What Is an Anhydrous Serum? Anhydrous vs. Water-Based Serums Explained

Every formula adds up to 100%, so when a serum is made without water, something else has to fill that space. That's what "anhydrous" means: formulated without water. In a lipid-based anhydrous serum, oils, esters and other lipids take over, forming the base that carries the active ingredients. The base decides which ingredients a serum can carry and how it helps your skin hold on to moisture, so when you're choosing one, start with what your other products already provide.

What takes the water's place?

Removing the water leaves more of the formula for lipids and the oil-soluble actives that dissolve in them. Each active still needs a concentration the formula can dissolve, keep stable and deliver comfortably to skin, so more room doesn't mean unlimited amounts. The lipid-rich base ends up doing two jobs at once, softening skin through its emollients and carrying oil-soluble ingredients.¹ It's an efficient trade, since the space water would have taken now does part of the moisturizing.

That only works for ingredients that dissolve in oil. Some ingredients dissolve in oil and others in water, and the base has to suit what it carries. Tetrahexyldecyl ascorbate, usually shortened to THD ascorbate, is an oil-soluble form of vitamin C that can go into a lipid-based serum.² Niacinamide dissolves in water, which makes it suited to a water-based serum.³

Water-based serums aren't limited to water-soluble ingredients, though. They can dissolve water-soluble ingredients and combine humectants, ingredients that bind water, with oils and other moisturizing materials.¹ They can also include an oil phase to carry oil-soluble ingredients. The vitamin C comparison explains how different forms of vitamin C shape the choice of base.

What changes about keeping it stable?

A cream or serum that combines oil and water needs emulsifiers to keep the two dispersed together, much the way mustard keeps a vinaigrette from separating.¹ A lipid-based anhydrous serum has no water phase, so there's no oil-water emulsion to hold together, and it carries its oils and compatible actives without one.

Preservation changes too, because how well microbes grow depends partly on how much usable water a product has. That's measured as water activity, and reducing it can inhibit or slow the growth of bacteria and fungi.⁴ Honey works this way. It contains water, but so little of it is available that microbes struggle to grow in it.

Low water activity may allow an anhydrous formula to be made without a traditional broad-spectrum preservative system. That's decided product by product, through a risk assessment of the finished formula that covers how it's made, packaged and used. ISO guidance identifies low microbial risk the same way, one product at a time.⁵ Water-free still isn't contamination-proof. Dipping your fingers into a jar can introduce microorganisms, so a water-free formula still needs packaging and handling that limit contamination.⁶

Oxidation is a separate problem, and taking the water out doesn't solve it. Laboratory research shows that THD ascorbate can break down under oxidative stress, so protecting vulnerable ingredients from oxidation is part of keeping a water-free formula stable.²

Can a serum without water help skin hold moisture?

It can, by helping your skin keep the water it already has. Occlusive ingredients slow water loss from the surface, while emollients help skin feel softer and smoother.¹ Your skin's outermost layer is built a little like a brick wall, with flattened cells as the bricks and organized lipids as the mortar between them. Ceramides, cholesterol and free fatty acids are major parts of that mortar, which helps limit water loss.^{1, 7}

The lipids in your skin's barrier are different from sebum, the oil on your skin's surface, which includes squalene and wax esters.⁸ Some skincare ingredients turn out to be close chemical relatives of sebum. Squalane is the saturated form of squalene,⁹ and jojoba oil consists mainly of wax esters.¹⁰ It's a tidy connection between what's in the bottle and what your skin already makes.

A formula can pair emollients for softness with ceramides, one of the barrier's own lipid classes, and water-based creams can combine emollients and ceramides too.¹ Binding water is a different job, done by humectants such as glycerin in the outer skin.¹ Humectants can complement the softness and moisture retention of a lipid-rich layer, and the guide to humectants, emollients and occlusives explains how the three roles work together.

Anhydrous and water-based serums, side by side

Neither base does everything. A water-based serum can combine water, humectants, emollients and occlusives in one product, while an anhydrous one gives up the water phase in exchange for more room for lipids and oil-soluble actives.

How to choose for your routine

Start with what you want the serum to do and what your current products already provide.

1. Choose the ingredient or moisturizing role you want. A water-based serum is a straightforward option for niacinamide. An anhydrous serum can offer a lipid-rich base for oil-soluble actives.
2. Check your current serum and moisturizer. Look for humectants such as glycerin alongside oils or other emollients. A water-based moisturizer may already combine water binding, softness and moisture retention.
3. Fill the gap. A humectant product adds water-binding ingredients. A lipid-rich product can help soften skin and reduce water loss.

If you use a water-based glycerin serum and want more softness and moisture retention, a lipid-rich product can add them. When the products' directions allow, you can apply the humectant serum first and follow with the lipid-rich layer. If your moisturizer already binds water, softens skin and slows water loss, it may cover all three in one step.

Why we made Omnia anhydrous

Omnia is an anhydrous solid serum built around a lipid-rich base of squalane, jojoba esters and other emollient esters, with ceramide NP and a phytosterol-derived lipid. It also carries THD ascorbate. We made Omnia anhydrous to devote more of the formula to its lipids and compatible oil-soluble actives. Omnia's role is to help soften skin and retain moisture while carrying its oil-soluble actives, and if you also want water-based hydration, a suitable humectant serum can go underneath it.

The Omnia page has more about its ingredients and how to use it.

Sources

1. Sethi A, et al. Moisturizers: The slippery road. Indian Journal of Dermatology, 2016.
2. Swindell WR, et al. Tetrahexyldecyl ascorbate (THDC) degrades rapidly under oxidative stress but can be stabilized by acetyl zingerone to enhance collagen production and antioxidant effects. International Journal of Molecular Sciences, 2021.
3. Wohlrab J, Kreft D. Niacinamide - mechanisms of action and its topical use in dermatology. Skin Pharmacology and Physiology, 2014.
4. Kerdudo A, et al. Optimization of cosmetic preservation: water activity reduction. International Journal of Cosmetic Science, 2015.
5. International Organization for Standardization. ISO 29621:2017. Cosmetics: Microbiology: Guidelines for the risk assessment and identification of microbiologically low-risk products.
6. U.S. Food and Drug Administration. Microbiological safety and cosmetics.
7. van Smeden J, et al. The important role of stratum corneum lipids for the cutaneous barrier function. Biochimica et Biophysica Acta, 2014.
8. Picardo M, et al. Sebaceous gland lipids. Dermato-Endocrinology, 2009.
9. Pandarus V, et al. Solvent-free chemoselective hydrogenation of squalene to squalane. ACS Omega, 2017.
10. Gad HA, et al. Jojoba oil: an updated comprehensive review on chemistry, pharmaceutical uses, and toxicity. Polymers, 2021.