Abstract
Indomethacin (IM) easily forms an amorphous complex with lidocaine (LC) by heat treatment. To know the mechanism involved in the formation of this complex, we studied temperature-dependent phase changes in mixtures of IM and lidocaine hydrochloride (LH), in which the cationic form of LC forms a salt with Cl−, in various molar ratios by using DSC and NMR. Although heating of the mixture of IM and LC (IM+LC), formed a eutectic mixture, that of IM and LH (IM+LH) did not, and IM in the IM+LH mixture was dissolved into fused LH. Cooling of the fused IM+LH showed the glass transition in all of the samples containing various amounts of IM, suggesting that fused IM+LH took a homogenous amorphous state (IM/LH) below its glass transition temperature, in contrast to the fused IM+LC, which formed the rubber state and/or glass state depending on the molar content of IM. The results of the NMR study showed that IM in IM/LH caused the electronic structure of LH to change in such a way as to become similar to that of LC, but this effect was limited. Hence, mode of interaction of LH with IM is different from that of LC with IM.
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