N-(dimethylvinylsilyl)-1,1-dimethyl-1-vinylsilylamine

Endpoint summary

Description of key information

The registered substance, N-(dimethylvinylsilyl)-1,1-dimethyl-1-vinylsilylamine, is not stable in water, which affects the approach to the determination of physicochemical properties. The significance of this for read-across is discussed in relevant sections of the dossier.

 

N-(Dimethylvinylsilyl)-1,1-dimethyl-1-vinylsilylamine is a liquid at standard temperature and pressure, with a measured melting point of <-100 °C at 1013 hPa and a measured boiling point of 164 °C at 1013 hPa. It has a relative density (D20/4) of 0.822 at 20°C, a measured kinematic viscosity of 0.8361 mm2/s at 20 °C and a predicted vapour pressure of 120 Pa at 25 °C.

 

The substance is classified as Flammable Liquid Category 3 (H226: Flammable liquid and vapour) according to Regulation (EC) No. 1272/2008 on the basis of a measured flash point value of 37°C at 1013 hPa.

It has a measured auto-ignition temperature of 270 °C at 968.4-976.2 hPa and is not explosive and not oxidising on the basis of structural examination.

 

In contact with water, N-(dimethylvinylsilyl)-1,1-dimethyl-1-vinylsilylamine hydrolyses very rapidly (half-life <<1 minute at 25 °C and pH 4, pH 7 and pH 9) to produce dimethylvinylsilanol and ammonia according to the following equation:

 

CH2=CHSi(CH3)2NSi(CH3)2CH=CH2 + 2H2O → 2CH2=CHSi(CH3)2OH + NH3

 

Therefore, requirements for testing of water-based physicochemical properties for the substance are waived on the basis of instability in water. The properties of the silanol hydrolysis products, dimethylvinylsilanol and ammonia are assessed instead.

 

The silanol hydrolysis product, dimethylvinylsilanol, may undergo condensation reactions in solution to give siloxane dimer and a dynamic equilibrium is established. The overall rate and extent of condensation is dependent on nominal loading, temperature, and pH of the system, as well as what else is present in the solution.

 

The condensation reactions of monosilanols may be modelled as an equilibrium between monomer and dimer. The reaction is reversible unless the dimer concentration exceeds its solubility; in this case, the dimer forms a separate phase, driving the equilibrium towards the dimer. At loadings above 250 mg/L of dimethylvinylsilanol, the concentration of the dimer is predicted to exceed its solubility, resulting in the formation of separate phase. In addition, the dimer is expected to have a high volatility from water and this may cause losses from water under some conditions. Further information is given in a supporting report (PFA 2016am) attached in Section 13.

 

Dimethylvinylsilanol has a predicted water solubility of 3700 mg/L at 20 °C and a predicted log Kow value of 1.5 using validated QSAR estimation methods. It is not surface active and has a predicted vapour pressure of 250 Pa at 25 °C. The first dissociation constant of a structurally analogous silanol (trimethylsilanol) has been reported to be around pKa = 11.

 

Ammonia is very soluble in water (510-530 g/L) and will ionise to form NH4+ under most environmental conditions. It has a high vapour pressure (861 kPa at 20 °C); log Kow is not relevant because ammonia is inorganic, but an indicative value of 0.23 is predicted. It has a pKa of 9.25 at 25 °C. Under ambient environmental conditions, ammonia is a stable substance that shows normal acid/base chemical activity with the following equilibria:

NH4+ + H2O ↔ NH3 + H3O+

NH3 + H2O ↔ NH4+ + OH-

The ammonia/ammonium ion in aqueous solution exists in equilibrium between NH3 and NH4+, depending on the pH. Where the term ammonia is used throughout this dossier, it refers to the equilibrium mixture of ammonia/ammonium unless otherwise stated. In general, as pH increases, the fraction of the total ammonia which is unionised increases. The fraction of unionised ammonia can be calculated using the following equation:

 

fractionunionised = 1/(10pKa-pH+1)

 

At pH 8.5, the proportion of unionised ammonia is approximately 10 times that at pH 7.5. The concentration of unionised ammonia will be lower at higher ionic strengths of very hard freshwater or saltwater environments. This effect can be significant in estuarine and marine waters. Moreover, the pKa is reciprocally related to temperature. For every 9 °C increase in temperature, the proportion of unionised ammonia approximately doubles (EA 2007).

 

Reference:

Environment Agency (2007). Proposed EQS for Water Framework Directive Annex VIII substances: ammonia (un-ionised). Science Report: SC040038/SR2; SNIFFER Report: WFD52(ii).

 

PFA (2016am). Peter Fisk Associates, Silanols and aquatic systems, 404.105.003