Dear Valerie: I am reverse engineering a moisturizing body lotion and I see both lactic acid and sodium lactate in the ingredient list. Do I need to include sodium lactate for some reason, or can I get adequate moisturization from lactic acid alone? I’m trying to avoid adding a new ingredient to our inventory.
—Enough Is Enough

Dear Enough:
Lactic acid can be moisturizing, but usually in its salt form, sodium lactate. A 1981 International Journal of Cosmetic Science article¹ outlined the composition of our skin’s Natural Moisturizing Factor (NMF) and found that 12% of our NMF is composed of lactates. Knowing this, you now may be tempted to reach for sodium lactate instead of lactic acid when looking for skin moisturization.

However, lactic acid is just as good at skin moisturization. When we formulate with lactic acid, most of the lactic acid is already going to be in its salt form once added to the formulation. This is because at a pH above an acid’s pKa, a large portion is not a free acid. It exists as a salt. For example, a lotion at a pH of 5, formulated with 10% lactic acid, only has about 0.70% of the lactic acid in free form. The rest exists as a salt, a lactate. (By the way, we learned all this in our General Chemistry courses in college; I probably would have enjoyed pKa problems more if they had been written in terms figuring out how much glycolic acid was present in my skincare).

Nonetheless, you still need sodium lactate in your formulation, in addition to the lactic acid. This is not because you need to supplement moisturization. When using lactic acid alone, even if you adjust the pH to 5 with sodium hydroxide, the formula will experience a downward pH drift over time. Adding sodium lactate will create a buffer system, stop the downward drift and maintain your pH. Your stability will be much better. 

References:

1. Clar, E.J., and A. Fourtanier. “L’acide pyrrolidone carboxylique (PCA) et la peau.” International Journal of Cosmetic Science 3.3 (1981).

---

Valerie George
[email protected]
 
Valerie George is a cosmetic chemist, science communicator, educator, leader, and avid proponent of transparency in the beauty industry. She works on the latest research in hair color and hair care at her company, Simply Formulas, and is the co-host of The Beauty Brains podcast. You can find her on Instagram at @cosmetic_chemist or showcasing her favorite ingredients to small brands and home formulators at simply-ingredients.com