Salicylic Acid: Origins, Formulation Strategies & Delivery Forms

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Salicylic Acid (SA)

Salicylic acid (Hydroxybenzoic acid) is a white, colorless crystalline powder that occurs naturally in white willow. The active extract of the bark, salicin, is a glucoside of salicylic acid.

The first recorded use of salicylates dates back 4,000 years to the Sumerians who used willow extracts for pain relief. Ancient civilizations in Mesopotamia used willow extracts to treat fever, pain, and inflammation. Both Chinese and Greek civilizations employed willow bark for medical use more than 2,000 years ago, and the Chinese also used poplar bark and willow shoots to treat rheumatic fever, colds, hemorrhages, and goiter. Native Americans were known to have widely used SA in the form of willow bark, to treat pain, fever, and inflammation.

In 1828 Johann Buchner was the first to distill from willow bark the substance he called “Salicin” after the Latin name for the willow tree, Salix alba. In 1829, French chemist Henri LaRoux perfected the extraction technique. In 1853, Charles Gerhardt, another Frenchman, invented Aspirin, though it wasn’t marketed until the Bayer company patented the formula in 1899.

SA is commercially prepared by treating sodium phenolate (the sodium salt of phenol) with carbon dioxide at high pressure (100 atm.) and high temperature (390K), a method known as the Kolbe-Schmitt reaction. Acidification of the product with sulfuric acid gives SA.

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Key chemical properties

Molecular weight 138.12
Sublimes at 70C
.18% soluble in water @20C, .41% @40C, 1.7% @75C
13.8% soluble in Ethanol @ 20C
Solubility in Glycerin 1.63%, 20% Ethanol will solubilize ~.5% SA

  • The solubility of salicylic acid in cold water is increased by the presence of neutral salts. Three parts of Sodium Phosphate will solubilize one part of SA in fifty parts of water
  • Spray drying of SA dispersed in acacia solutions can result in as much as a 50% improvement in its solubility.
  • PH of a saturated water solution is 2.4
  • Pka 2.97 (50% ionized)
  • Sodium Salicylate solubility in water 125 grams/100 grams
  • C log P 2.26

Typical OTC personal care applications are for the treatment of acne where it is monographed from .5-2% and dandruff 1.8-3%. Triethanolamine salicylate is also used as a topical analgesic, but this indication has not been approved by the FDA. It is also monographed for the removal of warts at 15%. Cosmetic applications include chemical peels and antiaging products to increase cell turnover and reduce wrinkles. The key mode of action for SA is increased keratolytic activity in these applications.

Formulation strategies

When formulating clear, water-based solutions, the key consideration is solubilization. SA can be solubilized using ethanol, glycols, and nonionic ethoxylated fatty alcohols.

The PH of the formulation is also a critical factor since it dramatically affects both solubility and efficacy. When formulating 2% SA at a PH of 3, the best strategy is to solubilize the SA using 42% ethanol and isoceteth 20 (Brij IC20 – Croda). Since SA easily penetrates skin, adding a material like Polyolprepolymer-15 (PEG-8 / SMDI Copolymer – Barnet Products) can be useful to increase the deposition of salicylic acid in the upper layers of the stratum corneum and to reduce skin irritation.

Another consideration when formulating at a PH of 3 is to avoid using ester-based materials that can hydrolyze. SA formulations should contain a good chelant to prevent discoloration due to reacting with trace iron. SA can also be formulated into alcohol-free, oil-in-water emulsions by first solubilizing the SA using an emollient like Arlamol PS15E (PPG-15 Stearyl Ether – Croda). This oil phase can then be emulsified into an aqueous gel containing acid-stable thickeners like Sepigel 305, Aristoflex AVC, or Synthalen CR. (Polyquaternium 37 – 3V).

Novel salicylic acid delivery forms

  • Curcylic 40 (Salicylic acid, Cocamidopropylamine – Vantage Specialties) is water soluble at a PH of 3 without needing any solvents.
  • CC Salicylic Acid 50% (Salicylic Acid, Cyclodextrin – Rahn)
  • Genti-Fol SA (Betaine Salicylate – Lonza, 50% SA) is a salicylic acid/betaine complex with improved water solubility.
  • Microsponge P078A (Salicylic Acid, Methyl Methacrylate/Glycol Dimethacrylate Crosspolymer – Amcol) provides sustained release of salicylic acid on skin.
  • SalSphere Salicylic Acid (Cocamidopropyl Dimethylamine, Salicylic Acid, Aqua, Polysorbate 80, Phenoxyethanol, Ethylhexylglycerin, Carthamus Tinctorius (Safflower) Seed Oil, Olea Europaea (Olive) Fruit Oil – Salvona) is a time release delivery system designed to treat acne from a clear rinse-off application.

References

  • http://www.chemheritage.org/discover/media/magazine/articles/27-2-aspirin.aspx 
  • https://en.wikipedia.org/wiki/Salicylic_acid
  • Rhein L., Chaudhuri B., Jivani N., Fares H., Davis A., Targeted delivery of salicylic acid from acne treatment products into and through skin’ Role of solution and ingredient properties and relationships to irritation, J. Cosmet. Sci., 55, 65-80 (January/February 2004).
  • Zheng Y., Wan M., Chen H., Ye C., Zhao Y., Yi J., Xia Y., Lai W., Clinical evidence on the efficacy and safety of an antioxidant optimized 1.5% salicylic acid (SA) cream in the treatment of facial acne: an open, baseline-controlled clinical study, Skin Res Technol. 2013 May;19(2):125-30.
  • Polyolprepolymer-15 Barnet Products product bulletin

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17 Responses to “Salicylic Acid: Origins, Formulation Strategies & Delivery Forms”

  4. Hi George,

    Very helpful article.

    I am wondering if you have any information on Sal acid degradation and discoloration due to UV exposure.

    Thanks,

  5. Sal acid is UV stable but can sometimes turn red in solution if trace iron is present. A chelator like Disodium EDTA can prevent this.

    George

  7. Hi George,
    Will you please send me a guideline for psoriasis medications by email!

    Thanks!

  8. This is not a simple question that I can quickly answer.

    Sorry

    George

  10. To get good efficacy the PH of the formulation should be around 3. SA has no stability issues.

  11. Very Informative, thank you. Octyldodecanol is also a good solvent for SA. Can you get the same anti acne activity in an anhydrous product (e.g. stick) that you do in a water/alcohol based product?

  12. Hi, Carl.

    SA will not partition out of Octyldodecanol into skin as easily as it does from ethanol/water however it still is probably better than formulating SA at a PH above 3 since the salt form isn’t very effective. My guess is you still will get efficacy since normally skin penetration isn’t an issue for unneutralized SA.

    Thank you,
    George

  13. Dear Concern,
    Will the SA decompose if the SA dissolved in the mixture of Glycerine & propylene glycol at 80°C?

  15. What concentrations do you recommend using for PPG-15 Stearyl Ether and Polyquat 37? Can I substitute PPG-15 stearyl ether with PPG-14 butyl ether?

  16. Dear Trang:

    I do not recommend using PPG 14 Butyl Ether due some eye irritation issues. To solubilize 1% Sal acid takes around 8% PPG 15 Stearyl Ether. Add Polyquaternium 37 to your desired viscosity. A gel will take around 1-2% active polymer.

    Thanks for reading!
    George

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