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The substance, 1,1-dimethyl-N,N'-bis(1-methylpropyl)silanediamine is not stable in water, which affects the approach to the determination of physicochemical properties.

1,1-Dimethyl-N,N'-bis(1-methylpropyl)silanediamine is a liquid at standard temperature and pressure, with a measured melting point of <-100°C, and a measured boiling point of 197.1°C. It has a measured relative density of 0.813 at 20°C, a kinematic viscosity value of 1.6 mm2/s at 20°C. It is not expected to be surface active. The substance has a measured vapour pressure of 6 Pa at 20°C and 140 Pa at 50°C.

The substance is not classified for flammability according to Regulation (EC) No. 1272/2008 on the basis of a flash point of 64.5± 0.5°C at 1013 hPa and a measured boiling point of 197.1°C. It has a measured auto-ignition temperature of 226°C at 960 -968 hPa, and is not explosive and is not oxidising on the basis of structural examination.

In contact with water, 1,1-dimethyl-N,N'-bis(1-methylpropyl)silanediamine hydrolyses very rapidly (half-life <2 minutes at pH 4, pH 7 and pH 9 and 25°C) to produce dimethylsilanediol (1 mole, CAS No. 1066-42-8) and sec-butylamine (2 moles, CAS No. 13952-84-6) according to the following equation:

(C8H18NH)2Si(CH3)2 + 2H2O → (CH)3Si(OH)2 + 2C4H9NH2

Therefore, the 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 product, dimethylsilanediol and sec-butylamine are assessed instead.

Sec-butylamine has a reported water solubility of 1.1E+05 mg/L at 20°C (Yalkowsky 1992), has low log Kow of 0.74 (Hansch et al. 1995) and a vapour pressure of approximately 24000 Pa at 25°C (Daubert and Danner 1989). The reported pKa of 10.6 (Perrin 1965) indicates that it has the potential to protonate when in the environment and will exist almost entirely in cationic form while in the environment.

The silanol hydrolysis product, dimethylsilanediol, may undergo condensation reactions in solution to give siloxane dimers, and linear and cyclic oligomers 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 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 dimethylsilanediol, a solution at 100 mg/l (often the top loading used in ecotox 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 of the silanol hydrolysis product is predicted to exceed its solubility, resulting in formation of a separate phase. In addition, the cyclic tetramer 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.

The saturation concentration in water of the silanol hydrolysis product dimethylsilanediol 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) with a predicted low log Kow of -0.4. It is not surface active and has a predicted vapour pressure of 7 Pa at 25°C).

References:

Daubert, T.E., R.P. Danner (1989). Physical and Thermodynamic Properties of Pure Chemicals Data Compilation. Washington, D.C.: Taylor and Francis

Hansch, C., Leo, A., D. Hoekman (1995). Exploring QSAR - Hydrophobic, Electronic, and Steric Constants. Washington, DC: American Chemical Society., 1995., p. 10

PFA (2016am). Silanols and aquatic systems. Reference 404.105.003

Perrin D D (1965). Dissociation constants of organic bases in aqueous solution. IUPAC Chem Data Ser, Buttersworth, London.

Yalkowsky SH, Dannenfelser RM (1992). The AQUASOL dATAbASE of Aqueous Solubility. Fifth Ed, Tucson, AZ: Univ Az, College of Pharmacy

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