Thermochemical studies of Rhodamine B and Rhodamine 6G by modulated differential scanning calorimetry and thermogravimetric analysis

Heat capacity is a very important thermodynamic parameter of any compound. How to precisely measure the heat capacity is a long-sought task. In this study, the low-temperature molar heat capacities of Rhodamine B (225–320 K) and Rhodamine 6G (225–440 K) were determined on a modulated differential scanning calorimetry (MDSC). There is no thermal anomaly or phase transition observed for Rhodamine B from 225 to 320 K. However, a phase transition appears for Rhodamine 6G with a peak temperature of 373 K, and the onset temperature is at 349.4 ± 0.4 K. The molar enthalpy ΔHtrans and molar entropy ΔStrans for this phase transition are determined to be 2.310 ± 0.011 kJ mol−1 and 6.611 ± 0.032 J K−1 mol−1, respectively. According to the measured molar heat capacities, the entropy and enthalpy referenced to the temperature of 298.15 K for Rhodamine B and Rhodamine 6G are derived. Moreover, thermogravimetric analysis (TG) is employed to investigate their thermal stability. The initial decomposition temperature for Rhodamine B and Rhodamine 6G is 326.5 and 492.1 K, respectively. There are two steps of mass loss observed for Rhodamine 6G and three steps for Rhodamine B.