Characterization of camphor:thymol or dl-menthol eutectic mixtures; Structure, thermophysical properties and lidocaine solubility

The low solubility of drugs in aqueous media is a major problem for the pharmaceutical industry. Among the solutions proposed, the use of eutectic mixtures can be highlighted. In them, the components used can also increase the therapeutic advantages of the active principle. In this work, we analyzed the solubility of lidocaine in mixtures of camphor + thymol or dl-menthol. These binary eutectic solvents were also characterized. For all this, several NMR techniques were used and different thermophysical properties (density, speed of sound, refraction index, isobaric molar heat capacity, surface tension, and kinematic viscosity) were measured. The results indicated that the interaction of camphor with thymol was stronger than with dl-menthol, providing a more compact fluid. On the contrary, the steric hindrance was greater in the mixture with dl-menthol, giving rise to a more viscous liquid. Furthermore, the lidocaine was significantly more soluble (up to 540 times) in these mixtures than in water. The lidocaine showed the strongest interactions with thymol molecules, somewhat weaker with dl-menthol, and the weakest were with camphor ones.