As part of our commitment to reduce and eventually eliminate plastic waste - we've been re-visiting old theories about airless packaging and digging deep into the science behind them.
Our Technical Manger - Sam, discusses some of the chemistry around oxidation.
What is oxidation?
Oxidation is a chemical reaction which causes fundamental changes in the structure of chemical substances. It is an essential part of life - without it, many of life’s physical processes would be impossible, e.g. the ability to generate energy from food.
As the name suggests, the first substance observed to cause this phenomenon was oxygen, but it is by no means the only substance which can cause oxidation. Other examples include hydrogen peroxide, ozone and chlorine.
Oxidising agents come in many different strengths; the mildest ones are completely benign, whereas the most powerful ones can destroy organic matter, and induce spontaneous combustion in flammable materials.
Can the pigments in makeup be affected by oxidation?
In a word, no - these are oxidatively stable materials. Any change in colour between batches is most likely because the pigment has not been milled consistently from batch to batch.
How does oxidation affect skincare products?
For the most part, the effects (if any) are very minor - the vast majority of materials used in skincare are not susceptible to oxidation. The most likely oxidant to be encountered by a skincare product is oxygen from the air, and it is not reactive enough to oxidise most skincare materials on its own.
Two exceptions to this are retinol (vitamin A) and ascorbic acid (vitamin C), which are more susceptible to oxidation than most other materials, and undergo a variety of reactions from discolouration to chemical degradation into other, non-active substances.
How is vitamin A affected by oxidation?
Retinol contains five unsaturated carbon-carbon bonds, marked as double lines on the diagram below. Polyunsaturated vegetable oils, e.g. linseed, sunflower and avocado oil, contain multiple bonds of this kind, and turn yellow over time as these bonds are oxidised; the same occurs with retinol, but because the bonds are right next to each other (conjugated), this makes them more reactive than those in vegetable oils, so retinol turns yellow more quickly.
As long as the molecule retains its general shape and the -OH group at the end of the molecule, i.e. the part which enables it to work, remains unaffected, the yellowing does not reduce the activity of retinol.
However, because the -OH group is located next to conjugated unsaturated bonds, this makes it more susceptible to oxidation than it would be otherwise; the compound which results from oxidation of the -OH group (retinal) is unstable and reacts to form products which have no effect on the skin.
The oxidation to retinal is very slow compared to the yellowing reaction, and it does not result in any obvious visual or olfactory change.
The -OH group is most effectively protected by chemically converting it to an ester, which cannot be oxidised by air. When applied to the skin, retinol is released from the ester by esterase enzymes in the skin; although the reaction in the skin is slower than with retinol itself, the overall effect is the same. The palmitate (R = C15H31) and acetate (R = CH3) esters are the most widely available.
Another approach is to protect the retinol by encapsulating it, i.e. physically prevent oxidising agents from reaching it. This was the approach taken in the Regenecalm Pro and Pro Retinol creams; encapsulated retinol is commercially available for use in cosmetics, and in this case, the encapsulation medium is a phospholipid (a surface-active fat) similar to those found in the skin.
How is vitamin C affected by oxidation?
The structure of ascorbic acid is shown in the left-hand diagram. The highlighted bond is inherently unstable, and vulnerable to oxidation by air when dissolved in water or polar solvents. The radical formed from oxidation (centre diagram) then loses the two hydrogens on the -OH groups to form dehydroacetic acid (right-hand diagram), which then fractures and decomposes into coloured by-products, rendering the ascorbic acid ineffective and turning the product brown.
The most effective way to prevent this is to block one or both of the -OH groups next to the unstable bond. Then, the unstable intermediate is unable to form dehydroacetic acid, and reverts back to the starting material. A number of vitamin C derivatives with blocked groups have been developed for this purpose, and are commercially available.
The derivatives used in our products are sodium ascorbyl 2-phosphate (water-soluble, left-hand diagram) and 3-ethyl ascorbate (oil-soluble, right-hand diagram.)
Alternatively, stable vitamin C products can be formulated as a dispersion in a medium in which ascorbic acid isn’t soluble, e.g. oil. The disadvantage is that the undissolved dispersed vitamin is less bioavailable than a solution of the vitamin, so a larger dose is required.
What can be done to prevent oxidation?
There is no way to prevent oxidation altogether, barring complete exclusion of air from manufacturing and filling, and filling the product into an airless pump. In practise though this is unrealistic and it's why formulators use a variety of techniques like anti-oxidants or stabilised versions of these actives.
A number of metal salts, for instance those of iron and copper, can catalyse oxidation reactions. Adding a metal chelator means any metals present are rendered incapable of acting in this way. Examples include ethylene diamine tetraacetic acid (EDTA), disodium glutamate triacetate and sodium phytate.
Another way is to add an antioxidant, which acts as a trap for radicals like oxygen. A naturally-occurring example is alpha-tocopherol, a major component of vitamin E. It forms relatively unreactive and long-lived radicals. The highlighted bond is where the molecule reacts; the radical is stabilised by the adjacent benzene ring, and the adjacent -CH3 groups on the ring make the reactive centre more difficult for reactants to reach.
The synthetic antioxidants butylated hydroxytoluene (BHT, left-hand diagram) and butylated hydroxyanisole (BHA, right-hand diagram) mimic the functional section of alpha-tocopherol, and are more efficient, as they form more stable radicals, since the bulky groups next to the radical centre make the reactive centre even more difficult to reach. Both are effective at very low levels (0.1% w/w or less.)
Being oil-soluble, vitamin E, BHT and BHA are most effective at protecting oil-based materials from oxidation. Similarly, water-soluble antioxidants are most effective at protecting water-soluble materials. Examples of water-soluble antioxidants include sodium sulphite, sodium isoascorbate and glutathione - the latter occurs naturally in plants, animals and fungi.
Which cosmetic materials are most severely affected by oxidation?
Oxidative (permanent / demi-permanent) hair dyes undergo oxidative polymerisation in air, turning from near-colourless to brown or black over a period of hours, which is why they are filled in aluminium tubes; metal acts as a much more effective barrier to air than plastic does.
The oxidative polymers are in fact the final forms of these hair dyes. The second part of the dye, the activator/developer, accelerates the reaction timescale from hours to minutes, and when the mixture is applied to the hair, the oxidation reaction proceeds in a controlled fashion and the colour develops within the hair. The reaction only becomes a problem when it occurs during storage.