Measured solid state and subcooled liquid vapour pressures of nitroaromatics using Knudsen effusion mass spectrometry

Abstract

Abstract. Knudsen effusion mass spectrometry (KEMS) was used to measure the solid state saturation vapour pressure (PSsat) of a range of atmospherically relevant nitroaromatic compounds over the temperature range from 298 to 328 K. The selection of species analysed contained a range of geometric isomers and differing functionalities, allowing for the impacts of these factors on saturation vapour pressure (Psat) to be probed. Three subsets of nitroaromatics were investigated: nitrophenols, nitrobenzaldehydes and nitrobenzoic acids. The PSsat values were converted to subcooled liquid saturation vapour pressure (PLsat) values using experimental enthalpy of fusion and melting point values measured using differential scanning calorimetry (DSC). The PLsat values were compared to those estimated by predictive techniques and, with a few exceptions, were found to be up to 7 orders of magnitude lower. The large differences between the estimated PLsat and the experimental values can be attributed to the predictive techniques not containing parameters to adequately account for functional group positioning around an aromatic ring, or the interactions between said groups. When comparing the experimental PSsat of the measured compounds, the ability to hydrogen bond (H bond) and the strength of the H bond formed appear to have the strongest influence on the magnitude of the Psat, with steric effects and molecular weight also being major factors. Comparisons were made between the KEMS system and data from diffusion-controlled evaporation rates of single particles in an electrodynamic balance (EDB). The KEMS and the EDB showed good agreement with each other for the compounds investigated.

Measured Solid State and Sub-Cooled Liquid Vapour Pressures of Benzaldehydes Using Knudsen Effusion Mass Spectrometry

“…This work builds on previous work by Booth et al (2012) [12], Dang et al (2019) [13] and Shelley et al (2020) [14] investigating the impacts of functional group positioning and the interaction of functional groups within a molecule on P sat . In previous work by Shelley at al.…”

“…Therefore, it is necessary to convert the P sat S determined by the KEMS system into a P sat L . As with previous KEMS studies [4,14,24] the melting point (T m ) and the enthalpy of fusion (∆H fus ) are required for the conversion. These values were measured with a TA Instruments DSC 2500 Differential Scanning Calorimeter (DSC) sourced from TA Instruments UK, Elstree, UK.…”

“…KEMS is an established vapour pressure measurement technique capable of measuring vapour pressures from 10 1 to 10 −8 Pa. The KEMS system is the same instrument that has been used in previous studies [4,14,24,25] and a summary of the measurement procedure will be given here. For a more detailed overview see Booth et al (2009) [25].…”

“…To calibrate the KEMS, a reference compound of known P sat is used. In this study the polyethylene glycol series (PEG series), PEG-3 (P 298 = 6.68 × 10 −2 Pa) and PEG-4 (P 298 = 1.69 × 10 −2 Pa) [26] were used as was implemented in Booth et al (2017) [27], Bannan et al (2019) [28], and Shelley et al (2020) [14].…”

“…In previous work by Shelley at al. (2020) [14] large absolute differences between experimental and estimated P sat were observed, especially for nitrophenol compounds. One of the major reasons for these differences was due to the lack of previous experimental P sat data for compounds with similar functionalities.…”