Influence of organic acid on the thermal behavior of dimethyl sulfoxide

Dimethyl sulfoxide (DMSO) has been extensively used as a solvent because of its high-performance characteristics. However, there have been accidents when DMSO has been used in some chemical processes (Lam et al. in J Therm Anal Calorim 85:25-30, 2006; http://riscad.db.aist.go.jp/PHP_EN/index.php). Although the decomposition mechanism of DMSO under acidic conditions is known (Santosusso et al. in Tetrahedron Lett 48:4255-4258, 1974), the number of studies about thermal behavior of DMSO under acidic condition is not large. The purpose of this investigation is to understand the decomposition thermal behavior of DMSO under acidic conditions. Thermal analysis using an accelerating rate calorimeter (ARC) was carried out with acids chosen for their acid dissociation degree constant (pKa), which is a typical parameter of acid strength. ARC data show that the self-heat rate increased and the exothermic onset temperature decreased with decreasing pKa. Additionally, the exothermic onset temperature and pKa had good linear correlation. To verify the initial exothermic reaction, differential scanning calorimetry (DSC) analysis was carried out. DSC results showed that the exothermic onset temperature was due to decomposition of DMSO. The activation energy of the decomposition reaction was calculated from the ARC results (self-heat rate). Our data indicate that the activation energy of the decomposition reaction decreased with decreasing pKa. Therefore, thermal hazards of DMSO increase with the decrease in pKa, with good linear correlation.