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Physical & Chemical properties

Vapour pressure

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vapour pressure
Type of information:
Adequacy of study:
key study
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 is an adaptation of the existing SRC model MPBPVPWIN v 1.43, which is a component of the EPI Suite. This existing model uses the normal boiling point as input. Whilst this method is good in principle, the model was developed using a wide range of organic chemicals and only a few organosilicon compounds. Therefore, a validation procedure was undertaken to assess the applicability of the model to organosilicon compounds. It was noted that the MBBPVPWIN model gave a systematic error; therefore, the current model was developed to correct this. The current model is a linear regression based QSAR, with the vapour pressure prediction from MPBPVPWIN as the descriptor. The adapted model applies to organosilicon substances
Key result
Test no.:
25 °C
Vapour pressure:
0.11 Pa
A vapour pressure value of 0.11 Pa at 25°C has been determined for the substance using an appropriate estimation method. The result is considered to be reliable.

Description of key information

Vapour pressure [triethoxy(octyl)silane]: 0.11 Pa at 25°C (QSAR)

Vapour pressure [octylsilanetriol]: 2.7E-05 Pa at 25°C (QSAR)

Vapour pressure [ethanol]: 7910 Pa at 25°C

Key value for chemical safety assessment

Vapour pressure:
0.11 Pa
at the temperature of:
25 °C

Additional information

A measured vapour pressure of 170 Pa at 20°C was determined for the substance using the static method in accordance with OECD Test Guideline 104 and in compliance with a known quality system (ISO 9001:2008) (Haas 2012). This measured result is considered too high for triethoxy(octyl)silane and is therefore disregarded due to the following reasons:


- fewer temperature determinations over a very narrow range was performed, three temperature points in total were used


- the requirement to repeat the vapour pressure determinations at low temperatures was not performed


- no evidence that the system is completely degassed, since the method used (static method) is susceptible to overestimate vapour pressure of a substance if volatile impurities are not properly degassed before the measurements are taken.


Similarly, vapour pressure values of <10 Pa at 20°C and 2900 Pa at 150°C were obtained for the substance using a dynamic method (AQura 2010). However, the reliability of the study is unassignable because (i) the purity of the test substance was not reported (ii) vapour pressure was obtained at ambient temperature by extrapolation from higher temperature of 128-259°C. Extrapolation from such high temperature is prone to significant error.


The structural analogue of the substance; trimethoxy(octyl)silane (CAS 3069-40-7) has a measured vapour pressure of 2.1 Pa at 20°C using the dynamic method according to EU Method A.4. Therefore, the measured results for triethoxy(octyl)silane (CAS 2943-75-1) are not used for assessment purposes.


A predicted vapour pressure of 0.11 Pa at 25°C was determined for the registered substance using a validated QSAR estimation method; the result is considered to be reliable and is selected as key study. The QSAR value has been validated using reliable measurements for structural analogues reflecting the main constituent in the registered substance.


In the other available secondary sources to which no reliability could be assigned, vapour pressure values of <133 Pa and ca. 10 Pa at 20°C were reported for the substance. Differences in vapour pressure determination could be as a result of differences in the purity of the commercial samples tested and the test method used.


In contact with water, triethoxy(octyl)silane will hydrolyse at a moderate rate to form octylsilanetriol and ethanol.


The vapour pressure of the silanol hydrolysis product, octylsilanetriol was determined to be 2.7E-05 Pa at 25°C using a validated QSAR estimation method.


Ethanol has a reported vapour pressure of 7910 Pa at 25°C (Daubert, T E and Danner R P (1985))




Daubert, T E and Danner R P (1985).Physical and Thermodynamic properties of Pure Chemicals. Data Compilation)