Sodium cocoamphoacetate with preservatives - unstable solution

leftoneleftone Member
Hello Everyone,
I'm working on a household cleaning product. It is an acidic one with pH below 4.

Ingredients:
water
sodium cocoamphoacetate
citric acid
sodium salicylate
sodium benzoate

At a pH about 4,3 I have a clear liquid with a viscosity of water. When I lower the pH my clear liquid becomes more viscous (pH 4,2 - 3,9). At a pH 3,7 the solution becomes cloudy and a sediment starts to appear... So my solution is unstable.

I made some more tests.
When I have no preseravites (only water, sodium cocoamphoacetate and citric acid) the problem doesn't occur - I can drop the pH even below 3 and there is no viscosity changes or sediment.
When I use only sodium benzoate the problem occurs in lower pH (about 3,2).
When I use preservatives and switch cocoamphoacetate to CAPB the phenomenon does NOT appear. Only with cocoamphoacetate.

Can anyone explain what is happening?

Comments

  • ketchitoketchito Member
    @leftone Since Cocoamphoacetate is a basic material, and at enough low pH both Benzoate and Salicylate are present in their acid forms, you might be experiencing and acid-base reaction, which results in an insoluble salt that lately sediments. You can either use an anionic surfactant instead, or switch (as you did) to a more compatible amphoteric.
  • chemicalmattchemicalmatt Member, Professional Chemist
    @leftone By reducing the pH that low, you have reached the solubility limit of the free base ions both salicylic and benzoic. Unless you want to add alcohols and/or glycols to this or reduce those salts, that pH will need to stay above 4.5. That formula is begging for a glycol ether. Perhaps a Dowanol addition is in order?
  • leftoneleftone Member
    Thank you guys for your answers. @chemicalmatt I guess you might be right - the sediment may consist of preservatives in their acid forms. When I add isopropanol sediment dissolves. Can you explain why cocoamphoacetate impacts significantly preservatives solubility and CAPB (or coco glucoside) doesn't?

    Important thing about this formula is to be scentless so using alcohols or glycols is rather problematic...
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