Potassium methylsilanetriolate

Endpoint summary

Additional information

The substance is a greater than 35% (35 -55%) w/w solution of potassium methylsilanetriolate in water.

The substance (tested as the aqueous solution) has a melting point of <-75°C, a boiling point of 112°C, a relative density of 1.388g/cm³ at 20°C and a measured kinematic viscosity of 27.94 mm²/s at 20°C.

There is no evidence to suggest that the substance should be classified for flammability according to Regulation (EC) No. 1272/2008. Testing is waived because it is only stable in aqueous solution at high pH. It has a measured auto-ignition temperature of >600°C at 1013 hPa.

The substance is not expected to be surface active, it is not explosive and does not possess oxidising properties on the basis of structural examination.

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 reactions are reversible as long as the concentrations are low and all species remain soluble; thus, a dynamic equilibrium is established. At higher concentrations one or more of the oligomers may exceed their solubility limit and form a separate phase, or insoluble cross-linked polymers may form, driving the equilibrium towards the oligomer/polymers.

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.

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 be important at loadings above 1000 mg/l.

Methylsilanetriol 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.

In summary, methylsilanetriolate, methylsilanetriol and the dimers and oligomers formed by condensation reactions exist in dynamic equilibrium in solution. The position of this equilibrium is influenced by concentration and pH (as well as temperature and what else is present in solution). Under comparable conditions of concentration and pH, methylsilanetriolate is equivalent to methylsilanetriol.

• At close to neutral pH and low concentration (conditions relevant for the environment), the neutral monomer (methylsilanetriol) predominates (>99%).
• At concentrations in the grams per litre range, dimers, and oligomers and polymers are formed, unless the pH is very high (>12).
• At pH >12 (the substance as sold), the ionised monomer (methylsilanetriolate) predominates.

The n-octanol/water partition coefficients of methylsilanetriol and its dimer and trimer are predicted as -2.4, -2.2 and -2.7 respectively. Similarly, methylsilanetriol, its dimer and trimer have a predicted vapour pressure of 0.05 Pa, 5.9E-05 Pa and 3.0E-07 Pa at 25°C respectively.