Choosing the Right Additive: Solubilizers 

Dear Valerie: Should solubilizers be added to foaming surfactant formulations? After adding them, I have noticed they reduce foam. Some claim they are a “superfatting” agent and if a fragrance needs to be solubilized, the surfactants can already do so. I would love your expertise on it.

—Foamy Whether

Dear Foamy,

May I grouse for a moment? I dislike broad sweeping statements about ingredients as the answer is often complex. In the case of fragrance solubilizers, the answer to your question highly depends on the solubilizer choice, fragrance being solubilized, surfactant selection, surfactant ratios, and overall surfactant load. To state they reduce foam or are superfatting (or don’t need to be used at all) is a little convoluted and requires context.

It’s correct to say that fragrance solubilizers can alter surfactant systems, since they alter micelle interactions. For example, Polysorbate 20 is notorious for viscosity reduction in sulfate free systems, but I have seen instances where—puzzlingly—viscosity has increased. Another popular counterpart, PEG-40 hydrogenated castor oil, can also decrease viscosity, but I’ve more so seen it increase in the chassis I formulate, and primarily in sulfated systems.

Foam suppression? I mostly notice this with PEG-40 HCO, not P20. Although PEG-40 HCO is ethoxylated, allowing the fatty hydrogenated castor oil molecules to become more water soluble, I find its intrinsic oiliness to negatively impact the speed of foam formation. It’s not like it kills all foam, so I mostly respond by adjusting my surfactant ratios or levels to accommodate its incorporation. 

I must mention I don’t use large quantities of solubilizer in formulations, which probably explains some of the above. Not revelatory, I rarely add more than 2 parts solubilizer to 1 part fragrance, and my goal is to keep the ratio 1 to 1. I personally don’t like the haptics of most solubilizers or stability challenges they can present, particularly with PEG-40 HCO or some of the polyglyceryl-based solubilizers (although they do have a time and a place). Maybe some people like how they feel, not me.

If you must use 5X or 10X solubilizer to fragrance, you’re using the wrong solubilizer. Not every surfactant system accommodates a solubilizer the same every time, and the qualities of the individual fragrance molecules themselves will additionally play a role in incorporation. Fragrances are complex mixtures and some fragrances are intrinsically more polar or non-polar than other fragrance materials. Anything citrus based? Oily and will wreak havoc over time on your surfactant system! And essential oils? Absolutely. Many fragrances even use essential oils, so a good solubilizer, meaning the right solubilizer, is a must.

Another critical factor I recommend: asking your fragrance supplier what the solvent is for the fragrance. Dipropylene glycol? Triethyl citrate? Polysorbate 20? This plays a huge role in how your fragrance is going to go incorporate and how your system is going to behave. I am actually shocked how many people aren’t asking this question. It can save a lot of headache and even help you select the right solubilizer.

When working with a new formula or new fragrance, play with some solubilizer options to see which can be used in the lowest concentration or which provides the most clarity for your product. Expand your repertoire to include other solubilizers like glycolipids (my preferred for essential oils) or one of the TEGO Solve type materials.

Many times, I don’t use a solubilizer, but again, it depends on what the fragrance is, how heavy the load needs to be, what the fragrance character is, and how my surfactant system is thickened. If I am working with SLS or SLES, and the solvent is a good one, I can likely drop in the fragrance with minimal impact to rheology or clarity. If I’m using a “sulfate free” chassis with optimal surfactant ratios, I will try just popping the fragrance in, if the fragrance character is right. If I am using a “sulfate free” chassis which requires the use of polymers or other structuring agents for viscosity, I probably would use a solubilizer right away. However, all this comes with experience, and even then, requires some experimentation. The only right answer is what works for your formula!

On the last topic of solubilizers being super fatting, you must be talking about PEG-40 hydrogenated castor oil or another PEG material like PEG-7 glyceryl cocoate, not Polysorbate 20. The former are intrinsically oily feeling and have the benefit of refatting the skin (or so they say). Not all solubilizers are refatting agents, and not all refatting agents are good solubilizers. In my opinion, these also impact the end feel of the skin or hair, and whichever phenomenon it is could be favorable or adverse. Pick wisely for the application—these may be suitable for skin, but not for hair.

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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 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