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EC number: 221-906-4
CAS number: 3277-26-7
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
+ H2O → 2HSi(CH3)2OH → 2(HO)2Si(CH3)2+
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
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=
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