Hexachlorodisilane

Hydrolysis

Description of key information

Hydrolysis: Half-life approximately 5 seconds as a worst case at 25°C and pH 4, 7 and 9 (analogue read-across). This half-life relates to degradation of the parent substance to give hexahydroxydisilane and HCl.

The Si-Si bonds may also react to produce monosilicic acid. At concentrations above about 100-150 mg/l (measured as SiO₂ equivalents), condensation products of monosilicic acid can also form.

Hexahydroxydisilane is also likely to form condensation products at similar concentrations (in terms of SiO₂ equivalents).

Additional information

The hydrolysis of hexachlorodisilane, and of the read-across substances used to fulfil other endpoint data requirements in this dossier, is discussed below.

A standard hydrolysis study with hexachlorodisilane is considered technically unfeasible because the substance is expected to react violently with water.

Hexachlorodisilane hydrolyses very rapidly, with hydrolysis half-lives of ≤10, ≤17, ≤9 seconds at 1.5°C and pH 4, 7 and 9 (analogue read-across).These half-lives relate to degradation of the parent substance to give hexahydroxydisilane and HCl.The estimated half-lives of the substance at 25ºC and pH 4, 7 and 9 are approximately 5 seconds as a worst case.

Further hydrolysis of the Si-Si bonds in hexahydroxydisilane is expected to happen rapidly and produces monosilicic acid. At concentrations above about 100-150 mg/l (measured as SiO₂equivalents), condensation products of monosilicic acid can also form. At concentrations >100-150 mg/l of SiO₂, monomeric monosilicic acid condenses into colloidal particles of polysilicic acid (silica sol) or a highly cross-linked network (silica gel).Hexahydroxydisilane is also likely to form condensation products (polyhydroxy-polysilanes) at similar concentrations (in terms of SiO₂equivalents). The structure and predicted properties of the Si-Si containing hydrolysis products (polyhydroxy-polysilanes) and (poly)silicic acid are very similar, and distinguishing between them would be very difficult analytically.

The condensation rate is dependent on temperature, concentration, and acidity/alkalinity (as in the pH) of the system. A dynamic equilibrium is established between monomeric monosilicic acid, oligomers and insoluble amorphous polysilicic acid. The composition of a solution is dependent upon conditions such as pH, temperature and the presence of ions.

Monosilicic acid and its condensation products are naturally occurring substances that are ubiquitous in the environment. Reaction mechanisms of this substance, and related substances, are discussed in the document attached to Section 13 of IUCLID (PFA, 2015ao, Peter Fisk Associates, The aquatic chemistry of inorganic silicic acid generators, PFA.404.001.001).

A non-standard hydrolysis study is available for a structurally-related substance to the registration substance, decachlorotetrasilane (CAS 13763-19-4) (AQura analytical solutions, 2014). This study was a process safety study and not a standard hydrolysis study.

The test substance was found to react very rapidly in water with the formation of a white precipitate (solid like white snow) directly after dosing. Directly after dosing of DCTS, formation of a relatively great amount of solid like white snow was observed. Dosing procedure was accompanied by low production of white fog. A maximum temperature of 42°C resulted during a time period of 6 min (initial temperature 24.2°C). After the end of hydrolysis (reaching of room temperature) the pH-value of the liquid phase was measured to be 1. This observation is consistent with hydrolysis of the Si-Cl bonds, and subsequent polymerisation of the silanol hydrolysis products.

Si-Si hydrolysis:

The cleavage of the Si-Si bond to give two Si-O bonds is highly thermodynamically-favoured. (Si-Si 340 kJ/mol; Si-O 452 kJ/mol. Greenwood and Earnshaw (1995))

Si-Cl hydrolysis

No measured hydrolysis data are available for the registered substance. Therefore, data are read-across from other dichlorosilanes and trichlorosilanes.

Reliable hydrolysis studies according to OECD 111 are available for the related substances: dichloro(dimethyl)silane, dichloromethyl(3,3,3-trifluoropropyl)silane, dichloro(diphenyl)silane and trichloro(methyl)silane, and1,1,2,2-tetrachloro-1,2,dimethyldisilane.These substances are fully hydrolysed within less than a minute at pH 4, 7 and 9 and 1.5°C.

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	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	2a
75-78-5	Dichloro(dimethyl)silane	10	17	7	1.5 ± 0.5˚C	2a
75-79-6	Trichloro(methyl)silane	7	9	6	1.5 ± 0.5˚C	2a
80-10-4	Dichloro(diphenyl)silane	6	10	8	1.5 ± 0.5˚C	2a
675-62-7	Dichloromethyl(3,3,3-trifluoropropyl)silane	8	12	9	1.5 ± 0.5˚C	2a
5578-42-7	Dichlorocyclohexylmethylsilane	<<27 min[1]	<<27 min[2]	<<27 min[3]	27°C	2a
4518-98-3	1,1,2,2-tetrachloro-1,2-dimethyldisilane	8	7	7	1.5 ± 0.5˚C	2a
13154-25-1	Chlorotri(3-methyl-propyl)silane	Not quantified[4]	Not quantified[5]	Not quantified[6]	50˚C	1a

[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.

 

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).

Hydrolysis half-lives of 10 seconds at pH 4, 17 seconds at pH 7 and 7 seconds at pH 9 and 1.5°C were determined for dichloro(dimethyl)silane in accordance with OECD 111 (Dow Corning Corporation 2001).

Hydrolysis half-lives of 8 seconds at pH 4, 12 seconds at pH 7 and 9 seconds at pH 9 and 1.5°C were determined for dichloromethyl(3,3,3-trifluoropropyl)silane in accordance with OECD 111 (Dow Corning Corporation 2001).

Hydrolysis half-lives of 6 seconds at pH 4, 10 seconds at pH 7 and 8 seconds at pH 9 and 1.5°C were determined for dichloro(diphenyl)silane in accordance with OECD 111 (Dow Corning Corporation 2001).

Hydrolysis half-lives of 8 seconds at pH 4, 7 seconds at pH 7 and 7 seconds at pH 9 and 1.5°C were determined for 1,1,2,2-tetrachloro-1,2,dimethyldisilane 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 t1/2. Half-lives were estimated as 0.1t, where t=time for complete hydrolysis.

Measured hydrolysis half-lives of <<27 mins at pH 4, pH 7 and pH 9 and 27°C were determined for dichlorocyclohexylmethylsilane in a study conducted according to generally acceptable scientific principles (Haas, 2012).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 part of the weight-of-evidence.

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

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

DT₅₀(XºC) = DT₅₀(T°C) *e^(0.08.(T-X))

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

Using the longest half-life measured for the dichlorosilanes and trichlorosilanes at 1.5ºC and pH 7 (17 seconds) the estimated hydrolysis half-life at 25ºC and pH 7 is 2.6 seconds. However, it is likely that factors such as diffusion become rate-determining 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 37.5ºC and pH 7 (relevant for lungs and blood and in vitro and in vivo (intraperitoneal administration) assays) is 1 second, so worst case approximately 5 seconds.

Using the longest half-life measured for the dichlorosilanes and trichlorosilanes at 1.5ºC and pH 4 (10 seconds)the estimated hydrolysis half-life at 37.5ºC and pH 4 is 0.6 second, so worst case 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.

k_obs= k₀+ k_H3O+[H3O⁺] + k_OH^-[OH^-] + k_a[acid] + k_b[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:

k_obs≈k_H3O+[H3O⁺]

At pH 4 [H₃O⁺] = 10^-4mol dm^-3and at pH 2 [H₃O⁺] = 10^-2mol dm^-3; therefore, k_obsat pH 2 should theoretically be approximately 100 times greater than k_obsat pH 4.

However, at 37.5ºC 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 37.5ºC and pH 4, and the hydrolysis half -life is therefore estimated to be approximately 5 seconds. At 37.5ºC 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 at37.5ºC, and thus also approximately 5 seconds as a worst case.

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 (Houghton 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.

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.

The significant extent of chlorosilane hydrolysis demonstrated in the studies with dichloro(dimethyl)silane 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 and 50% relative humidity would have more than 1000 times the amount of water necessary for complete hydrolysis of hexachlorodisilane:

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

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

Molecular weight of water = 18 g/mole; 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 hexachlorodisilane has 6 Cl groups, 8.33 ppm of the substance would produce 50 ppm HCl, so 8.33 ppm of hexachlorodisilane is the upper exposure limit for a repeated dose inhalation toxicity test.

Molecular weight of hexachlorodisilane   = 268.89 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 8.33ppm* 268.89 g/mol gives a mass of substance per mole of air, then dividing by 24.45 gives the mass of substance per volume of air.

So 8.33 ppm hexachlorodisilane                 ≡ 92 g/l (or 0.0003 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.

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

Trimethoxysilane (CAS 2487-90-3) has hydrolysis half-lives at 2°C of 14 seconds at pH 4, 17 seconds at pH 7, and 14 seconds at pH 9 (measured).

The estimated hydrolysis half-lives at 37.5ºC and pH 7 (relevant for lungs and blood and in vitro and in vivo (intraperitoneal administration) assays), at 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), and at 37.5ºC and pH 5.5 (relevant for dermal exposure), are approximately 5 seconds.

These half-lives relate to degradation of the parent substance to give silanetriol and methanol.

The half-life for Si-H reactivity of silanetriol is expected to be <12 hours at 25°C and pH 7, to produce hydrogen and monosilicic acid.

At concentrations above about 100-150 mg/l as SiO₂, condensation products of silanetriol and monosilicic acid can also form.

Hydrolysis of read-across substance tetraethylorthosilicate (CAS 78 -10 -4)

Tetraethyl orthosilicate (CAS 78-10-4) has hydrolysis half-lives at 25°C of 0.11 h at pH 4, 4.4 h at pH 7 and 0.22 h at pH 9 (measured).

The estimated hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood and in vitro and in vivo (intraperitoneal administration) assays) is 1.6 hours.

The estimated hydrolysis half-life at 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure) is approximately 5 seconds.

The estimated hydrolysis half-life at37.5ºC and pH 4 is approximately 5 seconds.

The hydrolysis half-life at 37.5ºC and pH 5.5 (relevant for dermal exposure) is estimated to be in between the half-lives at pH 4 and pH 7 at 37.5ºC.

The hydrolysis products are monosilicic acid and ethanol.

At concentrations above about 100-150 mg/l as SiO₂condensation products of monosilicic acid can also form.

Hydrolysis of read-across substance trichlorosilane (10025-78-2)

Trichlorosilane (CAS 10025-78-2) reacts rapidly and violently with water. Trichlorosilane, HSiCl3, contains two reactive groups: Si-Cl and Si-H. Strong evidence based on read-across within the category of chlorosilanes is available to indicate that the Si-Cl bonds will hydrolyse very rapidly to Si-OH with a half-life of ≤0.3 minutes at pH 4, 7 and 9 and 1.5°C. The products of hydrolysis are monosilicic acid and hydrogen gas.

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

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. The products of hydrolysis are monosilicic acid and ethanol.

References

AQura Analytical Solutions (2014) SPZ-Report No. 66b-14 Hydrolysis test of the sample "Decachlorotetrasilane (DCTS)"

Dow Corning Corporation (1997). An acute whole body inhalation toxicity study of dimethyldichlorosilane in Fischer 344 rats. Testing laboratory: Dow Corning Corporation, MI 48686-0994. Report no.: Internal Report No. 1997-I0005-43537. Report date: 1997-12-22.

Greenwood, N. N. and Earnshaw, A, 1984. Chemistry of the Elements. Butterworth-Heinemann, 1984., p. 389.