DIRECT ANALYSIS OF DIMETHICONE IN AQUEOUS EMULSIONS 211 Surprisingly, dimethicone is an exception: three of its absorption peaks at 1,260, 1,072, and 1,007 cm-1 rise above the water’s relatively low absorption at these frequencies, as seen in Figure 1. The single, well-defi ned peak at 1,260 cm-1 is due to the symmetric deformation vibration of the methyl groups attached to the silicon atom. This unique absorption peak of dimethicone is the basis for quantitative testing for pure dimethicone (20) and for dimethicone solutions in some organic solvents (16,17), whereas the double peaks at 1,072 and 1,007 cm-1 are preferred in methylene chloride (15). The single peak maximum at about 1,260 cm-1 differs slightly among previously published methods, ranging from 1,258 to 1,262 cm-1. These differences are caused by spectral shifts in sol- vents of differing polarities and instrumental differences such as transmission and attenu- ated total refl ectance (ATR). Infrared absorption frequencies for dimethicone are well characterized (22) and listed in Table I. We have ex ploited the use of an ATR cell with a long contact surface path length to measure the concentration of dimethicone in aqueous emulsions by integration of peak areas at 1,260 cm-1. Neat samples were analyzed without any sample preparation in the concen- tration ranges of 1–35% (g/100 g). EXPERIMENTAL REAGENTS Dime thicone r eference standards, Belsil® DM 350 (pure oil), and DM 5102E and DM 5700E (50% aqueous emulsions with different emulsifying surfactants), were from Wacker Chemie AG (Munich, Germany). ACS reagent-grade hexane and anhydrous ethanol were purchased from Fisher Scientifi c (Hampton, NH). PEG-100 stearate, sorbitan lau- rate, PPG-15 stearyl ether, Triton X-100, Tween 20, Tween 80, phenoxyethanol, DMDM hydantoin, sodium benzoate, methyl paraben, and propyl paraben were purchased from MilliporeSigma, Burlington, MA laureth-23 and laureth-4 were obtained from Rita Corporation, Crystal Lake, IL PPG-15 stearyl ether was purchased from Jeen Interna- tional, Fairfi eld, NJ and Kathon was purchased from Rohm and Haas, Philadelphia, PA. INSTRUMENTATIO N Analyses were performed using a Nicolet Avatar 470 FTIR spectrometer (Thermo Fisher Scientifi c, Waltham, MA) fi tted with a Smart Ark horizontal ZnSe ATR 45° multi-bounce Table I Dimethicone IR Absorption F requencies (22) Frequency (cm-1) Description 2,965 CH3 asymmetrical stretch 2,906 CH3 symmetrical stretch 1,410 CH3 asymmetrical bend 1,258 CH3 symmetrical bend 1,072 Si-O-Si asymmetrical bend 1,007 Si-O-Si asymmetrical stretch 864 Si-CH3 asymmetrical rock

JOURNAL OF COSMETIC SCIENCE 212 trough plate and controlled with OMNIC software (Thermo Fisher Scientifi c). This cell is much longer (approximately 72 mm in length with multi-bounce optics) than typical ATR cells (only 1.5 mm diameter with single bounce). This longer ATR plate yields enhanced sensitivity compared to those with shorter path lengths. The sample cell for transmission measurements was a demountable liquid cell with CaF2 windows and a 0.1-mm PTFE spacer. Quantitative scans for both ATR and transmission were limited to the expanded region of 1,239–1,279 cm-1 in the absorbance mode (Figure 2). Integrated peak areas for the absorption band centered at 1,260 cm-1 were then plotted as a function of dimethicone concentration to calibrate the instrument and to measure samples. METHOD 1: FTIR TRAN SMISSION Pure dimethicone st andards were prepared by dissolution in hexane at concentrations of 0.3–1.5 g/100 mL. Dimethicone emulsions were analyzed by extraction into hexane before FTIR scanning in transmission mode. Depending on the dimethicone concentration, be- tween 0.20 and 2.0 g samples of each liquid emulsion (carefully weighed to 0.001 g) were placed in separate 60-mL wide-mouth jars. The open jars were heated in an oven at 115°C Figure 2. Emulsifi ed dimethicone c a libration standards.