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Vapour pressure

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Reference
Endpoint:
vapour pressure
Type of information:
(Q)SAR
Adequacy of study:
key study
Study period:
03 April 2018
Reliability:
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:
1. SOFTWARE
EPI Suite Version 4.11

2. MODEL (incl. version number)
MPBPVP (v1.43)

3. SMILES OR OTHER IDENTIFIERS USED AS INPUT FOR THE MODEL
CAS: 73246-85-2: CCCCCCCCCCCCOC(=O)CS[Sn](CCCCCCCC)(CCCCCCCC)SCC(=O)OCCCCCCCCCCCC
CAS: 79330-84-0: O=C(CS[Sn](SCC(=O)OCCCCCCCCCCCCCC)(CCCCCCCC)CCCCCCCC)OCCCCCCCCCCCCCC

4. SCIENTIFIC VALIDITY OF THE (Q)SAR MODEL
MPBPWIN estimates vapour pressure (VP) by three separate methods: (1) the Antoine method, (2) the modified Grain method, and (3) the Mackay method. All three use the normal boiling point to estimate VP. Unless the user enters a boiling point on the data entry screen, MPBPWIN uses the estimated boiling point from the adapted Stein and Brown method.
- Antoine Method: Chapter 14 of Lyman et al (1990) includes the description of the Antoine method used by MPBPWIN. It was developed for gases and liquids. The Antoine equation used to estimate vapour pressure from the normal boiling (Tb) is:
ln Pvp = [(ΔHvb(Tb - C2)2) / (ΔZbRTb2] [(1 / (Tb - C2)) - (1 / (T - C2))]
The parameter ΔZb is assumed to have the value of 0.97.
The constant C2 is estimated via Thomson's rule by:
C2 = -18 + 0.19Tb
The heat of vaporisation at the boiling point is evaluated by:
ΔHvb / Tb = ΔSvb = Kf(8.75 + RlnTb)
The KF structural factors are available in chapter 14 of Lyman et al (1990); the variation of this parameter is related to chemical class and is small (roughly 0.99 to 1.2), so large errors in its selection are unlikely (Lyman, 1985). The value of R is 1.987 cal/mol-K.
MPBPWIN has extended the Antoine method to make it applicable to solids by using the same methodology as the modified Grain method to convert a super-cooled liquid VP to a solid-phase VP as shown below.

- Modified Grain Method: Chapter 2 of Lyman (1985) describes the modified Grain method used by MPBPWIN. This method is a modification and significant improvement of the modified Watson method. It is applicable to solids, liquids and gases. The modified Grain method equations are:
ln Pl = [(Kfln(RTb)) / ΔZb] [1 - ((3 - 2Tp)^m / Tp) - 2m(3 - 2Tp)^m-1 lnTp)]
Where Pl = liquid vapour pressure (atm); Kf = structural factor); R = gas constant (82.057 cm3 atm/mol K); ΔZ = compressibility factor (=0.97); Tb = normal boiling point (K); T = temperature (K) Tp = T/Tb; and m = 0.4133 - 0.2575 Tp.
For solids, a second term is added to the equation so that ln Ps = ln Pl + in ΔPs
Where:
ln ΔPs = 0.6ln (RTm) [1 - ((3 - 2Tpm)^m / Tpm) - 2m(3 - 2Tpm)^m-1 lnTpm)] and Ps = solid vapour pressure (atm); ΔPs = decrease in solid vapour pressure versus that of super-cooled liquid (atm); Tm = melting point (K; Tpm = T/Tm; and m = 0.4133 - 0.2575 Tpm.
The KF structural factors are available in chapter 14 of Lyman et al (1990); the variation of this parameter is related to chemical class and is small (roughly 0.99 to 1.2), so large errors in its selection are unlikely (Lyman, 1985). The modified Grain method may be the best all-around VP estimation method currently available.

- Mackay Method: Mackay derived the following equation to estimate VP (Lyman, 1985):
ln P = -(4.4 + ln Tb)[1.803(Tb/T - 1) - 0.803 ln(Tb/T)] - 6.8(Tm/T - 1)
Where Tb is the normal boiling pt (K), T is the VP temperature (K) and Tm is the melting pt (K). The melting point term is ignored for liquids. It was derived from two chemical classes: hydrocarbons (aliphatic and aromatic) and halogenated compounds (again aliphatic and aromatic).
MPBPWIN reports the VP estimate from all three methods. It then reports a "suggested" VP. For solids, the modified Grain estimate is the suggested VP. For liquids and gases, the suggested VP is the average of the Antoine and the modified Grain estimates. The Mackay method is not used in the suggested VP because its application is currently limited to its derivation classes.

Estimation Accuracy
The accuracy of MPBPWIN's "suggested" VP estimate was tested on a dataset of 3037 compounds with known, experimental VP values between 15 and 30 °C (the vast majority at 25 or 20 °C). The experimental values were taken from the PHYSPROP Database that is part of the EPI Suite. For this test, the CAS numbers were run through MPBPWIN as a standard batch-mode run (using the default VP estimation temperature of 25 °C) and the batch estimates were compared to PHYSPROP's experimental VP.
The estimation methodology uses the normal boil point to estimate the liquid-phase vapour pressure. For solids, the melting point is required to convert the liquid-phase vapour pressure to the solid-phase vapour pressure. VP estimation error can be introduced by poor Boiling Point estimates or values and poor Melting Point estimates or values (for solids).
The 3037 compound test set contains 1642 compounds with available experimental Boiling points and Melting points. For this subset of compounds, the estimation accuracy statistics are (based on log VP):
- number = 1642
- r² = 0.949
- std deviation = 0.59
- avg deviation = 0.32
These statistics clearly indicate that VP estimates are more accurate with experimental BP and MP data.
For maximum VP accuracy, good experimental Boiling Points and/or Melting Points should be entered on the data entry screen (or available from the experimental databases included with the EPI Suite).

The complete vapour pressure test is available at: http://esc.syrres.com/interkow/EpiSuiteData.htm
Substructure searchable data set of vapour pressure test is available at: http://esc.syrres.com/interkow/EpiSuiteData_ISIS_SDF.htm

- Supercooled (Subcooled) Vapour Pressure
A supercooled or subcooled liquid is a liquid that is cooled below its normal freezing point without solidification. For solid compounds, the vapour pressure of the solid is less than the vapour pressure of the subcooled liquid. The current version of the MPBPWIN program calculates the subcooled vapour pressure of subcooled liquids in addition to the normal vapour pressure of the solid.
By default, MPBPWIN calculates the subcooled vapour pressure using the Modified Grain Method for estimating vapour pressures as presented in chapter 2 of Lyman (1985). This method uses one equation for estimating a liquid vapour pressure (based upon boiling point) and a second equation for converting the liquid vapour pressure to a solid vapour pressure for solids.
For compounds with no experimental vapour pressure data in the experimental database, the subcooled vapour pressure is the liquid value calculated by Modified Grain Method. For solid compounds that have an experimental vapour pressure in the database, the solid-phase vapour pressure is converted to a subcooled vapour pressure using the following equation (Bidleman, 1988):
Ln (Pl/Ps) = 6.79 ((Tm - T)/T)
where Pl is the subcooled liquid vapour pressure, Ps is the solid vapour pressure, Tm is the melting point (kelvin), T is ambient temperature (kelvin) and the 6.79 factor is an approximation for the entropy of fusion divided by the gas constant (ΔS/R).
For solids (defined as compounds with melting point > 25 °C), when MPBPWIN is run as part of the EPI Suite interface program, a "User Entered" value for vapour pressure is used to calculate the subcooled vapour pressure in preference to the value in the experimental database.

5. APPLICABILITY DOMAIN
Currently there is no universally accepted definition of model domain. However, users may wish to consider the possibility that property estimates are less accurate for compounds outside the Molecular Weight range of the training set compounds, and/or that have more instances of a given fragment than the maximum for all training set compounds. It is also possible that a compound may have a functional group(s) or other structural features not represented in the training set, and for which no fragment coefficient was developed. These points should be taken into consideration when interpreting model results.
The complete training sets for MPBPWIN's estimation methodology are not available. Therefore, describing a precise estimation domain for this methodology is not possible.
The current applicability of the MPBPWIN methodology is best described by its accuracy in predicting vapour pressure as described in the Accuracy section.

6. ADEQUACY OF THE RESULT
A value was estimated for this substance using the Modified Grain method; the value is therefore considered to be acceptable for a screening assessment.
Qualifier:
according to guideline
Guideline:
other: REACH Guidance on QSARs R.6
Version / remarks:
May/July 2008
Deviations:
no
GLP compliance:
no
Type of method:
other: calculation
Specific details on test material used for the study:
- Molecular weight
CAS: 73246-85-2: 864.02
CAS: 79330-84-0: 920.13
Key result
Temp.:
25 °C
Vapour pressure:
0 mm Hg
Remarks on result:
other: CAS 73246-85-2
Key result
Temp.:
25 °C
Vapour pressure:
0 Pa
Remarks on result:
other: CAS 73246-85-2
Key result
Temp.:
25 °C
Vapour pressure:
0 mm Hg
Remarks on result:
other: CAS 79330-84-0
Key result
Temp.:
25 °C
Vapour pressure:
0 Pa
Remarks on result:
other: CAS 79330-84-0

CAS 73246-85-2

Vapour Pressure Estimations (25 °C): Using BP: 741.60 °C (estimated) ans Using MP: 325.46 °C (estimated)

VP: 4.36E-027 mm Hg, 5.81E-025 Pa (Antoine Method)

VP: 5.11E-018 mm Hg, 6.82E-016 Pa (Modified Grain Method)

VP: 2.71E-017 mm Hg, 3.61E-015 Pa (Mackay Method)

Selected VP: 5.11E-018 mm Hg, 6.82E-016 Pa (Modified Grain Method)

Subcooled liquid VP: 1.27E-014 mm Hg, 1.69E-012 Pa (25 °C, Mod-Grain method)

CAS 79330-84-0

Vapour Pressure Estimations (25 °C): Using BP: 788.02 °C (estimated) and Using MP: 347.14 °C (estimated)

VP: 3E-031 mm Hg, 4E-029 Pa (Antoine Method)

VP: 1.47E-019 mm Hg, 1.96E-017 Pa (Modified Grain Method)

VP: 8.8E-019 mm Hg, 1.17E-016 Pa (Mackay Method)

Selected VP: 1.47E-019 mm Hg, 1.96E-017 Pa (Modified Grain Method)

Subcooled liquid VP: 6.94E-016 mm Hg, 9.26E-014 Pa (25 °C, Mod-Grain method)

Conclusions:
The vapour pressures of the test material components were calculated to be 6.82E-016 Pa and 1.96E-017 Pa at 25 °C (Modified Grain method).
Executive summary:

The vapour pressure of the test material was calculated using MPBPVP v1.43 (Sept 2010) 2000 U.S. Environmental Protection Agency. Given that the substances are organic molecule within the Molecular Weight range of the training set compounds, the prediction is considered to be acceptable.

The vapour pressures of the test material components were calculated to be 6.82E-016 Pa and 1.96E-017 Pa at 25 °C (Modified Grain method).

Description of key information

The vapour pressures of the test material components were calculated to be 6.82E-016 Pa and 1.96E-017 Pa at 25 °C (Modified Grain method).

Key value for chemical safety assessment

Additional information

The vapour pressure of the test material was calculated using MPBPVP v1.43 (Sept 2010) 2000 U.S. Environmental Protection Agency. Given that the substances are organic molecule within the Molecular Weight range of the training set compounds, the prediction is considered to be acceptable.

The vapour pressures of the test material components were calculated to be 6.82E-016 Pa and 1.96E-017 Pa at 25 °C (Modified Grain method).