Penetration Enhancers in Vitamin C Serums: What Transcutol and Glycols Actually Do

Some vitamin C serum labels now name the ingredient meant to carry the vitamin C, not just the vitamin C itself. You may see "7% Transcutol" next to the vitamin C percentage, or an ingredient list that starts with propylene glycol, propanediol or pentylene glycol before water. These are solvents that can change how much of an active gets into the outer layer of skin. This guide explains what they do, what the research shows and where the safety limits sit.

Why vitamin C needs help getting in

The outermost layer of skin, the stratum corneum, is built to keep things out. It is made of flattened dead cells set in a mix of fats, and it lets oily molecules through more easily than water-loving ones. Vitamin C is strongly water-loving. Pure L-ascorbic acid only crosses this layer well in an acidic formula, which is why its serums are made at a low pH (see vitamin C serum pH explained).

Derivatives such as 3-O-ethyl ascorbic acid, the form used in many Indian serums, are more stable, but they are still water-loving. A 2019 laboratory study by Iliopoulos and colleagues at University College London measured its oil-water partition coefficient (log P) at about minus 1. That means it strongly prefers water to oil. So whether it gets into skin depends a lot on what it is dissolved in. For more on the different forms, see L-ascorbic acid vs vitamin C derivatives.

What a penetration enhancer is

In skincare, a penetration enhancer is usually a solvent that does one or both of two things:

• Dissolves more of the active, so more of it is available to move into skin.
• Changes the skin barrier slightly, for example by mixing into the fats between skin cells or by drawing water in or out, so the active can move through more easily.

The common ones in vitamin C serums are:

• Ethoxydiglycol, sold in purified form as Transcutol. Its chemical name is diethylene glycol monoethyl ether, or DEGEE.
• Propylene glycol and propanediol.
• Pentylene glycol (1,2-pentanediol) and 1,2-hexanediol, which also help preserve the formula.
• Glycerin, mainly a humectant, which can also affect how actives move through skin.

Liposomes and other encapsulation systems are a different approach. They wrap the active in a carrier, not a solvent.

What the studies show

The solvent matters, and more is not always deeper. In the 2019 study, the researchers applied 3-O-ethyl ascorbic acid dissolved in different single solvents to pig skin, a standard laboratory stand-in for human skin. The vitamin C derivative got through the skin from several solvents, including propylene glycol, glycerol, 1,2-pentanediol and 1,2-hexanediol, and propylene glycol delivered the most. With Transcutol it was taken up into the skin but did not pass through it in the test period. With two other glycols, almost none went in at all. These were single solvents on pig skin, not finished serums, but the study shows that the solvent can make a large difference.

Transcutol often keeps actives in the skin. A 2018 review in AAPS PharmSciTech by Osborne and Musakhanian brought together studies on more than 100 actives. The authors describe Transcutol as both a strong solvent and a substance that can swell the pathways between skin cells. Depending on the active, this can push more through the skin or hold more of it in the upper layers. The review also found that results change a lot with how much water is in the formula and how the experiment was set up. So a percentage on a label does not tell you how a serum will behave.

Most vitamin C may stay near the surface. A 2026 study in the International Journal of Pharmaceutics (Trzaska and colleagues) applied serums to the forearms of three volunteers, then removed thin layers of skin with tape strips and measured what was in each one. The signal from 3-O-ethyl ascorbic acid was strongest at the surface and fell with depth. It was found mainly around the edges of skin cells. Ascorbyl glucoside behaved differently and appeared to move into the cells. With three people this is a mechanism study, not proof of results, but it fits the idea that delivery is the weak point for vitamin C serums.

Safety and the EU limit

Glycols and Transcutol have a long history in cosmetics and medicines, and toxicology reviews describe Transcutol as well tolerated at normal exposure levels. The question is how much.

The EU's Scientific Committee on Consumer Safety (SCCS) has reviewed DEGEE several times. In a 2011 opinion it concluded that 5.5% in leave-on products did pose a risk to consumers. A revised opinion adopted in February 2013 found that up to 2.6% in cosmetic products does not pose a risk, alongside up to 10% in rinse-off products and set limits in hair dyes. It asked for the ethylene glycol impurity in DEGEE to be kept to 0.1% or less. The committee did not assess its use in eye or oral care products. The EU later wrote these limits into its cosmetics law.

EU rules do not govern products made for sale in India, and a higher percentage is not proof that a serum is harmful. But if a leave-on serum advertises more than 2.6% Transcutol, it is outside the range the EU committee judged safe. That is worth knowing, especially if you use it twice a day or have sensitive skin.

Practical points

• Read the ingredient list, not just the front label. Ethoxydiglycol is how Transcutol appears in an ingredient list. Ingredients above 1% are listed in order of amount, so a solvent near the top is a major part of the formula.
• Better absorption can mean more stinging. A solvent that helps vitamin C into skin can help other ingredients in too, such as fragrance or acids. If you have sensitive skin, patch test a new serum and introduce it slowly.
• Don't judge a serum by its solvent percentage. The lab work shows that the whole formula, including pH, water content and the form of vitamin C, decides delivery. A brand's own before-and-after photos are not independent evidence.
• Avoid the eye area with serums high in Transcutol, since that use has not been assessed in the EU.

To compare current vitamin C serums by form, strength, size and live prices, see our serum listings.