N,N-bis(trimethylsilyl)aminopropylmethyldiethoxysilane

Endpoint summary

Description of key information

The substance, N,N-bis(trimethylsilyl)aminopropylmethyldiethoxysilane is not stable in water, which affects the approach to the determination of physicochemical properties.

N,N-bis(trimethylsilyl)aminopropylmethyldiethoxysilane is a liquid at ambient temperature and pressure, with a melting point of -25°C, and a predicted boiling point of 260°C. In a boiling point study, the substance decomposes prior to boiling. It has a measured relative density of 0.88 at 20°C, and a predicted kinematic viscosity of 1.1 mm²/s at 20°C. The submission substance has a measured vapour pressure of 0.486 Pa at 20°C.

N,N-bis(trimethylsilyl)aminopropylmethyldiethoxysilane is not classified for flammability in accordance with Regulation (EC) No. 1272/2008. It has a measured flash point of 108°C at 1013 hPa and measured auto-ignition temperature of 261°C at 1015 mBar. In accordance with EU Test Method A.14, the substance does not meet the criteria for classification as explosive when subjected to heat or mechanical shock. It is not oxidising on the basis of structural examination.

In contact with water, the substance hydrolyses very rapidly (half-lives of <1 minutes at pH 4, pH 7 and pH 9 and 25°C) to form an intermediate hydrolysis product of 3-(diethoxymethylsilyl)propylamine and trimethylsilanol. The intermediate hydrolysis product, 3 -(diethoxymethylsilyl)propylamine undergoes further hydrolysis to form 3-(dihydroxymethylsilyl)propylamine and ethanol according to the following equations:

((CH₃)₃Si)₂N(CH₂)₃Si(CH₃)(OCH₂CH₃)₂ + 2H₂O → CH₃Si(CH₃CH₂O)₂(CH₂)₃NH₂ + 2(CH₃)₃SiOH

CH₃Si(CH₃CH₂O)₂(CH₂)₃NH₂ + 2H₂O → CH₃Si(OH)₂(CH₂)₃NH₂ + 2CH₃CH₂OH

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, 3-(dihydroxymethylsilyl)propylamine, trimethylsilanol and ethanol are assessed instead.

The unionised form of the intermediate hydrolysis product, 3-(diethoxymethylsilyl)propylamine has a predicted log K_ow of 2.5. In solution, the amine group will protonate; therefore, the estimated log K_ow for the unionised form was corrected for ionisation using the equation provided in the ECHA Guidance R.7a (2008). At pH 2, pH 4, pH 7 and pH 9, the corrected log K_ow values are -4, -3.5, -0.5 and 1.4 respectively. It has a predicted water solubility of 5900 mg/l at 20°C and an acid dissociation constant (pKa) of the amino group was estimated to be approximately 10.

Ethanol is miscible with water, has low log K_ow (-0.3) and high vapour pressure (7910 Pa at 25°C).

The silanol hydrolysis products; 3-(dihydroxymethylsilyl)propylamine and trimethylsilanol, may undergo condensation reactions in solution to give siloxane dimers, linear and cyclic oligomers and highly cross-linked polymeric particles (a colloidal suspension of small solid particles known as a sol) that may over time form an insoluble gel 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 silanediols and triols may be modelled as an equilibrium between monomer, dimer, trimer and tetramer, with the linear tetramer cyclising to the thermodynamically stable cyclic tetramer. The reactions are reversible unless the cyclic tetramer concentration exceeds its solubility; in this case, the cyclic tetramer forms a separate phase, driving the equilibrium towards the tetramer. For 3-(dihydroxymethylsilyl)propylamine and trimethylsilanol, a solution at 100 mg/l (often the top loading used in ecotoxicity tests) is predicted to contain >99% monomer, with small amounts of dimer, trimer, and cyclic tetramer. At loadings above about 1000 mg/l the concentration of the cyclic tetramer/dimer of the silanol hydrolysis products are predicted to exceed its solubility, resulting in formation of a separate phase. In addition, the cyclic tetramer/dimer are 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.

The saturation concentrations in water of the silanol hydrolysis products, 3-(dihydroxymethylsilyl)propylamine and trimethylsilanol are therefore limited by condensation reactions to approximately 1000 mg/l. However, they are very hydrophilic (calculated solubility is 1E+06 mg/l and 1.1E+04 mg/l respectively at 20°C using a QSAR method) and the ionised form of 3 -(dihydroxymethylsilyl)propylamine has a predicted log K_ow of -4 at pH 2, pH 4 and pH 7, at pH 9, the log K_ow is -1.9. 3-(Dihydroxymethylsilyl)propylamine has a vapour pressure of 0.022 Pa at 25°C and the first dissociation constant of a structurally analogous silanediol (dimethylsilanediol) has been reported to be around pKa= 12.

Trimethylsilanol has a predicted log K_ow value of 1.1 at 20°C, vapour pressure of 1400 Pa at 25°C and pKa of ca. 11.

Reference:

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