Investigating the thermal behavior of doxycycline and meclocycline

The thermal behavior of the antibiotics doxycycline (Dox) and meclocycline (Meclo) in the monohydrate and sulfosalicylate forms was investigated by TG/DTG, TG/DTA, DSC and TG-FTIR. Dox decomposed in three steps after its melting at 174 °C, with no evidence of crystallization on cooling. According to thermogravimetry associated with evolved gas analysis, it was suggested that decomposition begins with the release of: water, isocyanic acid, dimethylamine, as well as carbon dioxide and ammonia, resulting from the decomposition of isocyanic acid. At 535 °C carbon monoxide, methane and water were identified in gaseous phase. Meclo decomposed in two steps and did not presented melting and crystallization events. The gases released by Meclo during thermal decomposition were sulfur dioxide (from sulfosalicylate), carbon dioxide, isocyanic acid, carbon monoxide and ammonia. A mechanism for the thermal degradation of both antibiotics in nitrogen, has been proposed.