A few days ago I made an experiment. I diluted a few small chunks of bar soap with 20 ml of water. I then measured the ph of the solution which was around 10. I then added a tiny amount of citric acid. I immediately noticed that the solution stopped foaming. I measured the ph again and it was around 5.

I am interested what actually happened when I added the citric acid? Since the solution stopped foaming and there were no more bubbles even with continuous stirring was the final solution still soap? How does the ph of a solution affect its detergent abilities/properties?

Thank you.


Bar soap contains sodium salts of fatty acids, e.g. sodium stearate. The anions of the fatty acids have hydrophilic groups (the carboxylate group $\ce{COO-}$) as well as a hydrophobic tail ($\ce{R}$). This amphiphilic character of these anions is responsible for them to act as a surfactant.

If citric acid is being added to a solution of soap in water the carboxylate groups are being protonated.

$$\ce{H+ + R-COO- -> R-COOH}$$

The fatty acids are poorly soluble in water. They precipitate and the solution loses its ability to act as a surfactant. The foam collapses.

  • 1
    $\begingroup$ Yes, free fatty acids tend to dimerize and form lipidic drops. If there are any medium-chain fatty acids in you soap, you should notice it smells like HELL. Did the same "experiment" the other day. $\endgroup$ – julien Apr 13 '16 at 21:16
  • $\begingroup$ The difference is especially striking if you pour some acid on a solid bar of soap. All of a sudden, the thing that could wash out pretty much everything turns into a thing that can wash out pretty much nothing, and itself leaves greasy stains that need to be washed out. $\endgroup$ – Ivan Neretin Apr 13 '16 at 21:18
  • $\begingroup$ Lol, this question reminds me of when I was a kid and thought it would be cool to put orange juice and drain cleaner in a plastic bag. The mixture turned bright yellow, melted the bag, and got me in trouble for making a mess when I was in fact trying to clean one. I was working on an answer, but it turned out to be nearly identical to this one so I scrapped my answer. $\endgroup$ – Agriculturist Apr 13 '16 at 21:27
  • $\begingroup$ @julien Aren't most soaps these days sulfonic acids? $\endgroup$ – SendersReagent Apr 13 '16 at 21:39
  • $\begingroup$ @SendersReagent Of course, sulfonates, sulfates or other polar groups. But plain tradtionnal Aleppo soap is nothing but carboxylates and oil (the lipid drops might as well be unsaponified laurel oil) $\endgroup$ – julien Apr 14 '16 at 0:43

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