ARTICLE pubs.acs.org/JPCA
Solid/Liquid Phase Diagram of the Ammonium Sulfate/Maleic Acid/Water System Keith D. Beyer,* Jason R. Schroeder,† and Christian S. Pearson Department of Chemistry, University of Wisconsin—La Crosse, La Crosse, Wisconsin 54601, United States
bS Supporting Information ABSTRACT: We have studied the low temperature phase diagram and water activities of the ammonium sulfate/maleic acid/water system using differential scanning calorimetry and infrared spectroscopy of thin films. Using the results from our experiments, we have mapped the solid/liquid ternary phase diagram, determined the water activities based on the freezing point depression, and determined the ice/maleic acid phase boundary as well as the ternary eutectic composition and temperature. We also compare our results to the predictions of the extended AIM aerosol thermodynamics model and find good agreement for the ice melting points in the ice primary phase field of this system; however significant differences were found with respect to phase boundaries, maleic acid dissolution, and ammonium sulfate dissolution.
’ INTRODUCTION Tropospheric aerosols are often composed of internal mixtures of inorganic electrolytes and organics.19 The inorganic fraction is made up predominantly of aqueous ammonium and sulfate ions with the molar ratio of NH4+/SO42‑ ranging from 1 to 2.10,11 Additionally, upper tropospheric aerosols which are composed predominantly of aqueous sulfuric acid at high concentrations have been shown to contain NH3 which partially to completely neutralizes the H2SO4 molecules.12 These particles absorb and scatter solar radiation dependent upon their phase, thus contributing to the radiation balance.13 They may also play a significant role in heterogeneous chemistry in the troposphere,14 and can be found at cirrus cloud altitudes under strong convective conditions where they could serve as ice nuclei.15,16 Studies have shown that the incorporation of organic compounds into ammonium sulfate aerosols changes their deliquescence, efflorescence, hygroscopic properties, and potentially their crystallization properties.1721 This necessitates understanding the impact of organic substances on the phase transitions of aqueous systems that make up tropospheric aerosols. Some of the most abundant organic compounds found in aerosols are the dicarboxylic acids: oxalic, malonic, succinic, maleic, glutaric, and multifunctional acids such as malic.5,7 Very little is known about the thermodynamics of these systems in water at temperatures below 298 K. In particular, fundamental physical data is needed on these systems for incorporation into atmospheric models in order to better predict atmospheric cloud properties.22,23 Data, such as equilibrium freezing temperature of ice and solute saturation temperature as a function of solute concentration, are among the basic parameters that need to be experimentally determined. In this paper we focus on the ternary system composed of ammonium sulfate, maleic acid, and water. The binary systems of ammonium sulfate/water and maleic acid/water have been extensively studied with respect to solubilities of the solute and r 2011 American Chemical Society
solid/liquid phase equilibria.2426 With respect to the ternary system (NH4)2SO4/C4H4O4/H2O, Brooks et al.19 measured eutonic compositions (solution concentration where both maleic acid and ammonium sulfate are present) at 297, 277, and 263 K, and Wise et al.20 measured the water activity of these concentrations at 298 K. Several other groups have measured the deliquescence relative humidity (DRH) of dry (NH4)2SO4/C4H4O4 particles at various temperatures.2729 We have found no measurements in the literature for the melting temperatures of ice in this ternary system or measurements of the solubility of a single solid in a ternary solution; therefore, the data presented here for ice melting and individual solubilities of (NH4)2SO4 and C4H4O4 are completely new. We present here the results of our study of the low-temperature solid/liquid phase diagram of ammonium sulfate/maleic acid/ water and water activities in the ice primary phase region using thermal analysis and infrared spectroscopy techniques. We have coupled our experimental data with available literature data to construct a ternary phase diagram. Finally, we compare our results to the predictions of the extended AIM aerosol thermodynamics model (E-AIM).22,23,3032
’ EXPERIMENTAL SECTION Sample Preparation. Ternary samples were prepared by mixing 99 wt % ACS reagent grade (NH4)2SO4 supplied by Sigma-Aldrich and 99 wt % ACS reagent grade C4H4O4 supplied by Acros Organics with deionized water. The concentration of all samples is known to (0.40 wt %. Differential Scanning Calorimeter. Thermal data were obtained with both a Mettler Toledo DSC 822e with liquid nitrogen Received: September 2, 2011 Revised: October 18, 2011 Published: October 21, 2011 13842
dx.doi.org/10.1021/jp208488z | J. Phys. Chem. A 2011, 115, 13842–13851
The Journal of Physical Chemistry A cooling and a Mettler Toledo DSC 822e cooled via an intra cooler. Each DSC utilized an HSS7 sensor. Industrial grade nitrogen gas was used as a purge gas with a flow rate of 50 mL/min. The temperature reproducibility of these instruments is better than (0.05 K. Our accuracy is estimated to be (0.9 K with a probability of 0.94 based on a four point temperature calibration33 using indium, HPLC grade water, anhydrous, high purity (99%+) octane, and anhydrous, high purity heptane (99%+) from Aldrich, the latter three stored under nitrogen. The enthalpy/heat capacity measurement of each DSC was also calibrated using the same substances and the known enthalpy of fusion for each substance yielding an accuracy of (3% with a probability of 0.92. The sensitivity of our instruments to thermal signals is high. Previously, we have calculated our sensitivity to detecting a component undergoing a thermal transition to be
