Hexadecyltrimethoxysilane

Hydrolysis

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

test procedure in accordance with generally accepted scientific standards and described in sufficient detail

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 111 (Hydrolysis as a Function of pH)

Version / remarks:

Published method: NMR-Spectroscopic Investigations on the Hydrolysis of Functional Trialkoxysilanes, M. Brand, A. Frings, P. Jenkner, R. Lehnert, H. J. Metternich, J. Monkiewcz, J. Schram, Verlag der Zeitschrift für Naturforschung Tübingen 54b, 155-64, (1999)

Deviations:

yes

Remarks:

Due to the low solubility of the substance, the water/solvent ratio, was adjusted. Furthermore since the hydrolysis speed of the substance is low, the duration of the studies and the measurement intervals were prolonged to 14 d and daily measurement.

Principles of method if other than guideline:

- Principle of test: Three different series of measurements were carried out at pH 7, pH 10 and pH 4 according to the published method “NMR-Spectroscopic Investigations on the Hydrolysis of Functional Trialkoxysilanes, M. Brand, A. Frings, P. Jenkner, R. Lehnert, H. J. Metternich, J. Monkiewcz, J. Schram, Verlag der Zeitschrift für Naturforschung Tübingen 54b, 155-64, (1999)”.

GLP compliance:

no

Remarks:

The laboratory is accredited from the German Akkreditation Body according to DIN EN ISO/IEC 17025

Radiolabelling:

no

Analytical monitoring:

yes

Remarks:

1H-NMR spectra

Details on sampling:

- Sampling intervals for the parent/transformation products: At the start and then on a daily basis over two weeks.
- Sampling method: All tests were done directly within the NMR tubes.

Details on test conditions:

TEST SYSTEM
- Type, material and volume of test flasks, other equipment used: NMR tubes
- Is there any indication of the test material adsorbing to the walls of the test apparatus: Yes, therefore all NMR tubes were pretreated with Hexamethyldisilazane to prevent interactions with the glass surface, rinsed with water and dried overnight in a drying cabinet.

TEST PREPARATION
pH 7: 0.50 mL acetone-d6 and 0.10 mL H2O were transferred to a NMR tube and mixed. Using a syringe 10 µl of Hexadecyltrimethoxysilane were added (the resulting concentration of silane is 15 g/L or 0.043 mol/L).
pH 10: 0.50 mL acetone-d6 and 0.10 mL H2O were transferred to a NMR tube and mixed. The solution was adjusted to pH 10 with NaOH. Using a syringe 10 µl of Hexadecyltrimethoxy-silane were added (the resulting concentration of silane is 15 g/L or 0.043 mol/L).
pH 4: 0.50 mL acetone-d6 and 0.10 mL H2O were transferred to a NMR tube and mixed. The solution was adjusted to pH 4 with HCl. Using a syringe 10 µl of Hexadecyltrimethoxysilane were added (the resulting concentration of silane is 15 g/L or 0.043 mol/L).

OTHER TEST CONDITIONS
- Adjustment of pH: with NaOH or HCl

Duration:

14 d

pH:

4

Initial conc. measured:

94.2 other: mol%

Duration:

14 d

pH:

7

Initial conc. measured:

100 other: mol%

Duration:

14 d

pH:

10

Initial conc. measured:

98.7 other: mol%

Number of replicates:

Not reported

Positive controls:

not specified

Negative controls:

not specified

Preliminary study:

yes (feasibility study), see field "any other information on material and methods incl. tables"

Transformation products:

yes

No.:

#1

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

- Pathways for transformation: The three SiOMe-groups of the silane will react with water and deliberate methanol. The SiOH-groups are not stable and generate Si-O-Si bonding and crosslink eventually.

% Recovery:

59.9

pH:

4

Duration:

14 d

% Recovery:

84.4

pH:

7

Duration:

14 d

% Recovery:

82

pH:

10

Duration:

14 d

pH:

7

DT50:

> 14 d

Type:

not specified

pH:

4

DT50:

> 14 d

Type:

not specified

pH:

10

DT50:

> 14 d

Type:

not specified

Hydrolysis at pH 4:

After 14 days 60 mol% of the SiOCH₃groups remained intact and 40 mol% were hydrolyzed:

Hydrolysis at pH 7:

After 14 days 94.4 mol% of the SiOCH₃groups remained intact and only 5.6 mol% were hydrolysed.

Hydrolysis at pH 10:

After 14 days 82 mol% of the SiOCH₃groups remained intact and 18 mol% were hydrolysed.

Table 1: Hydrolysis speed at different pH values over time.

	% Hydrolysis (= mol % methanol)
Time [d]	pH 7	pH 10	pH 4
0	0	1.3	5.8
1	0,6	4.6	7.9
2	1	5.4	10.7
3	1.2	7.8	13.2
4	1.6	-	17.6
5	-	-	21.8
6	-	10.5	23.9
7	2.5	11.8	26.6
8	2.8	13.3	28.9
9	3.2	13.8	31.5
10	3.7	14.5	32.4
11	4.3	-	35
12	-	-	37.4
13	-	17.7	39.1
14	5.6	18	40.1

Validity criteria fulfilled:

not applicable

Reference 2

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

4 (not assignable)

Rationale for reliability incl. deficiencies:

documentation insufficient for assessment

Qualifier:

no guideline followed

Principles of method if other than guideline:

- Principle of test: the hydrolysis was determined by 1H-NMR-Spectroscopy. Since not all alcoxysilanes were water soluble, aceton was used as a solubility agent. The hydrolysis reaction was assumed to be a (pseudo) 1st order reaction.

GLP compliance:

no

Details on test conditions:

TEST SYSTEM
- Type, material and volume of test flasks, other equipment used: glass equipment and NMR tubes pretreated with hexamethyl disilazane

TEST MEDIUM
- Identity and concentration of co-solvent: in order to ensure sufficient solubility of the alkyl triakoxy silanes and avoid demixing during the hydrolysis tests, an organic solvent was added to the reaction mixture (acetone).

Transformation products:

not specified

pH:

4

DT50:

0.7 h

Type:

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

Remarks on result:

other: with HCl

Remarks:

K = 1.04/h, for further results see "any other information on results incl. tables"

pH:

4

DT50:

0.3 h

Type:

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

Remarks on result:

other: with HAc

Remarks:

K = 2.45/h, for further results see "any other information on results incl. tables"

pH:

7

DT50:

> 690 h

Type:

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

Remarks on result:

other: with H2O

Remarks:

K < 0.001, for further results see "any other information on results incl. tables"

Other kinetic parameters:

Rate constants (stirring test)
2% silane, pH 4 (HCl) K = 1.04/h
2% silane, pH 4 (HAc) K = 2.45/h
2% silane, pH 7 (H2O) K < 0.001

pH was found to be one of the key factors influencing the hydrolysis rate. The reaction rate was ten times higher in acetic acid solution on transition from pH 4 to 3. The reaction rate was even higher on transition from neutral to acidic medium. The authors interpreted this effect as an influence due to the catalytic effect of H+ ions.

Furthermore it is stated by the authors that the acid form (organic vs. inorganic acid) influences additionally the hydrolysis rate.

A mycelium formation was observed despite the solubilizing agent acetone. This is assumed to partly inhibit the water molecules to "attack" the Si-atom. It is assumed that in the tests performed with stirring, the micelle structures partly brake.

Validity criteria fulfilled:

not specified

Reference 3

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Qualifier:

no guideline available

GLP compliance:

not specified

Radiolabelling:

no

Transformation products:

yes

No.:

#1

Remarks on result:

other: for results see field: any other information on results incl. tables

RESULTS:

Test 1: Mixing test in glass flask and 1H-NMR Analytics

Result: During the first measurement 70 mg/kg of the parent substance was detected. During repeated measurements after one and two days (same sample) lower amounts of the parent substance were detected while an increase of Methanol was observed indicating hydrolysis. The signal of CH2-protons also decreased, which could be attributed to adsorption on the glass flask or phase separation.

Measurement via 1H-NMR	Parent substance	Methanol
After preparation of sample	67 mg/kg	< LOD
after 1 day	30 mg/kg	2 mg/kg
after 2 days	25 mg/kg	3 mg/kg

Test 2: Mixing in synthetic flasks and 1H-NMR Analytics

Result: during the first measurement 3 mg/kg of the parent substance was detected. The signal of the CH2-proton could still be detected. During the repeated measurement after three days (same sample) the signal of CH2-protons was not detectable. The disappearance of the CH2-signal implies either an adsorption of the substance on the teflon-inliner or to a phase separation. No silanol signal was detectable in the NMR-spectrum. 1 mg/kg methanol respects 3-4 mg/kg parent substance. (under the assumption of a full hydrolysis). This value is in the same order of magnitude with the 3 mg/kg of parent substance that was measured at the beginning. The result is obviously lower than the 70 mg/kg value found in the previous experiment (with glass flask). This could be attributed to the altered boundary conditions and fluctuations due to the manual mixing. The formation of micro-emulsions during shaking is not ruled out. 

 

Measurement via 1H-NMR	Parent substance	Methanol
After preparation of sample	3 mg/kg	< LOD
after 3 days	< LOD (1.5 mg/kg)	1 mg/kg

Test 3:  Slow stirring technique in glass flasks and 1H-NMR Analysis

Result after 3 d experimental duration: during the experiment no cloudiness or precipitation was observed. The substance could not be detected < 1 mg/kg. The determination of methanol (1 mg/kg) indicates hydrolysis. 

Result after 7 d experimental duration: during the experiment no precipitation was observed. An organic film was observed at the glass flask walls in the area of the substance (evidence for adsorption on the glass walls). The substance could not be determined in the water solution. The detection of methanol: slightly higher in comparison to the 3 days result.

Test 4:   Slow stirring technique in silylated glass flasks and 1H-NMR Analysis

Result: during the experiment no cloudiness or precipitation was observed. The substance could not be detected < 0.2 mg/kg (lower detection limits in comparison to the previous experiment due to higher number of scans).

Measurement via 1H-NMR	Parent substance (LOD = 0.2 ppm)	Methanol (LOD = 0.1 ppm)
After preparation of sample	< LOD	0.2 mg/kg
After 5 – 6 h	< LOD	0.3 mg/kg
1 d	< LOD	0.4 mg/kg
2 d	< LOD	0.4 mg/kg
5 d	< LOD	< LOD
7 d	< LOD	< LOD

Test 5:  Slow stirring technique in silylated glass flasks and Extraction/GC Analysis

Result: the substance could not be detected in any extracted sample (< 0.1 mg/kg). During the experiment no cloudiness or precipitation was observed.

Remark: the samples (in n-heptane) could not be measured directly after sampling due to the methodological development. Samples were measured 1-2 weeks later. Adsorption of the substance on glass is not excluded. 

Test 6:

Used D2O solution: 7.1 mg of TSP-d4 sodium saltin 4997.9 mg D2O

Substance	pH	Concentration at the beginning (wt%)	Sample addition	Start of the measurement	Half-life[sec]
229056 Silan 25013 VP Charge: QB15693	7	0.12	9:43:46	9:45:00	not determinable, as solubility too low

Conclusions:

- The hydrolysis at pH 7 is very slow

- The starting concentration of the silan decreases within 22 h to almost 0, which means that the "sample solution" decomposes/demixes

- An increase of the MeOH concentration is not determined

Remark: the weak signal occurring at the 3 ppm concentration is not assumed to be due to MeOH since this signal is expected to appear at 3.3 ppm. Assuming nonetheless that it was MeOH despite the unusual chemical shift of approx. 3 ppm, and a complete hydrolysis of the silan [(MeO)3SiR → (HO)3SiR], this would correspond to 3% hydrolysed silan. If (MeO)3SiR → (HO)(MeO)SiR, this corresponds to 9%. The occurred hydrolysis products should in this case be considered as precipitated.

- The silan is barely water soluble

In conclusion under the given conditions and the observation timeframe of 22 h, although a decrease of the CH3OSi-signal could be determined the formation of the respective amount of methanol could not be observed. However there was evidence for the occurence of a short amount of methanol. These observations were interpreted as a decomposition/demixing of a mikro-emulsion.

Overall conclusion:

The experiments indicate a slow hydrolysis process of the substance in neutral aqueous medium. Under the given conditions and an experimental duration of 22 h a reduction of the „CH3OSi“-signal was identified but not the formation of the respective amount of methanol. Only small amounts of methanol could be measured. The results are interpreted as a segregation process of a micro-emulsion. 

Validity criteria fulfilled:

not applicable

Description of key information

DT50 > 14 d at pH 4 -10

Additional information

One key experimental study is available testing the hydrolysis half-life of hexadecyl(trimethoxy)silane (CAS No. 16415-12-6) at pH 4, 7 and 10 according to the method developed by Brand et al. 1999.

1NMR-Spectra were measured during 14 d. The study showed that the hydrolysis of the substance at pH 7 is slow in comparison to other trialkoxysilanes. The degree of hydrolysis after 2 weeks was only 5.6%. Higher or lower pH values increased the velocity of hydrolysis but still only up to 40% of the original substance was hydrolysed at pH 4 and 18% at pH 10 after 14 days.

In conclusion the half-life of the substance at pH 4, 7 and 10 was > 14 d.

 

A supporting study is available, in which the hydrolysis kinetics was tested under various conditions and analytically monitored via 1NMR-spectroscopy or GC analysis (2003). A slow hydrolysis was observed, which confirms the outcome of the key study.

 

Furthermore, supporting information is available from a publication, in which the hydrolysis of functional trialkoxysilanes was studied under various conditions by 1H NMR spectroscopy in a water/acetone system (1999). The study showed that the reaction is first order and pH was found to be one of the key factors influencing the hydrolysis rate. The hydrolysis kinetics at pH 7 were slow and increased with decreasing pH. 

 

The hydrolysis products are hexadecylsilanetriol and methanol.