THERMODYNAMIC PROPERTIES OF PHASE-TRANSITIONS IN MALONIC-ACID AND ITS DEUTERATED ANALOG

The heat capacities of malonic acid and its fully deuterated analogue were measured from 13 to 371 K. Two first-order phase transitions were found in each compound, and their thermodynamic quantities were determined as follows: malonic acid, 47.3 K (14.80 J mol-1 and 0.313 J K-1 mol-1) and 352.2 K (1.837 kJ mol-1 and 5.217 J K-1 mol-1); malonic acid-d4, 60.0 K (23.86 J mol-1 and 0.411 J K-1 mol-1) and 348.0 K (1.810 kJ mol-1 and 5.201 J K-1 mol-1). The transition entropies suggest displacive mechanisms for both transitions. The entropy changes of the high temperature transitions are explained by vibrational frequency changes at the transitions. The low temperature transitions have a martensitic nature: the intermediate beta-phase supercools partially and coexists with the lowest temperature gamma-phase in a limited temperature range. Hysteresis was also found at the high temperature transitions. The deuteration effects on the transition temperatures are discussed with a thermodynamic model and compared with those of other hydrogen-bonded substances.