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Vapour pressure

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Description of key information

The vapour pressure of the substance is estimated to be <0.01Pa at 20 degC.

Key value for chemical safety assessment

Vapour pressure:
0.009 Pa
at the temperature of:
20 °C

Additional information

A test report is available for the vapour pressure of LAS-IPA. The report suggests a minimum boiling point for LAS-IPA of 340°C at atmospheric pressure (1001 hPa), based on Differential Scanning Calorimetry (DSC). It also presents results of a ThermoGravimetric Analysis (TGA) at 1009 hPa and results of a DSC analysis at reduced pressure (3.0 hPa). It applies the Modified Grain method to calculate vapour pressure at ambient temperature (20°C), referencing OECD Guideline 104, and Boethling and Mackay, Handbook of Property Estimation Methods for Chemicals. The report arrives at a vapour pressure at ambient temperature of ≤ 0.028 Pa.

 

However, the report does not present evidence to support the (implicit) claim therein that the temperature effect observed in the DSC (and TGA) is in fact associated with boiling. With reference to available information on LAS-salts including LAS-MIPA and LAS-TEA, as well as related substances (LAS free acid, LAS-Na, LAS-ammonium salt), it is deemed very likely that a true boiling point would be found only at much higher temperatures, and that the observed temperature effect is in fact associated with decomposition rather than boiling. For example, note in this respect that LAS-MIPA is reported to start thermal decomposition at 290°C. As such, vapour pressure is expected to be much lower than the reported maximum of 0.028 Pa.

 

It is also noted that when recalculating vapour pressure with the modified Grain algorithm, with an in-house implementation in MATLAB, a liquid with a boiling point of 340°C at 1001 hPa is predicted to have a vapour pressure of 0.018 Pa (rather than 0.028 Pa; this reflects the report’s omission of theDZbcompressibility factor in the calculation equation).

 

Finally the report states that ‘a modified version of the modified Grain method’ was used, as described in Boethling and Mackay, to support explicit treatment of solids (note that the modified Grain method calculates VP for a liquid or supercooled liquid - additional energy is required for solidification, which means that solids at room temperature have a lower vapour pressure than the corresponding supercooled liquids at the same temperature). However, it incorrectly claims that the equation defining the 'm' exponent of the modified Grain method “takes into account the aggregate state at the interesting temperature”; this is incorrect (possibly suggested by the fact that in the original definition of the modified Grain method in Lyman (1982), ‘m’ was not a continuous variable, but could only adopt a few discrete values based on the aggregation state of a substance). In fact, as can also be ascertained from the EPISuite documentation, a separate term needs to be introduced to account for the difference in vapour pressure between supercooled liquids and solids - this fact itself is already mentioned in Lyman (1982), although a much simpler term is suggested there. Note also that although the material tested is described as a pasty substance, LAS-IPA itself is solid at ambient temperature. The calculated value presented in the test report is in fact a supercooled liquid value. If we calculate a value for a solid with a boiling point at 340°C at 1001 hPa, a melting point for that solid of 56°C is sufficient to lower the resulting vapour pressure to below 0.01 Pa. It is deemed likely that the melting point of LAS-IPA exceeds 56°C. As such, given a reported minimum boiling point of 340°C at 1001 hPa (which is likely a gross underestimation of the true boiling point as pointed out already), the vapour pressure of LAS-IPA is very likely < 0.01 Pa.