Thermodynamics and kinetics of thermal decomposition of dibutylalkyl and dipentylalkyl phosphonate-nitric acid systems

Thermal stability of neat and nitric acid-solvated dibutylalkyl and dipentylalkyl phosphonates was studied under closed air ambience using an adiabatic calorimeter. Enthalpies and kinetic parameters for the decomposition reaction were derived wherever possible and are reported for the first time. The neat compounds were found to be stable up to 555 K while the nitric acid-solvated phosphonates decomposed at lower temperatures (370–403 K) with exothermic rise in temperature and pressure invariably producing incompressible gases. Decomposition of all these acid-solvates followed first order kinetics. The rise in temperature and pressure was found to be strongly dependent on nitric acid content of the acid-solvate. While the neat phosphonates produced solid black residue, the acid-solvates produced a viscous reddish-brown residue on decomposition. Heat capacities (Cp), of acid-solvates (DBPrP·0.6HNO3, DBBP·0.6HNO3 and DBPP·0.6HNO3) were measured at constant pressure using a heat flux type differential scanning calorimeter in the temperature range 305–350 K.