J. Cosmet. Sci., 70, 209–216 (July/August 2019) 209 Direct Analysis of Dimethicone in Aqueous Emulsions by Infrared Spectroscopy EDWARD B. WALKER and CHASE S. NAISBITT , Department of Chemistry, Weber State University, Ogden, UT (E.B.W., C.S.N.) Accepted for publication June 19, 2019 . Synopsis Dimethicone is a mixture of fully methylated linear siloxane polymers of various molecular weights. This water-insoluble ingredient provides exceptional skin protection and lubrication when incorporated into skin and hair care products. Quantitative analysis of dimethicone is often required to support quality assurance testing of fi nished products. We report a new, rapid analytical method based on Fourier transform infrared spectrometry, using a special attenuated total refl ectance cell to determine concentrations of dimethicone directly in aqueous emulsions present in personal care products. This rapid, simplifi ed method eliminates sample preparation to remove water, while providing reliable results across a wide range of dimethicone concentrations. INTRODUCTION Dimethicone, polydimethylsiloxane [-(CH3)2SiO-)n], is the most widely used silicon-based organic polymer, fi nding diverse applications ranging from industrial products to health care. It is a common ingredient in cosmetics (1), hair care products (2), and pharmaceu- ticals. Pure dimethicone is a highly effective pediculicide for treating head lice (3,4). The US Food and Drug Administration allows certain skin protection claims for over-the- counter (OTC) drug products containing concentrations of 1–30% (5). Rapid, robust analytical methods are needed to support quality assurance during manu- facturing. Dimethicone is no exception, especially when it is the active ingredient in drug products. This clear, water-insoluble polymer is often incorporated into personal care products as an aqueous emulsion. Commercially available dimethicone emulsions contain a variety of surfactants and preservatives. During compounding, more ingredi- ents are added to create fi nal products. These additives and dimethicone’s relatively low concentration in such aqueous emulsions can complicate its analysis. Sample preparations often focus on separating the dimethicone from water and other excipients before analysis. Many different types of instrumental techniques for analyzing dimethicone have been reported, including gas chromatography (6), gel permeation chromatography (7), Address all correspondence to Edward B. Walker at ewalker@weber.edu .
JOURNAL OF COSMETIC SCIENCE 210 high-pressure chromatography (HPLC), (8–11), X-ray fl uorescence (12,13), inductively coupled plasma optical emission spectroscopy (13), atomic absorption (14), and Fourier transform infrared (FTIR) spectroscopy (15–17). FTIR spectroscopy has been a particu- larly useful technique in quality testing laboratories for many applications over the past two decades due to its relatively low cost and ease of use (18,19). An identifi cation and quantitative method of analysis for pure dimethicone is described in the U.S. Pharmaco- peia (20) that uses infrared spectroscopy. Sabo et al. (15) described in this journal the application of FTIR spectroscopy with a fi xed- length transmission cell to the quantitation of dimethicone in lotions, following extraction into an organic solvent. In fact, all of the published FTIR methods we have reviewed describe the extraction of emulsifi ed dimethicone into a variety of organic solvents, including methylene chloride (15), toluene (16), carbon tetrachloride (17,20), and hexane (21), before analysis. Solvent extraction is often followed by additional steps to dry the solvent and fi lter out insoluble particulates, all of which increase sample preparation times and escalate the potential for errors. Some extraction solvents exhibit infrared absorption frequencies sim- ilar to dimethicone, requiring spectral subtraction to overcome these interferences (15). The primary purpose of these solvent extractions is to remove water before FTIR analysis. Aqueous solutions and emulsions typically cause problems with FTIR analysis because water’s intense, broad absorption spectrum overpowers most other analytes. Even small percentages of water in samples or high humidity can distort infrared spectra. Figure 1. FTIR-ATR absorption sp ec tra for water and aqueous emulsifi ed dimethicone.
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