Analyzing Silicone Oils

Carol was running silicone oil extracts of hair swatches on an AA. She noted that the 1% silicon oil standard was submitted in MIBK/toluene solvent. Carol had a certified silicone oil standard, but the mineral oil was interfering with the MIBK/toluene matrix (it burned differently in the flame). Carol hoped Paul could offer some assistance.

Dear Carol,

Silicone Oils are difficult to analyze because they contain light oligomers with significant vapor pressures at room temperature. These give higher signals than a solid or high boiling liquid (e.g., vaporization interference). Silicone Oils -- dimethyl silicones depolymerize to form volatile monomer units when heated (measure directly in an alcoholic KOH/xylene mixture where the sample is treated first with the KOH at 60-100 °C to "unzip" the Si-O-Si polymeric structure or digest with conc. H2SO4 / H2O2 followed by cooling and dissolution of the dehydrated silica with HF). Note that the direct analysis of silicone oils in an organic solvent will result in false high results due to high vapor pressure of volatile monomer units such as hexamethylcyclotrisiloxane. The KOH forms a nonvolatile silicone salt.

The best solvent to use is xylene. If you choose not to use the KOH preparation technique, have your standard be the same viscosity of silicone oil as is being analyzed and use xylene as your solvent.

Please contact me if you need further help.

Serving you in chemistry,

Paul R. Gaines, Ph.D.
CEO of Inorganic Ventures & Fellow Chemist

DISCLAIMER: Advice offered by the chemists at Inorganic Ventures is intended for the individual posing the question. Feel free to contact us to verify whether these suggestions apply to your unique circumstances.