Phase-Transition Thermodynamics of Tolbutamide

Abstract

Tolbutamide is an active pharmaceutical substance used in the treatment of type 2 diabetes. In this work, we report the first measurement of the vapor pressure above the liquid and crystalline phases of tolbutamide employing fast scanning calorimetry. From the temperature dependences of the vapor pressure, the vaporization and sublimation enthalpies were derived. The fusion enthalpy and the isobaric heat capacities of the crystalline and liquid phases were measured by differential scanning calorimetry, and the ideal gas-phase heat capacity was computed. The consistency between the values determined was checked, and the phase-transition thermodynamics of tolbutamide in a wide temperature range between 298 and 470 K was characterized. The obtained thermodynamic properties were compared with the calculations based on the group additive schemes that are often used to estimate unknown parameters of the pharmaceutical compounds.