Thermodynamic study of ibuprofen by adiabatic calorimetry and thermal analysis

Abstract

Molar heat capacities of ibuprofen were precisely measured with a small sample precision automated adiabatic calorimeter over the temperature range from 80 to 400 K. The polynomial functions of C p , m (J K −1 mol −1) versus T were established on the heat capacity measurements by means of the least fitting square method. The functions are as follows: for solid ibuprofen, at the temperature range of 79.105 K≤T≤333.297 K, C p, m =144.27+77.046 X+3.5171 X 2+10.925 X 3+11.224 X 4, where X=( T−206.201)/127.096; for liquid ibuprofen, at the temperature range of 353.406 K≤ T≤378.785 K, C p, m =325.79+8.9696 X−1.6073 X 2−1.5145 X 3, where X=( T−366.095)/12.690. A fusion transition at T=348.02 K was found from the C p – T curve. The molar enthalpy and entropy of the fusion transition were determined to be 26.65 kJ mol −1 and 76.58 J mol −1 K −1, respectively. The thermodynamic functions on the base of the reference temperature of 298.15 K, ( H T − H 298.15) and ( S T − S 298.15), were derived. Thermal characteristic of ibuprofen was studied by thermo-gravimetric analysis (TG–DTG) and differential scanning calorimeter (DSC). The temperature of fusion, the molar enthalpy and entropy of fusion obtained by DSC were well consistent with those obtained by adiabatic calorimeter. The evaporation process of ibuprofen was investigated further by TG and DTG, and the activation energy of the evaporation process was determined to be 80.3±1.4 kJ mol −1.