Sodium chloride function in bar soap

I have seen sodium chloride in the LOI in some bar soaps, such as Lever 2000, Dove, Irish Spring.

What is the purpose of NaCl in bar soap?

I know that stearic acid & sodium lactate are used to HARDEN a bar.

If HARDENING is the purpose, what is the chemical reaction that's happening (since I'm not a chemist, I ask this question).

Thanks very much for any assistance!

Comments

  • Hello Margaret, 

    In simple terms NaCl is used to harden the soap bar.

    In more complicated terms, the sodium ion (Na+) from the NaCl which is highly soluble in water (which is the main constituent of the liquid phase of the soap) will enter in a competition with the other salts (mainly fatty acid salts) present in this phase and cause the least soluble one to precipitate and transfer to the solid phase (usually the aim is to precipitate the sodium laurate). This will harden the soap and will help improve the quality of the foam. Sodium Lactate, glycerin and some other water soluble additives work in the same way to harden the soap. (you should note that the hardening is not linear, and there is always a sweet spot that the formulator should find where you have the best foam, the best hardness, and of course the ideal longevity)

    The stearic acid (or sodium stearate) is highly insoluble in water so it will be in the solid phase, so it will harden the soap and increase its longevity and help stabilise the foam.

    There are several mechanisms to harden the soap, and usually all of them are used to a certain extent.
  • @ChemicalPyros, I enjoyed the explanation! Reminded me of the time I was a general chemistry teacher.
  • Ussually we use salt (brine) in order to saturate the soap with electrolytes and precipitate all the "no-salt" compounds of the soap (sodium salt of fatty acid), the brine separates mainly the glycerin and impurities of the soap and brings they to the bottom of the reactor-tank.  this procedure is only made in old factories like mine that use crude vegetable-animal fats as raw material for soap. Some (trazes) of the salt could remain on the soap.

    newer factories splits this Fats (triglicerides) with high pressure steam (30-50bar) and remove the glycerin before the saponification reaction, they do not need the salt.
  •    I use sodium lactate when I make my soap, to harden it further and to help keep it from cracking on the surface. 
        I only make smallish batches, around 4 kgs.  in a slow cooker, on my deck, outside in the non-winter months. 
      I guess I can experiment by using a small % of NaCl added to the NaOH solution to see if that makes my soap as hard as the sodium lactate does.
      Thanks SO much for the explanations!!

       
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