Silicone vs. Silicon

Introduction

Silicone is the organosilicon polymer,  most commonly polydimethylsiloxane. Silicones are widely used in plastics, lubricants, hydraulic fluids, antifoaming agents, and cosmetics to name a few. 

Silicone
(polydimethylsiloxane)

 

Wikipedia - Silicone

Silicon (no "e") refers to the element in any form, most commonly found as silica or silicate minerals. 

Silicon, Si

Silica, SiO2 (sand, quartz)

Total dissolved silicon can be determined using an atomic spectroscopy technique such as ICP-OES or ICP-MS. If the material is an insoluble solid, total silicon can be determined by x-ray fluorescence. But because silicates are ubiquitous, these techniques are not good alternatives for measuring silicone, the organosilicon polymer. 

Wikipedia - Silicon

 


USP Identification of Simethicone

Simethicone, a water soluble silicone, is used in pharmaceuticals, and the quality of the material is governed by compendial methods such as the USP monographs.  Exova tests for Simethicone include:

Identification
Defoaming Activity
Loss on Heating
Heavy Metals
Content of Silicon Dioxide
Assay
Residual Solvents <467>
pH <791>

The current USP monograph requires extraction of the Simethicone water solution with toluene and analysis of the extract by FTIR.  Typically polydimethylsiloxane has strong absorptions at 1262, 1092, 1020, and 799 cm-1.  (Click here to see an FTIR spectrum of silicone rubber.)  However in a toluene solution the spectrum is dominated by the toluene.    A recent modification has been suggested in PF35(6) in which the toluene is evaporated onto an ATR crystal for analysis.

NMR of Polydimethylsiloxane

A superior identification method is based upon nuclear magnetic resonance (NMR).  Even in a water solution containing a few percent Simethicone, the methyl protons attached to silicon (Si-CH3) are generally well separated from other signals since they appear in a somewhat unique region of the spectrum near 0 ppm.  For a clearer spectrum of only the methyl protons, one may obtain a Si-filtered 1H spectrum or a 2D {1H,29Si} shift correlation spectrum. 

Besides the easy identification of silicones, one can determine other structural details such as chain length, end groups, and other functional groups. In addition to the somewhat broadened polymer signals, one normally also observes signals from smaller oligomers such as the cylo -trisiloxane and
-tetrasiloxane which appear as sharper signals.

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1H NMR Spectra of Simethicone Emulsion

Simethicone, 1H and Si-filtered NMR spectra


Analysis of Trace Silicones

Silicones present in percent levels can easily be identified by infrared spectroscopy or nuclear magnetic resonance. However, determining trace amounts (ppm) of silicone is a different story.

For several years we have been using a method adapted from Dow Corning (see L.G. Mahone, et al., Env. Tox. Chem. 1983, 307-313 and J.J. Kennan, et al., Anal. Chem. 1999, 71, 3054-3060) for low molecular weight silicones in wastewater.  Polydimethylsiloxane can be hydrolyzed in strong acid and derivatized to give a product analyzed by gas chromatography. This analysis is only offered to industrial clients as we have discontinued offering the test for breast milk or tissues for individuals.

While this test can detect low molecular weight silicones in water >10 ppm, most silicone oils used in industry are high molecular weight and water insoluble.  As such, this test usually gives negative results for water known to have been in contact with silicone. There are water soluble forms of silicone.  Simethicone is a water soluble form used in antacids and other drugs to break up gas in the stomach.  In fact Simethicone is used to calibrate the test above.

Other tests for trace silicones include GCMSD which is discussed below.

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GC-FID of dimethylsiloxane derviative

GCMS of Water Extracts

By extracting large volumes of water (up to 1 L) and concentrating the extracts, ppb-ppm levels of low molecular weight silicones can be determined in aqueous solutions. The chromatogram below shows silicone oligomers in the range of n = 3 - 17. 

The top selected ion chromatogram shows mass 73 which is the primary mass fragment characteristic of polydimethylsiloxane.  The total ion chromatogram (bottom) shows additional peaks which are not silicones. Note the drop in concentration with molecular weight reflects their relative water solubility.

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polydimethylsiloxane, GCMS of water extract


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Formerly West Coast Analytical Service (WCAS) and Bodycote Testing Group