Trimethylsilane

Water solubility

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

water solubility

Type of information:

(Q)SAR

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: The result was obtained by an appropriate predictive method.

Justification for type of information:

Please refer to attached justification documents

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 the publically available WaterNT v1.01 program (US EPA 2012). WaterNT is part of EPI Suite v4.11 and may be downloaded from http://www.epa.gov/oppt/exposure/pubs/episuite.htm. The 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.

US EPA 2012. WaterNT v. 1.01, US EPA. 2012. Estimation Programs Interface Suite™ for Microsoft® Windows, v 4.10. United States Environmental Protection Agency, Washington, DC, USA. Available at http://www.epa.gov/oppt/exposure/pubs/episuite.htm.

Water solubility:

1 400 mg/L

Temp.:

20 °C

pH:

7

Conclusions:

A water solubility value of 1.4E+03 mg/l at 20°C was obtained for the substance using a validated QSAR. The result is considered to be reliable.

Reference 2

Endpoint:

water solubility

Type of information:

experimental study

Adequacy of study:

supporting study

Study period:

2005-03-22 to 2005-04-22

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Remarks:

The study was conducted according to an/the appropriate OECD test guideline and in compliance with GLP.

Qualifier:

according to guideline

Guideline:

OECD Guideline 105 (Water Solubility)

GLP compliance:

yes (incl. QA statement)

Type of method:

flask method

Water solubility:

0.995 g/L

Temp.:

24 °C

pH:

6.9 - 7

Table 1: Preliminary result

Total amount of test substance [g]	Total amount of distilled water [g]	Stirring time [h]	Clear solution (yes/no)	Approximate solubility [g/l]
2.0112	100	24	No	<20
1.7602	100	24	No	<18
1.5152	100	24	No	<15
1.2515	100	24	No	<12
1.0157	100	24	No	<10
0.7515	100	24	No	<8
0.5046	100	24	No	<6
0.2569	100	24	No	<3
0.1089	100	24	Yes	1

Table 2: Main results of water solubility tests

	Preincubation time / h	Time at test temperature until measurement / h	Concentration / g/l^1)	pH-value
vessel 1	24	24	1.000	7.0
vessel 2	48	24	0.985	7.0
vessel 3	72	24	1.000	7.0

¹⁾ mean values from 2 measurements; 1 w% (NMR results) = 0,1 g/l

A value of 0.995 g/L for water solubility was obtained as an average of the three measurements with a standard deviation of 0.009 g/L.

The results from ¹H NMR are shown in Table 2.

Table 3: Results from the ¹H NMR spectra

sample ID	sample name	amount of the sample [g]	amount of internal standard [g]	Integral of Silanol M3	Integral of the internal standard	Silanol M3 [w%]
378136	BW 1/24h ^1)	1.1116	0.0474	0	100.0000	0
378137	Probe 1.1/24h	0.9789	0.0489	11.1166	100.0000	0.095
378138	Probe 1.2/24h	0.9932	0.0508	11.4323	100.0000	0.100
378139	BW 2/48h ^1)	1.044	0.0508	0	100.0000	0
378140	Probe 2.1/48h	1.0274	0.0521	11.2032	100.0000	0.099
378141	Probe 2.2/48h	1.0963	0.0533	12.7292	100.0000	0.098
378142	BW 3/72h ^1)	1.0836	0.0492	0	100.0000	0
378143	Probe 3.1/72h	1.1318	0.0497	12.9594	100.0000	0.097
378144	Probe 3.2/72h	0.9806	0.0562	10.522	100.0000	0.103

¹⁾ BW means blank

Without water suppression, the results for silanol M3 are 0.100 and 0.088 w%.

Conclusions:

A water solubility value of 995 mg/l was determined in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP.

Description of key information

Water solubility [trimethylsilane]: predicted water solubility of 1400 mg/l at 20°C was obtained using a validated QSAR.

Water solubility [trimethylsilanol]: at concentrations above 995 mg/L at 20°C (EU Method A.6); condensation may occur over time, limiting the concentration dissolved in water.

Key value for chemical safety assessment

Water solubility:

1 400 mg/L

at the temperature of:

20 °C

Additional information

Trimethylsilane is a gas with a boiling point of 6.7°C at standard atmospheric pressure; the vapour pressure at standard temperature and pressure is expected to be above the limit of measurement (>100000 Pa). Therefore, conducting a water solubility study is considered technically not feasible or necessary.

However, the water solubility of the substance has been estimated to be 1400 mg/l at 20°C using a validated QSAR estimation method. In practice this concentration in water is not expected to be achieved due to the very high volatility of the substance.

In contact with water, trimethylsilane hydrolyses at a slow rate to form trimethylsilanol and hydrogen.

The silanol hydrolysis product, trimethylsilanol may undergo condensation reactions in solution to give the siloxane dimer (hexamethyldisiloxane; CAS No: 107-46-0) and a dynamic equilibrium is established. The overall rate of condensation is dependent on nominal loading, temperature, and pH of the system, as well as what else is present in solution.

The condensation reactions of monosilanols may be modelled as an equilibrium between monomer and dimer. The reaction is reversible unless the dimer concentration exceeds its solubility; in this case, the dimer forms a separate phase, driving the equilibrium towards the dimer. For trimethylsilanol, a solution at 100 mg/l (the highest concentration often used in ecotoxicity tests) is predicted to contain >99.9% monomer. At loadings above about 500 - 1000 mg/l the concentration of the dimer is predicted to exceed its solubility, resulting in formation of a separate phase. In addition, the dimer is expected to have a high volatility from water and this may cause losses from water under some conditions. Further information is given in a supporting report (PFA 2016am) attached in Section 13 of the IUCLID dataset.

There are two measured water solubility results for trimethylsilanol. In the first study (Wieser 2005), a water solubility value of 995 mg/l at 20°C was obtained for trimethylsilanol using the flask method in accordance with OECD 105. In this first water solubility study, ¹H-NMR was used for the determination of the concentration of trimethylsilanol in water. This analytical method is specific and it is expected to give the dissolved concentration of trimethylsilanol in water. In addition, in the NMR spectrum attached to the study report, the formation of the dimer (hexamethyldisiloxane) was observed.

In the second study (Dow Corning 1999), a water solubility value of 20000 ± 700 mg/l at 20°C was obtained for trimethylsilanol using the flask method in accordance with OECD 105 test method. However, the reliability of this second result is unassignable due to deficiencies in the analytical method used (see reviewer's comment in Endpoint study record of Dow Corning 1999 for further details).

Therefore, for environmental exposure assessment, calculation of Henry's Law Constant and toxicokinetic modelling, the water solubility value of 995 mg/l will be used for trimethylsilanol.