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Diss Factsheets

Physical & Chemical properties

Water solubility

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Endpoint:
water solubility
Data waiving:
study scientifically not necessary / other information available
Justification for data waiving:
the study does not need to be conducted because the substance is hydrolytically unstable at pH 4, 7 and 9 (half-life less than 12 hours)
Endpoint:
water solubility
Type of information:
(Q)SAR
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
results derived from a valid (Q)SAR model and falling into its applicability domain, with adequate and reliable documentation / justification
Justification for type of information:
See attached QMRF and QPRF
Principles of method if other than guideline:
The result was obtained using an appropriate QSAR method (see attached QMRF and QPRF for details).

The model is an adaptation of the existing SRC model WaterNT v 1.01, aka WaterFrag, which is a component of the EPI Suite. This model is based on fragment values. That is, the chemical structure is broken down into its constituent functional groups, and the contribution of each group toward the overall solubility is calculated. Certain correction factors may also apply. Whilst this method is good in principle, it suffers from not having reliable fragment values for substances containing Si-O bonds. Therefore, multiple linear regression on a set of organosilicon compounds has been used to generate new fragment values for silicon and oxygen bound to silicon (with separate values for –OH and other oxygen). The adapted model applies to substances containing Si-O bonds.
GLP compliance:
no
Water solubility:
2 800 mg/L
Temp.:
20 °C
Conclusions:
A water solubility of 2800 mg/l at 20 °C was obtained for the test substance using an appropriate calculation method. The result is considered to be reliable.

Description of key information

Water Solubility [3-(trimethoxysilyl)propane-1-thiol]: 2.8E+03 mg/l at 20 °C (QSAR)


 


Water Solubility [3-(trihydroxysilyl)propanethiol]: above approximately 1000 mg/l condensation reactions can occur over time, limiting the concentration dissolved in water. The calculated solubility is 1.0E+06 mg/l at 20 oC (QSAR)


 


Water solubility [methanol]: miscible at 20 °C

Key value for chemical safety assessment

Water solubility:
2 800 mg/L
at the temperature of:
20 °C

Additional information

The requirement to conduct a water solubility study for the substance is waived on the basis that it hydrolyses rapidly to form 3-(trihydroxysilyl)propanethiol and methanol.


 


However, the water solubility of the parent [3-(trimethoxysilyl)propane-1-thiol] has been calculated with the use of a validated QSAR method to be 2.8E+03 mg/l at 20 °C.


 


The silanol hydrolysis product, 3-(trihydroxysilyl)propanethiol, may 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. 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 silanetriol may be 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 500 mg/l of 3-(trihydroxysilyl)propanethiol, the soluble monomer is expected to predominate in solution (> 99 %) with small amounts of dimer and oligomers. Condensation reactions are expected to become important at loadings above 1000 mg/l causing the formation of insoluble polymeric particles (sols) and gels over time. Further information is given in a supporting report (PFA 2016am) attached in Section 13.


 


The hydrolysis product, 3-(trihydroxysilyl)propanethiol, 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 because it is considered to adequately describe the hydrophilicity of the substance and hence the partitioning behaviour.


 


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


 


References:


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


 


PFA (2016am). Peter Fisk Associates, Silanols and aquatic systems, 404.105.003