Evaluation of sublimation enthalpy by thermogravimetry: Analysis of the diffusion effects in the case of methyl and phenyl substituted hydantoins

Abstract

By using a thermogravimetric analyzer, and applying the Clausius-Clapeyron and Langmuir equations, the enthalpies of sublimation and/or vaporization of a set of organic compounds were obtained without determining the vapor pressures of the compounds. Although the Langmuir equation is strictly valid in vacuum conditions, its validity was tested here both considering and neglecting the diffusional factor. To test and validate these methodologies, molar enthalpies of vaporization or sublimation of a set of ten compounds, whose values have been reported in the literature were determined. The aforementioned compounds are: pyrene, phenanthrene, anthracene, benzoic acid, ferrocene, 2-furancarboxylic acid, 2-thiophenecarboxylic acid, methyl 4-hydroxybenzoate, hydantoin, and 2-thiohydantoin. The values obtained using both methodologies show that, the sublimation enthalpy increases by 0.9–2.5kJmol−1. The methodology that considers the diffusional effect was applied to a group of hydantoines methyl- and phenyl-substituted to determine their enthalpies of sublimation (the group is comprised by 1-methylhydantoin, 5-methylhydantoin, 5,5-diphenylhydantoin, and 5-methyl-5-phenylhydantoin). The enthalpies of these compounds have not been reported so far, and the values show low uncertainty. In addition, the heat capacity at 298.15K, the purity, melting temperature and enthalpy of melting of the compounds studied here were determined by differential scanning calorimetry.