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Octamethylcyclotetrasiloxane

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Not to be confused with 22-dihydroergocalciferol.
Octamethylcyclotetrasiloxane
Skeletal formula of octamethylcyclotetrasiloxane
Ball-and-stick model of the octamethylcyclotetrasiloxane molecule
Names
Preferred IUPAC name
Octamethyl-1,3,5,7,2,4,6,8-tetroxatetrasilocane
Other names
  • D4
  • D4
  • OMCTS
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.008.307 Edit this at Wikidata
EC Number
  • 209-136-7
UNII
  • InChI=1S/C8H24O4Si4/c1-13(2)9-14(3,4)11-16(7,8)12-15(5,6)10-13/h1-8H3
    Key: HMMGMWAXVFQUOA-UHFFFAOYSA-N
  • C[Si]1(O[Si](O[Si](O[Si](O1)(C)C)(C)C)(C)C)C
Properties
[(CH3)2SiO]4
Molar mass 296.616 g·mol−1
Density 0.956 g/mL
Melting point 17–18 °C (63–64 °F; 290–291 K)
Boiling point 175–176 °C (347–349 °F; 448–449 K)
56.2±2.5 ppb (23 °C)[1]
log P 6.98±0.13[2]
Vapor pressure 124.5±6.2 Pa (25 °C)[3]
Hazards
GHS labelling:
GHS08: Health hazard[4]
Warning
H361f, H410 M=10[4]
Related compounds
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Octamethylcyclotetrasiloxane, also called D4, is an organosilicon compound and one of a number of cyclic siloxanes (cyclomethicones). It is a colorless viscous liquid with a high boiling point. It has historically been used on a large scale in personal care products including cosmetics, hair conditioners and emollients (moisturising creams),[5] Global production in 1993 was 136,000 tons,[6] however it is now facing significant pressure from regulators. It is a substance of very high concern In the EU, where it was classified as a PBT and effectively banned in personal care products in 2018. The US EPA began reevaluating its risks in 2020.[7] Replacements include branched-chain alkanes (isoparaffins).

Production and polymerization

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Commercially D4 is produced from dimethyldichlorosilane. Hydrolysis of the dichloride produces a mixture of cyclic dimethylsiloxanes and polydimethylsiloxane. From this mixture, the cyclic siloxanes including D4 can be removed by distillation. In the presence of a strong base such as KOH, the polymer/ring mixture is equilibrated, allowing complete conversion to the more volatile cyclic siloxanes:[5]

[(CH3)2SiO]4n → n [(CH3)2SiO]4

D4 and D5 are also precursors to the polymer. The catalyst is again KOH.

Safety and environmental considerations

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D4 is of low acute toxicity. The LC50 for a single four hour inhalation exposure in rats is 36 mg/L. The oral LD50 in rats is above 4800 mg/kg and the dermal LD50in rats is above 2400 mg/kg.[8]

As the smallest cyclic dimethylsiloxane that does not experience considerable ring strain,[9] D4 is one of the most abundant siloxanes in the environment, e.g. in landfill gases.[10] D4 and D5 have attracted attention because they are pervasive. Cyclic siloxanes can be detected in some species of aquatic life.[11] An independent, peer-reviewed study in the US found "negligible risk from D4 to organisms"[12] while a scientific assessment by the Australian government stated, "the direct risks to aquatic life from exposure to these chemicals at expected surface water concentrations are not likely to be significant."[13]

In the European Union, D4 was characterized as a substance of very high concern (SVHC) due to its PBT and vPvB properties and was thus included in the candidate list for authorisation.[14][clarification needed] D4 shall not be placed on the market in wash-off cosmetic products in a concentration equal to or greater than 0.1% by weight of either substance, after 31 January 2020.[15] Conversely, a detailed review and analysis of the science by the State of Washington in 2017 led to the removal of D4 from their CHCC listing.[16][clarification needed] That decision prompted the State of Oregon to follow suit in 2018.[17]

See also

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References

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  1. ^ Sudarsanan Varaprath; Cecil L. Frye; Jerry Hamelink (1996). "Aqueous solubility of permethylsiloxanes (silicones)". Environmental Toxicology and Chemistry. 15 (8): 1263–1265. doi:10.1002/etc.5620150803.
  2. ^ Shihe Xu, Bruce Kropscott (2012). "Method for Simultaneous Determination of Partition Coefficients for Cyclic Volatile Methylsiloxanes and Dimethylsilanediol". Analytical Chemistry. 84 (4): 1948–1955. doi:10.1021/ac202953t. PMID 22304371.
  3. ^ Ying Duan Lei; Frank Wania; Dan Mathers (2010). "Temperature-Dependent Vapor Pressure of Selected Cyclic and Linear Polydimethylsiloxane Oligomers". Journal of Chemical & Engineering Data. 55 (12): 5868–5873. doi:10.1021/je100835n.
  4. ^ a b "Harmonised classification – Annex VI of Regulation (EC) No 1272/2008 (CLP Regulation)". Retrieved 2022-03-15.
  5. ^ a b Moretto, Hans-Heinrich; Schulze, Manfred; Wagner, Gebhard (2005). "Silicones". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a24_057. ISBN 978-3527306732.
  6. ^ Philippe P. Quevauviller; Patrick Roose; Gert Verreet (24 August 2011). Chemical Marine Monitoring: Policy Framework and Analytical Trends. John Wiley & Sons. p. 2010. ISBN 978-1-119-97759-9.
  7. ^ US EPA, OCSPP (29 October 2020). "Risk Evaluation for Octamethylcyclotetra- siloxane (D4)". www.epa.gov.
  8. ^ Brooke DN, Crookes MJ, Gray D, Robertson S (2009). Environmental Risk Assessment Report: Octamethylcyclotetrasiloxane (PDF) (Report). Environment Agency.
  9. ^ Brook, Michael A. (2000). Silicon in Organic, Organometallic and Polymer Chemistry. New York: Wiley. p. 262. ISBN 978-0-471-19658-7.
  10. ^ Franz-Bernd Frechen (2009). Odours and VOCs: Measurement, Regulation and Control Techniques. kassel university press GmbH. p. 287. ISBN 978-3-89958-609-1.
  11. ^ Wang, De-Gao; Norwood, Warren; Alaee, Mehran; Byer, Jonathan D.; Brimble, Samantha "Review of Recent Advances in Research on the Toxicity, Detection, Occurrence and Fate of Cyclic Volatile Methyl Siloxanes in the Environment" Chemosphere 2013, volume 93, pages 711–725, doi:10.1016/j.chemosphere.2012.10.041.
  12. ^ Josie B. Nusz, Anne Fairbrother, Jennifer Daley, G. Allen Burton (2018). "Use of multiple lines of evidence to provide a realistic toxic substances control act ecological risk evaluation based on monitoring data: D4 case study". Science of the Total Environment. 636: 1382–1395. Bibcode:2018ScTEn.636.1382N. doi:10.1016/j.scitotenv.2018.04.335. PMID 29913599.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  13. ^ "Cyclic volatile methyl siloxanes: Environment tier II assessment" Archived 2020-05-27 at the Wayback Machine Australian Department of Health. Retrieved 2018-12-27.
  14. ^ "Candidate List of substances of very high concern for Authorisation – Octamethylcyclotetrasiloxane". ECHA. Retrieved 2018-07-18.
  15. ^ "Commission Regulation (EU) 2018/35 of 10 January 2018 amending Annex XVII to Regulation (EC) No 1907/2006 of the European Parliament and of the Council concerning the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) as regards octamethylcyclotetrasiloxane ('D4') and decamethylcyclopentasiloxane ('D5')". Retrieved 2018-07-18.
  16. ^ "Concise Explanatory Statement: Children's Safe Products Reporting Rule" State of Washington Department of Ecology. Retrieved 2018-12-27.
  17. ^ "Notice of Proposed Rulemaking" State of Oregon. Retrieved 2018-12-09.
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