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Physical & Chemical properties

Water solubility

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Description of key information

Water solubility (2,4,6,8,10-pentamethylcyclopentasiloxane): 3.33 mg/l at 20°C (QSAR)
Water solubility (methylsilanetriol): 1 000 000 mg/l at 20°C (QSAR). At concentrations above approximately 1000-2000 mg/l condensation reactions can occur, forming dimers and higher oligomers, and limiting the concentration dissolved in water.

Key value for chemical safety assessment

Water solubility:
3.3 mg/L
at the temperature of:
20 °C

Additional information

In accordance with Column 2 of REACH Annex VII, the water solubility study does not need to be conducted as the substance has a hydrolytic half-life less than 12 hours at pH 4, 7 and 9.

Therefore a value of 3.3 mg/l has been calculated for 2,4,6,8,10-pentamethylcyclopentasiloxane using a validated QSAR model. This result is considered reliable and has been chosen as the key study.

The final hydrolysis product, methylsilanetriol, and the intermediate product, methylsilanediol, 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. 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 silanetriols and 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. At higher loadings, cross-linking reactions may occur for silanetriols. At loadings below 500 mg/l of methylsilanetriol or methylsilanediol, the soluble monomer is expected to predominate in solution (>99%), with small amounts of dimer and oligomers. Condensation reactions may become important at loadings above about 1000-2000 mg/l causing the formation of insoluble polymeric particles (sols) and gels over time. Further information is given in a supporting report (PFA 2013am) attached in Section 13 of the IUCLID dataset.

The dimer, trimer, tetramer and cyclic tetramer associated with the monomers methylsilanetriol and methylsilanediol are:

Monomer

Methylsilanetriol

Methylsilanediol

Dimer

1,3-dimethyl-disiloxane-1,1,3,3-tetrol
[Si](O[Si](O)(O)C)(O)O
H2L2-tetrol

1,3-dimethyl-disiloxane-1,3-diol
C[SiH](O[SiH](O)C)O
H2L2-diol

Trimer

1,3,5-trimethyl-trisiloxane-1,1,3,5,5-pentol
C[Si](O[Si](O)(O)C)(O[Si](O)(O)C)O
H3L3-pentol

1,3,5-trimethyl-trisiloxane-1,5-diol
C[SiH](O[SiH](O)C)O[SiH](O)C
H3L3-diol

Tetramer

1,3,5,7-tetramethyl-tetrasiloxane-1,1,3,5,7,7-hexol
O([Si](O[Si](O)(O)C)(O)C)[Si](O[Si](O)(O)C)(O)C
H4L4-hexol

1,3,5,7-tetramethyl-tetrasiloxane-1,7-diol
C[SiH](O[SiH](O)C)O[SiH](O[SiH](O)C)C
H4L4-diol

Cyclic tetramer

2,4,6,8-tetrahydroxy-2,4,6,8-tetramethylcyclotetrasiloxane
C[Si]1(O)O[Si](C)(O)O[Si](O[Si](O1)(C)O)(C)O
H4D4-tetrol

2,4,6,8-tetramethylcyclotetrasiloxane
[SiH]1O[SiH](C)O[SiH](O[SiH](O1)C)C
HD4, D4D4
CAS 2370-88-9

 

The monomer hydrolysis products, methylsilanetriol and methylsilanediol, are very hydrophilic and hence the calculated solubility is 1E+06 mg/l using a QSAR method. This QSAR method for water solubility cannot be validated for silanetriols because the saturation concentration of silanetriols in water is limited by condensation reactions rather than lack of true solubility, as discussed above. A prediction of 1 000 000 mg/l or 1000 g/l is indicative but has no practical meaning. The prediction is however considered valid for use in calculation of the Henry’s Law Constant and for environmental exposure modelling and toxicokinetics modelling because it is considered to adequately describe the hydrophilicity of the substance and hence the partitioning behaviour.