Home Cosmetic Science Talk Formulating Precipitation Nightmare!

  • Precipitation Nightmare!

    Posted by Tee_Marie on December 28, 2023 at 7:08 am

    I could really use some help on a formulation that keeps forming precipitation no matter what I do! I’m no chemist, but I’ve gotten pretty good at formulating and just cant fix this. I took a course on formulating this particular serum and the instructor seems to have left something out! I believe the SD is super sensitive to lower PH…everything is just peachy until I lower the PH. Can someone help PLeasssseeeeeee??

    Sodium Deoxycholate 1%

    Distilled Water 93.5%

    Caffeine Extract 5%

    Germall+ .5%

    I’ve tried various combinations, omitted, and added ingredients to include glycerin at the proper percentages. No matter what when I attempt to adjust PH to right around 5 the solution is fine for a short while and then begins to form precipitation. Im confused and this is driving me nuts! 🙃

    ketchito replied 1 month, 2 weeks ago 4 Members · 9 Replies
  • 9 Replies
  • ketchito

    Member
    December 28, 2023 at 7:38 am

    For whatvI found out, Sodium deoxycholate’s pKa is around 6.2. That means that a pH of 5, you’ll have its acid form. Organic acids have a lower solubility in water than their salts, so maybe that’s your issue. Make a sample with a pH around 6.5-7.0 to see if that solves the issue.

    • Bluebird

      Member
      January 14, 2024 at 7:25 pm

      Is this how it works, though?

      When you lower pH of the salt of an organic acid, wouldn’t you still expect higher solubility than its organic acid counterpart in low pH, because there is the balancing counter ion?

      Ex, sodium benzoate solubility in water is high even in low pH;

      but apparently the benzoic acid solubility in water is low, less than 0.2%.

  • Tee_Marie

    Member
    December 28, 2023 at 7:42 am

    Thanks so much! I tried that and it does solve the issue…problem is I really do need the formula right around 5-5.50. Dont really want to use the serum with ph in the suggested range. 😥

    • Tee_Marie

      Member
      December 28, 2023 at 7:45 am

      Would a solvent of some sort help?

  • ketchito

    Member
    December 29, 2023 at 5:41 am

    You could try replacing some water by ethanol. You’d need to test to see how much you’d need.

    • Tee_Marie

      Member
      January 2, 2024 at 2:39 am

      Thank you much! I’ll try that!

  • KacieLedner

    Member
    January 11, 2024 at 12:55 am

    Hello everyone, I am new here.

  • Bluebird

    Member
    January 14, 2024 at 10:22 am

    Caffeine 5% in water seems way too high to be stably soluble.

  • ketchito

    Member
    January 16, 2024 at 7:33 am

    @bluebird At low pH, you have a high concentration of H protons, which will stick to electron-rich centres like oxygen in benzoate. That’s why at low pH (blow its pKa), you have benzoic acid.

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