Trichlorosilane

Hydrolysis

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study with acceptable restrictions

Remarks:

The study was conducted according to an appropriate OECD test guideline but it was not conducted under GLP.

Qualifier:

according to guideline

Guideline:

OECD Guideline 111 (Hydrolysis as a Function of pH)

GLP compliance:

no

Analytical monitoring:

yes

Buffers:

o Buffer solutions were not sterilized.
o Co-solvent: <1% acetonitrile
o Buffer volume for hydrolysis: 50 mL

Target pH Buffer System Measured pH (before addition of test material
4.0 Acetic acid/sodium hydroxide 4.00
7.0 Sodium dihydrogen phosphate/sodium hydroxide 7.01
9.0 Boric acid/sodium hydroxide 8.99

Details on test conditions:

o Plastic, instead of glass containers were used since it is known that the SiOH layer on glass will react with SiCl compounds.
o Temperature: 1.5 ± 0.5°C
o Vessels: Low-density polyethylene bottles of 90-mL capacity with screw caps. Vessels were not sterilized.
o Co-solvent: <1% acetonitrile

Number of replicates:

One at pH 4, 7, and 9

Statistical methods:

• Data treatment: For given solution conditions, the hydrolysis of parent was followed to completion as indicated by a stable chloride ion concentration measurement (≤ 2% change between concentration readings). The elapsed time between the addition of the test material to the aqueous buffer solution and the observation of a stable chloride ion concentration was used to estimate an upper limit on t1/2 (seconds) assuming that 10 half-lives represents exhaustive hydrolysis (99.9% complete).

Transformation products:

yes

No.:

#1

No.:

#2

Key result

pH:

4

Temp.:

1.5 °C

DT50:

< 1 min

Key result

pH:

7

Temp.:

1.5 °C

DT50:

< 1 min

Key result

pH:

9

Temp.:

1.5 °C

DT50:

< 1 min

Details on results:

Nominal: 1x10-3 M (149 mg/L) Methyltrichlorosilane

Measured value (the value with units preferably mg/L):
3.0x10-3 M (106 mg/L) Chloride ion at pH 4
3.2x10-3 M (113 mg/L) Chloride ion at pH 7
2.7x10-3 M (96 mg/L) Chloride ion at pH 9

Half-life (t(1/2) in seconds at a specific pH (pH 4, 7, 9, or other) at 1.5±0.5 °C:
pH 4.0: 7 seconds
pH 7.0: 9 seconds
pH 9.0: 6 seconds

Remarks Field for Results
• Values of upper limit on t1/2 (shown above) refer to disappearance of test material, i.e. complete hydrolysis, from measurement of chloride ion concentration formed.
• Since the hydrolysis is so rapid, there is insufficient data to determine the rate constants (k1, k2, and k3) for the hydrolysis reactions by regression modeling.
• Rate constants and half-lives could not be determined quantitatively, although the data is certainly adequate for estimating the upper limit of t1/2.
• The chloride ion concentration measured was stoichiometrically equivalent to the methyltrichlorosilane concentration added to each buffer. This confirmed the quantitative completion of hydrolysis.
• First order or pseudo-first order behavior could not be confirmed because: a) sparse nature of the data during the critical portion of the process (20-70% hydrolyzed), b) the inherent limitation caused by measuring co-product concentration, and c) the relationship between k1, k2, and k3 is not known.
• Breakdown products from hydrolysis: Hydrogen chloride and silanol. For given solution conditions, the degradation product hydrogen chloride was observed to be stable during data collection. Consequently, HCl was considered stable. The stability of silanol was not measured, however silanols will undergo condensation reactions to form siloxanes. The stabilities of silanols lie in the order R3SiOH > R2Si(OH)2 > RSi(OH)3, with the bulkier R groups lending more stability to the SiOH function.2

Conclusions:

The test material was found to have a half-life of <1 min at pH 4, 7 and 9 and 1.5°C in a reliable study not conducted under GLP.

Reference 2

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study with acceptable restrictions

Remarks:

The study was conducted according to a test protocol that is comparable to the appropriate OECD test guideline method. It was not compliant with GLP.

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 111 (Hydrolysis as a Function of pH)

Deviations:

yes

Principles of method if other than guideline:

Studies were conducted at 1.5±0.5 °C to slow the hydrolysis reaction rate.

GLP compliance:

no

Analytical monitoring:

no

Details on sampling:

Not applicable

Buffers:

Buffers were selected because they had a very low or non-detectable chloride ion concentration. Buffers were prepared by titration of acetic acid, sodium phosphate monobasic or boric acid with sodium hydroxide. Constant ionic strength of 0.50 M was maintained by addition of appropriate volumes of 5 M sodium nitrate solution. Buffer solutions were made to known final volumes in polypropylene volumetric flasks with deionized water. Final pH adjustments were made by dropwise addition of a 2M sodium hydroxide solution using a calibrate pH meter. Prior to use, all buffer solutions were sparged with argon for at least 15 minutes to exclude oxygen and carbon dioxide. The buffers were not sterilized. The pH of each buffer solution was measured just prior to the kinetic experiment for which it was used.

- pH: Target: 4.0; Measured: 4.00
- Type and final molarity of buffer: Acetic Acid/Sodium Hydroxide, 0.20M
- Composition of buffer: 100mL 1.00 M Acetic Acid solution, 20 mL 1M Sodium Hydroxide solution, 46 mL 5M Sodium Nitrate solution. Total volume 500 mL.

- pH: Target: 7.0; Measured: 7.01
- Type and final molarity of buffer: Sodium Phosphate, monobasic/Sodium Hydroxide, 0.20M
- Composition of buffer: 100mL 1.00 M Sodium Phosphate, monobasic solution, 61.5 mL 1M Sodium Hydroxide solution, 5.4 mL 5M Sodium Nitrate solution. Total volume 500 mL.

- pH: Target: 9.0; Measured: 8.99
- Type and final molarity of buffer: Boric acid/Sodium hydroxide, 0.30M
- Composition of buffer: 9.27 g boric acid (neat reagent), 53 mL 1M Sodium Hydroxide solution, 39.5 mL 5M Sodium Nitrate solution. Total volume 500 mL.

Estimation method (if used):

Not applicable.

Details on test conditions:

TEST SYSTEM
- Type, material and volume of test flasks, other equipment used: The vessels used for the individual hydrolysis experiments were wide mouth low-density polyethylene bottles (Cole-Parmer/Bel-Art, 90 mL, 52 x 69 mm) with screw caps. Plastic, instead of glass containers were used since it is
known that the SiOH layer on glass will react with SiCl compounds (Smith, A.L., The Analytical Chemistry of Silicones (1991) 112, 29.). One screw cap was fitted with a rubber grommet to hold the chloride electrode securely in place and a rubber septa (Aldrich, Suba-Seal, i.d. 9.5 mm) to allow injection of the chlorosilane solution using a microliter syringe. The modified screw cap w as used with a new vessel for each pH and chlorosilane hydrolysis experiment after replacing the rubber septa and cleaning the chloride electrode with deionized water.
- Sterilisation method: The vessels were not sterilized.
- Lighting: No details given.
- Measures taken to avoid photolytic effects: No photolytic effects expected.
- Measures to exclude oxygen: Stock solutions of the test substance were prepared in a nitrogen-purged glove bag. Buffer solutions were purged with argon for 15 minutes before use.
- Details on test procedure for unstable compounds: The test substance is very unstable in contact with moisture. Acetonitrile was used to make up the stock solutions; it is considered a suitable solvent for chlorosilanes. Solutions of the test substance were prepared inside a nitrogen-purged glove bag and stored in 22-mL plastic vials having septum lined open-top caps (oven dried to remove trace moisture). When not in use, the kinetic solutions were stored in a secondary airtight container filled with Drierite.
- Details of traps for volatile, if any: None.
- If no traps were used, is the test system closed/open: Closed
- Is there any indication of the test material adsorbing to the walls of the test apparatus?: No (see comments above on selection of test vessels)

TEST MEDIUM
- Volume used: 450 µL of the test substance in acetonitrile was injected into 50 mL of the buffer solution.
- Preparation of test medium: A 0.1M stock solution of the test substance in acetonitrile was prepared. This was injected directly into the buffer solution at the start of the hydrolysis experiment.
- Renewal of test solution: Not applicable.
- Identity and concentration of co-solvent: Acetonitrile (99.93% purity), 0.9% in final test solution.

OTHER TEST CONDITIONS
- Adjustment of pH: No pH adjustment was carried out during the test.
- Dissolved oxygen: No details given.

Duration:

2.3 min

pH:

4

Initial conc. measured:

0.001 mol/L

Duration:

4 min

pH:

7

Initial conc. measured:

0.001 mol/L

Duration:

2.7 min

pH:

9

Initial conc. measured:

0.001 mol/L

Number of replicates:

Replicates:  One at pH 4, 7, and 9

Positive controls:

not specified

Negative controls:

not specified

Statistical methods:

Since the hydrolysis was so rapid, there was insufficient data to use statistical methods to interpret the results.

Preliminary study:

The substance is known to be unstable at environmentally relevant temperatures, therefore, no preliminary study was required.

Test performance:

There were a few instances where higher than expected [Cl-] readings (called spikes hereafter) were observed. The definitive reason for the spikes is unknown, however, possible causes were: (a) hydrolysis product precipitate physically striking the sensing membrane, or (b) the chlorosilane solution was injected into the buffer too fast causing a disturbance to the sensing membrane, or (c) the chloride ion concentration was temporarily concentrated near the sensing membrane prior to achieving a homogeneous solution. The spikes had no effect on the hydrolysis results.

Transformation products:

yes

No.:

#1

No.:

#2

Details on hydrolysis and appearance of transformation product(s):

- Formation and decline of each transformation product during test: Increase in chloride ion concentration was measured during the test.  For given solution conditions,the degradation product hydrogen chloride was observed to be stable during data collection. Consequently, HCl was considered stable. The total concentrations of chloride ion as a percentage of the theoretical concentration at the end of the tests were 99%, 105% and 95% at pH 4, 7 and 9, respectively.
The stability of silanol was not measured, however silanols will undergo condensation reactions to form siloxanes (Smith, A. L., The Analytical Chemistry of Silicones 1991, 112, 12).
- Pathways for transformation: Due to the limitation imposed by the response time of the ion selective electrode, only the total hydrolysis could be studies potentiometrically. It was not possible to differentiate between first, second and third chloride ion replacement by a hydroxyl group from the aqueous buffer, producing a silanol.

pH:

4

Temp.:

1.5 °C

DT50:

ca. 0.2 min

Type:

not specified

Remarks on result:

other: This value represents an estimated upper limit of the hydrolysis half-life. It refers to disappearance of the test material (based on chloride ion concentration) assuming the first hydrolysis step is rate limiting.

pH:

7

Temp.:

1.5 °C

DT50:

ca. 0.3 min

Type:

not specified

Remarks on result:

other: This value represents an estimated upper limit of the hydrolysis half-life. It refers to disappearance of the test material (based on chloride ion concentration) assuming the first hydrolysis step is rate limiting.

pH:

9

Temp.:

1.5 °C

DT50:

ca. 0.1 min

Type:

not specified

Remarks on result:

other: This value represents an estimated upper limit of the hydrolysis half-life. It refers to disappearance of the test material (based on chloride ion concentration) assuming the first hydrolysis step is rate limiting.

Other kinetic parameters:

None determined.

Details on results:

TEST CONDITIONS
- pH, sterility, temperature, and other experimental conditions maintained throughout the study: Yes

MAJOR TRANSFORMATION PRODUCTS
The chloride ion concentration was measured over the course of the hydrolysis. Measured concentrations are given in Tables 1-3.

INDICATION OF UNSTABLE TRANSFORMATION PRODUCTS:
- The silanol hydrolysis product is known to undergo condensation to siloxanes, however, the test report does not indicate that this was observed during the hydrolysis test.

PATHWAYS OF HYDROLYSIS
- Description of pathways: Hydrolysis is thought to proceed via consecutive replacement of Si-Cl with Si-OH. Due to the rapidity of the hydrolysis
further study of reaction pathways was not possible.
- Figures of chemical structures attached: No

Since the hydrolysis was so rapid, there was insufficient data to determine rate constants for the hydrolysis reactions by regression modelling. First order or pseudo-first order behaviour could not be confirmed because: (a) sparse nature of the data during the critical portion of the process (20 -70%) hydrolyzed), b) the inherent limitation associated with measuring co-product concentration for consecutive reactions, and (c) the relationship between k1, k2 and k3 is not known. Although rate constants and half-lives could not be determined quantitatively, the data was adequate for estimating the upper limit of t1/2.

Tables 1 - 3 show the results of the chloride ion measurements during the hydrolysis experiments. Table 4 illustrates the calculation of the hydrolysis half-lives at each pH.

Table 1. Results at pH 4

Time (sec)		[Cl-] (mM)	Blank Corrected [Cl-] (mM)*	% of Theoretical [Cl-]
10		0.699	0.667	33
20		0.748	0.716	35
30		1.08	1.05	52
40		1.22	1.19	59
50		1.43	1.40	69
60		1.59	1.56	77
70		1.78	1.75	87
80		1.95	1.92	95
90		2.02	1.99	99
100		2.03	2.00	99
110		2.03	2.00	99
120		2.03	2.00	99
130		2.03	2.00	99
140		2.03	2.00	99

* Subtracted chloride ion concentration measured in buffer blank = 0.0325 mM.

Table 2. Results at pH 7

Time (sec)		[Cl-] (mM)	Blank Corrected [Cl-] (mM)*	% of Theoretical [Cl-]
10		0.134	0.107	5
20		0.665	0.638	32
30		0.844	0.817	41
40		1.04	1.01	50
50		1.21	1.18	59
60		1.34	1.31	65
70		1.44	1.41	70
80		1.54	1.51	75
90		1.63	1.60	79
100		1.73	1.70	84
110		1.82	1.79	89
120		1.87	1.84	91
130		1.92	1.89	94
140		1.97	1.94	96
150		2.02	1.99	99
160		2.07	2.04	101
170		2.10	2.07	103
180		2.12	2.09	104
190		2.14	2.11	105
200		2.14	2.11	105
210		2.14	2.11	105
220		2.14	2.11	105
230		2.15	2.12	105
240		2.15	2.12	105

* Subtracted chloride ion concentration measured in buffer blank = 0.0275 mM.

Table 3. Results at pH 9

Time (sec)		[Cl-] (mM)	Blank Corrected [Cl-] (mM)*	% of Theoretical [Cl-]
10		1.68	1.64	81
20		1.21	1.17	58
30		1.45	1.41	70
40		1.64	1.60	79
50		1.81	1.77	88
60		1.87	1.83	91
70		1.89	1.85	92
80		1.91	1.87	93
90		1.92	1.88	94
100		1.93	1.89	94
110		1.93	1.89	94
120		1.93	1.89	94
130		1.95	1.91	95
140		1.94	1.91	94
150		1.95	1.91	95
160		1.95	1.91	95

* Subtracted chloride ion concentration measured in buffer blank = 0.0405 mM.

Table 4. Overall results

pH		Time to total hydrolysis	% Theoretical [Cl-] at this temperature	t1/2 / s*
4		100	99	10
7		170	102	17
9		70	92	7

* Time to total hydrolysis / 10

Validity criteria fulfilled:

yes

Conclusions:

A hydrolysis half-life of ca. 0.3 minutes at pH 7 was determined in a reliable study conducted according to an appropriate test protocol. It was not conducted according to GLP.

Reference 3

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 111 (Hydrolysis as a Function of pH)

GLP compliance:

yes

Radiolabelling:

no

Analytical monitoring:

yes

Details on sampling:

- Sampling intervals for the parent/transformation products: Immediately after addition of the test substance to the buffer and then at 1 minute intervals.
- Sampling method: Approximately 800µL of test solution was quickly transferred to an NMR tube using a clean dried 1000µL syringe.

Buffers:

- pH: Target: 4.0; Measured: 4.02
- Type and final molarity of buffer: Formic Acid/Sodium Hydroxide, 0.05M
- Composition of buffer: 0.237 g Acetic Acid, 1.24 mL 2M Sodium Hydroxide solution, 1.323 g NaCl. Total volume 100 mL.

- pH: Target: 7.0; Measured: 7.00
- Type and final molarity of buffer: Sodium Phosphate, monobasic/Sodium Hydroxide, 0.05M
- Composition of buffer: 0.600 g Sodium Phosphate, monobasic, 0.92 mL 2M Sodium Hydroxide solution, 0.953 g NaCl. Total volume 100 mL.

- pH: Target: 9.0; Measured: 9.00
- Type and final molarity of buffer: Boric acid/Sodium hydroxide, 0.30M
- Composition of buffer: 0.312 g boric acid, 0.440 mL 2M Sodium Hydroxide solution, 1.413 g NaCl. Total volume 100 mL.

- Buffer solutions were sparged with argon gas for a minimum of 5 min to exclude oxygen prior to conducting the test.
- The temperature of each buffer was 2.0±0.1°C
- The pH meter was calibrated for use with D2O.

Estimation method (if used):

Not applicable.

Details on test conditions:

oTemperature:  2.0   0.1 oC.  The rationale for performing the hydrolysis at one temperature was to provide the best opportunity to 
slow down the hydrolysis rate and obtain kinetic information.
oThe hydrolysis reactions employed an initial test substance concentration of 1.0   10-3 M. oDue to the hydrolytically unstable nature 
of trimethoxysilane (Kallos et al., 1991), a stock solution in acetonitrile-d3/acetonitrile was used.  oConstant ionic strength of 0.25 M 
was maintained for buffers by the addition of sodium chloride.
o0.05 M buffer solutions were prepared using deuterated water (99.9 atom % D).  Deuterated water (D2O) rather than H2O was used to 
provide a reference frequency lock for the NMR spectrometer and minimize the dynamic range problem introduced by a large solvent peak.
oThe relationship between the pH and pD scales has been established (Glasoe and Long, 1960).  For solutions of comparable acidity or 
basicity, the pH meter reading in D2O solutions is 0.40 pH units lower than in H2O solutions when calibrated against aqueous buffer 
standards.  Therefore, pD = pH meter reading + 0.40 pH units.  The relationship is independent of whether the internal solution of the
electrode contains H2O or D2O.
oBuffer solutions were sterilized by filtering through 0.20 um cellulose nitrate membrane. oThe pH of each buffer solution was measured 
with a calibrated pH meter (using aqueous pH buffers) at 2.0   0.1 oC and then converted to pD values.  This provided a D+ concentration 
equivalent to the H+ concentration at pH 4, 7, and 9. oVessels:  50-mL sterile polypropylene centrifuge tubes. oCo-solvent:  
<1% acetonitrile (~50:50 mixture of deuterated and non-deuterated acetonitrile).

TEST SYSTEM
- Type, material and volume of test flasks, other equipment used: 50 mL sterile polypropylene centrifuge tubes with caps.
- Sterilisation method: Nalgene sterile filtration units with 0.2µm cellulose nitrate membrane were used to sterilize the buffers.
- Measures to exclude oxygen: Prior to use, all buffer solutions were sparged with argon gas for a minimum of 5 min to exclude oxygen and carbon dioxide.
- Details on test procedure for unstable compounds: The test substance is unstable with respect to moisture. Items used to prepare the test substance stock solution were oven dried to remove trace moisture. Syringes were dried in a Hamilton syringe heater with aspirator. The acetonitrile used to make up the test substance solution was distilled over P2O5 and stored over molecular sieves to remove trace moisture. The stock solution was prepared inside a nitrogen gas purged glove bag and stored in 22-mL plastic vials with septum lined open-top caps. When not in use, the stock solution was stored in a secondary air tight container with Drierite. The test substance solution in acetonitrile was added directly to the buffer solution at the start of the hydrolysis experiment.
- Details of traps for volatile, if any: None
- If no traps were used, is the test system closed/open: Closed, although caps were removed for sampling.
- Is there any indication of the test material adsorbing to the walls of the test apparatus?: No

TEST MEDIUM
- Volume used/treatment: 800µL of the 0.1035 M solution of the test substance in acetonitrile-d3/acetonitrile was added to 25 mL of each buffer.
- Kind and purity of water: Deuterated water, D2O.
- Preparation of test medium: A 0.1M stock solution of the test substance in acetonitrile was prepared. This was injected directly into the buffer solution at the start of the hydrolysis experiment because the test substance is highly unstable in water.
- Renewal of test solution: Not applicable.
- Identity and concentration of co-solvent: Acetonitrile (0.98%)

Number of replicates:

Replicates:  Two at pD 4.02, 7.00, and 9.00.

Positive controls:

no

Negative controls:

yes

Remarks:

The stock solution of the test substance in acetonitrile was analyzed by 1H-NMR before and after the hydrolysis kinetic experiments to ensure the purity and integrity of trimethoxysilane in the solvent. Stock solution integrity was maintained throughout t

Statistical methods:

Descriptive statistics were performed.

Preliminary study:

The substance is known to be unstable at environmentally relevant temperatures, therefore, no preliminary study was required.

Transformation products:

yes

No.:

#1

No.:

#2

Details on hydrolysis and appearance of transformation product(s):

- Formation and decline of each transformation product during test: Decrease in the 1H-NMR peak for MeO-Si (in the test substance) and increase in the peak due to methanol (transformation product) were measured during the test. Complete disappearance of the test substance was observed.
- Pathways for transformation: Due to the limitation imposed by the rapid hydrolysis, only the total hydrolysis could be studied.

% Recovery:

0

pH:

4

Temp.:

2 °C

Duration:

1.5 min

% Recovery:

0

pH:

7

Temp.:

2 °C

Duration:

2 min

% Recovery:

0

pH:

9

Temp.:

2 °C

Duration:

1.5 min

pH:

4

Temp.:

2 °C

DT50:

<= 0.2 min

Remarks on result:

other: This value represents an estimated upper limit of the hydrolysis half-life. It refers to disappearance of the test material.

pH:

7

Temp.:

2 °C

DT50:

<= 0.3 min

Remarks on result:

other: This value represents an estimated upper limit of the hydrolysis half-life. It refers to disappearance of the test material.

pH:

9

Temp.:

2 °C

DT50:

<= 0.2 min

Remarks on result:

other: This value represents an estimated upper limit of the hydrolysis half-life. It refers to disappearance of the test material.

Other kinetic parameters:

None determined.

Details on results:

TEST CONDITIONS
- pH, sterility, temperature, and other experimental conditions maintained throughout the study: Yes

MAJOR TRANSFORMATION PRODUCTS
The increase in methanol during the hydrolysis experiment was observed but not quantified.

In all kinetic experiments, trimethoxysilane was completely hydrolyzed by the time the first 1H NMR spectrum was acquired.  Initial spectra were acquired after 90-144 seconds.

Rate constants and half-lives could not be determined quantitatively, although the data is certainly adequate for
estimating the upper limit of t1/2. The half-life was estimated based on the elapsed time to the initial spectrum.

Table 1 shows the results for each experiment.

Table 1. Results

pH		Replicate	Elapsed time	Estimated half-life*	Average half-life
4		A	98	14.0	14.1
4		B	99	14.1	14.1
7		A	144	20.6	17.0
7		B	94	13.4	17.0
9		A	103	14.7	13.8
9		B	90	12.9	13.8

* [elapsed time / s] / 7

Since the hydrolysis was so rapid, there was insufficient data to determine rate constants (k1, k2, and k3) for the
sequential hydrolysis reactions of each methoxy group by regression modeling.

First order or pseudo-first order behavior could not be confirmed because:  a) the analytical method was unable to
follow the decrease of the parent peak intensity from the test substance or the increase of peak intensity from the
hydrolysis co-product (methanol) due to a rapid hydrolysis reaction, b) no data points were obtained during the
critical portion of the hydrolysis process (20   70% hydrolyzed), and c) the relationship between k1, k2, and k3
is not known.

Validity criteria fulfilled:

yes

Conclusions:

A hydrolysis half-life of ≤17 s at pH 4, 7 and 9 and 2°C was determined in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP.

Reference 4

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 111 (Hydrolysis as a Function of pH)

GLP compliance:

yes

Analytical monitoring:

yes

Details on sampling:

The rates of hydrolysis were followed by an extraction method. Individual kinetic experiments were executed by staggered starts of multiple reaction aliquots, one for each unique reaction time to be sampled. At a predetermined time, the hydrolysis reaction was effectively stopped by addition of the extraction solution. This approach was used to minimize the time required to complete an experiment and achieve convenient sampling intervals to collect hydrolysis data spanning 2-3 half-lifes.
- Sampling intervals for the parent/transformation products: Time intervals between samples were: 0.3 - 5 min at pH4 and 25°C; 0.3 - 15 min at pH9 and 25°C; 0.3-1 min at pH 7 and 25°C; 2-15 min at pH 4 and 10°C; 2-25 minutes at pH 9 and 10°C; 0.3 - 4 min at pH 7 and 35°C; 0.3-30 min at pH 9 and 35°C. On average data was collected 12 discrete times for each kinetic endpoint.
- Sampling method: At the appropriate time, each sample was removed from the water bath and the extraction solution added using a volumetric syringe. The time was recorded. The sample was vortexed for ca. 1 min to separate the aqueous and organic phases. An aliquot of the organic phase was transferred to a GC vial for analysis.
- Sampling intervals/times for pH measurements: pH was not monitored during or after the kinetic experiments as the reaction products were not expected to affect the pH of the solutions.
- Sample storage conditions before analysis: Not applicable

Buffers:

- pH: 4.00
- Type and final molarity of buffer: Formic acid; ionic strength 0.25M
- Composition of buffer: Formic acid; sodium hydroxide; sodium chloride

- pH: 7.00
- Type and final molarity of buffer: Sodium phosphate monobasic; ionic strength 0.25M
- Composition of buffer: Sodium phosphate monobasic; sodium hydroxide; sodium chloride

- pH: 9.00
- Type and final molarity of buffer: Boric acid; ionic strength 0.25M
- Composition of buffer: Boric acid; sodium hydroxide; sodium chloride

Details on test conditions:

TEST SYSTEM
- Type, material and volume of test flasks, other equipment used: Vessels used for individual hydrolysis kinetic experiments were 15-mL sterile polypropylene centrifuge tubes.
- If no traps were used, is the test system closed/open: Open
- Is there any indication of the test material adsorbing to the walls of the test apparatus? No
TEST MEDIUM
- Preparation of test medium: Nominally 0.10 M stock solutions of the test substance in acetonitrile were prepared in a nitrogen purged glove bag and stored in 20 mL HDPE vials having septum lined open-top caps. The solution of the test substance in acetonitrile was added to the appropriate buffer solution at the start of the experiment.
- Identity and concentration of co-solvent: Acetonitrile; 40 - 45µL of the solution of TEOS in ACN was added to 5 mL of the buffer.

Duration:

25 min

pH:

4

Temp.:

25

Initial conc. measured:

289.85 other: µg/g

Duration:

10 min

pH:

7

Temp.:

25

Initial conc. measured:

288.61 other: µg/g

Duration:

60 min

pH:

9

Temp.:

25

Initial conc. measured:

308.07 other: µg/g

Duration:

60 min

pH:

4

Temp.:

10

Initial conc. measured:

310.84 other: µg/g

Duration:

150 min

pH:

9

Temp.:

10

Initial conc. measured:

322.4 other: µg/g

Duration:

14 min

pH:

4

Temp.:

35

Initial conc. measured:

282.4 other: µg/g

Duration:

30 min

pH:

9

Temp.:

35

Initial conc. measured:

292.3 other: µg/g

Number of replicates:

1; except pH 4 25°C where 2 replicates were used.

Positive controls:

no

Negative controls:

yes

Remarks:

The integrity of the test solution in acetonitrile was checked after each kinetic experiment to check that TEOS had not hydrolysed.

Statistical methods:

The hydrolysis of TEOS in dilute aqueous solution was observed to follow first-order kinetics. The natural logarithm of the concentration was plotted as a function of time. The observed rate constant, k, for the hydrolysis reaction is equal to the slope of a first-order regression line fitted to the data. The half-life of the hydrolysis reaction was calculated from the estimated rate constant according to the following equation: t1/2 = ln 2/k, where k is the reaction rate constant and t1/2 is teh half-life of the test substance.

The observed rate constants as a function of temperature at the two extremes of pH were used to construct an Arrhenius diagram for each catalytic condition by plotting ln k against 1/T for constant pH (4 or 9), where T is the temperature in K. According to the logarithmic form of the Arrhenius equation, ln k = -(Ea/RT) + ln A, the activation energy, Ea, is readily obtained from the slope of the aforementioned plot. Using the values of the Arrhenius parameters, the rate constant can be calculated at any temperature. Descriptive statistics such as average, standard deviation, relative standard deviation (RSD) and linear regression analysis were also performed.

Preliminary study:

The preliminary study was not carried out as TEOS was expected to be hydrolytically unstable.

Transformation products:

not measured

pH:

4

Temp.:

10 °C

Hydrolysis rate constant:

3.55 h-1

DT50:

0.2 h

Type:

(pseudo-)first order (= half-life)

pH:

4

Temp.:

25 °C

Hydrolysis rate constant:

6.16 h-1

DT50:

0.11 h

Type:

(pseudo-)first order (= half-life)

Remarks on result:

other: Rep I

pH:

4

Temp.:

25 °C

Hydrolysis rate constant:

6.69 h-1

DT50:

0.1 h

Type:

(pseudo-)first order (= half-life)

Remarks on result:

other: Rep II

pH:

4

Temp.:

35 °C

Hydrolysis rate constant:

9.55 h-1

DT50:

0.07 h

Type:

(pseudo-)first order (= half-life)

pH:

7

Temp.:

25 °C

Hydrolysis rate constant:

0.16 h-1

DT50:

4.4 h

Type:

(pseudo-)first order (= half-life)

pH:

9

Temp.:

10 °C

Hydrolysis rate constant:

0.91 h-1

DT50:

0.76 h

Type:

(pseudo-)first order (= half-life)

pH:

9

Temp.:

25 °C

Hydrolysis rate constant:

3.17 h-1

DT50:

0.22 h

Type:

(pseudo-)first order (= half-life)

pH:

9

Temp.:

35 °C

Hydrolysis rate constant:

5.88 h-1

DT50:

0.12 h

Type:

(pseudo-)first order (= half-life)

Other kinetic parameters:

k(H3O+): A = 0.0000614 h-1; Ea = 28.4 kJ/mol; R2 = 0.9948
k(OH-): A = 0.000000000105 h-1; Ea = 54.5 kJ/mol; R2 = 0.9971

Average solvent extraction efficiencies ranged from 87.9% for pH 4 at 35 °C to 98.3% for pH 7 at 25 °C.

The hydrolysis of TEOS was observed to follow pseudo first-order kinetics, was pH dependent and increased as a function of temperature.  The half-life of TEOS at 25 °C was observed to be 0.11, 4.4, and 0.22 hrs at pH 4, 7, and 9, respectively.  At 10 °C, the half-life of TEOS was observed to be 0.20 and 0.76 hrs at pH 4 and 9, respectively.  At 35 °C, the half-life of TEOS was observed to be 0.073 and 0.12 hrs at pH 4 and 9, respectively.  The catalytic constants for the hydronium and hydroxide ion catalyzed hydrolysis reactions were determined from the observed rate constants at pH 4 and 9 at 25 °C.  The values of the catalytic constants, k(H3O+) = 61,560 M(-1)hr(-1) and k(OH) = 317,400 M(-1)hr(-1) demonstrate that base catalysis is faster than acid catalysis.  Based on these values, the predicted rate constant at pH 7 and 25 °C of 0.0379 hr(-1), was within a factor of 5 of the observed rate constant, 0.1575 hr(-1). This indicated that there was minimal buffer catalysis, though no further hydrolysis experiments were conducted to confirm this finding.  Using the catalytic constants, the minimum hydrolysis rate is predicted to occur at pH of 6.64. Additional experiments were conducted at 10 and 35 °C. These experiments were conducted at both pH 4 and 9, using 0.05 M buffer concentration employed for the 25 °C experiments.  The information of each pH value was used to construct a linear Arrhenius plot from which the Arrhenius constant (A) and activation energy (Ea) were determined for the specific acid and base catalyzed reactions. The results show that the activation energy is 2 fold greater for the hydroxide ion catalyzed hydrolysis reaction than the hydronium ion catalyzed hydrolysis reaction, although a much greater preexponential factor, A, is associated with the former.

Validity criteria fulfilled:

yes

Conclusions:

A hydrolysis half life of 4.4 h was determined at pH 7 and at 25°C in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP.

Reference 5

Endpoint:

hydrolysis

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/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 for hydrolysis at pH 7 has been developed for, and applies specifically to, di- and tri-alkoxysilanes. It is a multiple linear regression based model with descriptors representing (i) steric effects of the alkoxy group, (ii) steric effects of the side-chain(s), and (iii) electronic effects of the side-chain(s).

The models for hydrolysis at pH 4, 5 and 9 have been developed for, and applies specifically to, organosilicon compounds. They are linear regression based models where the descriptor is the half-life at pH 7.

pH:

4

DT50:

0.1 h

Remarks on result:

other: 20 - 25°C

pH:

5

DT50:

0.2 h

Remarks on result:

other: 20-25°C

pH:

7

DT50:

0.7 h

Remarks on result:

other: 20 - 25°C

pH:

9

DT50:

0.02 h

Remarks on result:

other: 20 - 25°C

Conclusions:

Hydrolysis half-lives of 0.1 h at pH 4, 0.2 h at pH 5, 0.7 h at pH 7 and 0.02 h at pH 9 and 20-25°C were obtained for the substance using an appropriate calculation method. The result is considered to be reliable.

Reference 6

Endpoint:

hydrolysis

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Justification for type of information:

See endpoint summary for justification of read-across

Reason / purpose for cross-reference:

read-across source

Transformation products:

yes

No.:

#1

No.:

#2

No.:

#2

pH:

4

Temp.:

1.5 °C

DT50:

< 1 min

pH:

7

Temp.:

1.5 °C

DT50:

< 1 min

pH:

9

Temp.:

1.5 °C

DT50:

< 1 min

Description of key information

Half-life approximately 5 seconds as a worst-case at 25°C and pH 4, pH 7 and pH 9 (analogue read-across).

This half-life relates to degradation of the parent substance to give silanetriol and HCl.  The Si-H bond also reacts giving monosilicic acid and hydrogen. A rate of hydrogen formation of >10 l/(kg min) is equivalent to a half-life for Si-H reactivity of a few minutes or less under the conditions of UN Test Method N.5 for flammability. Therefore, the half-life is much less than 12 hours at pH 7 and 25°C.

Additional information

Hydrolysis testing with trichlorosilane is considered to be technically unfeasible because it reacts violently and releases hydrogen in contact with water. It is classified as Water React. 1 (H260: In contact with water releases flammable gases which may ignite spontaneously) and EUH014 (Reacts violently with water).

Trichlorosilane [HSiCl₃] reacts immediately in the presence of water and undergoes subsequent condensation reactions according to the secondary literature reported in PFA 2015ao. Trichlorosilane contains two reactive groups, [Si-Cl] (producing hydrogen chloride (HCl) which then dissolves in water to give hydrochloric acid), and [Si-H], producing hydrogen. A non-standard hydrolysis study (Argonne National Laboratory, 2009) reported the immediate formation of a white precipitate (probably polysilicic acid according to the reviewers) and a gas when trichlorosilane was mixed with a small amount of water.

Strong evidence based on read-across within the category of chlorosilanes is available to indicate that the Si-Cl bonds hydrolyse very rapidly to Si-OH with a half-life of ≤17 seconds at pH 4, 7 and 9 and 1.5°C (see below).

Si-Cl bonds hydrolyse more rapidly than Si-H resulting in the theoretical intermediate hydrolysis product of silanetriol: HSi(OH)₃. This species has so far not been isolated due to combination of the following:-

1.    The reactivity of the Si-H bond and results in the production of monosilicic acid Si(OH)₄. Hydrogen is produced as the by-product according to the equation HSi(OH)₃+ H₂O → Si(OH)₄ + H₂. Monosilicic acid condenses readily in water to form polysilicic acid (the white precipitate as shown in the Argonne 2009 report).

2.    The HSi(OH)₃monomer rapidly condenses. Holleman-Wiberg (2001) suggests that the product of trichlorosilane hydrolysis would be initially silanetriol and followed immediately by elimination of water. In a similar manner to monosilicic acid, Si(OH)₄, concentrations of silanetriol above about 100-150 mg/L as SiO₂ would condense to form insoluble polymeric species.

SiHCl₃+3H₂O             →        HSi(OH)₃+3HCl

Trichlorosilane + water                      silanetriol + hydrochloric acid

Then almost immediately

2HSi(OH)₃         →        HSi(OH)₂OSi(OH)₂H + H₂O  →        →    (HSiO_1.5)_n

 

The condensation reactions of monosilicic acid are discussed in detail in a report (PFA 2015ao).

 

The substance is self–classified by the registrants as Water React. 1 (H260: In contact with water releases flammable gases which may ignite spontaneously) according to the criteria of Regulation (EC) No. 1272/2008 (CLP Regulation). In a test according to UN Test Method N.5 (BAM, 2014a), trichlorosilane produced a flammable gas in contact with water at a maximum rate of more than 10 l/(kg min), refer to IUCLID Section 4.13. Due to this hazard, hydrolysis testing with trichlorosilane is considered to be technically unfeasible. In addition, appropriate analytical methods for quantification of the intermediate and final products are not available.

 

The flammability in contact with water study indicates that hydrogen (the product of Si-H hydrolysis) is formed rapidly under the conditions of this study. A rate of hydrogen formation of >10 l/(kg min) is equivalent to a half-life for Si-H reactivity of a few minutes or less. However, the concentrations used in this study are much higher than those of OECD 111, and no buffering of the solution was used.

 

The Si-H bond is therefore expected to react giving monosilicic acid and hydrogen. A rate of hydrogen formation of >10 l/(kg min) is equivalent to a half-life for Si-H reactivity of a few minutes or less under the conditions of UN Test Method N.5 for flammability. Therefore, the half-life is much less than 12 hours at pH 7 and 25°C.

 

Si-Cl hydrolysis

Trichlorosilane contains the Si-Cl (chlorosilane) reactive group. Strong evidence based on read-across within the category of chlorosilanes is available to indicate that the Si-Cl bonds will rapidly hydrolyse to Si-OH with a half-life of ≤17 seconds at pH 4, pH 7 and pH 9 and 1.5°C (see below).

For six substances, quantitative half-life data at 1.5°C are available; the measured half-lives at pH 4, pH 7 and pH 9 and 1.5°C are all ≤17 s. Two studies at 25°C and 27°C gave limit half-lives, one study with one further substance at 50°C found no parent substance to be present at t₀, indicating extremely rapid hydrolysis.

This read-across is made in the context of evidence from other available data for chlorosilane structural analogues, as shown in the following table.

Table: Hydrolysis data for chlorosilanes

CAS No	Name	Result – half-life at pH 4 (seconds)	Result – half-life at pH 7 (seconds)	Result – half-life at pH 9 (seconds)	Temperature	Klimisch
75-77-4	Chlorotrimethylsilane	7	11	8	1.5 ± 0.5°C	2
75-78-5	Dichloro(dimethyl) silane	10	17	7	1.5 ± 0.5°C	2
75-79-6	Trichloro(methyl) silane	7	9	6	1.5 ± 0.5°C	2
80-10-4	Dichloro(diphenyl) silane	6	10	8	1.5 ± 0.5°C	2
675-62-7	Dichloromethyl(3,3,3-trifluoropropyl) silane8	12	9		1.5 ± 0.5°C	2
5578-42-7	Dichlorocyclohexylmethylsilane	<<27 min[1]	<<27 min[2]	<<27 min[3]	27°C	2
18379-25-4	Trichloro(2,4,4-trimethylpentyl)silane	<<2 min[7]	<<2 min[7]	<<2 min[7]	27°C	2
4518-98-3	1,1,2,2-tetrachloro-1,2-dimethyldisilane	8	7	7	1.5 ± 0.5°C	2
13154-25-1	Chlorotri(3-methyl-propyl) silane	Not quantified[4]	Not quantified[5]	Not quantified[6]	50°C	1

[1] No parent substance was detected when the first measurement was taken.

[2] No parent substance was detected when the first measurement was taken.

[3] No parent substance was detected when the first measurement was taken.

[4] In this test the t₀analysis (50°C) showed a recovery <LOD, suggestive of an extremely rapid reaction.

[5] In this test the t₀analysis (50°C) showed a recovery <LOD, suggestive of an extremely rapid reaction.

[6] In this test the t₀analysis (50°C) showed a recovery <LOD, suggestive of an extremely rapid reaction.

[7] Determination of hydrolysis kinetics was not possible, very fast hydrolysis and precipitation of solids were observed.

Hydrolysis half-lives of 7 seconds at pH 4, 9 seconds at pH 7 and 6 seconds at pH 9 and 1.5°C were determined for trichloro(methyl)silane in accordance with OECD 111 (Dow Corning Corporation 2001).

Since the hydrolysis was so rapid relative to the timescale of the analytical measurement, there was insufficient data to determine rate constants for the hydrolysis reactions of these chlorosilanes using regression modelling. However, the data was adequate for estimating the upper limit of t_1/2. Half-lives were estimated as 0.1t, where t=time for complete hydrolysis.

Measured hydrolysis half-lives of <<2 mins at pH 4, pH 7 and pH 9 and 25°C were determined for trichloro(2,4,4 -trimethylpentyl)silane in a study conducted according to generally acceptable scientific principles (Altmann 2015). Only a preliminary study was carried out and a more precise knowledge of the half-life is needed for use in the chemical safety assessment. Therefore, this data is not used as evidence in the read-across approach.

Given the very rapid hydrolysis rates in water (≤17 seconds at 1.5°C and pH 4, pH 7 and pH 9) observed for all tested chlorosilanes, and the lack of significant variation in the half-lives for the different substances, it is considered appropriate to read-across this result to trichlorosilane.

Reaction rate increases with temperature, and therefore hydrolysis will be faster at 25°C and at physiologically-relevant temperatures. Under ideal conditions, hydrolysis rate can be recalculated according to the equation:

DT₅₀(X°C) = DT₅₀(T°C) *e^(0.08.(T-X))

Where T = temperature for which data are available and X = target temperature.

Using the half-life measured for the trichlorosilane analogue at 1.5°C and pH 7 (9 seconds) the estimated hydrolysis half-life at 25°C and pH 7 is 1.4 seconds. However, it is not appropriate or necessary to attempt to predict accurately when the half-life is less than 5-10 seconds. As a worst-case it can therefore be considered that the half-life of the substance at pH 7 and 25°C is approximately 5 seconds.

The estimated hydrolysis half-life at 37.5°C and pH 7 (relevant for lungs and blood and in vitro and in vivo (intraperitoneal administration) assays) is 1 second, so as a worst-case can be considered to be approximately 5 seconds.

Using the half-life measured for the trichlorosilane analogue at 1.5°C and pH 4 (7 seconds), the estimated hydrolysis half-life at 37.5°C and pH 4 is 0.4 second, so worst-case can be considered to be approximately 5 seconds.

The hydrolysis reaction may be acid or base catalysed, and the rate of reaction is expected to be slowest at around pH 7 and increase as the pH is raised or lowered. For an acid-base catalysed reaction in buffered solution, the measured rate constant is a linear combination of terms describing contributions from the uncatalysed reaction as well as catalysis by hydronium, hydroxide, and general acids or bases.

 

kobs= k0+ kH3O+[H3O+] + kOH-[OH-] + ka[acid] + kb[base]

 

At extremes of pH and under standard hydrolysis test conditions, it is reasonable to suggest that the rate of hydrolysis is dominated by either the hydronium or hydroxide catalysed mechanism.

 

Therefore, at low pH:

kobs≈kH3O+[H3O+]

 

At pH 4 [H3O+] = 10-4 mol dm-3 and at pH 2 [H3O+] = 10-2 mol dm-3; therefore, kobs at pH 2 should theoretically be approximately 100 times greater than kobs at pH 4. However, at 37.5°C and pH 2 (relevant for conditions in the stomach following oral exposure), it is not appropriate to apply any further correction for pH to the limit value at 37.5°C and pH 4 and the hydrolysis half -life is therefore estimated to be approximately 5 seconds. At 37.5°C and pH 5.5 (relevant for dermal exposure), the hydrolysis half -life is estimated to be in between the half-lives at pH 4 and pH 7 at 37.5°C, and thus also approximately 5 seconds as a worst-case.

The rate of hydrolysis of trichlorosilanes has been found to decrease with increasing size of organic constituent (Jones et al., 2000)

                                               H > methyl > ethyl > propyl

Trichlorosilane had the fastest measured rate constant of 28 chlorosilanes tested (including trichloro(methyl)silane) in the Argonne study. Therefore, it can be concluded that trichlorosilane will have a hydrolysis half-life at least as fast as that of the read-across substances. It is concluded that the chlorosilane bonds of trichlorosilane will hydrolyse with a half-life of ≤17 seconds (0.3 minutes) at pH 4, 7 and 9 and 1.5°C. The half-life will be <17 seconds (0.3 minutes) at pH 4, 7 and 9 and 25°C.

 

Hydrolysis in air

The above hydrolysis studies were carried out with the substance dissolved in water.

Consideration of the rates of reaction with moisture in air is relevant for inhalation exposure assessment. Experience in handling and use, as well as the extremely rapid rates observed in the available water-based studies, would suggest that rates of reaction in moist air will also be rapid. If any unreacted chlorosilane were to reach the airways, it would rapidly hydrolyse in this very moist environment.

A simulated nose-only exposure study (Dow Corning Corporation 2013) has been conducted to determine hydrolytic stability of dichloro(dimethyl)silane under conditions typical of nose-only vapour inhalation exposures. The vapour generation was on 1 day for 3 hrs 14 minutes; concentrations of parent material were measured at 30 minute intervals using gas chromatography (GC). The nominal concentration was 50 ppm. The mean temperature was 21.6°C and the relative humidity (RH) was 57%. 24% parent concentration remaining in the test atmosphere relative to nominal concentration was measured by GC. This indicates 76% hydrolysis of the parent substance had taken place by the time the test atmosphere reached the GC. It was concluded that at least 20-29% of the parent test article would be present in the breathing zone relative to the nominal concentration under typical conditions used for nose-only inhalation exposure of rats. It is therefore possible to expose rats in a nose-only study to parent chlorosilane, because the transit time from the substance reservoir to the nose is very rapid (<1 second), however, this is not considered to be representative of human exposure conditions.

This represents extremely rapid hydrolysis because the time taken for the test substance to reach the GC was very short (13-15 seconds).The authors of this summary have used the information from this study to estimate a half-life for dichloro(dimethyl)silane in air of approximately 7 seconds (95% confidence limit = 3 -11 seconds), which is comparable to the half-life in water.

In a study of the acute toxicity to rats via the inhalation route (Dow Corning Corporation 1997), dichloro(dimethyl)silane was quantified in the exposure chamber using Thermal Conductivity Detection and identification was confirmed using GC/MS. The relative humidity (RH) in the exposure chamber was 30 -35%. The mean measured concentrations in the exposure chambers during exposure (1 hour) was only about 15% of the nominal concentration of dichloro(dimethyl)silane. The test atmosphere contained an amount of chloride consistent with the nominal concentration of test article as determined via electrochemical detection. Thus, the majority of parent had hydrolysed in the test atmosphere at only 30-35% relative humidity.

Similarly, in a study to assess stability of dichloro(dimethyl)silane vapour in air using gas-sampling FTIR (Dow Corning 2009), dichloro(dimethyl)silane was observed to be extremely unstable in high relative humidity atmospheres. At 75% relative humidity (RH) level, a stable test atmosphere of the substance could not be generated. In dry air (<5% RH), the substance had achieved 28% loss after 1 hour and 71% loss after 3.2 hours.

In a 9 day repeated exposure inhalation study (Union Carbide Corporation, 1989), trichlorosilane and HCl were quantified in the exposure chamber. Gas chromatographic analysis of the chamber indicated no trichlorosilane in any of the exposure chambers. Mean metered concentrations (as trichlorosilane) were 96, 51 and 25 ppm for target concentrations of 100, 50 and 25 ppm, respectively. HCl gas was present in the chamber atmospheres. The mean + SD HCl concentrations were 262 + 14, 127 + 16, and 52 + 5 ppm for the target concentrations of 100, 50, and 25 ppm trichlorosilane, respectively. These HCl concentrations agree well with the stoichiometric hydrolysis of trichlorosilane, that being 3 moles of HCl produced for each mole of trichlorosilane. HCl was not detected in the control chamber atmosphere. During exposures, the daily mean chamber temperatures for the control group ranged from 23.3 to 24.5°C, and the relative humidity ranged from 46 to 58%. 

The significant extent of trichlorosilane hydrolysis demonstrated in this study is in good agreement with the theoretical capacity for hydrolysis in air under conditions typical of a rat repeated exposure test.

Theoretically, air at 20°C at 50% relative humidity would have more than 100 times the amount of water necessary for complete hydrolysis of trichlorosilane:

Water content of air at 20°C and 100% relative humidity = 17.3 g/m³

Assuming a 50% humidity, the water content would be 8.65g water/m³= 8.65 mg water/L

Molecular weight of water = 18 g/mol; So 8.65 mg water/L = 0.48 mmol water/L

50 ppm HCl is the estimated upper exposure limit based on HCl corrosivity for a repeated dose inhalation toxicity exposure test.

As trichlorosilane has three Cl groups, this would be equivalent to 17 ppm.

Molecular weight of trichlorosilane = 135.45 g/mol;

1 mole of an ideal gas under relevant conditions (standard pressure and temperature 25°C) has a volume of 24.45 l. So 17 ppm * 135.45 g/mol gives mass of substance per mole of air, then dividing by 24.45 gives the mass of substance per volume of air.

So 17 ppm trichlorosilane ≡ 94.2 g/L (or 0.0007 mmol/L).

Therefore, it can be concluded that the registered substance will hydrolyse very rapidly under conditions relevant for environmental and human health risk assessment and no further testing is necessary. It is not possible or necessary to attempt a quantitative prediction of rate or half-life because the chemical safety assessment is not sensitive to this uncertainty within this range. Additional information is given in a supporting report (PFA 2013ab) attached in Section 13.

The hydrolysis products are silanetriol (intermediate hydrolysis product), monosilicic acid (final hydrolysis product) and hydrochloric acid.

The hydrolysis half-life of substances used for read-across in other areas are discussed below:

Hydrolysis of the read-across substance trimethoxysilane (CAS 2487-90-3)

Data for the substance trimethoxysilane (CAS 2487-90-3) are read-across to the submission substance trichlorosilane for the following endpoints, short-term toxicity to fish, short-term toxicity to aquatic invertebrates, toxicity to aquatic algae, bacteria mutagenicity, mammalian cytogenicity and in vivo mammalian micronucleus. The hydrolysis half-lives and the silanol hydrolysis products of the two substances are relevant to this read-across, as discussed in the appropriate sections for each endpoint.

Trimethoxysilane, HSi(OMe)₃, is very unstable in the presence of water. The substance contains two reactive groups: Si-OMe and Si-H. The rate of Si-OMe hydrolysis has been measured in a reliable study; half-lives of ≤0.2 minutes at pH 4 and pH 9, at pH 7, half-life of ≤0.3 minutes and 2°C were obtained for trimethoxysilane (Dow Corning Corporation 2002). Methanol is produced by this reaction. If Si-OMe is hydrolysed, but Si-H is not, silanetriol (HSi(OH)₃) would be formed. This is the same as the product of Si-Cl hydrolysis for trichlorosilane.

 

The Si-H bond of silanetriol reacts in water, forming monosilicic acid, Si(OH)₄. The rate of this reaction is uncertain when the parent compound is a trimethoxy. Neither silanetriol nor monosilicic acid have been isolated; they only exist in dilute aqueous solution. They readily and rapidly (within minutes) condense to give insoluble polymeric species. Depending on the pH and concentration, solutions will contain varying proportions of monomeric silanol species, cyclic and linear oligomers and polymeric species of three-dimensional structure.

 

The presence of HCl could in theory influence the reactivity of the Si-H bond or the condensation reactions by changing the pH. However, this should not be an issue in suitably buffered media (such as the conditions of the OECD 111 hydrolysis study or the natural environment or in vivo).

 

At 0.01 M trimethoxysilane (the maximum concentration for the OECD 111 hydrolysis study), trimethoxysilane is present at 0.94 g/L (molecular weight 94 g/mol). Three moles of methanol are generated per mole of test substance giving 0.96 g/L methanol (molecular weight 32 g/mol). This is approximately 0.1% v/v; well below the 1% co-solvent allowed by OECD 111.

 

Therefore, it can be concluded that neither HCl nor methanol will significantly influence the hydrolysis and condensation reactions of the silanetriol species formed by initial hydrolysis of both trichlorosilane and trimethoxysilane.

 

Therefore, regardless of the rates of these further reactions, the final silicon-containing products of trimethoxysilane and trichlorosilane hydrolysis are equivalent and produced on an equivalent timescale.

For trimethoxysilane, hydrolysis half-lives at 2°C of ≤0.2 minute at pH 4, ≤0.3 minute at pH 7, and ≤0.2 minute at pH 9 were determined in accordance with OECD 111 (Dow Corning Corporation 2002).

The half-lives at pH 2 and 25°C, at pH 7 and 37.5°C and at pH 2 and 37.5°C, are as a worst case considered to be approximately 5 seconds. 

The ultimate hydrolysis products are silicic acid, methanol and hydrogen.

Hydrolysis of the read-across substance triethoxysilane (CAS 998-30-1)

Data for the substance triethoxysilane (CAS 998-30-1) are read-across to the submission substance trichlorosilane for the mammalian mutagenicity endpoint. The silanol hydrolysis product of the two substances is relevant to this read-across, as discussed in the appropriate section for the endpoint.

For triethoxysilane, hydrolysis half-lives at 20-25°C of 0.1 h at pH 4, 0.2 h at pH 5, 0.7 h at pH 7 and 0.02 h at pH 9 were obtained using QSAR estimation methods.

For triethoxysilane the hydrolysis half-life at 37.5°C and pH 7 (relevant for lungs and blood) is 0.26 hours.

The ultimate hydrolysis products are silicic acid, ethanol and hydrogen.

Hydrolysis of the read-across substance tetraethyl orthosilicate (CAS 78-10-4)

Data for the substance tetraethyl orthosilicate (CAS 78-10-4) are read-across to the submission substance trichlorosilane for toxicity to microorganisms endpoint. For the short-term toxicity to fish, short-term toxicity to aquatic invertebrates and toxicity to aquatic algae endpoints, data for tetraethyl orthosilicate are used as supporting data. The hydrolysis half-lives and the silanol hydrolysis products of the two substances are relevant to this read-across, as discussed in the appropriate sections for each endpoint.

For tetraethyl orthosilicate, hydrolysis half-lives at 25°C of 0.1 h at pH 4, 4 h at pH 7 and 0.22 h at pH 9 were determined in accordance with OECD 111 (Dow Corning Corporation 2003).

The hydrolysis products are silicic acid and ethanol.

Hydrolysis of the read-across substance silicic acid (H4SiO4), tetraethyl ester, hydrolysed (CAS 68412-37-3)

Data for the substance, silicic acid (H4SiO4), tetraethyl ester, hydrolyzed (CAS 68412-37-3) are read-across to the submission substance trichlorosilane for the toxicity to aquatic microorganisms endpoint. The silanol hydrolysis product of trichlorosilane is relevant to this read-across, as discussed in the appropriate section for the endpoint.

Silicic acid (H4SiO4), tetraethyl ester, hydrolysed is a complex multiconstituent reaction mass containing hydrolysed and partially hydrolysed products of tetraethyl orthosilicate, including Si(OH)₄. The half-lives of tetraethyl orthosilicate are 0.1 h at pH 4, 4 h at pH 7, and 0.22 h at pH 9 at 25°C (Dow Corning Corporation, 2003). The reaction of the first alkoxysilane group is the rate determining step in alkoxysilane hydrolysis; therefore, partially hydrolysed products, (HO)_nSi(OEt)_4-n, are expected to hydrolyse more quickly.

Hydrolysis of the read-across substance synthetic amorphous silica (CAS 112926-00-8)

Data for the substance, synthetic amorphous silica (CAS 112926-00-8) are read-across to the submission substance trichlorosilane for the repeated dose toxicity oral, toxicity to reproduction and developmental toxicity endpoints. The formation of the same silanol hydrolysis product is relevant to this read-across, as discussed in the appropriate section for each endpoint.

 

Trichlorosilane undergoes very rapid hydrolysis in contact with water to form monosilicic acid and hydrochloric acid. Monosilicic acid condenses to insoluble polysilicic acid [equivalent to synthetic amorphous silica (SAS)] at concentrations higher than 100-150 mg/L ‘SiO₂ equivalent’ in water. At very high concentration, polysilicic acid can condense to silicon dioxide (SiO₂).

Hydrolysis of the read-across substance dichloro(dimethyl)silane (CAS 75-78-5)

Data for the substance dichloro(dimethyl)silane (CAS 75-78-5) are read-across to the submission substance trichlorosilane for the repeated dose toxicity inhalation endpoint. The rate of hydrolysis and the silanol hydrolysis products of the two substances are relevant to this read-across, as discussed in the appropriate section for the endpoint.

For dichloro(dimethyl)silane, hydrolysis half-lives at 1.5°C of <1 minute at pH 4, pH 7 and pH 9 were obtained from hydrolysis study on dichloro(dimethyl)silane and other analogue dichlorosilane substances.

 

The half-lives at pH 2 and 25°C, at pH 7 and 37.5°C and at pH 2 and 37.5°C may be calculated in the same way as for the registration substance above. This gives a half-lives of approximately 5 seconds.

 

The hydrolysis products for dichloro(dimethyl)silane are dimethylsilanediol and hydrochloric acid.