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Dimethicone

INCI name: dimethicone
Synonyms: poly(dimethylsiloxane)
Molecular formula: (C₂H₆OSi)_nC₄H₁₂Si
Molecular weight: variable
IUPAC name: poly(dimethylsiloxane)
CAS number: 63148-62-9

Silicones are manufactured from quartz, which contains silicon in the form of SiO₂. Through a number of processing steps a variety silicone entities may be synthesized including silicone polymers, such as dimethicone. Silicones have been used in cosmetic products for more than half a century. They are commonly found in personal care formulations that are applied to skin and hair due to their ease of spreadibilty as well as many other characteristics. Their unique surface properties arise from their low surface energy (surface tension). Some applications of silicones in cosmetics consist of: skin lotions (desoaping, rub-out, protection, feel), skin cleanser (lubricity and wetting), antiperspirant (anti-whitening and detackification), pre-shave lotion (lubricity), aftershave lotion (feel), makeup (water resistance), hair care (conditioning), and shaving cream (reduce razor drag).¹

The low surface energy of silicones may be explained by their structural characteristics. For example, dimethicone contains a siloxane backbone (high surface energy) with pendant methyl groups (low surface energy). It is believed that the methyl groups are the predominant species present at interfaces (or the surface), allowing the low surface energy portion of the molecule to dominate its interactions. Further, it should be noted that methyl groups contribute to lower surface energy than methylene groups. This would explain why most oils (dominated by methylene groups) are higher in surface tension than silicones, such as a dimethicone, which contain mostly methyl groups (see Table 2).

From a structural standpoint, dimethicone is one of the simplest molecules of the silicones, yet it still remains a very popular molecule that is utilized in a variety of cosmetic formulations. The backbone of dimethicone is very flexible due to the freedom of the pendant methyl groups to rotate about the Si-O bonds. As a result, it has a relatively low glass transition temperatura (Tg).²

Properties
Dimethicone is a polymer. Therefore, many of its properties will depend on its molecular weight.
Surface tension: 21.3 mN/m
Tg: 150 °C²

Table 1: Various grades of dimethicone available in different molecular weights ( characterized by viscosity) and the corresponding specific gravity and refractive index values. Data from Reference 3.^†

 

Table 2: Liquid surface tension values of various oils and dimethicone obtained at 25 °C. Data from Reference 2.

Solubilities^††
Soluble: mineral spirits, cyclohexane, n-hexane, ethyl ether, isopropyl laurate, isopropyl palmitate, lauryl alcohol
Partially soluble: acetone, 2-ethylhexanol, butanol
Insoluble: methanol, ethanol, ethylene glycol, glycerin, propylene glycol, liquid paraffin, petrolatums, fatty acids, animal/vegetable oils, water

Footnotes
†Specific gravity is the ratio of the density of a particular substance to the density of a reference material (normally water). If the reference is water, a specific gravity > 1 indicates that a compound will sink in water (since it will be more dense than water) while a substance with specific gravity < 1 will float in water (since it will be less dense than water).⁴
††For a complete list of solubilities, the reader should consult Reference 3.

References
1. Anthony J. O´Lenick, Jr., Silicones for Personal Care, 2nd Ed., Allured Books: Carol Stream, IL (2008).
2. M.S. Ferritto and M.J. Owen, “Silicone wettability and its significance in beauty products” in Polymers for Personal Care and Cosmetics, Eds. A. Patil and M.S. Ferritto, ACS Symposium Series, American Chemical Society: Washington, D.C. (2013).
3. Silicone Products for Personal Care, Shin-Etsu Chemical Co., Ltd., trade brochure (2008).
4. A.J. O’Lenick, Comparatively speaking: density vs. specific gravity, Cosmet. Toil. Science Applied, Apr. 26 (2011).