Cyclomethicones are also favored by formulators because they spread easily on the hair and skin

Because of the high interest in silicones, we periodically take a closer look at some of the many silicones found in hair-care products. This month we'll be looking at Cyclomethicone.
Cyclomethicone is the INCI (International Nomenclature for Cosmetic Ingredients) name for a category of silicones used in many personal-care products. Cyclopentsiloxane, cyclotetrasiloxane, and cyclohexasiloxane are also designations for the same class of molecules. In contrast with linear silicones such as dimethicone, these molecules have a cyclic, or ring-like architecture comprised of only four to six repeat units of the dialkyl siloxane group. These are much smaller molecules than the polymeric silicones, such as polydimethyl siloxane (dimethicone) which may have hundreds of thousands of siloxane repeat units.

Cyclic silicones (or siloxanes) are frequently used as solvents for fragrance and essential oils, and also as carrier fluids for higher molecular weight silicones such as dimethicone and dimethiconol. This aids the formulator in preparing her final product because she can prepare her oil phase by combining cyclomethicone and the silicone and/or fragrance oil and add her emulsifier, prior to addition of the entire oil phase to the aqueous phase.

Cyclomethicones are also favored by formulators because they spread easily on the hair and skin, and the lubrication they provide isn't greasy or tacky. Because of their low vapor pressure, they evaporate easily from hair or skin at room temperature, and therefore aren't prone to build-up or an oily residue. For this reason, they are sometimes used as an additive in products such as spray leave-in conditioners to help speed drying time. Another benefit of their low vapor pressure is that it provides a way for the desired ingredients to penetrate into the hair shaft or skin. The cyclic silicone is too large to penetrate the skin or hair itself, but is small enough to dissolve the beneficial ingredient, deposit it onto the hair surface, and then evaporate, leaving behind the beneficial ingredient.

Cyclic silicones may appear on labels by several different names, such as:

  • Cyclomethicone (can be a single cyclic silicone or a mixture of two or more)

  • Cyclopentasiloxane

  • Cyclotetrasiloxane

  • Cyclohexasiloxane

  • Octamethyl cyclotetrasiloxane

Several curly-haired consumers on NaturallyCurly have reported very unfavorable results with products containing cyclopentasiloxane in particular. These problems were attributed to build-up on the surface or build up due to penetration of the smaller silicone into the hair shaft. However, this seems unlikely because of the chemical and physical properties of these molecules. I believe that evaporation of the cyclic silicones creates a ruffled hair cuticle that creates a rough, unpleasant texture on the hair surface in the same manner as evaporation of ethyl alcohol or isopropyl alcohol. The mildly acidic pH of a shampoo and conditioner will help restore the sealed, flat surface of the cuticle, thereby improving the texture and gloss of the hair.