Thermodynamic study of acetamides

Abstract

An extensive thermodynamic study of acetamide and its derivatives, N-methylacetamide, and N,N-dimethylacetamide, is presented in this work. Phase behavior was investigated by a heat-flux differential scanning calorimetry (DSC) from 183 K confirming the previously reported polymorphism of acetamide and N-methylacetamide. Two different polymorphic forms of N,N-dimethylacetamide were observed for the first time. Two static apparatus were employed for the vapor pressure measurements over a wide temperature range focusing on the ambient temperatures. Vapor pressures of metastable phase of acetamide were determined for the first time. Heat capacities of liquid and crystalline phases were measured by Tian-Calvet calorimetry at temperatures between 265 and 355 K. The ideal-gas thermodynamic properties were calculated using a combination of the density functional theory (DFT) and statistical thermodynamics. A consistent thermodynamic description was developed using the method of simultaneous correlation, where the experimental data for phase behavior, vapor pressures, and condensed phase heat capacities and calculated ideal-gas heat capacities are treated together to ensure overall thermodynamic consistency of the results. The observed trends in the thermodynamic properties of liquid phase were rationalized based on molecular interactions.