1,1,1,3,3,3-hexamethyldisilazane

Vapour pressure

Link to relevant study record(s)

Reference

Endpoint:

vapour pressure

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 using a relevant method, but not in compliance with GLP

Principles of method if other than guideline:

Vapour pressure data were obtained for hexamethyldisilazane (999-97-3) by Wiltec’s low residence time flow apparatus.

GLP compliance:

not specified

Type of method:

other: Wiltec’s low residence time flow apparatus

Key result

Test no.:

#1

Temp.:

20 °C

Vapour pressure:

1 900 Pa

Key result

Test no.:

#2

Temp.:

25 °C

Vapour pressure:

2 400 Pa

Key result

Test no.:

#3

Temp.:

50 °C

Vapour pressure:

7 400 Pa

The results of the critical point measurements are given in Table 1 below. This table lists the measured and estimated
critical temperature (Tc) and pressure (Pc). The estimated uncertainties are also listed with each measured value. The
estimated Tc and Pc are based on atomic contributions (see the first order method of Wilson and Jasperson, as given in
Poling et al., 2001) using the boiling point obtained as part of this project.

Table 1: Results of the critical point measurements for hexamethyldisilazane:

	Critical temperature, Tc (K)	Pressure, Pc (Psia)
Measured	556.3±1.5	277.9±2
Estimated (a)	545	282

Tc,K:
Measured = 556.3 +/- 1.5
Estimated (a) = 545

Pc, psia:
Measured = 277.9 +/- 2%
Estimated (a) = 282

Where 
(a) = Estimated by the first order method of Wilson and Jasperson (Poling et al., 2001) using the boiling point obtained as part of this project.

Table 2 reports the measured vapor pressure data for hexamethyldisilazane. This table lists the measured temperatures and the measured and correlated vapor pressures. Measured literature values (DIPPR 2003) are also
included in this table. The data are fitted to the Riedel equation using four parameters, and the resulting parameters
are given at the bottom of Table 2. The Riedel equation has the following form.

Ln(P, psia) = A + (B/T, K) + Cln(T,K) + D(T,K)^6

where A, B, C, and D are parameters used to obtain the best fit of the measured data. Both the data measured at Wiltec
and the literature data were used to obtain the reported correlation. The vapor pressures measured at Wiltec
exhibited more scatter than is usually obtained with this apparatus. The vapor pressures measured at Wiltec are
estimated to be accurate to within +/- 1.5 K in the temperature and +/-5% in the pressure.

Table 2: Vapor Pressure of Hexamethyldisilazane
     

T (K)	Pressure measured (pisa)	Correlated	% Dev
556.3	277.9	280.0	0.76% Tc, Pc
556.0	275.0	278.9	1.44% This work
555.3	270.4	275.9	2.02%
547.9	200.6	246.0	-1.68%
531.6	200.6	191.0	-4.77%
524.3	172.5	170.4	-1.22%
494.7	102.6	105.5	2.88%
457.5	52.41	54.22	3.46%
410.9	19.91	19.59	-1.58% DIPPR, 2003
405.7	17.40	17.19	-1.20%
399.4	14.69	14.56	-0.86%
390.9	11.59	11.54	-0.43%
377.4	7.723	7.735	0.16%
368.4	5.793	5.813	0.34%
356.7	3.865	3.882	0.46%
349.0	2.901	2.923	0.77%
338.7	1.937	1.942	0.25%
333.3	1.550	1.548	-0.09%
330.2	1.357	1.356	-0.05%
326.7	1.163	1.160	-0.29%
322.8	0.969	0.969	-0.08%

Where:
Reidel parameters:
        A = 83.9199
        B = -8098.77
        C = -10.1938
        D = 0.240239 x 10^(-16)
        Normal boiling point: 399.7 K

Extrapolation of the data presented in this robust summary indicates: Vapor pressure at 25°C of approximately 0.32 PSIA, 2.2x10E3 Pa, 16.5  Torr, 0.022 Atm, 16.5 mmHg (22 hPa)

Conclusions:

Vapour pressure values of 1900 Pa, 2400 Pa and 7400 Pa at 20°C, 25°C and 50°C respectively were determined for the substance in a reliable study conducted according to generally accepted scientific principles. The result is considered reliable.

Description of key information

Vapour pressure [1,1,1,3,3,3-hexamethyldisilazane]: 1900 Pa at 20°C, 2400 Pa at 25°C

Vapour pressure [trimethylsilanol]: 1300 Pa at 20°C, 1900 Pa at 25°C (EU Method A.4)

Vapour pressure [ammonia]: 861100 Pa at 20°C

Key value for chemical safety assessment

Vapour pressure:

2 400 Pa

at the temperature of:

25 °C

Additional information

Measured vapour pressure values of 1900 Pa at 20°C, 2400 Pa at 25°C and 7400 Pa at 50°C were determined for the substance using a relevant test method. The results are considered to be reliable and is selected as key study.

The key study is supported by a vapour pressure value of 10500 Pa at 50°C which was reported for the substance in a handbook or collection of reliable data which has been subject to peer-review and in which the original sources are traceable. Another study reported a vapour pressure of 10100 Pa at 20°C using the Carius tube method, however, this result is considered high for this substance considering the molecular weight (161.4 g/mol) and boiling point (125°C) of the substance.

In other secondary sources to which reliability could not be assigned, vapour pressure values of 1200 - 3066 Pa at 20 - 25°C were reported for the substance. These values indicate that the substance is a moderately volatile substance. The presence of volatile impurities/reaction products (ammonia) may cause vapour pressure measurements to be higher than the true value for the substance. Therefore, the key study was selected as the study that gives a true indication of the vapour pressure of the substance with reference to other available data.

The submission substance hydrolyses very rapidly in contact with water to form trimethylsilanol and ammonia. Therefore, for the purpose of chemical safety assessment, the vapour pressure of the silanol hydrolysis products is also relevant.

Vapour pressure values of 1300 Pa at 20°C and 1900 Pa at 25°C were determined for trimethylsilanol in accordance with the Method EU Method A.4.

Ammonia has a reported vapour pressure of 861100 Pa at 20°C [OECD SIDS (2007). SIDS Initial Assessment Report for SIAM 24, Paris, France, 17-20 April 2007, Ammonia Category]

Reference:

OECD SIDS (2007). SIDS Initial Assessment Report for SIAM 24, Paris, France, 17-20 April 2007, Ammonia Category