At the very mention of the term “dimethicone” most makeup and skincare enthusiasts will immediately think of foundations and primers with a high silicone content which leave a heavy and occlusive film of the skin and most often contribute to worsening the condition of skin prone to acne and comedones. However, not many people know that, depending on its molecular weight, dimethicone may have a completely different effect on the skin. More on the difference between high molecular weight and low molecular weight dimethicone in today’s article by Lucija Božičević.
Silicones or polysiloxanes are polymer compounds that are used in a large number of different industrial sectors. One of those sectors is the cosmetics industry and silicones are a very common ingredient in numerous formulations available on the market. Polysiloxane is a polymer with a molecular formula [R2SiO]n where the polymer chain is composed of interlinked atoms of oxygen and silicon one after the other, while R2 represents two strands that are most often various organic substituents. The N in the formula represents the number of repetitions of siloxane units in the polymer. The properties of different silicones depend on the length of the polymer, the structure of substituents (R) and the way in which the links are mutually interconnected.
Silicones are hydrophobic compounds characterised by excellent resistance to aging and chemical reagents. Cosmetic formulations most often use dimethicone (polydimethylsiloxane) which has two methyl groups as substituents. Dimethicone is an inert and non-toxic compound which exhibits exceptional rheological properties, that is, apart from its functional role in preserving the skin’s barrier or for example softening hair, it also contributes to easier spreading and application of product. Nevertheless, despite all its good properties, lately dimethicone has found itself on the “black list” of ingredients used in cosmetic products. However, it is important to understand that the dimethicone used in cosmetic formulations most often does not have an occlusive but rather an emollient effect.
The difference is great – emollients are substances that are rich in hydrophobic components which will fill out the damaged and peeled spots on the skin and so restore the skin’s natural barrier function. Natural skin functioning implies that there is free exchange of gases and water which means that pores are not clogged. Emollients actually assume the role of lipids naturally present in our skin and prevent excessive transepidermal loss of water. Excessive transepidermal loss of water is present in damaged skin because there is no normal lipid system to prevent it from happening. Occlusive effect means creating a barrier on the skin that prevents the water from evaporating which is why it is used in cases of extreme skin damage or skin disorders characterised by extremely dry and irritated and flaky skin. Mix up of these two categories and functions most often happens because some extremely hydrophobic emollients rich in fatty components may also exhibit an occlusive effect. That is where the misconceptions on emollients come from. It is actually completely understandable. To be exact, certainly silicones will have an occlusive effect. In that case we are talking about high molecular silicones. This complicated word hides a very simple explanation – high molecular silicones have a large molecular weight, that is, a high number of repetitions of units of siloxane (previously mentioned as n) and/or size of substituents (R). Low molecular silicones that are most often used in cosmetic formulations have a small number of repetitions of the basic units that make up the polymer and are therefore less hydrophobic. In the case of dimethicone the advantage is also that the natural substituent on siloxane is a small group (methyl group, -CH3) which makes it even less hydrophobic in relation to other representatives from this group.
Low molecular silicones, including low molecular dimethicone, are in most cases volatile liquids. So when low molecular dimethicone is applied on the skin, part of it will evaporate but a film will remain to enable the skin’s future exchange of gases as well as loss of water. Such an effect has been proven by numerous in vitro (on cell culture) and in vivo (on live organisms) studies comparing transepidermal water loss after application of silicones and other known occlusives.1,2 For these very reasons low molecular silicones are the most common choice in cosmetic product formulations and it really isn’t necessary to raise such a panic whenever there are dimethicones present in a product. Taking into account numerous independent studies made with the aim of testing the effect of low molecular soluble silicones on the skin and its permeability in relation to water and gases, it can be said that the fear of their application is to a great extent unjustified. Of course, the presence of certain high molecular silicones may result in an occlusive effect, but one must keep in mind that not every occlusive is necessarily comedogenic. Things are never exclusively black or white and the situation is the same with silicones. However, low molecular silicones are compounds that may be present in cosmetic formulations for a whole range of reasons: to improve the product’s „slip and glide“ property, to dissolve active substances, while in sunscreen they serve for dispersing (uniformly distributing throughout the volume) mineral protection factors. In addition, as studies show, they help preserve the normal protective barrier function of the skin, not hindering water evaporation and gas exchange, so one should not fear using products whose formulations contain them.
- Da Paepe K et al. Silicones as Nonocclusive Topical Agents. Skin Pharmacol Physiol, 2014, 27, 164-171.
- Glombitza B. Müller-Goymann CC. Investigation of interactions between silicones and stratum corneum lipids. Int J Cosmet Sci, 2001, 23, 25-34.