Octamethylcyclotetrasiloxane
From Wikipedia, the free encyclopedia
 | |
 | |
| Names | |
|---|---|
| Preferred IUPAC name
Octamethyl-1,3,5,7,2,4,6,8-tetroxatetrasilocane | |
Other names
| |
| Identifiers | |
3D model (JSmol) |
|
| ChEBI | |
| ChemSpider | |
| ECHA InfoCard | 100.008.307 |
| EC Number |
|
PubChem CID |
|
| UNII | |
CompTox Dashboard (EPA) |
|
| |
| |
| 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: | |
 [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).
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
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]