Registration Dossier
Registration Dossier
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EC number: 250-807-9 | CAS number: 31795-24-1
Water solubility
Administrative data
Link to relevant study record(s)
Description of key information
Water solubility [potassium methylsilanetriolate in aqueous solution]: miscible at ambient temperature
Water Solubility (methylsilanetriol): the concentration dissolved in water is limited to about 1000 mg/L at neutral pH by condensation reactions
Key value for chemical safety assessment
Additional information
In accordance with Section 1 of REACH Annex XI, the study does not need to be conducted because the composition of the substance (35-55% w/w solution of potassium methylsilanetriolate in water) indicates that potassium methylsilanetriolate is completely miscible with water.
It should be noted that:
-The substance as sold is a solution of 35-55% potassium methylsilanetriolate in water, with a very alkaline pH of >12. Potassium methylsilanetriolate dissociates completely into potassium and methylsilanetriolate ions in aqueous solution.
-The substance as sold is only stable at high pH (pH>12). When the pH is reduced, the concentration of the non-ionised form (methylsilanetriol) increases. The first pKa of methylsilanetriol is approximately 10. At pH 9.0 and below, the substance is no longer in the ionised form and methylsilanetriol predominates.
- Methylsilanetriol undergoes reversible condensation reactions in solution to give siloxane dimers, linear and cyclic oligomers. The condensation reactions have been 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 a few hundred mg/l of methylsilanetriol at around neutral pH, the soluble monomer predominates in solution (>99%) with small amounts of dimer and oligomers. Condensation reactions are expected to become important by the time the concentration reaches 1000 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.
- If the pH of the registered substance is lowered without a large amount of dilution, polymerisation occurs. This is illustrated by H-NMR and Si-NMR spectra, which show the presence of dimers and oligomers. The substance is diluted with D2O, reducing the pH and thereby allowing condensation reactions to occur.
- The potassium methylsilanetriolate and the subsequent methylsilanetriol formed may appear to be completely miscible with water on mixing but may form highly cross-linked polymeric particles (sol) then the insoluble gel upon standing over a number of hours. The overall rate and extent of condensation is dependent on the amount of substance present (the nominal loading), temperature, pH of the system and what else is present in the solution.
- Methylsilanetriol, its dimer and trimer, are extremely 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 1E+06 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.
Information on Registered Substances comes from registration dossiers which have been assigned a registration number. The assignment of a registration number does however not guarantee that the information in the dossier is correct or that the dossier is compliant with Regulation (EC) No 1907/2006 (the REACH Regulation). This information has not been reviewed or verified by the Agency or any other authority. The content is subject to change without prior notice.
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