Features of heat-induced amorphous complex between indomethacin and lidocaine

We studied temperature-dependent phase changes in the mixture of indomethacin (IM) and lidocaine (LC) in various molar ratios by differential scanning calorimetry (DSC). DSC studies were carried out between -40°C and 180°C heating/cooling/heating cycle at a rate of 10°C/min under a nitrogen gas flow. Although LC in the liquid state was crystallized, fused IM showed a glass transition signal upon cooling. Hence, cooling of fused IM caused it to assume the amorphous state below its glass transition temperature. Heating of the mixture of IM and LC resulted in formation of a eutectic mixture between them, and cooling of the fused mixture caused formation of the amorphous state at any molar ratio examined. It is noteworthy that the amorphous solid of the fused complex of IM and LC was in the rubber state and glass state in the mixtures containing IM between 20% and 40%, whereas there was only the glass state in the samples containing 50% IM and higher. In addition, crystallization of LC took place by cooling the fused sample containing 10% IM.