Thermodynamic studies of mixtures for topical anesthesia: Lidocaine–salol binary phase diagram

The lidocaine–salol binary system has been investigated by differential scanning calorimetry, direct visual observations, and X-ray powder diffraction, resulting in a temperature-composition phase diagram with a eutectic equilibrium. The eutectic mixture, found at 0.423 ± 0.007 lidocaine mole-fraction, melts at 18.2 ± 0.5 °C with an enthalpy of 17.3 ± 0.5 kJ mol−1. This indicates that the liquid phase around the eutectic composition is stable at room temperature. Moreover, the undercooled liquid mixture does not easily crystallize. The present binary mixture exhibits eutectic behavior similar to the prilocaine–lidocaine mixture in the widely used EMLA® topical anesthetic preparation.