Investigation of thermal behavior of nicotinic acid

The thermal behavior of nicotinic acid under inert conditions was investigated by TG, FTIR and TG/DSC-FTIR. The results of TG/DSC-FTIR and FTIR indicated that the thermal behavior of nicotinic acid can be divided into four stages: a solid-solid phase transition (176-198 degrees C), the process of sublimation (198-232 degrees C), melting (232-263 degrees C) and evaporation (263-325 degrees C) when experiment was performed at the heating rate of 20 K min(-1). The thermal analysis kinetic calculation of the second stage (sublimation) and the fourth stage (evaporation) were carried out respectively. Heating rates of 1, 1.5, 2 and 3 K min(-1) were used to determine the sublimation kinetics. The apparent activation energy, pre-exponential factor and the most probable model function were obtained by using the master plots method. The results indicated that sublimation process can be described by one-dimensional phase boundary reaction, g(alpha)=alpha. And the 'kinetic triplet' of evaporation process was also given at higher heating rates of 15, 20, 25, 30 and 35 K min(-1). Evaporation process can be described by model of nucleation and nucleus growing, G(alpha)=[-LN(1-alpha)](1/3).