Registration Dossier

Physical & Chemical properties

Endpoint summary

Administrative data

Description of key information

Additional information

The substance, 1,1,3,3-tetramethyldisiloxane (H2-L2), 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 Section 1.4.1 of the CSR.

H2-L2 is a liquid at ambient temperature and pressure, with a melting point of <-78°C and a measured boiling point of 70.3°C. It has a predicted density of 0.87 g/cm3 at 20°C, a predicted kinematic viscosity of 0.7 mm2/s at 20°C, and measured vapour pressure values of 15000 Pa at 20°C and 18000 Pa at 25°C.

The substance is classified as a highly flammable liquid (Flammable Liquid Category 2), according to Regulation (EC) No. 1272/2008, on the basis of measured flash point values of -27.3°C and -25°C and a measured boiling point of 70.3°C. It has a measured auto-ignition temperature of 245°C, and is not explosive and not oxidising on the basis of examination of the structure.

In contact with water, H2-L2 hydrolyses very rapidly (half-life 11.3 minute at 25°C and pH 7). H2-L2 undergoes two consecutive reactions in water. The half-life mentioned above is for the removal of parent substance to form dimethylsilanol. Complete reaction to the ultimate hydrolysis product, dimethylsilanediol and hydrogen gas as co-product of hydrolysis, takes longer.The reaction is thought to proceed according to the following chemical equation:

HSi(CH3)2OSi(CH3)2H + H2O → 2HSi(CH3)2OH  →  2(HO)2Si(CH3)2+ H2

Therefore, requirements for testing of water-based physicochemical properties for the submission substance (H2-L2) are waived on the basis of instability in water. The properties of the silanol hydrolysis products, dimethylsilanol and dimethylsilanediol are assessed instead.

However, key physicochemical properties of H2-L2 are available. A measured log Kow value of 4.5 was obtained for H2-L2 using OECD 117. In addition, H2-L2 has a predicted water solubility value of 13 mg/l at 20°C.

The final hydrolysis product, dimethylsilanediol, and the intermediate product, dimethylsilanol, may undergo condensation reactions in solution.  

The condensation reactions of dimethylsilanediol may be modelled as an equilibrium between monomer, dimer, trimer and tetramer, with the linear tetramer cyclising to the thermodynamically stable cyclic tetramer. 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 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. At loadings below 100 mg/l of dimethylsilanediol, the soluble monomer is expected to predominate in solution (>99%), with small amounts of dimer and oligomers. 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. The intermediate hydrolysis product, dimethylsilanol, may also undergo condensation reactions at high concentrations in solution re-forming the parent siloxane dimer. The overall rate of condensation is dependent on nominal loading, temperature, and pH of the system, as well as what else is present in 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. For dimethylsilanol, a solution at 100 mg/l (the highest concentration often used in ecotoxicity tests) is predicted to contain >99% monomer. At loadings above about 1500 mg/l the concentration of the dimer is predicted to exceed its solubility, resulting in formation of a separate phase. Further information is given in a supporting report (PFA 2016am) attached in Section 13 of the IUCLID dataset.

The saturation concentration in water of the final hydrolysis product, dimethylsilanediol, is therefore limited by condensation reactions to approximately 1000 mg/l, and the intermediate hydrolysis product, dimethylsilanol, is limited to approximately 1500 mg/l. However, dimethylsilanol and dimethylsilanediol are predicted to be very soluble in water at 20°C (1.9E+04 mg/l and 1E+06 mg/l respectively) and have low log Kow (0.6 and -0.38 respectively). They are not surface-active and are less volatile than the parent substance (vapour pressure = 770 Pa and 7 Pa at 25°C respectively). The first dissociation constant of dimethylsilanediol has been reported to be around pKa= 12.