Naphthenic acids

Vapour pressure

Link to relevant study record(s)

Reference

Endpoint:

vapour pressure

Type of information:

(Q)SAR

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study with acceptable restrictions

Justification for type of information:

QSAR prediction

Qualifier:

according to guideline

Guideline:

other: REACH guidance on QSARs R.6, 2008

Qualifier:

according to guideline

Guideline:

other: Guidance on information requirements and chemical safety assessment. Chapter R.7a: Endpoint specific guidance, R.7.1.5 Melting/freezing point, table R.7.1-18

Principles of method if other than guideline:

Acidic fraction: Vapour pressure was estimated using the validated EPISuite modelling tool (US EPA). The module MPBPWIN v 1.43 is used. The vapour pressure is the average of the vapor pressure obtained by the Antoine method and the modified Grain method.
Estimation of the vapour pressure is based on the boiling point.
Non-acidic faction: The vapour pressure of the non-acidic fraction is estimated in the PETRORISK® tool for petroleum substances, developed by CONCAWE. PETRORISK® needs a 2D-matrix as input for composition (see IUCLID section 1.4).
Naphthenic acids: The vapour pressure is calculated from the vapour pressure of the acidic and the non-acidic fraction and the mole fraction.
Details are described in the document ‘Vapour pressure of naphthenic acids.pdf’ under ‘Attached background material’.

GLP compliance:

no

Remarks:

not applicable

Type of method:

other: MPBPWIN v 1.43 and Petrorisk®

Specific details on test material used for the study:

Acidic fraction: For Smiles notation and C-number please see under Any other information on materials and methods incl. tables

Temp.:

ca. 25 °C

Vapour pressure:

< 37.1 Pa

Remarks on result:

other: for the acidic fraction

Temp.:

ca. 25 °C

Vapour pressure:

ca. 190 Pa

Remarks on result:

other: for the non-acidic fraction

Temp.:

ca. 25 °C

Vapour pressure:

ca. 31.4 Pa

Non-acidic fraction and naphthenic acids: results are described in the document ‘Vapour pressure of naphthenic acids.pdf’ under ‘Attached background material'. The vapour pressure of the non-acidic fraction and of the naphthenic acids are 190 Pa and 31.4 Pa respectively.

Acidic fraction: since naphthenic acids can adopt a wide variety of structures, QSARs are performed on a large number of different structures. The structures are chosen in order to represent the composition of the mixture of naphthenic acids.

In the EPISUITE program, an experimental vapour pressure is indicated if present in the database. Therefore three experimental vapour pressures are also given in some cases. By comparing the experimental vapour pressure with the calculated vapour pressure, it seems that the calculated vapor pressures give an overestimation.

Validity of model.

1. Defined endpoint: vapour pressure

2. Unambigous algorithm: estimation of vapour pressure by three methods: Antoine method, modified Grain method and Mackay method. MPBPWIN reports a suggested vapour pressure based on the modified Grain method for solids, an average of the Antoine method and modified Grain method for gases and liquids. The boiling point is used as a descriptor in the Antoine and Mackay method. The melting point is used as a descriptor in the modified Grain method.

3. Applicability domain: There is no real applicability domain defined, but the applicability is expected to be better for molecules with

molecular weights within in the test set (16.04 - 943.17) and for substances with the same functional groups or structural features as the test set. The estimation error increases as the vapour pressure decreases, especially for values lower than 1.33E-04 Pa.

4. Statistical characteristics: N = 3037, the coefficient of determination, R2 = 0.914

5. Mechanistic interpretation: related to stuctures/functional groups in the molecule affecting the boiling point. Based on equations in which the estimated boiling point and melting point are entered, the vapour pressure is calculated.

Adequacy of the prediction: naphthenic acids fall within the applicability domain described above and (the molecular weight range of the mixture = 116.16 - 432.74) and the substance does not contain any other functional groups than the molecules in the test set), therefore, the predicted value can be considered reliable taking into account that the standard deviation error of prediction of log(vapour pressure) is 141 Pa.

Conclusions:

The vapour pressure for different structures of the acidic fraction of naphthenic acids ranges from 37.1 to 1.41E-21 Pa, for smaller and larger molecules respectively. The vapour presuure of the non-acidic fraction is 190 Pa. The vapour pressure of naphthenic acids is calculated by using partial vapour pressure and mole fractions; the resulting vapour pressure for naphthenic acids is 31.4 Pa.

Executive summary:

Acidic fraction: To gain an understanding of vapour pressure characteristics of naphthenic acids, various different structures are estimated for vapour pressure using the EPISuite computer model (US EPA, 2009). Values obtained ranged from 1.41 E-21 to 37.1 Pa at 25°C. The vapour pressure of the acidic fraction of naphthenic acids can be concluded to be <37.1 Pa at 25°C. All values obtained are below 37,1 Pa indicating that naphthenic acids do not have the tendency to evaporate to air. The average vapour pressure of the most prominent structures (C8-C20) is 0.35 Pa (for the vapour pressure of one C8 structure, an experimental value is used).

Acidic fraction: The vapour pressure is calculated in Petrorisk®from a 2D matrix resulting from GCxGC analyses (mean of 8 samples). The vapour pressure of the non-acidic fraction is 190 Pa.

The vapour pressure of naphthenic acids is calculated from the vapour pressure and the mole fraction of the acidic and non-acidic fractions. The resulting vapour pressure of naphthenic acids is 31.4 Pa.

Description of key information

The vapour pressure of naphthenic acids was estimated using the EPISuite model (US EPA, 2009) and was considered reliable with restrictions (K2). Values  ranged from 1.41E-21 to 37.1 Pa at 25°C. The vapour pressure of the non-acidic fraction is 190 Pa, estimated in Petrorisk® from the GCxGC analyses results (n=8). The vapour pressure of naphthenic acids is calculated by using partial vapour pressure and mole fractions; the resulting vapour pressure for naphthenic acids is 31.4 Pa.

Key value for chemical safety assessment

Vapour pressure:

31.4 Pa

at the temperature of:

25 °C

Additional information

Acidic fraction: because naphthenic acids are mixtures of many different isomers of cycloalkyl carboxylic acids, physicochemical properties vary according to the proportions of the individual compounds in their composition. Therefore it is not possible to set a specific value for vapour pressure. To gain an understanding of vapour pressure characteristics of naphthenic acids, various hydrocarbon acidic structures which are shown to predominate in naphthenic acids were estimated for vapour pressure at 25°C using the EPISuite computer model (US EPA, 2009). All values obtained were below 37.1 Pa indicating that naphthenic acids do not have the tendency to evaporate to air.

Non-acidic fraction: The vapour pressure is calculated in Petrorisk® from a 2D matrix resulting from GCxGC analyses (mean of 8 samples). For each cell of the matrix, representative structures with experimental values for vapour pressure are present in the large library of Petrorisk®. The vapour pressure of the non-acidic fraction is 190 Pa.

The vapour pressure of naphthenic acids is calculated from the vapour pressure and the mole fraction of the acidic and non-acidic fractions. The resulting vapour pressure of naphthenic acids is 31.4 Pa.