Dimethicone

Overview

Dimethicone is a synthetic silicone polymer — a chain of silicon and oxygen atoms with methyl groups attached. It is sold as a clear, odourless, slightly slick liquid that ranges from water-thin to honey-thick depending on the chain length. You will see this expressed as a viscosity number on the supplier label: dimethicone 5, dimethicone 50, dimethicone 350, dimethicone 1000, dimethicone 12500, all the way up to dimethicone 100000.

The number is the viscosity in centistokes (cSt) and it dictates everything about how the product feels:

• Dimethicone 5-50 cSt — water-thin, fast-absorbing, used in primers and serums for slip
• Dimethicone 100-350 cSt — light cream feel, the most common viscosity for face creams
• Dimethicone 500-1000 cSt — silky, slightly draggy, common in conditioners and body lotions
• Dimethicone 5000-12500 cSt — gel-like, used in scar treatments and stretch mark products
• Dimethicone 60000+ cSt — extremely thick, used in specialty hair smoothing products

Shelf life is essentially indefinite. Silicones do not oxidise, do not feed microbes, and do not lose their function on a shelf.

What it does in a formula

Dimethicone forms a thin, breathable, water-resistant film on whatever you put it on. Unlike oils, the film does not feel greasy — it feels smooth and “blurred,” which is why dimethicone is the workhorse of every face primer and pore-blurring serum on the market.

Specifically, it:

• Reduces transepidermal water loss by 30-50% in measured studies, but without the suffocating feel of petrolatum or beeswax
• Smooths over fine lines and pores optically, by filling micro-irregularities with a flat film
• Conditions hair by depositing a smooth coating on the cuticle, reducing friction and frizz
• Prevents stretch marks and supports scar healing (the cosmetic-grade silicones used in scar gels are dimethicone-based and have human-trial evidence)

It is also famously the ingredient that lets a moisturiser glide under makeup without pilling.

How to use

Add to the cool-down phase, ideally below 50 C, after the emulsion has formed. Dimethicone is oil-phase compatible but does not need to be heated; in fact, holding it at high temperature for long can dull the silky feel slightly.

For emulsions, you need either:

• An emulsifier that handles silicones (most standard cosmetic emulsifiers like olivem 1000, ECO mulse, or polysorbate-based ones work)
• A silicone-specific emulsifier like cetyl PEG/PPG-10/1 dimethicone for high-silicone water-in-silicone systems

Usage rates by product type:

• Face creams and lotions: 1-5% (5 cSt to 350 cSt)
• Primers: 5-15% (5-100 cSt for skin-feel; 350+ for blur effect)
• Hair conditioners (rinse-off): 1-5% (1000-5000 cSt)
• Hair serums (leave-on): 1-10% (5-100 cSt)
• Scar gels: 50-95% (12500 cSt, often blended with 5 cSt cyclomethicone)
• Stretch-mark butters: 3-10%
• Sunscreen vehicles: 2-8%

Best for / Worst for

Best for: primers, makeup-ready moisturisers, anti-frizz hair products, scar treatments, stretch-mark prevention products, sport and athletic body lotions (it is sweat-resistant), legs and elbows where you want a long-lasting smooth feel.

Worst for: “100% natural” certifications (dimethicone is synthetic — not from petroleum, but not plant-derived either), people who insist on plant-based formulating, products marketed to anti-silicone audiences (a real subset of customers actively avoid it). For environmental concerns, the data is mixed: dimethicone is not bioaccumulative and degrades in soil, but volatile cyclic silicones like D4 and D5 have stricter EU restrictions.

Common pitfalls

Build-up on hair. Lower-cost dimethicones do not rinse out well and can build up on the hair shaft, especially fine or low-porosity hair. This is the source of the “silicone-free” hair-care movement. The fix is using a clarifying shampoo every 2-3 weeks, or choosing water-soluble silicones (PEG-dimethicone) for leave-in products.

Pilling under makeup. If you layer a high-silicone primer over a high-acrylate moisturiser, the two films cannot bond and rub off in little balls. Stick to silicone-on-silicone or water-on-water layering for the same product family.

Confusing the viscosities. Buying dimethicone 12500 thinking it will absorb like a face serum is a disappointment. Always note the viscosity number when ordering.

Substitutes

• Squalane — for the slip and skin-feel; loses the long-wear water-resistant film and the optical blur
• Coco-caprylate — for the dry-touch finish in face creams; lighter, less occlusive
• Isoamyl laurate — closest plant-derived feel to a light silicone
• Cetiol Ultimate (undecane and tridecane) — synthetic but hydrocarbon-based, similar fast-drying feel
• Polyglyceryl-3 dicocoate — for water-soluble silicone replacement in hair products