Polymorphism and thermophysical properties of l- and dl-menthol

•Phase behavior and kinetics of polymorphic transformations of menthol were studied.•Vapor pressures and heat capacities of several phases were measured.•Ideal-gas thermodynamic properties were obtained by statistical thermodynamics.•Phase diagrams of l-menthol and dl-menthol were suggested. The thermodynamic properties, phase behaviour, and kinetics of polymorphic transformations of racemic (dl-) and enantiopure (l-) menthol were studied using a combination of advanced experimental techniques, including static vapour pressure measurements, adiabatic calorimetry, Tian-Calvet calorimetry, differential scanning calorimetry (DSC), and variable-temperature X-ray powder diffraction. Several concomitant polymorphs (α, β, γ, and δ forms) were observed and studied. A continuous transformation to the stable α form was detected by DSC and monitored in detail using X-ray powder diffraction. A long-term coexistence of the stable crystalline form with the liquid phase was observed. The vapour pressure measurements of both compounds were performed using two static apparatus over a temperature range from 274 K to 363 K. Condensed-phase heat capacities were measured by adiabatic and Tian-Calvet calorimetry in the wide temperature interval from 5 K to 368 K. Experimental data of l- and dl-menthol are compared mutually as well as with available literature results. The thermodynamic functions of crystalline and liquid l-menthol between 0 K and 370 K were calculated from the calorimetric results. The thermodynamic properties in the ideal-gas state were obtained by combining statistical thermodynamics and quantum chemical calculations based on a thorough conformational analysis. Calculated ideal-gas heat capacities and experimental data on vapour pressure and condensed-phase heat capacity were treated simultaneously to obtain a consistent thermodynamic description. Based on the obtained results, the phase diagrams of l-menthol and dl-menthol were suggested.