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Double Freezing of (NH4)2SO4/H2O Droplets below the Eutectic Point and the Crystallization of (NH4)2SO4 to the Ferroelectric Phase A. Bogdan Institute of Physical Chemistry, UniVersity of Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria, Laboratory of Polymer Chemistry, Department of Chemistry, P.O. Box 55, UniVersity of Helsinki, FI-00014 Finland, Department of Physics, P.O. Box 48, UniVersity of Helsinki, FI-00014 Finland ReceiVed: March 24, 2010; ReVised Manuscript ReceiVed: August 12, 2010
This paper presents the differential scanning calorimetry (DSC) results obtained from measurements of single droplets of different subeutectic concentrations ( Tf > Te, where Tf is the freezing temperature, the viscosity of a dilute (NH4)2SO4/H2O solution is small and NH4+ and SO42- can be easily expelled. However, at Tf < Te the mobility of particles would be small and NH4+ and SO42- may be buried within the ice lattice. If this is the case, then this could account for why only one freezing event has been observed during the cooling of emulsified micrometerscaled (NH4)2SO4/H2O droplets that freeze at Tf , Te.28 Thus it is important to improve our knowledge about the temperature conditions at which the phase separation into ice and a residual solution can occur in (NH4)2SO4/H2O. In this paper, we present the differential scanning calorimetry (DSC) results of the measurements of 5, 15, 25, and 38 wt % (NH4)2SO4 droplets of diameters of ∼1-1.5 mm. Such droplets have a size intermediate between micrometer-scaled droplets
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and bulk samples and therefore freeze at a temperature not very much below the Te. The goal of the measurements is to find out whether the phase separation into ice and a residual solution can occur at Tf < Te. We find that in contrast to bulk (NH4)2SO4/ H2O, the phase separation always occurs at Tf < Te. We also find that the residual solution always freezes and, depending on whether it freezes below or above the ferroelectric “Curie” temperature of Tc ≈ 223 K, (NH4)2SO4 crystallizes into the paraelectric or the ferroelectric phase, respectively. Our results add to our knowledge about the freezing behavior of (NH4)2SO4/ H2O below the Te and can be useful for the study of other aqueous systems. The obtained results can also be useful for industrial implications, for example, eutectic freeze crystallization,29 and for better understanding of the paraelectric-toferroelectric transition of (NH4)2SO4, which is not fully understood yet.30 Experimental Section We prepared aqueous solutions of the concentration of 5, 15, 25, and 38 wt % (NH4)2SO4, which are smaller than the eutectic concentration of ∼40 wt % (NH4)2SO4. The solutions were prepared by mixing 99.99% (NH4)2SO4 crystals (Sigma Aldrich) with the corresponding amount of ultrapure deionized water. The phase transformations of (NH4)2SO4/H2O droplets were studied using a differential scanning calorimeter Mettler Toledo DSC 822. The calorimeter is well calibrated and reproduces the melting points of indium (429.75 K), water (273.15 K), and heptane (182 K) to accuracy better than (0.3-0.4 K. The DSC measurements were performed at a scanning rate of 3 K/min between 278 and 133 K. For measurement, (NH4)2SO4/H2O droplets were placed and then cold sealed in standard aluminum (Al) crucibles of volume 40 µL. It is known that metallic Al, on exposure to atmospheric oxygen, very rapidly becomes covered with an oxide Al2O3 film of ∼4 nm thickness. Therefore, (NH4)2SO4/H2O droplets were in fact in contact with the Al2O3 surface. Being placed on an Al (or Al2O3) surface, (NH4)2SO4/H2O droplets always form approximately a halfsphere. The diameter and weight of droplets were ∼1-1.5 mm and ∼5-15 mg, respectively. To be sure that the Al2O3 surface is not responsible for the phase separation (for example, by triggering heterogeneous ice nucleation), we also performed measurements of (NH4)2SO4/H2O droplets placed on a film of hydrophobic halocarbon grease of series 28LT (Halocarbon Products Corp.) spread on the bottom of Al crucibles. On a grease film, (NH4)2SO4/H2O also forms approximately a halfsphere. The measurements were performed repeatedly to verify the reproducibility of results. The repeated measurements were of two types: (i) measurements performed on 10-15 different droplets of the same composition; (ii) two to three measurements performed on the same droplet. Results and Discussion Phase Behavior of (NH4)2SO4/H2O Droplets during Freezing. Figure 1 demonstrates cooling and warming thermograms obtained from 5 and 15 wt % (NH4)2SO4 droplets placed on Al2O3. In the upper cooling thermogram of 5 wt % droplet, two exothermic (pointing upward) peaks at ∼252 and 207 K are formed by the heat of fusion released during freezing. In the cooling thermogram of 15 wt % droplet, the two freezing peaks are at ∼248 and 213 K. The existence of two freezing events indicates that two solids are formed during the cooling of each droplet. On warming, the solids produce two endothermic melting peaks at ∼257 and 273 K in the lower warming thermogram of the 5 wt % (NH4)2SO4 droplet and at ∼257 and
Figure 1. DSC thermograms obtained from single 5 and 15 wt % (NH4)2SO4 droplets placed on an Al2O3 substrate. The horizontal arrows show the direction of temperature change. In the upper cooling thermograms (blue lines), the two sharp exothermic peaks are due to the heat of fusion released during the freezing out of ice (large peaks) and the freezing of a residual solution (small peaks). In the lower warming thermograms (red lines), the two endothermic peaks are due to the heat of fusion absorbed during the melting of the eutectic mixture of ice/(NH4)2SO4 and ice, respectively. The temperature of ∼254.5 K is the eutectic melting temperature of ice/(NH4)2SO431 (see text for details) The magnified parts show a ferroelectric-to-paraelectric (F f P) transition30 at ∼223 K in crystalline (NH4)2SO4. The weight of droplets is 6.52 and ∼9.2 mg. The programmed scanning cooling and warming rates are 3 K/min. The scaled bar indicates heat flow through the droplets.
270 K in the warming thermogram of the 15 wt % (NH4)2SO4 droplet. It should be noticed that two or more melting events have been observed during the warming of binary and ternary aqueous solutions.28,31-33 However, the appearance of two freezing events during the cooling of bulk aqueous solutions to my best knowledge has not been reported before. The presence of two freezing events has only been observed during the cooling of emulsified micrometer-scale droplets of H2SO4/H2O,34 H2SO4/ HNO3/H2O with an excess of H2SO4,35 and aqueous multicomponent solutions containing H2SO4, HNO3, and ammonium salts.36 The types of solids that crystallize on cooling cannot be identified from the equilibrium phase diagram of (NH4)2SO4/ H2O, because the freezing process is not an equilibrium phenomenon. The phase diagram of (NH4)2SO4/H2O was measured by Beyer et al.31 and calculated using the thermodynamic E-AIM model of the system of H+-NH4+SO42--NO3--H2O.37 However, using the phase diagram, one can identify the type of solids which melt on warming. Below we will analyze the thermograms of a 15 wt % (NH4)2SO4 droplet. Similar analysis can be applied to the thermograms of any droplet of subeutectic concentration. According to the phase diagram of (NH4)2SO4/H2O,31 the temperature of 270 K is the melting point of ice of 15 wt % (NH4)2SO4 solution. The intersection of a baseline with the line of the largest slope of the low-temperature side of the cold melting peak gives the temperature of ∼254.5 K (Figure 1). This temperature is the eutectic melting temperature of ice/(NH4)2SO4 in the phase diagram of (NH4)2SO4/H2O. Thus the two solids, whose melting produces the endothermic peaks at ∼257 and ∼270 K, are the eutectic mixture of ice/(NH4)2SO4 and ice, respectively. Figure 1 shows that the melting of ice begins far below peak temperature. The long low-temperature tail is due to the melting
Double Freezing of (NH4)2SO4/H2O Droplets of ice, which is in contact with a concentrated residual solution. As the temperature increases, more ice melts, the solution becomes less concentrated, and the ice melts at warmer temperature. The purity of ice is greater if it freezes out from less concentrated solutions and at a distance far from the ice/ solution interface. Ice surrounded by 5 and 15 wt % (NH4)2SO4 solutions, which can be considered as pure ice, melts at ∼273 and 270 K, respectively. The interfacial layer of ice, which is in contact with a residual solution, is mostly contaminated by NH4+ (see Introduction). Literature data on the solubility of NH4+ in ice is scattered. For example, Gross et al.10 reported that, depending on concentration, the distribution coefficient of the ammonium ion can be as large as 0.1, whereas in ref 18, it is reported that the concentration of dissolved NH4+ in ice is less than 0.001 M. Considering that during the freezing of concentrated (NH4)2SO4/H2O (>10 wt % (NH4)2SO4) a larger amount of NH4+ may be trapped in the interfacial advancing ice layer, one may assume that a sulfate rich residual solution would be formed on the perimeter of ice. In this case similar to H2SO4/H2O,38 the freezing of a sulfate rich solution could have produced the eutectic mixture of ice/SAT (sulfuric acid tetrahydrate, H2SO4 · 4H2O) and/or SAO (sulfuric acid octahydrate, H2SO4 · 8H2O). If so, then on warming, we could have observed the eutectic melting of ice/SAT and/or the melting of SAO at ∼199 and 200 K, respectively.38 However, Figure 1, as well as figures below, clearly demonstrates the absence of such melting events. Thus, the amount of incorporated NH4+ ions in the interfacial ice layer is very small. The majority of NH4+ ions are in a residual solution. The measurements on the spatial distribution of impurities (or impurity gradient) in ice crystals formed during the eutectic freeze crystallization (EFC) showed that the ice crystals were almost pure except for the thin interfacial layer.39 Thus, ice crystals formed within aqueous solutions may be considered to consist of pure ice except for a thin interfacial layer. It is reasonable to assume that the first freezing event in the thermograms in Figure 1 is due to the crystallization of ice, not the eutectic mixture of ice/(NH4)2SO4, because the amount of water is greater than that of (NH4)2SO4. The confirmation for the assumption can be easily obtained experimentally. In Figure 2, the thermograms obtained from three droplets of the composition of 25 wt % (NH4)2SO4 are shown. The complete blue (1) and black (2) thermograms are similar to those in Figure 1; i.e., they also contain two freezing and two melting events. However, the red truncated thermograms (3), which were obtained during the measurement in which the cooling of a droplet was terminated after the first freezing event, contain only one freezing and one melting event. Since in the warming truncated thermogram, the temperature of the melting peak at ∼265 K exactly matches the melting point of ice in the phase diagram of (NH4)2SO4/H2O,31 one can conclude that the only solid formed during cooling is ice. Similar truncated thermograms were also obtained from (NH4)2SO4/H2O droplets of other compositions. Thus during the cooling of subeutectic (NH4)2SO4/ H2O droplets the warm freezing event is due to the freezing out of ice, Ti. The remaining cold freezing event, Tr, is due to the freezing of a residual solution which is formed by the expulsion of NH4+ and SO42- from the ice lattice during the nucleation and growth of ice. From what has been discussed above and from the phase diagram of (NH4)2SO4/H2O, one can see that a residual solution may possess the eutectic concentration of ∼40 wt % (NH4)2SO4. Since this concentration is larger than that of the mother droplets, the residual solution is called a freeze-concentrated residual solution. Finally, in all our
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Figure 2. Complete (blue 1 and black 2) and truncated (red 3) thermograms obtained from three 25 wt % (NH4)2SO4 droplets. The truncated thermograms demonstrate that the warm transition peak is due to the freezing out and melting of pure ice. Thermogram 1 was obtained from a droplet placed on hydrophobic halocarbon grease film, and thermograms 2 and 3 were obtained from the droplets placed on Al2O3. The magnified parts show F f P transition at ∼223 K. The numbers denote the weight of droplets. The scanning rate is 3 K/min.
measurements (of which there were more than 50), the two freezing events are always observed during the cooling of droplets. This fact suggests that the millimeter-scaled (NH4)2SO4/ H2O droplets of subeutectic concentration always produce the phase separation into pure ice and a residual freeze-concentrated solution below the Te. Since in our measurements, (NH4)2SO4/H2O droplets are placed on an Al2O3 substrate, their freezing is heterogeneous; i.e., it is triggered by the topological (cracks, irregularities, dislocations, etc.) and/or chemical (attached chemical impurities) surface properties of the substrate. To verify whether the specific surface properties of Al2O3 are solely responsible for the phase separation, we performed several DSC measurements of 25 wt % (NH4)2SO4 droplets placed on a film of hydrophobic halocarbon grease (see Experimental Section). Clearly, the surface properties of grease and Al2O3 are different. In Figure 2, thermograms 1 and 2 were obtained from the droplets placed on the film of grease and Al2O3, respectively. It is seen that they are almost identical. The repeated measurements of different 25 wt % (NH4)2SO4 droplets placed on grease and Al2O3 show a small scattering of Ti with an average temperature of Ti,ave ) 244 ( 6 K, whereas the repeated measurements of the same droplets produce almost identical Ti. In contrast to Ti the scattering of Tr is much larger and can reach up to 25 K between the coldest and warmest Tr. The cold Tr’s are shown in Figure 2. The warmest Tr can be seen in close vicinity to the corresponding Ti (not shown). What is interesting is that the repeated measurements for the same droplets sometimes produce a Tr whose value differs from the previous one by as much as ∼10 K. Although the measurements on hydrophobic grease and Al2O3 surfaces always demonstrate the presence of two freezing events, we cannot be sure that the droplets placed on other surfaces and subjected to other cooling rates will behave similarly. Further measurements, including other experimental techniques, are needed to elucidate the matter. Crystallization of (NH4)2SO4 to Ferroelectric Phase. It is known that, as crystalline (NH4)2SO4 is cooled, a transition
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Figure 3. DSC thermograms obtained from a single (NH4)2SO4 crystal. The exothermic and endothermic peaks are due to heat released and absorbed during P f F and F f P transitions. The weight of crystal is ∼1.47 mg. The scanning rate is 3 K/min.
occurs from the paraelectric phase with the space group of 16 ) to the ferroelectric phase with the space group of Pnam(D2h Pna21(C92V) at the ferroelectric “Curie” temperature of Tc ≈ 223 K.30,40-44 On warming, a reverse ferroelectric-to-paraelectric transition (F f P) occurs at the same temperature. Figure 3 demonstrates an example of paraelectric-to-ferroelectric (P f F) and F f P transitions that occur in a single (NH4)2SO4 crystal. It is seen that the onset of transitions is at ∼223 K, which can serve as an additional argument for our DSC device being well calibrated. From what has been discussed above, as a residual freezeconcentrated solution freezes, the eutectic mixture of ice/ (NH4)2SO4 is formed. In Figures 1 and 2, the magnified parts show the absence of the P f F transition in cooling thermograms and the presence of the F f P transition in warming thermograms. This indicates that if the residual solution freezes below 223 K then (NH4)2SO4 crystallizes directly to the ferroelectric phase. To be sure that this is not an artifact, we performed additional measurements on 38 wt % (NH4)2SO4 droplets that would produce more pronounced transition peaks at ∼223 K because of the larger amount of (NH4)2SO4 present than was in the less concentrated droplets. Figure 4 shows thermograms obtained from four different 38 wt % (NH4)2SO4 droplets. The thermograms demonstrate two situations. The residual solution freezes (i) above 223 K and (ii) below 223 K. The thermograms (1) and (4) show that if the freezing out of pure ice and the freezing of residual solution take place above 223 K, then the further cooling of the droplets produces an P f F transition at ∼223 K. On warming, the reverse F f P transition also occurs at ∼223 K. Thermograms 2 and 3 demonstrate situation ii, which is similar to that presented in Figures 1 and 2. Thus our DSC measurements of (NH4)2SO4/ H2O droplets of different compositions show that (NH4)2SO4 crystallizes to the ferroelectric phase if a residual (NH4)2SO4/ H2O solution freezes below the ferroelectric “Curie” temperature of Tc. This finding, to my best knowledge, is reported for the first time. The fact that below the Tc ammonium sulfate crystallizes directly to the ferroelectric phase may find implication in the theory of the ferroelectric transition which despite a long study is not fully understood yet.30,40,44 Conclusions This paper has provided for the first time experimental evidence on the phase separation into ice and a residual freezeconcentrated solution that occurs during the freezing of millimeter-scaled (NH4)2SO4/H2O droplets below the eutectic
Figure 4. DSC thermograms obtained from four droplets of the composition 38 wt % (NH4)2SO4. In the first and fourth thermograms, the freezing out of pure ice and the freezing of a residual solution occur above 223 K. In these completely frozen droplets, the P f F transition occurs at ∼223 K. In the second and third thermograms, the residual solution freezes below 223 K. In these partly droplets, there is no P f F transition. The warming thermograms contain the F f P transition at ∼223 K. The broad endothermic peak at ∼257 K is due to the overlapping of the melting peaks of pure ice and the eutectic mixture of ice/(NH4)2SO4. The weight of the droplets is between 5 and 15 mg. The scanning rate is 3 K/min.
temperature of Te ≈ 254.5 K. The finding is contradictory to the behavior of a bulk solution, which below the Te exists as a solid mixture of ice/(NH4)2SO4. The residual solution is formed by the expulsion of NH4+ and SO42- from the ice lattice and always freezes on further cooling, producing in such a way a second freezing event. The fact that the two freezing events are observed on a hydrophobic grease surface and solid Al2O3 suggests that the phase separation may not be induced by some specific surface properties but is an intrinsic property. However, to be sure that the phase separation below the Te is indeed an intrinsic property of (NH4)2SO4/H2O (and possibly of other aqueous binary solution droplets), measurements on hydrophilic and metallic surfaces as well as measurements at other cooling rates are needed. The applied cooling rate may impact the character of freezing behavior below the Te. Our DSC measurements also show that if residual freeze-concentrated (NH4)2SO4/ H2O freezes below the ferroelectric “Curie” temperature of Tc ≈ 223 K, then (NH4)2SO4 crystallizes directly into the ferroelectric phase, a finding that has not been reported before. The obtained results can be useful for industrial implication, for example, eutectic freeze crystallization29 and for the better understanding of the paraelectric-to-ferroelectric transition of (NH4)2SO4, which is not fully understood yet.30 The finding that millimeter-scaled (NH4)2SO4/H2O droplets produce two freezing events below the Te contradicts the past works that have reported the measurements on micrometerscaled (NH4)2SO4/H2O droplets. These previous measurements performed using different techniques, including DSC, never revealed the phase separation and consequently the two freezing events now observed. The inconsistency is not completely clear. It may be accounted for by the large difference between the heterogeneous freezing temperatures of millimeter-scaled droplets and the homogeneous freezing temperature of micrometerscaled droplets. At Tf , Te the viscosity of (NH4)2SO4/H2O may be large enough for NH4+ and SO42- to be expelled from the ice lattice during the freezing out of ice in very small droplets. However, taking into account the fact that our recent DSC
Double Freezing of (NH4)2SO4/H2O Droplets measurements revealed the phase separation during the freezing of micrometer-scaled droplets of H2SO4/HNO3/H2O with an excess of H2SO434 or HNO335 and multicomponent micrometerscaled droplets containing H2SO4, HNO3, and ammonium salts,36 the large-viscosity argument looks unconvincing. Another reason could be that there are limits to the volumes of (NH4)2SO4/ H2O that do not have an effect on the freezing behavior of the solution, and when these limits are crossed, larger volumes of the solution will start to play a role in the freezing behavior. Further measurements are needed to elucidate the matter. Acknowledgment. The author thanks T. Loerting for discussions. Financial support by the ERC (Starting Grant SULIWA) is gratefully acknowledged. References and Notes (1) Water - A ComprehensiVe Treatise; Franks, F., Ed.; Plenum Press: New York, 1982; Vol. 7. (2) Svishchev, I. M.; Kusalik, P. G. J. Am. Chem. Soc. 1996, 118, 649. (3) Schwerdtfeger, P. J. Glaciol. 1963, 4, 789. (4) Pruppacher, H. R.; Klett, J. D. Microphysics of Clouds and Precipitation; Kluwer: Dordrecht, 1997. (5) Baicu, S. C.; Taylor, M. J. Cryobiology 2002, 45, 33. (6) Cao, E. H.; Chen, Y. H.; Cui, Z. F.; Foster, P. R. Biotechnol. Bioeng. 2003, 82, 684. (7) Yamamoto, S. A.; Harris, L. J. Food Protection 2001, 64, 1315. (8) Vaessen, R.; Seckler, M.; Witkamp, G. J. Ind. Eng. Chem. Res. 2003, 42, 4874. (9) Gross, G. W.; Svec, R. K. J. Phys. Chem. B 1997, 101, 6282. (10) Gross, G. W.; Wu, C.-ho.; Bryant, L.; McKee, C. J. Chem. Phys. 1975, 62, 3085. (11) Wilson, P. W.; Haymet, A. D. J. J. Phys. Chem. B 2008, 112, 11750. and citation therein. (12) Bauerecker, S.; Ulbig, P.; Buch, V.; Vrbka, L.; Jungwirth, P. J. Phys. Chem. C 2008, 112, 7631. (13) Franks, F. Biophys. Chem. 2003, 105, 251. (14) Nagashima, K.; Furukawa, Y. J. Phys. Chem. B 1997, 101, 6174. (15) Cheng, J.; Soetjipto, C.; Hoffmann, M. R.; Colussi, A. J. J. Phys. Chem. Lett. 2010, 1, 374. (16) Vrbka, L.; Jungwirth, P. Phys. ReV. Lett. 2005, 95, 148501–1. (17) Moore, J. C.; Wolff, E. W.; Clausen, H. B.; Hammer, C. U.; Legrand, M. R.; Fuhrer, K. Geophys. Res. Lett. 1994, 21, 565.
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