Vapour pressures of selected organic compounds down to 1 mPa, using mass-loss Knudsen effusion method

• A recently described mass-loss Knudsen apparatus was used for measurements of vapour pressures down to around 1 mPa.
• Complementary calorimetric studies were performed in a Calvet-type calorimeter.
• New vapour pressures are given for benzoic acid and benzanthrone, in ranges in which no consistent data existed.
• Vapour pressures for solid n-octadecane are presented, correcting existing values from literature.

A recently developed Knudsen effusion apparatus was improved and used for measurements of vapour pressures of selected organic compounds. Calorimetric studies were conducted using a Calvet-type calorimeter, complementing the information obtained for the vapour pressures and facilitating the modelling and analysis of the data.Vapour pressures of benzoic acid, a reference substance, were determined at temperatures between 269 K and 317 K, corresponding to a pressure range from 2 mPa to 1 Pa, extending the range of results available in the literature to lower pressures. Benzanthrone was studied between temperatures 360 K and 410 K (5 mPa–1 Pa) in order to test the apparatus at higher temperatures.Values presented in the literature for the vapour pressure of solid n-octadecane, one of the most promising compounds to be used as “phase change material” for textile applications, were found inconsistent with the triple point of the substance. Sublimation pressures were measured for this compound between T = 286 K and 298 K (2–20 mPa) allowing the correction of the existing values. Finally, vapour pressures of diphenyl carbonate, a compound of high industrial relevance for its use in the production of polycarbonates, were determined from T = 302 K to 332 K (0.02–1 Pa).

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