Experimentally Determined Thermochemical Properties of the Malonic Acid/Water System: Implications for Atmospheric Aerosols

Abstract

The liquid/solid phase diagram, solution heat capacities, water activities, enthalpies of fusion, and eutectic temperature of the malonic acid/water binary system have been investigated using differential scanning calorimetry and infrared (IR) spectroscopy of thin films. We report here measurements of the ice melting envelope, malonic acid dissolution envelope, and the ice/malonic acid eutectic temperature and composition in this binary system. We also report the first observation of a malonic acid hydrate, possibly C3H4O4·6H2O, using both thermal analysis and IR spectroscopy. We have observed the formation of this hydrate over a large range of concentrations and determined that it can be a significant fraction of samples within that region. We have also determined the enthalpy of fusion of malonic acid, as well as the constant pressure heat capacities of solutions in the concentration range of 5−55 wt % malonic acid from 323 K down to the freezing point of each solution. Water activities have also been determined using the freezing point depression of ice. We compare our results to those recently reported in the literature for bulk solutions and aerosols.