Dear Valerie: I am trying to emulsify stearic acid in a formula but I’m not sure how to calculate the amount of sodium hydroxide I need to make sure everything is neutralized. Where’—Neutral State

Dear Neutral State:
Calculating neutralization ratios is one of my favorite exercises when preparing to make a lab batch. It takes me back to high school when I first learned about stoichiometry. Other students found stoichiometry challenging, but I loved it. Don’t tell anyone, but I used to make up my own problems and try to solve them. Actually, stoichiometry is what sent me down the chemist career path; I was otherwise dreaming of medical school. Stoichiometry has come in handy outside of the lab, like when making cookies and the recipe must be scaled down because you only have a partial portion of an ingredient.

Neutralization is about the molar ratios of the chemical being neutralized and the alkali. Start by writing down the chemical equation for what you’re neutralizing, then balance it (if needed).

The equation below is already naturally balanced, and we can see that for every mole of NaOH, there is one mole of stearic acid. Therefore, we would be neutralizing one mole of one, with the other. Start the stoichiometry problem by listing how many grams of stearic acid you are using. Then, convert it to moles of stearic acid by using its molecular weight. Plug in the 1:1 molar ratio of stearic acid to NaOH, so we are talking in units of NaOH. Finally, convert from moles of NaOH to grams of NaOH by using its molecular weight. This is how much pure NaOH is needed to neutralize 3g of stearic acid.

This theory works for pretty much anything you’re looking to neutralize, with any alkali, such as carbomers (although some suppliers will tell you the neutralization ratios). Write down the chemical equation, balance it, figure out how many grams you need and batch away.

Dear Valerie: How do I know what ingredients to choose for a project? There are so many options, it’—Too Many Choices

Dear Too Many:
I get anxiety every time I formulate. Like you, I also wonder if I’m picking the best ingredients to deliver what is being asked. The thing is, chemists approach a formulation differently, based on their experience and what they have access to. Although approaches may differ, the outcome could be the same. Focus on the performance you’re looking to achieve and what you know, and not on the deluge of ingredients in the cabinets. I’m sure you’ll continue to create beautiful things! When in doubt, ask a chemist friend to suggest a favorite ingredient.

Dear Valerie: I’m making a lotion with 5% glycolic acid and I’—Low Cost Provider

Dear Low Cost:
These are three solid emulsifier choices. It’s so hard to pick just one, although I suppose we must for your formula!
All are appreciable for different reasons. My first choice would be glyceryl stearate and PEG-100 stearate. This is the original, tried and true, robust emulsifier. It practically emulsifies itself in production with little to no input. It’s incredibly stable and can pretty much handle just about anything—including your AHAs. It’s also cost-effective, which comes in handy when you’re looking to add a bunch of goodies to a formula.

I also find cetearyl alcohol and cetearyl glucoside a pleasure to work with. This emulsifier can easily handle low pH situations, as well as emulsify a high amount of oil. It feels drastically different from the first emulsifier. It’s lightweight and has excellent skin feel. Glyceryl stearate and PEG-100 stearate can feel a little old-fashioned because it’s been around for such a long time, plus, it contains ethoxylated chemistry. The glucoside is more elegant than the stalwart glyceryl stearate and PEG-100 and I could definitely imagine using a night cream with it! However, I do find cetearyl alcohol and cetearyl glucoside can be fussy when using too high a concentration of polar oils; it is on the pricier side, comparatively, so you must budget for it.

Behentrimonium methosulfate and cetearyl alcohol has a very special skin feel. I like how it glides onto skin and leaves the skin feeling nearly refatted. This is a valuable attribute when formulating with AHAs. Truth be told, I also may be partial to it because it’s great in hair care. It’s relatively affordable and has somewhat escaped the trolling by clean beauty fanatics because of its quat status. It’s also reasonably priced, when not in short supply or on allocation, which it currently is as of this writing.

Because you said pick one, I probably would pick glyceryl stearate and PEG-100 stearate. It’s not going to fail you. 

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Valerie George
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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 and is the co-host of The Beauty Brains podcast. You can find her on Instagram at @cosmetic_chemist. Do you have a formulation question you want answered? Email her at the address above.