Humectants and glycols in cosmetics
Chemically speaking, these are alcohols, as they possess multiple hydroxy groups (OH groups) in their molecule, which makes them highly water-soluble, even water-loving (hydrophilic), or even water-attracting (hygroscopic). They have an oily, syrupy consistency and act as emollients (softeners) without feeling greasy.
Incidentally, sugars also count as alcohols and are likewise used as humectants; just like sugar, other humectants are also sweet and sticky.
But what are the differences? Here is an overview of the most important humectants and their specific properties:
Glycerin
The traditional, proven, and ubiquitous humectant par excellence. It has a chain length of 3 carbon atoms with 3 OH groups and is therefore particularly hygroscopic. Glycerin is produced during soap manufacture, as it occurs in plant and animal oils, naturally bound as a triglyceride. Nowadays, it is obtained as a byproduct of biodiesel production and is therefore an inexpensive, readily available raw material.
Glycerin is considered extremely moisturizing, highly compatible, and mild to the skin and mucous membranes. It tastes sweet and is therefore popular for use in toothpastes and mouthwashes. In skincare products, however, it feels somewhat sticky and therefore cannot be used in high doses. It is not surface-active and therefore does not have a thinning or preserving effect in formulations.
Does glycerin have a drying effect?
Contrary to what is sometimes claimed, glycerin or humectants in general do not have a drying effect per se. Yes, they are hygroscopic; they attract moisture and bind it to the skin's surface. In dry climates (our latitudes are considered dry in this context), water is also drawn from the skin to the surface. However, our skin is constantly replenished with moisture from underneath via the blood vessels.
The idea that humectants dry out deeper layers of the skin is therefore a myth!
However, glycerin is not only used as a humectant: it is often found in surfactant formulations to improve the product's skin compatibility, and it often serves as a carrier and solvent for active ingredients.
Typical concentration for use: 4 - 6 %
INCI: Glycerin
Examples: Glycerin 99%, CollAdvance, Hydrachrysum
Glycols (Diols)
Glycols are alcohols that contain 2 OH groups in their molecular structure, hence their name "diol" (di=2, -ol = the suffix for alcohols). They serve as moisturizers and solvents and help mix various ingredients and create a homogeneous texture. Due to their solvent effect, glycols can also contribute to better skin absorption of other active ingredients, and they are also popular as extraction agents for plant extracts, as they not only dissolve water-soluble ingredients but also a portion of the fat-soluble ones. Furthermore, the glycol extracts obtained in this way are self-preserving.
Due to their (co-)preserving properties, diols prevent the growth of microorganisms such as bacteria and fungi in the products. This extends the shelf life of cosmetic products and ensures their quality.
Glycols have long been used in cosmetics and pharmaceutical preparations and were for the longest time of petrochemical origin. However, thanks to new technologies in the field of fermentation, it is now possible to produce many glycols from plant-based sources as well. Since then, they have also enjoyed increasing popularity in natural cosmetics.
They are not only used directly in formulations, but also as raw materials for the synthesis of other raw materials, such as emulsifiers or ester oils.
Glycols are considered very safe and well-tolerated; however, they have varying irritation potentials and can also facilitate the penetration of other substances. Therefore, very sensitive individuals can sometimes react sensitively to them.
With increasing chain length, the molecule's surface activity increases, as does the preservative performance, the penetration enhancement (for active ingredients), and the irritation potential.
Something that attacks bacteria can per se also irritate our skin, but bacteria are waaaay more thin-skinned than we humans are - so no need to fear glycols and preservatives!
Propylene Glycol (1,2 Propanediol)
A very commonly used glycol in cosmetics and pharmaceutical products is propylene glycol. It has the same chain length as glycerin (3 carbon atoms = C3), but one less hydroxy group. This gives it a slight lipophilicity (fat-loving property). It is therefore highly water-soluble, but also possesses a certain potential to dissolve lipophilic substances and is thus slightly surface-active (like a weak surfactant). This makes it not only a humectant but also a good solvent and a co-preserving substance.
Propylene glycol is much less sticky than glycerin. On the skin, it feels rather silky and soft, even at high concentrations. At concentrations around 20%, an end product sometimes does not even require any further preservative.
Typical concentration for use: 6 - 20 %
INCI: Propylene Glycol
Propanediol (1,3 Propanediol)
Very similar to propylene glycol is propanediol. It also has a chain length of C3 and 2 OH groups, but these are located at the first and third positions. It becomes even more confusing because the chemically correct name for both molecules is "propanediol," but in the INCI designation on cosmetic products, propanediol always refers to the 1,3-variant. For the sake of simplicity, we therefore distinguish between propanediol and propylene glycol in cosmetics.
The properties of propanediol are also very similar to those of propylene glycol; however, due to the different distribution of the hydroxy groups, the surface-active properties are somewhat less pronounced. Propanediol therefore has a slightly weaker preservative-boosting property and also feels slightly richer on the skin, not as silky as propylene glycol. Nevertheless, it is hardly sticky and creates a rather light and soft skin feel. At usage concentrations around 25%, an end product sometimes does not require any further preservative.
Typical concentration for use: 8 - 20 %
INCI: Propanediol
Examples: Propandiol green, Sunwell, Preblend Triple, Paralift
It appears bound in a molecule here: Liposens
In natural cosmetics, there were no glycols for a long time, as these could only be produced synthetically. Propanediol was the first natural glycol and has meanwhile been used for a long time as a natural moisturizer and active ingredient carrier in the cosmetics industry. It is still always from natural sources (e.g., corn starch or palm oil), but some qualities are not free of GMO raw materials and are therefore not approved for certified natural cosmetics.
Butylene Glycol (1,3 Butanediol)
Another well-known glycol is butylene glycol. It has similar moisturizing properties to propylene glycol or propanediol, but is one carbon atom longer (C4) and therefore feels a touch richer and more enveloping on the skin. The surface-active properties are also somewhat more pronounced - and thus, in turn, so are the preservative performance and irritation potential.
With its chain length, it falls between propanediol and pentylene glycol and is therefore in an interesting application range: it can be used in higher concentrations and is still well tolerated by the skin, but it also does not thin emulsions as strongly and provides a great skin feel!
There are also different isomers of butylene glycol, i.e., molecules that have the same chain length but have the hydroxy groups in different positions than the first and third.
Interestingly, butylene glycol has always been used in Asian skincare products. In the Asian market, particularly moisturizing (sometimes slightly sticky), light, non-greasy products are preferred that make the skin appear soft and "nourished" without feeling too heavy. This is partly due to the warm, humid climate conditions. Since Korea remains a trendsetter in the global cosmetics industry, "Asian cosmetics" are becoming increasingly popular here as well, and butylene glycol will likely be used more and more frequently in the future.
Typical concentration for use: 5 - 15 %
INCI: Butylene Glycol
Examples: Butylene Glycol green, Sea Lavender HC, Delisens
Pentylene Glycol (1,2 Pentanediol)
Pentylene glycol has a chain length of C5 (penta = 5) and the two hydroxy groups distributed at the ends - at the first and second positions. It is still well water-soluble, but it is the first of the glycols that also dissolves reasonably well in oil. Due to these amphiphilic (lipophilic + hydrophilic) properties, it is distinctly surface-active and an excellent solvent. It helps to better dissolve raw and active ingredients in water but can strongly thin or destabilize formulations, especially emulsions.
It is still an excellent humectant, but is predominantly used due to its pronounced antibacterial and preservative-boosting properties. Its irritation potential is also somewhat pronounced, and it should not be dosed too high - although it is generally very well tolerated by the general public. As a sole preservative, it should be used at over 5% - better 6 - 8%. In this range, however, it can already be slightly irritating, which is not as suitable for sensitive skin types or, for example, the eye area. Also, due to its destabilizing effect, it can only be used to a limited extent in emulsions at higher dosages. Propanediol and butylene glycol have a much less thinning effect.
Typical concentration for use: 2 - 8 %
INCI: Pentylene Glycol
Caprylyl Glycol (1,2 Octanediol)
This substance should be mentioned for the sake of completeness. Caprylyl glycol has a chain length of C8 (octa = 8) and is therefore far longer than the typical humectants; it is also no longer well water-soluble, but is distinctly surface-active. It acts similarly to pentylene glycol, but much more strongly. Its application spectrum is less as a humectant and more as a preservative booster to boost other preservative substances and bring them into better solution. The irritation potential and the destabilizing effect on emulsions are also far more pronounced, but small amounts such as 0.3 or 0.5% are sufficient for a significant antibacterial effect.
Caprylyl glycol is often found in preservative mixtures or as part of a preservative system in active ingredients, which themselves often need to be preserved if they contain water.
Typical concentration for use: 0.3 - 1 %
INCI: Caprylyl Glycol
Examples: Preblend Triple, Preblend Easy, AvenaGlucan
Area of application of the most important humectants:
Glycerin Propanediol Butylene Glycol Pentylene Glycol Chain length C3 C3 C4 C5 Hydroxy groups 3 2 2 2 Surface-active None Very slight Slight Pronounced Irritation potential Hardly any Hardly any Very slight Moderate Preserving None Supportive Slight Pronounced Application Humectant Humectant/Booster Humectant/Booster Booster/Humectant Skin feel Rich, sticky Light, hardly sticky Moisturizing, soft Moisturizing, somewhat sticky
