Evolved gas analysis of aminophylline and related compounds containing theophylline

Abstract A comparative study on thermal behaviour of a commercial aminophylline ( 1 ) and three theophylline (th) compounds containing ethylenediamine (en) analogues, has been carried out by using simultaneous TG/DTA and evolved gas analysis (TG–FTIR). Theophylline ethylenediamine carbamate ( 2 ), 1,4-diammoniumbutane bis(theophyllinate) ( 3 ) and a physical mixture containing both 1,6-diaminohexane carbamate and lattice compound of 1,6-diaminohexane with theophylline ( 4 ) have been prepared and heated to about 500 °C with 10 °C min −1 rate. The thermal behaviour of the four samples has shown various similarities. Based on simultaneous TG/DTA curves and stoichiometric calculation, it can be seen that all the molecules of the solubilizers are released below 250 °C in flowing air atmosphere. The evolution rate of water of crystallization from aminophylline reaches a maximum at lower temperature than that of ethylenediamine, so water of crystallization leaves easier from the molecular compound. Ethylenediamine carbamate is released as ethylenediamine and carbon dioxide in one TG step from compound 2 as observed by TG–FTIR. The coupled TG–FTIR analysis has also revealed that 1,4-diaminobutane is released without degradation from lattice 3 . During the first step of decomposition of 4 , the 1,6-diaminohexane carbamate evolved in form of carbon dioxide and diamine. In the second step 1,6-diaminohexane is released from the targeted 1,6-diammoniumhexane bis(theophyllinate). In all cases, in the last step after a slight sublimation, the anhydrous theophylline melts and evaporates totally.