Phase diagram and aqueous solubility of the lidocaine-prilocaine binary system.

The phase behavior of the lidocaine-prilocaine binary system has been studied by X-ray diffraction, differential thermal analysis, hot-stage microscopy, and IR spectrometry. No intermediate compounds or solid solubilities have been detected. The eutectic composition is close to 1:1, and the eutectic temperature is 18 +/- 1 degrees C. Aqueous solubility studies show that the lidocaine heat of solubility from the eutectic mixture is different from that of the pure drug, whereas it is the same for prilocaine. Investigations of various lidocaine-prilocaine ratios indicate that the two local anesthetics decrease the solubility of each other. The total solubility, however, is affected only to a minor extent.