Solvents

Solvents: propylene glycol, water and more

A solvent is a liquid used to dissolve a powder, or, in a more precise definition from Wikipedia: “A solvent (from Latin solvō , “I loosen, untie, I solve”) is a substance that dissolves a solute (a chemically different liquid, solid or gas), resulting in a solution.” The solute dissolves because its molecules interact with the molecules of the solvent. Example: water will dissolve sugar but oil will not.

In skin care, solvents are ingredients that are used to dissolve other ingredients. They include water, vegetable or animal oils, silicones, alcohols, etc. When I plan a formulation, my objective is to use actives at the optimal concentration. If the active is not soluble in water or in oil, I have to find an alternative solvent, because the skin will not absorb an un-dissolved active. In most SAS products, there are so many ingredients that I like to have a good mix of solvents to give a chance to all those fanciful powders to dissolve and stay dissolved and stable.

The physical properties of an ingredient, including the capacity to dissolve in a particular solvent is fixed. The capacity to dissolve depends on the relations that the molecule can establish with the molecules of the solvent/s. If you want an active to retain its activity, you shouldn’t modify its chemistry, and for this reason its solubility is also fixed. Finding the right solvent (or mix of solvents) is a craft, tables in chemical indexes provide solubility information on a few solvents that are useful in the chemistry lab (water, ethanol, acetone), but of these only water is suitable for skin care use. It is up to the formulator to experiment until he/she gets a nice solution.

Do I have preferences? I prefer to use solvents that are also actives, like oil or glycerol. Conversely, I would never use a solvent that could produce unwanted effects, like alcohol denatured with methanol. I even avoid pure ethanol, which can be used, but will dry the skin.

Using “old” solvents that have been in use for a very long time is the safest path, because they are well understood and we know exactly where we stand. We should assume that anything we apply to our skin will reach the bloodstream, even if only a tiny fraction. Can our body “manage it?” Will the body metabolize it or eliminate it? How long until it is eliminated from the body? In which form? Any unwanted side effects? All of this information is published in scientific and medical journals and is available to us through scientific databases which collect information from reputed journals.

The longer a solvent has been in use, the longer the track it will leave in the literature, including unwanted effects on humans and animals. Other considerations are ecological: energy and environmental cost of obtaining the ingredient and disposing of it.

Because of the difficulty in finding useful solvents, it is very annoying when a “rumor of the week” on the internet scares people away from a useful solvent, just as they have scared them off good preservatives.

Rumor of the week: propylene glycol is toxic

Another name for propylene glycol is 1,2 propanediol, which describes the structure of this chemical: it has three carbon atoms, and two of them have an alcohol group, while the last C has just hydrogen atoms attached to it. Its structure determines its properties, which are as follows: hygroscopic, viscous liquid miscible with water, acetone, chloroform, soluble in ether; will dissolve many essential oils, but it is inmiscible in fixed oils. And propylene glycol properties determine its uses in the pharmaceutical and skin care industry as a humectant, solvent for drugs, inhibitor of fermentation and mold growth, and as a mist to disinfect air. In the food industry, it is used as an emulsifier.

The compound propylene glycol is sometimes called α-propylene glycol to distinguish it from the isomer 1,3 propylene glycol (β-propylene glycol).

Because propylene glycol is non-toxic, it is used as an antifreeze (instead of the toxic-for-humans ethylene glycol) in breweries and dairy establishments. Environmental organizations urged the replacement of ethylene glycol by non-toxic propylene glycol, because there is no way to avoid leakage and run-off and ethylene glycol that ends up in lakes and aquifers.

How do we know that propylene glycol in non-toxic? Because of the evidence produced by hundreds of experiments, but more importantly, because after many decades of using it in the food and pharmaceutical industry there is no evidence of toxicity. Moreover, it is known how propylene glycol is metabolized after it is ingested or absorbed through the skin. An enzyme, similar to the enzyme that oxidizes the ethanol in alcoholic drinks, oxidizes propylene glycol. The product goes through the Krebs cycle and it is converted into carbon dioxide (which we exhale) and water.

How rumors originate

If the absence of toxicity of propylene glycol is so well established, why the rumor? We can only guess, but it possible that the name, propylene glycol, sounds similar to that of another chemical, ethylene glycol, which is very toxic. Ethylene glycol also has two alcohol groups (hence the name “glycol”) BUT it has only 2 carbon atoms. Why is ethylene glycol toxic then? Although ethylene glycol is not toxic by itself, the products of its breakdown inside the body are very toxic. Another source for the rumor could be that propylene glycol is not safe for cats.

Regarding the source of propylene glycol, there are several ways of obtaining it: partial oxidation of glycerol, reduction of hydroxyacetone by yeast, or from propylene oxide by hydration. As with other chemicals, the chemical industry uses different methods and sources of raw materials as it looks at the economics. Whatever the source and the method, the result will be a very pure chemical suitable for use in foods or pharmaceuticals, and able to comply with FDA requirements.

This story is a reminder of how important chemical structure is to properties: we can use propylene glycol safely, but not ethylene glycol. We can enjoy a glass of wine with ethanol in it, but methanol will kill us. One carbon atom makes a world of a difference!

About 1,3 propylene glycol

As described above, in this compound the second alcohol group of the glycol is o
n a different carbon. But the main difference is that marketing has chosen the alternate name (propanediol) rather than the “scary” word, glycol. A rose, by any other name, is a rose. And propanediol is a glycol.

Conclusions

Do not use internet gossip to guide you. These rumors of the week have a ruinous effect: they push people to search for alternatives when no alternative is needed. This “push” eventually gets to the chemical companies that manufacture ingredients for skin care. If there is an internet rumor that X is bad for you, eventually a substitute will have to be found even if the rumor was false.

Why do I mind the “internet trolls” that go after ingredients or chemicals? Because when gossip achieves a critical mass, manufacturers are pushed towards finding an alternative, quick. New ingredients bring uncertainty because of the lack of a long history of use and data about their side effects.