Thermogravimetric method for a rapid estimation of vapor pressure and vaporization enthalpies of disubstituted benzoic acids: an attempt to correlate vapor pressures and vaporization enthalpies with structure

Abstract

A rapid estimation of vapor pressure and vaporization enthalpies of some disubstituted benzoic acids (2,4-dihydroxybenzoic acid (2,4-DHBA), 2,6-dihydroxybenzoic acid (2,6-DHBA), 3,4-dihydroxybenzoic acid (3,4-DHBA), 2,4-dinitrobenzoic acid (2,4-DNBA), 3,4-dinitrobenzoic acid (3,4-DNBA), 2,5-dibromobenzoic acid (2,5-DBBA), and 3,5-dibromobenzoic acid (3,5-DBBA)) was made using a simultaneous TG/DSC apparatus operating with aluminum open crucibles under inert atmosphere in both isothermal and non-isothermal mode. No evidence of thermal decomposition (in the form of endo or exothermic effect) was found during each experiment. Vapor pressure was obtained in the range from some tenth to some hundreds of Pa after calibration with benzoic acid. All operative conditions (sample mass, temperature rage, and purge gas flow) were carefully checked in order to obtain reliable results. Internal consistency of the results obtained was checked by comparing the sublimation enthalpy obtained by the sum of the vaporization enthalpies derived by the global NITG and ITG data, the melting enthalpies from DSC adjusted at 298.15 using the molar isobaric heat capacities of both solid and liquid estimated according to a group additivity approach and that obtained from the sublimation enthalpies determined by torsion effusion corrected at 298.15 K using the same approach. Finally, some comments concerning the relationship between energetics and structure (substituent effect) are also reported.