How to keep salicylic acid from crystallizing out of my gel peel?

I've formulated a  gel peel intended for estheticians with:

1% Hydroxyethylcellulose
10% Alcohol
20% Glycolic Acid
5% Salicylic Acid
20% Propylene Glycol

Theoretically this amount of propylene glycol and alcohol should be enough to solubilize the salicylic acid, and it does when formulated, but over time a lot of the SA form crystals in the gel.

Advice on how to fix this?

Comments

  • Is it completely solved when you are batching? If even one crystal is not solved it will start to crystalize and build from there. Even if you cannot see it.
  • I do agree with Mike, you can try hexandiol as a solvent.
  • Are you sure that "over time" all the alcohol is not evaporating away?
    Design of anti-aging creams, gels, and serums; shampoos; and therapeutic cosmetics. In-house label and box design capability.
  • edited March 2016
    it's also worth bearing in mind that the figure for salicylic acid solubility is most likely based on the assumption that it's in an isotropic, Newtonian solution, which your product is not (because it contains HEC, which hinders the motion of solvent molecules and alters the flow properties)

    also: is crystallisation more severe at low temperature than it is at room temperature?
    UK based formulation chemist. Strongest subjects: hair styling, hair bleaches, hair dyes (oxidative and non-oxidative) I know some stuff about: EU regulations, emulsions (O/W and W/O), toothpaste, mouthwash, shampoos, other toiletries
  • Mike_M: it is heated in propylene glycol to 60C under stirring, at which point I can't see any crystals. It's possible this is inadequate. 

    Belassi: Over time = overnight, It's in a sealed container so I doubt evaporation is the cause.

    Bill_Toge: Good point. I have not measured to see if crystallization is more severe at lower temperature.


  • If you have a temp controlled oven you could test to see at what point it forms crystals. I suspect that you are making what's called a supersaturated solution.
    Design of anti-aging creams, gels, and serums; shampoos; and therapeutic cosmetics. In-house label and box design capability.
  • What is the pH of your concoction?

    (1)  Try adjusting the pH to 3.5 ... I would suspect that your solution's unadjusted pH is probably around 2.5.  Too low of a pH can cause salicylic acid to precipitate from solution.
    (2)  Try adding some sodium citrate to your formulation
    (3)  You can also try using Methyl Gluceth-20 Benzoate as a solvent
    Chemist/Microbiologist formulating in the Organic & Naturals arena under ECOCert/Natural Products Assn/Whole Foods/National Organic Program guidelines focused skincare & haircare products. 

    Provides Formulation Development and Lab-Scale Contract Manufacturing Services.  See website for details www.desertinbloomcosmeticslab.com

  • edited March 2016
    Belassi: You are probably correct, will test. Any issue with just adding more propylene glycol btw?

    MarkB: Correct pH is 2.5. I was not aware of this. Any advantages for Methyl Gluceth-20 over propylene glycol or hexandiol? How much sodium citrate do you suggest I add?

    Thanks for the very useful feedback so far! 

  • I wish I could comment further but I have very little experience with SA. If you want something even more difficult try glycyrrhizic acid!
    Design of anti-aging creams, gels, and serums; shampoos; and therapeutic cosmetics. In-house label and box design capability.
  • Add 1% Sodium Citrate.  Methyl Gluceth-20 Benzoate may improve the sensorial over PG or Hexanediol.
    Chemist/Microbiologist formulating in the Organic & Naturals arena under ECOCert/Natural Products Assn/Whole Foods/National Organic Program guidelines focused skincare & haircare products. 

    Provides Formulation Development and Lab-Scale Contract Manufacturing Services.  See website for details www.desertinbloomcosmeticslab.com

  • Thanks Mark, I'll try 1% Sodium Citrate first!
  • Hey curious what's the rest of your formula just water? I would increase the alcohol concentration.
  • Rest is just water. Increasing alcohol is an option, but afraid it could make it more irritating. 
  • Your problem is most likely the water
  • @MarkBroussard ;

    I'll try adjusting ph to 3.5 first using sodium lactate. If inadequate I'll add 1% citric acid first then adjust pH. I'll also keep the SA propylene glycol solution at 70C for 30 min to ensure adequate solubilizing of SA.

    If this doesn't work I'll look into trying different solvents, e.g. Methyl Gluceth-20, Hexanediol or possibly Cocamidopropyl Dimethylamine.
  • edited April 2016
    preface: i am a complete neophyte and formulate only for myself (and do lots and lots of reading and experimenting)

    i do recall i had added water directly into the beaker containing my SA/prop gly mix and was rewarded with an opaque, crystal-laden, shard soup.. not fun..

    i found a ratio (on a supplier's site) of 16 g prop. gly : 2 g SA.. this, along with NOT heating to dissolve the SA (which means lots and lots o' stirring) = no crystallization issues in my final products.. done it this way many times.. 

    ive got an azelaic acid 8%, SA 2%, 10% prop gly micellar water here and it is lovely.. i added the AA/SA/prop gly to small amount of sea buckthorn berry co2 oil which was previously combined with a liquid emulsifier (PEG-6 Caprylic/Capric Glycerides (and) Polyglycerol-6 Dioleate (and) Glyceryl Caprylate/Caprate).. topped it off with a pre-combined water phase/extracts/preservative system of phytocide aspen bark at 3%, japanese honeysuckle and an active antimicrobial silver citrate at 0.3%.. checked pH at the end..

    please, when should i test my pH? test the AA/SA/prop gly first? or testing the end formula is okay? also i would like to know, please, where in the process does the SA decide whether or not to precipitate back out from the prop. gly.? does my theory about not applying heat .. ahem.. hold any water? or is there a degree of 'superstition' being adhered to in my avoidance of heating the SA/prop gly? 

    i have been called loquacious.. thank you all for reading/responding..
  • Does anyone have any recommendations for anhydrous? Primarily Isopropyl Alcohol and a high % of Sal Acid? It's crystallizing after a few days-
  • Try SD Alcohol and propylene glycol
  • You may try glycerol+PG+alcohol to dissolve SA
    Not sure if sodium lactate can increase the pH to 3.5, I guess you will need a bigger quantity. I'm curious what results did @Zink achieve. 
  • edited January 12
    I use trimethylglycine (betain), as a solubilizing aid for salicylic acid. Betain also has skin benefits. No need for alcohol etc. Also works for allantoin (solubilizing higher % of it). I use 3% of it and it perfectly keeps 2-2.5% SA (or 1-1.5% allantoin) solubilized (along with 5% pentylene glycol and 10% PG in an aqueous product) I never have recrystallization problems anymore, even if I put it in the fridge!
  • You could try Finsolv EMG-20 or one of the other benzoic esters.
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