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The submission substance, N-(3-(trimethoxysilyl)propyl)ethylenediamine is not stable in water, which affects the approach to the determination of physicochemical properties.

N-(3-(trimethoxysilyl)propyl)ethylenediamine is a liquid at ambient temperature and pressure, with a melting point of <-20°C, and a predicted boiling point of 240°C. It has a predicted density of 1.0 g/cm3 at 20°C, and a predicted kinematic viscosity of 3.1 mm2/s at 20°C. The substance has a very low vapour pressure of 0.3 – 0.4 Pa at 20°C.

The substance is not oxidising or explosive on the basis of structural examination. It is not classified as flammable according to Regulation (EC) No. 1272/2008 on the basis of a measured flash point of 120°C and a predicted boiling point of 240°C. It has a measured auto-ignition temperature of 301°C at 1013 hPa.

In contact with water, N-(3-(trimethoxysilyl)propyl)ethylenediamine reacts very rapidly (half-life of 0.025 h at pH 7 and 25°C) to produce N-[3-(trihydroxysilyl)propyl]ethylenediamine and methanol according to the following equation:

(CH3O)3Si(CH2)3NHCH2CH2NH2+ 3H2O → (HO)3SiCH2CH2CH2NHCH2CH2NH2+ 3CH3OH

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 hydrolysis products, N-[3-(trihydroxysilyl)propyl]ethylenediamine and methanol are assessed instead.

However, the properties of the parent submission substance have been estimated using appropriate QSAR methods. The unionised form of the substance has a predicted log Kow of -0.3. In solution, the amine group will protonate; therefore,the estimated log Kow for the unionised form was corrected for ionisation using the equation CORR = 1/1 +10A(pH-pKa) [where A = 1 for acids, -1 for bases; pH = pH-value of the environment; pKa = acid/base dissociation constant].

At pH 2, pH 4, pH 7 and pH 9, the corrected log Kow values are -4.0, -4.0, -3.3 and -1.3 respectively. It has a predicted water solubility of 1.0E+06 mg/l at 20°C. On the basis of structural examination, it is not surface-active and the acid dissociation constants (pKa) of the di-amine group are estimated to be 7.2 and approximately 10.

Methanol is miscible with water, has low log Kow (-0.82 to -0.64) and high vapour pressure (12790 Pa at 20°C) (OECD 2004).

The silanol hydrolysis product, N-(3-(trihydroxysilyl)propyl)ethylenediamine may undergoes condensation reactions in solution to give siloxane dimers, linear and cyclic oligomers and highly cross-linked polymeric particles (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 silanetriol may be modelled as an equilibrium between monomer, dimer, trimer and tetramer cyclising to the thermodynamically stable cyclic tetramer. At higher loadings, cross-linking reactions may occur. The reactions are reversible unless cyclic tetramer forms a separate phase, driving the equilibrium towards the tetramer. At loadings below 500 mg/l of N-(3-(trihydroxysilyl)propyl)ethylenediamine, the soluble monomer is expected to predominate in solution (>99%) with small amounts of dimer and oligomers. Condensation reactions are expected to become important at loadings above 1000 mg/l causing the formation of insoluble polymeric particles (sols) and gels over time. Further information is given in a supporting report (PFA 2016am) attached in Section 13.

The saturation concentration in water of the silanol hydrolysis product N-(3 -(trihydroxysilyl)propyl)ethylenediamine is therefore limited by condensation reactions to approximately 1000 mg/l. However, it is very hydrophilic (calculated solubility is 1E+06 mg/l at 20°C using a QSAR method) and the ionised form of the silanol hydrolysis product has a predicted log Kow of -4.0 at pH 2, pH 4, pH 7 and pH 9. The silanol hydrolysis product is much less volatile than the parent substance (vapour pressure = 6.8E-08 Pa at 25°C, predicted). The first dissociation constant of a structurally analogous silanetriol (phenylsilanetriol) has been reported to be around pKa= 10.

Reference:

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

Additional information