Triethoxy(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)silane

Hydrolysis

Link to relevant study record(s)

Reference

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

key study

Study period:

2015-11-23 to 2016-02-10

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)

Version / remarks:

2004

Deviations:

no

Qualifier:

according to guideline

Guideline:

EU Method C.7 (Degradation: Abiotic Degradation: Hydrolysis as a Function of pH)

Version / remarks:

Council Regulation (EC) No. 440/2008

Deviations:

no

GLP compliance:

yes (incl. QA statement)

Radiolabelling:

no

Analytical monitoring:

yes

Details on sampling:

Samples were taken at test start and additionally at 9 to 11 spaced points, respectively, normally between 10 and 90% of hydrolysis. At each sampling one further replicate was retained (-18±2°C) and stored for 6 months after study finalization for possible determination of degradation products. All test item containing samples were analysed immediately (max. 1% of total incubation time until start of analyses) via LC-MS. To avoid adsorption, test containers were emptied and 1 mL acetonitrile was added. Afterwards the test containers were strongly shaken, treated with ultrasound for 2 minutes and analysed.

The incubation temperature was checked automatically once per hour and at least once per day manually

Buffers:

Sterile buffer solutions with pH values 4, 7 and 9 (20, 30 and 50°C).


Buffer solution pH 4 0.18 g of NaOH and 5.7555 g of mono potassium citrate were dissolved in 500 mL double distilled water.

Buffer solution pH 7 0.7358 g of NaOH and 4.3012 g of KH2PO4 were dissolved in 500 mL double distilled water.
The usage of TRIS buffer is not feasible due to the immiscibility of the buffer with organic solvents.

Buffer solution pH 9 0.426 g NaOH, 1.8638 g KCl and 1.5458 g H3BO3 were dissolved in 500 mL double distilled water.

Sterile buffer solutions were prepared from chemicals with analytical grade or better quality following the composition guidance given in “KÜSTER-THIEL, Rechentafeln für die Chemische Analytik” and the OECD Guideline No. 111, respectively, by direct weighing of the buffer components. Buffers were purged with nitrogen for 5 min and then the pH were checked to a precision of at least 0.1 at the test temperatures. Sterilisation of buffer solutions was not deemed necessary due to the rapid hydrolysis

Details on test conditions:

Co-solvent: Acetonitrile, 5% (v/v); due to the test item properties
Test container: HPLC vials, polypropylene, volume: 1.5 mL
Test volume: 1 mL
Test concentration / solution 800 µg/L The water solubility was determined to be at least 3.7 mg/L (non GLP preliminary test).

Application: Via stock solution the test item was dissolved in dried acetonitrile. Sterile buffer solutions were spiked to a test item concentration of 800 µg/L and filled into the test containers. After the vials were sealed they were transferred into the thermostat. The time between test item application and transfer to thermostat / analysis did not exceed 30 min.

Incubation time: The test was conducted for at least two half live periods, optimally until 90% degradation is reached or 30 days, whichever was shorter.

Temperature: 20, 30 and 40±0.5°C

Light: Photolytic effects were precluded by avoidance of direct sunlight and the use of opaque water baths

Sterility: No sterility check was deemed necessary due to the short incubation times

Equipment:
Autoclave, HST 32/25, ZIRBUS
Balances, SARTORIUS and KERN
Clean bench, NUNC
Multipette xstream, EPPENDORF
pH-Meter, Lab850, SCHOTT
Piston stroke pipette, 200–1000 µL, THERMO SCIENTIFIC
Sterile vacuum filtration system 0.2 µm, SARSTEDT
Thermometers, GMH 3750 and temperature sensing devices, GREISINGER ELECTRONIC GMBH
Thermostat, LAUDA and MEDINGLAB TEMPERIERTECHNIK
Standard laboratory equipment

Sample Preparation:
Standards: For calibration a stock solution of 1 g test item/L in dried acetonitrile was diluted with dried acetonitrile to 8 concentrations, respectively in the range of 10 to 1000 µg/L.

Preparation of the samples: All samples were analysed directly. To avoid adsorption test containers were emptied and 1 mL acetonitrile was added. Afterwards the test containers were strongly shaken, treated with ultrasound for 2 minutes and analysed.

Preparation of fortified Samples: Five replicates of each buffer solution were fortified with test item stock solutions to concentrations of 1 x LOQ and 10 x LOQ and analysed. For further information concerning the preparation see “Preparation of the samples.

Blank samples: Two blank samples were prepared in the same manner of the fortified samples without the spiking step and analysed

Duration:

9.98 h

pH:

4

Temp.:

20 °C

Initial conc. measured:

704 µg/L

Duration:

7.58 h

pH:

4

Temp.:

30 °C

Initial conc. measured:

671 µg/L

Duration:

2.72 h

pH:

4

Temp.:

40 °C

Initial conc. measured:

940 µg/L

Duration:

76.3 h

pH:

7

Temp.:

20 °C

Initial conc. measured:

808 µg/L

Duration:

32.4 h

pH:

7

Temp.:

30 °C

Initial conc. measured:

808 µg/L

Duration:

23.8 h

pH:

7

Temp.:

40 °C

Initial conc. measured:

920 µg/L

Duration:

24.2 h

pH:

9

Temp.:

20 °C

Initial conc. measured:

937 µg/L

Duration:

10.9 h

pH:

9

Temp.:

30 °C

Initial conc. measured:

659 µg/L

Duration:

6.45 h

pH:

9

Temp.:

40 °C

Initial conc. measured:

845 µg/L

Number of replicates:

Duplicates per pH value and sampling time, single injected

Positive controls:

no

Negative controls:

no

Transformation products:

not measured

% Recovery:

9.53

pH:

4

Temp.:

20 °C

Duration:

10 h

% Recovery:

7.35

pH:

4

Temp.:

30 °C

Duration:

7.58 h

% Recovery:

7.71

pH:

4

Temp.:

40 °C

Duration:

2.72 h

% Recovery:

7.54

pH:

7

Temp.:

20 °C

Duration:

76.3 h

% Recovery:

11.1

pH:

7

Temp.:

30 °C

Duration:

32.4 h

% Recovery:

11

pH:

7

Temp.:

40 °C

Duration:

23.8 h

% Recovery:

13.1

pH:

9

Temp.:

20 °C

Duration:

24.2 h

% Recovery:

8.94

pH:

9

Temp.:

30 °C

Duration:

10.9 h

% Recovery:

4.69

pH:

9

Temp.:

40 °C

Duration:

6.45 h

Key result

pH:

4

Temp.:

20 °C

Hydrolysis rate constant:

0 s-1

DT50:

1.44 h

Type:

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

Remarks on result:

other: Confidence interval: 0.788 to 1.96; half-life reported for the solvated fraction

Key result

pH:

4

Temp.:

30 °C

Hydrolysis rate constant:

0 s-1

DT50:

0.9 h

Type:

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

Remarks on result:

other: Confidence interval: 0.747 to 1.04; half-life reported for the solvated fraction

Key result

pH:

4

Temp.:

40 °C

Hydrolysis rate constant:

0 s-1

DT50:

0.474 h

Type:

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

Remarks on result:

other: Confidence interval: 0.249 to 0.642; half-life reported for the solvated fraction

Key result

pH:

7

Temp.:

20 °C

Hydrolysis rate constant:

0 s-1

DT50:

12.5 h

Type:

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

Remarks on result:

other: Confidence interval: 10.6 to 14.3; half-life reported for the solvated fraction

Key result

pH:

7

Temp.:

30 °C

Hydrolysis rate constant:

0 s-1

DT50:

5.25 h

Type:

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

Remarks on result:

other: Confidence interval: 4.37 to 6.12; half-life reported for the solvated fraction

Key result

pH:

7

Temp.:

40 °C

Hydrolysis rate constant:

0 s-1

DT50:

2.74 h

Type:

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

Remarks on result:

other: Confidence interval: 2.37 to 3.11; half-life reported for the solvated fraction

Key result

pH:

9

Temp.:

20 °C

Hydrolysis rate constant:

0 s-1

DT50:

5.18 h

Type:

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

Remarks on result:

other: Confidence interval: 4.37 to 6.12; half-life reported for the solvated fraction

Key result

pH:

9

Temp.:

30 °C

Hydrolysis rate constant:

0 s-1

DT50:

2.61 h

Type:

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

Remarks on result:

other: Confidence interval: 1.31 to 3.63; half-life reported for the solvated fraction

Key result

pH:

9

Temp.:

40 °C

Hydrolysis rate constant:

0 s-1

DT50:

1.38 h

Type:

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

Remarks on result:

other: Confidence interval: 1.15 to 1.59; half-life reported for the solvated fraction

Method validation:

Linearity:

The analytical system gave linear response for the test item in the range of 10 – 1000 µg/L. The coefficients of determination (r²) of all calibration curves were > 0.992

System quantification limit:

The system quantification limit was fixed at 10 µg/L for the analytical standardverified by an S/N of 271.

Limit of quantification (LOQ):

The limit of quantification of theanalytical method(LOQ) was fixed at 40 µg/L. The 1 x LOQ as well as the 10 x LOQ were checked by means of accuracy.

Accuracy and precision:

The results in Table 1 to Table 3 demonstrate the validity of the analytical method. Mean recovery rates of 94% at the 1 x LOQ and 113% at the 10 x LOQ for the test item at pH 4, mean recovery rates of 110% at the 1 x LOQ and 118% at the 10 x LOQ at pH 7 as well as mean recovery rates of 102% at the 1 x LOQ and 109% at the 10 x LOQ at pH 9 were obtained, showing sufficient accuracy of the analytical method.

The corresponding coefficients of variation were ≤ 20% and showing a sufficient precision of the analytical method.

Table 1: Recovery Rates of Fortified Samples at pH 4

Fortified concentrations:40 µg/L (1 x LOQ) and 400 µg/L (10 x LOQ)

Replicate	triethoxy(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)silane
	1 x LOQ		10 x LOQ
	Calc. conc. [µg/L]	RR [%]	Calc. conc. [µg/L]	RR [%]
1	36.2	90	465	116
2	35.4	88	444	111
3	44.6	111	450	113
4	33.7	84	445	111
5	37.4	93	462	115
Mean	37.4	94	453	113
SD	4.2		10
CV %]	11.3		2.1

 

Table 2: Recovery Rates of Fortified Samples at pH 7

Fortified concentrations:40 µg/L (1 x LOQ) and 400 µg/L (10 x LOQ)

Replicate	triethoxy(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)silane
	1 x LOQ		10 x LOQ
	Calc. conc. [µg/L]	RR [%]	Calc. conc. [µg/L]	RR [%]
1	42.9	107	474	118
2	45.2	113	483	121
3	46.7	117	474	119
4	44.5	111	470	117
5	41.0	103	467	117
Mean	44.1	110	474	118
SD	2.2		6
CV %]	4.9		1.3

 

Table 3: Recovery Rates of Fortified Samples at pH 9

Fortified concentrations:40 µg/L (1 x LOQ) and 400 µg/L (10 x LOQ)

Replicate	triethoxy(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)silane
	1 x LOQ		10 x LOQ
	Calc. conc. [µg/L]	RR [%]	Calc. conc. [µg/L]	RR [%]
1	42.1	105	457	114
2	37.3	93	447	112
3	41.1	103	436	109
4	41.1	103	423	106
5	43.0	108	425	106
Mean	40.9	102	438	109
SD	2.2		15
CV %]	5.3		3.3

Calc.Conc. = Calculated concentration (sum of aqueous phase and acetonitrile phase)

RR = Recovery rate regarding to the fortified concentration of the test item

SD = Standard deviation

CV= Coefficient of variation

Specificity:

For each sample, two transitions of the same precursor ion (one quantifier used for evaluation and one secondary used for confirmation of the analyte identity) were measured, confirming the test item identity. No significant signal was observed for the blank samples, confirming the specificity of the analytical method.

Check of pH values:

Table 4: pH-Value of the Test Systems, measured before start of hydrolysis

Intended pH value	Measured pH value at 20°C	Measured pH value at 30°C	Measured pH value at 50°C
4.0 ± 0.1	4.03	4.03	4.04
7.0 ± 0.1	7.00	7.00	7.00
9.0 ± 0.1	9.01	9.01	9.01

Temperature monitoring:

Table 5: Incubation Temperature, measured every hour

pH value	Intended temperature	measured temperature
		Mean ± SD	Min. / Max
4	20.0 ± 0.5 30.0 ± 0.5 40.0 ± 0.5	20.0 ± 0.03 30.0 ± 0.08 40.0 ± 0.04	19.9/20.0 29.9/30.1 39.9/40.0
7	20.0 ± 0.5 30.0 ± 0.5 40.0 ± 0.5	20.0 ± 0.05 30.1 ± 0.07 40.0 ± 0.09	19.9/20.3 29.7/30.1 39.7/40.2
9	20.0 ± 0.5 30.0 ± 0.5 40.0 ± 0.5	20.1 ± 0.07 30.0 ± 0.02 40.0 ± 0.02	20.0/20.4 30.0/30.1 40.0/40.1

SD – Standard deviation

 

The additional manually taken values confirm the results of the automated temperature recording of the datalogger.

Hydrolysis results:

Hydrolysis was evaluated based on the sum of the test item in the solved (aqueous) and adsorbed fractions from duplicates. Where necessary, extrapolated values below the calibration range were taken into account. This was deemed to be acceptable due to the usage of calibration curves forced through zero.

 

Table 6: Hydrolysis Results for the Test Item at pH 4 and 20°C

Hydrolysis Time [h]	Concentration* [µg/L]			Ln Concentration
	Solved Fraction	Adsorbed Fraction	Sum
0.000	590	115	704	6.56
0.467	600	177	777	6.66
1.05	509	168	677	6.52
1.85	400	170	570	6.35
3.20	245	88.6	333	5.81
4.88	67.4**	172	240	5.48
5.95	37.5**	133	171	5.14
6.82	6.61***	104	111	4.71
7.28	1.17***	173	175	5.16
8.42	0.202***	119	119	4.78
9.98	0.600***	75.6	76.2	4.33

*Mean value of duplicates

** At least one replicate < LOQ (40 µg/L), but above 120% (12 µg/L) of the lowest calibration standard

***At least one replicate below 120% of the lowest calibration standard (extrapolated value)

Table 7: Hydrolysis Results for the Test Item at pH 4 and 20°C, solvated fraction

Hydrolysis Time [h]	Concentration^*[µg/L]	Ln Concentration
	Solvated Fraction
0.000	590	6.38
0.467	600	6.40
1.05	509	6.23
1.85	400	5.99
3.20	245	5.50
4.88	67.4**	4.21
5.95	37.5**	3.62
6.82	6.61***	1.89
7.28	1.17***	0.157
8.42	0.202***	-1.60
9.98	0.600***	-0.511

Table 8: Hydrolysis Results for the Test Item at pH 4 and 30°C

Hydrolysis Time [h]	Concentration* [µg/L]			Ln Concentration
	Solved Fraction	Adsorbed Fraction	Sum
0.000	467	205	671	6.51
0.333	486	172	659	6.49
0.617	398	173	571	6.35
6.35	346	177	524	6.26
1.15	269	174	443	6.09
1.42	217	174	391	5.97
1.80	134	191	325	5.78
2.58	76.9**	162	239	5.48
2.93	61.7	132	193	5.27
3.50	8.77***	119	128	4.85
4.73	0.00***	97.6	97.6	4.58
7.58	0.204***	58.6	58.8	4.07

Table 9: Hydrolysis Results for the Test Item at pH 4 and 30°C, solvated fraction

Hydrolysis Time [h]	Concentration* [µg/L]	Ln Concentration
	Solvated Fraction
0.000	467	6.15
0.333	486	6.19
0.617	398	5.99
0.883	346	5.85
1.15	269	5.59
1.42	217	5.38
1.80	134	4.90
2.58	76.9**	4.34
2.93	61.7	4.12
3.50	8.77***	2.17
4.73	0.00***	-
7.58	0.204***	2.17

 

Table 10: Hydrolysis Results for the Test Item at pH 4 and 40°C

Hydrolysis Time [h]	Concentration* [µg/L]			Ln Concentration
	Solved Fraction	Adsorbed Fraction	Sum
0.000	768	171	940	6.85
0.183	638	105	743	6.61
0.450	594	115	709	6.56
0.750	405	132	537	6.29
1.02	233	117	350	5.86
1.32	163	103	266	5.58
1.62	63.2	69.0	132	4.88
2.03	5.46***	79.7	85.2	4.44
2.37	0.501***	63.5	64.0	4.16
2.72	1.37***(	60.3	61.7	4.12

Table 11: Hydrolysis Results for the Test Item at pH 4 and 40°C, solvated fraction

Hydrolysis Time [h]	Concentration^*[µg/L]	Ln Concentration
	Solvated Fraction
0.000	768	6.64
0.183	638	6.46
0.450	594	6.39
0.750	405	6.00
1.02	233	5.45
1.32	163	5.09
1.62	63.2	4.15
2.03	5.46**	1.70
2.37	0.501**	-0.691
2.72	1.37**	0.315

Table 12:       Hydrolysis Results for the Test Item at pH 7 and 20°C

 

Hydrolysis Time [h]	Concentration^* [µg/L]			Ln Concentration
	Solved Fraction	Adsorbed Fraction	Sum
0.000	652	156	808	6.70
0.800	590	190	780	6.66
1.80	590	171	761	6.63
3.70	494	196	690	6.54
7.23	396	211	606	6.41
10.1	328	154	482	6.18
23.4	194	193	387	5.96
29.3	148	277	424	6.05
33.1	123	276	400	5.99
48.9	33.4^***	140	173	5.15
76.3	0.191^***	60.1	60.3	4.10

Validity criteria fulfilled:

yes

Conclusions:

Hydrolysis half-lives of the test substance has been determined using a relevant test method and in compliance with GLP. The result is considered to be reliable.

Executive summary:

 

Table 24: Reaction Rate Constants and Half-Lives at pH4

 

	pH 4
	20°C	30°C	40°C
Slope of regression graph	-0.239x	-0.359x	-1.12x
Correlation factor [r^2]	0.964	0.951	0.974
Reaction rate constantkobs[1/s]	6.63 x 10^-5	9.97 x 10^-5	3.12 x 10^-4
Half-lifeT½[h]	2.90	1.93	0.617
Confidence interval of half-lifeT½[h]	2.33 to 3.40	1.58 to 2.26	0.457 to 0.754
Half-lifeT½[d]	0.121	0.0805	0.0257
Confidence interval of half-lifeT½[d]	0.0971 to 0.142	0.0658 to 0.0942	0.0190 to 0.0314

 

Table 25: Reaction Rate Constants and Half-Lives at pH 4, Solvated Fraction

 

		pH 4
		20°C		30°C		40°C
Slope of regression graph		-0.483x		-0.771x		-1.46x
Correlation factor [r^2]		0.958		0.976		0.937
Reaction rate constantkobs[1/s]		1.34 x 10^-4		2.14 x 10^-4		4.06 x 10^-4
Half-lifeT½[h]		1.44		0.900		0.474
Confidence interval of half-lifeT½[h]		0.788 to 1.96		0.747 to 1.04		0.249 to 0.642
Half-lifeT½[d]		0.0600		0.0375		0.0198
Confidence interval of half-lifeT½[d]		0.0328 to 0.0817		0.0311 to 0.0433		0.0104 to 0.0268

 

 

Table 26: Reaction Rate Constants and Half-Lives at pH 7

 

	pH 7
	20°C	30°C	40°C
Slope of regression graph	-0.0315x	-0.0700x	-0.0922x
Correlation factor [r^2]	0.955	0.968	0.939
Reaction rate constantkobs[1/s]	8.76 x 10^-6	1.95 x 10^-5	2.56 x 10^-5
Half-lifeT½[h]	22.0	9.90	7.52
Confidence interval of half-lifeT½[h]	18.2 to 25.8	8.53 to 11.3	6.25 to 9.01
Half-lifeT½[d]	0.915	0.412	0.313
Confidence interval of half-lifeT½[d]	0.758 to 1.08	0.355 to 0.471	0.260 to 0.375

 

 

Table 27: Reaction Rate Constants and Half-Lives at pH 7, Solvated Fraction

 

		pH 7
		20°C		30°C		40°C
Slope of regression graph		-0.0553x		-0.132x		-0.253x
Correlation factor [r^2]		0.981		0.976		0.972
Reaction rate constantkobs[1/s]		1.54 x 10^-5		3.67 x 10^-5		7.03 x 10^-5
Half-lifeT½[h]		12.5		5.25		2.74
Confidence interval of half-lifeT½[h]		10.6 to 14.3		4.37 to 6.12		2.37 to 3.11
Half-lifeT½[d]		0.521		0.219		0.114
Confidence interval of half-lifeT½[d]		0.442 to 0.596		0.182 to 0.255		0.0988 to 0.130

 

Table 28: Reaction Rate Constants and Half-Lives at pH 9

 

	pH 9
	20°C	30°C	40°C
Slope of regression graph	-0.0924x	-0.207x	-0.457x
Correlation factor [r^2]	0.996	0.939	0.981
Reaction rate constantkobs[1/s]	2.57 x 10^-5	5.76 x 10^-5	1.27 x 10^-4
Half-lifeT½[h]	7.50	3.34	1.52
Confidence interval of half-lifeT½[h]	7.06 to 7.93	2.53 to 4.06	1.27 to 1.75
Half-lifeT½[d]	0.312	0.139	0.063
Confidence interval of half-lifeT½[d]	0.294 to 0.330	0.105 to 0.169	0.0519 to 0.0729

 

Table 29: Reaction Rate Constants and Half-Lives at pH 9, Solvated Fraction

 

	pH 9
	20°C	30°C	40°C
Slope of regression graph	-0.134x	-0.266x	-0.504x
Correlation factor [r^2]	0.975	0.889	0.978
Reaction rate constantkobs[1/s]	3.72 x 10^-5	7.39 x 10^-5	1.40 x 10^-4
Half-lifeT½[h]	5.18	2.61	1.38
Confidence interval of half-lifeT½[h]	4.37 to 6.12	1.31 to 3.63	1.15 to 1.59
Half-lifeT½[d]	0.216	0.109	0.0575
Confidence interval of half-lifeT½[d]	0.193 to 0.240	0.0546 to 0.151	0.0479 to 0.0663

 

Overall conclusion

Hydrolysis as a function of pH was determined according to OECD Guideline No. 111 and Council Regulation (EC) No. 440/2008, Method C.7 for the test itemtriethoxy(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)silane(batch number:2951503091) from 2015-11-23 to 2016-02-10 at the test facility in 31157 Sarstedt, Germany.

 

Analyses of the test item triethoxy(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)silane were performed via LC-MS/MS using the test item as external standard. The analytical method was validated with satisfactory results regarding linearity, accuracy, precision and specificity.

 

The test was conducted with a test item concentration of 800µg/L in buffer solution of pH 4, 7 and 9 at temperatures of 20, 30 and 40°C . Samples were taken at test start (0 h) and additionally at 9 to 11 spaced points until test end. Buffer solutions were analysed at test start and test end and there was no analytical interference with the test item. After analysis of the test solution, all test vessels were emptied and extracted with acetonitrile to determine the adsorbed fraction. The sum of the test item amounts analysed in the buffer solution and the adsorbed fraction were used for evaluation. Reaction rate constants and half-lives were calculated from the analysed samples based on a first / pseudo first order reaction kinetics model and are presented.

 

The test item showed a fast hydrolysis rate (t1/2≤ 2.4 h) for pH 4 at 30 and 40°C and for pH 9 at 40°C. For pH 4 at 20°C, pH 7 at 20, 30, and 40°C and pH 9 at 20 and 30°C, a moderate hydrolysis (t1/2≥ 2.4 h but ≤ 30 d) was observed.

 

In addition to the sum of the test item, the solvated fraction was evaluated separately. The solvated fraction of the test item showed a fast hydrolysis rate (t_1/2≤ 2.4 h) for pH 4 at 20, 30 and 40 °C and for pH 9 at 40 °C. For pH 7 at 20, 30 and 40 °C and pH 9 at 20 and 30 °C, a moderate hydrolysis (t_1/2≥ 2.4 h but ≤ 30 d) was observed.

Description of key information

Hydrolysis half-life: 1.44 h at 20°C, 0.9 h at 30°C, 0.474 h at 40°C and pH 4, 12.5 h at 20°C, 5.25 h at 30°C, 2.74 h at 40°C and pH 7, 5.18 h at 20°C, 2.61 h at 30°C, 1.38 h at 40°C and pH 9 (OECD 111)

Key value for chemical safety assessment

Half-life for hydrolysis:

12.5 h

at the temperature of:

20 °C

Additional information

The hydrolysis half-lives of the substance have been measured in accordance with OECD Test Guideline 111 and in compliance with GLP. Due to the low solubility of the registration substance, the half-lives were measured for the sum of solvated and adsorbed fractions (whole substance), and for the solvated fraction of the substance in aqueous solution. The half-life of the solvated fraction is used for evaluation.

 

The hydrolysis half-lives of the solvated fractions are:

pH 4.0 = 1.44 h at 20°C, 0.9 h at 30°C, 0.474 h at 40°C

 

pH 7.0 = 12.5 h at 20°C, 5.25 h at 30°C, 2.74 h at 40°C

 

pH 9.0 = 5.18 h at 20°C, 2.61 h at 30°C, 1.38 h at 40°C

 

As the hydrolysis reaction may be acid or base-catalysed, 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^-4 mol dm^-3 and at pH 2 [H₃O⁺] = 10 ^-2 mol dm^-3; therefore, k_obs at pH 2 should be approximately 100 times greater than k_obs at pH 4.

 

The half-life of a substance at pH 2 is calculated based on:

 

t_1/2(pH 2) = t_1/2(pH 4) / 100

 

The calculated half-life of the substance at pH 2 is therefore 0.0144 hours (approximately 52 seconds) at 20°C.

 

Reaction rate increases with temperature therefore hydrolysis will be faster at physiologically relevant temperatures compared to standard laboratory conditions. 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.

 

Thus, for [2-(perfluorohexyl)ethyl]triethoxysilane the hydrolysis half-life at 37.5°C and pH 7 (relevant for lungs and blood) is approximately 3.1 hours. At pH 7 and 40°C, measured half-life is 2.7 hours. At 37.5°C and pH 2 (relevant for conditions in the stomach following oral exposure), the half-life is 0.0036 hours (approximately 13 seconds).

 

The hydrolysis products in this case are [2-(perfluorohexyl)ethyl]silanetriol and ethanol.

 

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

 

Hydrolysis of the read-across substance [2-(perfluorohexyl)ethyl]dichloro(methyl)silane (CAS 73609-36-6)

Data for the substance [2-(perfluorohexyl)ethyl]dichloro(methyl)silane (CAS 73609-36-6) are read across to the registration substance [2-(perfluorohexyl)ethyl]triethoxysilane (CAS 51851-37-7) for appropriate endpoints. The hydrolysis half-life and the silanol hydrolysis product of the two substances are relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

 

For [2-(perfluorohexyl)ethyl]dichloro(methyl)silane, hydrolysis half-lives are read-across from analogous dichlorosilane substances. 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 (CAS 75-78-5) 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 (CAS 675-62-7) 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 (CAS 80-10-4) in accordance with OECD 111 (Dow Corning Corporation, 2001).

 

Measured hydrolysis half-lives of <<27 mins at pH 4, pH 7 and pH 9 and 27°C were determined for dichloro(cyclohexyl)methylsilane (CAS 5578-42-7) in accordance with EU method C.7 in analogy to OECD Test Guideline 111 (Haas, 2012). However, the substance was fully hydrolysed when the first NMR spectrum of each pH series was measured. Therefore, only a preliminary study was carried out on the registration substance. A more precise knowledge of the half-life is needed for use in the chemical safety assessment.

 

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 the lack of significant variation in the half-lives for the different substances, it is considered appropriate to read-across this result to [2-(perfluorohexyl)ethyl]dichloro(methyl)silane.

 

Since rate of hydrolysis is faster at increased temperature, at ambient conditions (20-25°C), relevant to the environment, the hydrolysis half-lives are expected to be faster. Additionally, the hydrolysis rate is consistently very rapid across the pH range of 4-9 and it is expected that the variations in the hydrolysis rate will be minimal across different pH values compared to the increased hydrolysis rate at higher temperatures. Under physiologically-relevant temperatures (37.5°C), and conditions relevant for oral (pH 2) and dermal exposure (pH 5.5), the hydrolysis rate is estimated to be << 17 seconds. However, it is not appropriate or necessary to attempt to predict accurately when the half-life of this magnitude. As a worst-case it can therefore be considered that the half-life of the substance at 25°C and pH 4, 7 and 9 (and all relevant physiologically conditions), is << 1 minute.

 

The hydrolysis products are [2-(perfluorohexyl)ethyl]methylsilanediol and hydrogen chloride.