERIN*, ALOE BARBADENSIS EXTRACT*, TOCOPHERYL ACETATE (VITAMIN E), CITRIC ACID, SODIUM CHLORIDE*, SODIUM CITRATE*, METHYLISOTHIAZOLINONE, METHYLCHLOROISOTHIAZOLINONE, FRAGRANCE (PARFUM: LINALOOL, LYRAL), EXTERNAL VIOLET 2 (CI 60730), BLUE 1 (CI 42090).

  2. 2/3 is equate anti-bacteria foaming soap with ingredients in the picture. equate foaming

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    $\begingroup$ The answer to your last question is that that's not really possible. If it foams then the washing properties of the soap should be ok. Also, stability (or instability as you observed) depends on the particulars of composition, and not knowing what it is you mixed it's not possible to say what exactly happened. $\endgroup$
    – Buck Thorn
    Oct 10, 2020 at 5:46
  • $\begingroup$ To the level of accuracy required by the question certainly your mix is still working and I agree with the comment above. $\endgroup$
    – Alchimista
    Oct 10, 2020 at 12:57
  • $\begingroup$ Thank you @BuckThorn. I added the ingredients list of two foaming soap I mixed. $\endgroup$
    – ksing
    Oct 10, 2020 at 23:23
  • $\begingroup$ Thank you @Alchimista. I added the ingredients list of two foaming soap I mixed. $\endgroup$
    – ksing
    Oct 10, 2020 at 23:23
  • $\begingroup$ The point is that you most likely ruin the optimized formulations but the mix will certainly have washing and antibacterial properties. However from a chemical point of view you didn't mix two soaps. See the answer by James Gaidis. Say that the next time you might well use each single product to the last drop. - Let's stay safe everybody. $\endgroup$
    – Alchimista
    Oct 11, 2020 at 9:36

2 Answers 2

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The soap solutions were originally transparent. An interesting experiment, if you still have some of the original soaps, is to shine a light thru the solutions and look for a Tyndall effect:

enter image description here

The light source is on the far right, out of the picture. The beaker on the right shows light scattering from particles comparable in size to the wavelength of light due to the Tyndall effect; the beaker on the left shows no scattering, even tho the light beam is passing thru it. Its contents are unknown, but there seem to be no particles of sufficiently different index of refraction and size to scatter light.

The tiny particles which scatter the light (just like in a smoky room) are micelles, aggregates of several hundred or thousand soap molecules. In the original soap solutions, this scattering is small, due to selection of manufacturing conditions (exact formulation, concentration, pH, other ingredients, etc., etc.), so the solutions appear transparent. What is optimal for each soap separately may not be optimal for the combination: perhaps the mixing produced larger micelles which scattered more, so the mixture scattered more light and became cloudy to the eye. I'm going to suggest that both soap solutions show a Tyndall effect, and the mixed solution shows a much larger Tyndall effect. It may change with time.

Now the question becomes: is the combination as effective as either solution alone. Well, it is different (cloudy). It could be worse; it could be better! It might not be observably different.

The foaming test could cast some light on the effectiveness: If you can test the foaming (height of foam and time stability) of the original solutions and of the mixture, and if they are all the same (+/- ~10-20%), the combination is probably OK.

There is a case where this foaming test will indicate immediately that the combination is not good. The molecules in soaps and detergents have a hydrophilic end and a hydrophobic end. Soaps are salts of fatty acids, and have a negative (hydrophilic) carboxyl group at one end and a (hydrophobic) hydrocarbon at the other end, and a soluble alkali metal cation. There are nonionic detergents which have no charge; they are compatible with anionic or cationic soaps and detergents. Cationic detergents (usually amine salts) have a hydrophilic positively charged nitrogen at one end and a hydrophobic hydrocarbon at the other, and some anion, usually chloride. If you should mix solutions of a cationic detergent with an anionic soap, the positive and negative charges associate to produce a neutralized ion-pair with two hydrophobic hydrocarbon groups - together this looks like a big fatty molecule with a little polarity in the middle. Overall, this molecule is not very soluble in water and precipitates out (but it floats like scum). And since it is not soluble in water, it won't foam.

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  • $\begingroup$ This is so detailed. Thank you @JamesGaidis. I added the ingredient list and hopefully that can provide more information to answer the question. Do I need any tool to test the foaming? Thank you. $\endgroup$
    – ksing
    Oct 10, 2020 at 23:25
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The extra information from the ingredient list helps suggest some reactions that may be occurring.

The antibacterial foaming soap contains four detergents (not salts of fatty acids), probably totaling 10-15% by weight of the product. The antibacterial action is provided by the benzalkonium chloride at 0.13%. The label tells you to avoid contact with the eyes, but it is used as a preservative in eye drops (up to 0.01%). So this product contains foamers (considered inactive ingredients) and the very dilute, but aggressive, cationic antimicrobial surfactant.

The amine oxides have positive polarity on the nitrogen, but the attached oxygen, which is negatively charged, creates a dipole, unlike the unambiguous cation in benzalkonium chloride. A betaine has its anion chained into the molecule already, so its availability to connect with another anion (or cation) is minimal. So the antibacterial foaming soap is internally compatible: the benzalkonium chloride does not develop a tight bond to any other of the fatty molecules in this product. Everything is soluble, and there is plenty of surfactant to give good foaming, without an excess of benzalkonium chloride, which at higher concentrations would be more likely to cause skin irritation.

There are two primary surfactants in the foaming hand soap. Neither is a fatty acid soap. The cocamidopropyl betaine is present in both products, so should not cause an incompatibility. The issue of sodium lauryl sulfate is not clear to me (apparently not clear to the mixture of solutions either). On the one hand, the negative charge on the oxygens of the sulfate group could be so delocalized that it does not bind tightly with an amine cation. On the other hand, perhaps there is some interaction, but so little benzalkonium that the combination, a precipitate (which should float) may be emulsified by the remaining surfactants, of which there are plenty.

The extra cloudiness suggests that the benzalkonium chloride has reacted with the sodium lauryl sulfate. The combination might separate and float, or might just be incorporated into an emulsion. The surfactant concentration is so great that it could easily emulsify ~0.13% of scum. However, that suggests that the benzalkonium ion is not available for antibacterial action, even if there is still plenty of surfactant available for foaming!

The bottom line is that using the products separately is more certain to do what they claim to do. You can't combine the claims.

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  • $\begingroup$ Thank you @JamesGaidis. Based on your analysis, is it correct to conclude that the claimed 'anti-bacteria' action is reduced but the regular soap's amphiphile and ability to wash away virus/bacteria isn't so negatively affected? $\endgroup$
    – ksing
    Oct 15, 2020 at 1:25
  • $\begingroup$ It would seem that way. Theoretically, the antibacterial action is neutralized, but there's plenty of foaming and washing capability remaining, so it's not bad, just not as good as using the separate soaps separately. $\endgroup$ Oct 15, 2020 at 13:48

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