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salicylic acid recrystallisation
Hi everyone.
I have a serum with the below formula
Salicylic acid 2%
Propanediol 20%
DMI 4%
Lactic/Malic/Mandelic 10%
Aloe vera powder 1%
Caprylyl glycol/phenylpropanol 2.5%
Water Q.S.
I predissolve salicylic in propanediol and DMI. Heat to 50C .. Leave it to cool to around 27-28C before ading to the rest of the acids and water. N.B. I adjust the pH of the rest of the rest of acids and water to pH around 3.2 , so when I add the salicylic acid mixture, the overall pH is around 3.5
Starting Freeze/thaw testing on the above , i can see salicylic acid needle crystals from second cycle..
Can someone please help me out..
N.B. I don’t want to use ethanol
TIA
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9 Replies
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Have you tried the formula with betaine?
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So you have adjusted pH of acids, added solution of SA with PG and DMI and got higher pH? How this even possible?
SA is known to be stubborn. 20% of PG is definitely not enough.
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You might want to add 20% Methylpropanediol or increase your Propanediol to 35% to 40%. A touch of Polysorbate 80 - 1% to 2% would help. Finally, adjust your final pH to 4.0 will also help. As @Graillotion mentioned, you may also try adding Betaine or Sodium Citrate at 1%.
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I had success by mixing salicylic acid with propylene glycol and forming a paste, then adding sodium lactate and let it sit until it is completely dissolved. I use betaine and xanthan gum in my formula, as well. It’s been about a year since I experimented with how much I could solubilize and I believe I got to 20% but the amount of sodium lactate I had to use increased the pH too much. I think 16% was the highest where I could add it to my formula without having to lower the pH. I wasn’t using the full amount in my formulas just a total of 2% salicylic acid.
There is an article about using hydrotropic agents to dissolve salicylic acid that helped me a lot and I can’t seem to find it in my files or online. But I believe someone posted in the forum before.
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The reason why Sodium Lactate works is that it “neutralizes” part of Salicylic Acid.
SA + Sodium Lactate -> Sodium Salicylate + Lactic Acid.
Sodium Salicylate is very soluble in water. So you don’t have much of a free acid in your solution.
Same with Sodium Citrate.
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Does it stay neutralized even if the pH is low around 3.5? I thought as long as the pH was in the optimal range it would free up those acids. A lot of the times when I make a product with salicylic acid I will add an AHA, like lactic acid. Would that counteract the neutralization from the sodium lactate?
I remember Urea, sodium lactate, sodium citrate, and peg 400 mentioned in that study about using hydrotropes to dissolve salicylic acid. Out of those, would peg 400 be a better option to use with propylene glycol? I thought about trying the urea, but it’s a pain by itself with the pH drift.
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Does it stay neutralized if the pH is low around 3.5?
pKa is a pH value under which an acid is 50% free and 50% is “neutralized” or better to say is a conjugated base.
pKa of Salicylic Acid is 2.97. So if your solution has pH 2.97 - 50% of added acid is free and 50% is a conjugated base. The higher pH - the less free acid and more conjugated base.
Under pH 3.5 around 23% of added SA is free and 77% is conjugated base (in your case it is Sodium Salicylate).
So “neutralization” is not some point, it is continuous along pH scale. The higher pH - the less free acid and wise versa.
Would that counteract the neutralization from the sodium lactate?
When you mix SA with Sodium Lactate some part of SA replaces Lactic Acid in Sodium Lactate and becomes Sodium Salicylate (+ released Lactic Acid). Sodium Salicylate is highly soluble in water.
Adding more acids will lower pH and release some of SA. But since SA is not soluble in water it will crystalize.
I remember the study you mention, it is here https://www.researchgate.net/publication/44298504_Mixed-solvency_approach_-_Boon_for_solubilization_of_poorly_water-soluble_drugs
Would peg 400 be a better option
I don’t know. I use only PG. You should now worry that much about neutralization. If you can make solution with right pH 3.0-3.5 with Sodium Lactate - that’s fine.
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I have had success with glycol content of around 25-35%. I have used both 1, 3 propanediol and propylene glycol with a final pH of 3.8.
@MarkBroussard I finally got my hands on some methylpropanediol and butylene glycol. What ratio would you recommend for this combination of glycols? I’m not sure if this is normal but the butylene glycol has a strong alcohol scent, and I would prefer to minimize what comes through the final product. I am aiming for stability at pH 3.2 - 3.6.
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Try using 20% Methylpropanediol + 10% Butylene Glycol for starters. If the smell is unpleasant to you then increase the methylpropanediol and lower the butylene glycol
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