ARTICLE pubs.acs.org/JPCA
Solid/Liquid Phase Diagram of the Ammonium Sulfate/Malic Acid/ Water System Jason R. Schroeder,† Christian S. Pearson, and Keith D. Beyer* 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/malic acid/water system using differential scanning calorimetry (DSC) and infrared spectroscopy (IR) of thin films. Using the results from our experiments we have mapped the ice primary phase region of the solid/liquid ternary phase diagram. In our DSC and IR experiments we observe ice nucleation in all samples and ammonium sulfate in some samples, which were cooled to 183 K. However, we only observed malic acid nucleation in IR experiments, where the sample was in contact with ZnSe windows. We also compare our results to the predictions of the Extended AIM Aerosol Thermodynamics Model (E-AIM) and find good agreement for the ice melting points in the ice primary phase field of this system; however, the E-AIM has difficulty predicting malic acid crystallization.
’ INTRODUCTION The inorganic fraction of tropospheric aerosols usually contains a significant amount of ammonium and sulfate ions with the molar ratio of NH4+/SO42 ranging from 1 to 2.1,2 Additionally, upper tropospheric aerosols composed predominantly of aqueous sulfuric acid at high concentrations have been shown to contain NH3, which partially to completely neutralizes the H2SO4 molecules.3 These particles absorb and scatter solar radiation dependent upon their phase, thus contributing to the radiation balance.4 They may also play a significant role in heterogeneous chemistry in the troposphere5 and can be found at cirrus cloud altitudes under strong convective conditions where they could serve as ice nuclei (IN).6,7 Recent field measurements have also shown that a significant fraction of tropospheric aerosols in many regions is organic.814 Studies have shown that the incorporation of organic compounds into ammonium sulfate aerosols changes their deliquescence, efflorescence, and hygroscopic properties, and potentially their crystallization properties.1519 Some of the most abundant organic compounds found in aerosols are the dicarboxylic acids.11,13 Very little is known about the thermodynamics of these systems in water at temperatures below 298 K. In particular, fundamental physical data are needed on these systems for incorporation into atmospheric models to better predict atmospheric cloud properties.20,21 Data, such as the melting temperature of ice and the temperatures at which a solution is saturated with respect to differing amounts of solute, are among the basic parameters that need to be experimentally determined. In this paper we focus on the ternary system composed of ammonium sulfate ((NH4)2SO4), dl-malic acid (C4H6O5), and water. The binary systems of ammonium sulfate/water and dlmalic acid/water have been extensively studied with respect to solubilities of the solute and solid/liquid phase equilibria.2224 With respect to the ternary system (NH4)2SO4/C4H6O5/H2O, r 2011 American Chemical Society
Brooks et al.17 measured eutonic compositions (solution concentration where both malic acid and ammonium sulfate are present, which was found by saturating the solution with respect to both solids while measuring the amount of each solid added to solution) at 297, 277, and 263 K, and Wise et al.18 measured the water activity of these concentrations at 298 K. However, we have found no measurements in the literature for the melting temperature of ice in this ternary system; therefore, the data presented here for ice melting are completely new. We present here the results of our study of the low-temperature solid/liquid phase diagram of ammonium sulfate/malic acid/ water 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).20,21,2527
’ 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 dl-C4H6O5 supplied by Acros Organics with deionized water. To check the ice melting points in the l-malic acid/water binary system, samples were prepared by mixing 99 wt % l-malic acid supplied by Acros Organics with deionized water. The concentration of all samples is known to (0.40 wt %. Differential Scanning Calorimeter (DSC). Phase transition temperatures and enthalpies were obtained with a Mettler Toledo DSC 822e with liquid nitrogen cooling and a Mettler Received: June 28, 2011 Revised: December 9, 2011 Published: December 12, 2011 415
dx.doi.org/10.1021/jp206101v | J. Phys. Chem. A 2012, 116, 415–422
The Journal of Physical Chemistry A 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 calibration28 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 20 μL) would be less likely to crystallize.
’ RESULTS (NH4)2SO4/C4H6O5/H2O and C4H6O5/H2O Data. The (NH4)2SO4/H2O and dl-C4H6O5/H2O binary phase diagrams have been studied previously by our group,23,24 and those data are incorporated into our figures and analysis for the ternary system. In the studies of Brooks et al.17 and Wise et al.18 on the ternary system, l-malic acid was used rather than dl-malic acid. However, Clegg and Seinfeld20 treated l-malic acid and dl-malic acid in their model as having the 416
dx.doi.org/10.1021/jp206101v |J. Phys. Chem. A 2012, 116, 415–422
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Figure 1. Ternary diagram for the (NH4)2SO4/C4H6O5/H2O system indicating concentrations studied in our lab and solids formed in each solution when cooled to 183 K in both DSC and IR experiments. The legend indicates solids that formed in each type of experiment: circles are for DSC experiments where only one solid formed/melted, squares represent DSC experiments where two solids formed/melted/dissolved, black symbols indicate no IR experiment was performed at that concentration, red symbols indicate ice was observed to form and melt in the IR, green indicates ice and solid (NH4)2SO4 was observed to form and melt/dissolve in the IR, blue indicates ice and solid C4H6O5 was observed to form and melt/dissolve in the IR. Data from our group for the C4H6O5/H2O binary system24 and (NH4)2SO4/H2O binary system23 are included in the figure for completeness.
Figure 2. IR spectra of 10/30 wt % (NH4)2SO4/C4H6O5 sample undergoing a cooling and heating cycle. Cooling: red (268.9 K), completely liquid sample; blue (192.8 K), ice and malic acid have frozen as seen in the shift of the large water OH peak from 709 to 831 cm1, and the splitting of the CdO peak of malic acid from 1718 to 1722/ 1712 cm1, shifts in NH4+ and SO42 that would normally be observed if (NH4)2SO4 crystallized are absent (see text). Warming: green (259.4 K), malic acid has dissolved and some ice melted; purple (263.2 K), ice completes melting. IR spectra shown for reference in black: (a) dry, solid dl-malic acid in an ATR accessory; (b) 30 wt % C4H6O5 in water (273.1 K) completely liquid sample; (c) completely frozen sample (228.9 K) with ice and solid malic acid (note the bifurcated CdO peak at 1722/ 1712 cm1). Spectra are offset by the following absorbance units for clarity: (a) black, 0.05; blue, +0.12; green, +0.17; purple, +0.20; (b) black, +0.30; (c) black, +0.45.
same thermodynamic properties. We used dl-malic acid in our study because it is more atmospherically relevant (malic acid in the atmosphere would be expected to be a racemic mixture), but we also experimentally determined the ice melting point depression in l-malic acid/water to be identical (within experimental error) to that previously reported for the dl-malic acid/water system24 by determining the ice melting points for several concentrations. These results are listed in Table 1. Therefore, throughout this paper we will use “malic acid” and “C4H6O5” for both dl-malic acid and l-malic acid except where we specifically mean one of the two. Figure 1 shows the ternary concentrations studied utilizing DSC and FTIR techniques. We have sorted our observations into several categories: number of melting transitions observed in DSC experiments and identity of solids formed in IR experiments. In the DSC experiments we observed either one or two melting transitions (never three). The IR experiments were correlated with the DSC results in terms of transition temperatures, which showed that the transitions were either ice melting, ice melting, and solid (NH4)2SO4 dissolution or ice melting and solid C4H6O5 dissolution. (Throughout we will refer to the transition H2O(s) f H2O(l) as “melting” and the transitions (NH4)2SO4(s) f (NH4)2SO4(aq), C4H6O5(s) f C4H6O5(aq) as “dissolution” because clearly water is the solvent and (NH4)2SO4 and C4H6O5 are the solutes either dissolving into or crystallizing out of solution.) As mentioned above, there are a number of concentrations for which only DSC experiments were performed (generally high water content). All of the results are summarized in Figure 1 using symbol and color coding. If a ternary sample completely freezes, at least three solids will be present: ice, solid (NH4)2SO4, and solid C4H6O5.
However, there may be a nucleation barrier or kinetic hindrance to growth of one or more solids in a sample of a particular concentration. In these cases only two or possibly one solid may form—those that do not suffer from these hindrances to solid formation. It is seen from Figure 1 that at most two solids formed, and for many of our samples only one solid formed (ice). We observed a disparity between our DSC and IR experiments in terms of the number of solids that formed for samples of the same composition falling in the range 4 wt % < [(NH4)2SO4] < 35 wt %, [C4H6O5] > 20 wt %. In this region, samples at 46 different concentrations were studied, as indicated in Figure 1. In the DSC experiments all samples showed only one melting transition (indicated by colored circles in Figure 1), which was ice in all cases as determined either by matching melting temperatures in with the IR spectra or by plotting trends in the final meting temperature in DSC experiments as given in Figure 2 (discussed below). IR experiments were performed on 32 of these samples with concentrations indicated in Figure 1 (red, green, and blue symbols). Of those, in nine experiments only ice formed (red circles in Figure 1), in one experiment ice and ammonium sulfate formed (green circles in Figure 1), and in 22 experiments both ice and malic acid formed (blue circles in Figure 1). Therefore, in this concentration region (rich in malic acid) there is a clear difference between DSC and IR experiments in terms of forming malic acid from solution. Nucleation of solids from solution depends on multiple variables such as solution volume, cooling rate, presence of heterogeneous nuclei, etc. Sample sizes in the DSC experiments are much larger than the IR experiments; therefore, nucleation probabilities would be expected to be higher in the 417
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Table 2. Ice Melting Temperature Polynomial Coefficients from eq 1a A1
A0
range [(NH4)2SO4]
3
0.2652
273.8
035
5 10
7.705 103 9.259 103
0.2007 0.2250
272.7 272.1
037 035
15
7.586 103
0.2.976
271.2
035
20
6.975 103
0.3688
270.5
035
25
1.351 102
0.2462
268.5
030
30
1.127 102
0.3734
267.8
030
35
1.332 102
0.3509
265.3
020
40
1.756 102
0.3525
263.1
020
45b 50b
2.253 102 4.106 104
0.3729 1.021
260.2 258.0
020 020
[C4H6O5] 0
A2 5.625 10
a
[(NH4)2SO4] and [C4H6O5] are in wt %. b For these malic acid concentrations, the ice melting temperatures at 0 wt % (NH4)2SO4 are metastable because the eutectic [C4H6O5] = 42.4 wt % in the binary C4H6O5/H2O system. Thus ice would melt at the binary eutectic temperature of 262.5 K.24
Figure 3. Plot of ice final melting temperatures as determined by DSC experiments and compositions of ternary samples as a function of constant malic acid concentration with malic acid concentrations as given in the legend. Lines represent fits to the data using eq 1 and parameters as given in Table 1.
DSC samples. Cooling rates are about equal in the two types of experiments, though cooling was not specifically controlled in the IR experiments. However, the samples are in contact with different surfaces in the two experiments: either platinum or aluminum in the DSC, and ZnSe in the IR. It would appear that sample contact with the ZnSe surface significantly enhanced nucleation of malic acid in the IR experiments. The surface contact factor is enhanced due to the fact that in these experiments the 2 μL sample is compressed between two ZnSe plates to a thickness that is estimated to be a few micrometers.30 Thus, the sample is pressed into a very high surface area to volume ratio. On the basis of these observations, we believe the ZnSe surface is enhancing nucleation of malic acid in our IR experiments, which is not the case in the DSC experiments. Because our samples are large compared to atmospheric aerosols, and in contact with a surface that (at least in the case of ZnSe/malic acid) should enhance nucleation of solids, it would seem that our experiments are a “best case” approximation of the solids that could form in atmospheric aerosols (within the time constraints of our experiments, 30 wt %) and low ammonium sulfate concentrations (
