2,11-Dioxa-3,10-disiladodecane, 3,3,10,10-tetramethoxy-

Water solubility

Description of key information

Water solubility [1,6-Bis(trimethoxysilyl)hexane]: 0.526 g/l at 20°C  (EU Method A.1)
Water Solubility [1,6-Bis(trihdroxysilyl)hexane]: 1E+06 mg/l at 20°C (QSAR); however, above approximately 100 mg/l condensation reactions can occur over time, limiting the concentration dissolved in water
Water solubility [methanol]: miscible with water at 20°C

Key value for chemical safety assessment

Water solubility:

0.526 g/L

at the temperature of:

20 °C

Additional information

The requirement to conduct a water solubility of the substance is waived because in contact with water the substance hydrolyses rapidly to form [1,6-Bis(trihydroxysilyl)hexane] and methanol. The hydrolysis half-lives of the substance are: 0.08 h at pH 5, 5.2 h at pH 7 and 0.15 h at pH 9 and 25.1°C.

However, the water solubility of the substance has been measured using a modified form of EU Test Method A.6. Mixtures of the test substance and distilled water were added to three separate flasks, flask 1, flask 2 and flask 3. The samples were shaken at 30°C for 1 hour, 3 hours and 24 hours respectively with all samples being equilibrated for 1 hour at 20°C. Following filtration of the samples, the concentration in water were measured using GC-FID. The mean water solubility result for samples/flasks 1 and 2 is 0.526 g/l at 20±0.05°C and pH 6.3-6.4. The result is considered to reflect that of the parent compound because of minimal hydrolysis that is considered to have occurred in flasks 1 and 2. The result for flask 3 was discounted as extensive hydrolysis was observed to have occurred.

The silanol hydrolysis product, 1,6-bis(trihydroxysilyl)hexane, undergo condensation reactions in solution to give siloxane dimers, linear and cyclic oligomers and highly cross-linked polymeric particles (sol) that may over time form an insoluble gel and 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.In the water solubility study, the formation of a white solid precipitate in the sample solution was observed following the equilibration period at 20°C.

 

The condensation reactions of silanetriol may be modelled as an equilibrium between monomer, dimer, trimer and tetramer cyclising to the thermodynamically stable cyclic tetramer. However, with silanetriol groups at both ends of the molecule cross linking reactions are highly probable creating insoluble particles. Condensation reactions are expected to become important at loadings above 100 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 hydrolysis product, 1,6-bis(trihydroxysilyl)hexane, is 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 1E+06 mg/l is indicative but has no practical meaning. The prediction is however considered to be 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.

Methanol was reported to be miscible in water at 20°C (OECD 2004).

 

PFA, 2013am, Peter Fisk Associates, Silanols and aquatic systems, 350.001.003

 

OECD (2004): SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 18-20 October 2004, Methanol, CAS 67-56-1.