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Polydimethylsiloxane analysis using quantitative spectroscopy

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  • 1. Polydimethylsiloxane Determination in Lotionized Formulations Using Quantitative FTIR Part 1: Linearity Studies Luke Shokere, Ph.D.
  • 2. Characteristics of Dimethicone
    • Chemical name is polydimethylsiloxane.
    • Cultural uses: cosmetics (skin protectant / emollient), machinery (lubricant), manufacturing (antifoam).
    • Physical properties: optically clear fluid, rheologically ease of flow, smooth/friction-less texture.
    • Chemical properties: polymer, inert, non-flammable, soluble in aliphatic hydrocarbon solvents (e.g., Hexane).
    • Structural properties: terminal trimethyls covalently bonded to Silicon atom (trimethylsiloxy) which are covalently bonded to multiple repeating units of methylsiloxys:
    • [--(CH 3 ) 2 —Si—O--] n
    • Analytical properties: extractable from matrices with aliphatic hydrocarbon solvents, displays distinct absorbance in the mid-infra red regions 1260, 1080, and 1015 cm -1 , the signal at 1260 cm -1 is commonly used in FTIR quantitations.
  • 3. Introduction
    • Previously, dimethicone type polymers were quantified by gel permeation chromatography (GPC). The analytical procedure described herein is a convenient and inexpensive alternative to the more time-consuming and expensive GPC method without compromising accuracy and precision.
    • The capability of the analytical method to deliver a “dose-dependent response” for dimethicone was investigated by spiking varying amounts of dimethicone into aliquots of placebo lotionized formulation. The dimethicone was extracted from the samples and its concentration was measured by recording the absorbance at 1260cm -1 , which correlates to the H 3 C-Si bond vibration (stretching action) in the siloxane polymer.
    • It was hypothesized that a linear relationship would be observed between the amount of dimethicone recovered and the FTIR absorbance values.
  • 4. Experimental
    • The test samples were prepared according Table 1, extracted with Hexane, purified using anhydrous salt and ultra-centrifugation. The purified extracts were quantified with a 100µm path length semi-demountable liquid flow cell fitted with NaCl windows and an FTIR (Perkin Elmer or suitable vendor). Data were acquired using Spectrum ™ software which accomplished background subtraction, peak integration, and conversion of measurements from % Transmission to Absorbance units.
  • 5. Table 1. Sample preparation. 1.80 3928 72 120 1.65 3934 66 110 1.50 3940 60 100 1.35 3946 54 90 1.20 3952 48 80 0 4000 0 0 Dimethicone Placebo Dimethicone Sx wt. % wt. mg mg % Normal
  • 6. Figure 1a. FTIR spectral results for the 1260cm -1 peak. Blue 120%; Red 110%; Black 100%; Brown 90%; Green 80%; Black, Placebo (no peak).
  • 7. Figure 1b . Method of peak integration at 1260cm-1 . The signature peak that corresponds to the H 3 C-Si bond vibration (stretching action) is integrated. Smaller peak on right shoulder is the H 3 C bond vibration of the Hexane solvent.
  • 8. Figure 2. Linearity plot.
  • 9. Table 2. Peak integration absorbance results 0.1922 120 0.1706 110 0.1516 100 0.1302 90 0.1077 80 0.0000 0.0 Absorbance Dimethicone FTIR % Normal
  • 10. Conclusion
    • The analytical method delivered a dosage-dependent response with acceptable predictability. The acceptance criterion was R 2 ≥ 0.9500; the observed value R 2 = 0.9992. It was concluded that:
    • Greater than 99% of the variance in the absorbance response was explained by differences in the amount of recovered dimethicone.
    • The method was specific for polydimethylsiloxane because no peak at 1260 cm -1 peak was observed in the placebo.