Double Freezing of (NH₄)₂SO₄/H₂O Droplets below the Eutectic Point and the Crystallization of (NH₄)₂SO₄ to the Ferroelectric Phase

Abstract

This paper presents the differential scanning calorimetry (DSC) results obtained from measurements of single droplets of different subeutectic concentrations (<40 wt % (NH4)2SO4) and diameters of 1-1.5 mm. The measurements show that despite the fact that the freezing of the droplets takes place below the eutectic temperature of Te ≈ 254.5 K, a phase separation into ice and a residual freeze-concentrated solution occurs. The residual solution is formed by the expulsion of NH4+ and SO42- ions from the ice lattice during the nucleation and growth of ice and may possess the eutectic concentration of 40 wt % (NH4)2SO4. On further cooling, the residual solution freezes to the eutectic solid mixture of ice/(NH4)2SO4 at a temperature that is either above or below the ferroelectric "Curie" temperature of Tc ≈ 223 K. If the freezing of the residual solution takes place below the Tc, then (NH4)2SO4 crystallizes directly into the ferroelectric phase.

A review of air–ice chemical and physical interactions (AICI): liquids, quasi-liquids, and solids in snow

“…Cho et al (2002) argued that kinetic limitations leading to supercooled solutions could not explain the observations, because the amount of liquid closely followed the temperature during warming periods toward the eutectic temperature. Freezing of emulsions that resemble water droplets in the atmosphere can be kinetically hindered so that a meta-stable liquid phase exists well below the eutectic temperature (Koop et al, 2000;Bogdan, 2010). Using surface-sensitive, synchrotron-based X-ray spectroscopy, Křepelová et al (2010a) found no evidence for liquid on the surface of frozen solutions of NaCl below the eutectic.…”

Different freezing behavior of millimeter- and micrometer-scaled (NH4)2SO4/H2O droplets

“…It is seen in figure 1(a) that the millimeterscaled droplets produce sharp freezing peaks, whereas in figure 1(b) the micrometer-scaled droplets produce broad bellshaped freezing peaks which are due to the freezing of a large number ( 10 5 ) of size-distributed droplets. In both cases, in the warming thermograms (lower lines) there are two melting events which are produced by melting of the eutectic solid mixture of ice/(NH 4 ) 2 SO 4 (colder melting peaks at constant temperature) and pure ice, respectively [17]. It is also seen that as the concentration increases, the peak melting temperature of ice approaches the eutectic melting of ice/(NH 4 ) 2 SO 4 .…”

“…We prepared 5,10,15,20,25,28,30,33,35, and 38 wt% (NH 4 ) 2 SO 4 solutions by mixing 99.99% (NH 4 ) 2 SO 4 crystals (Sigma Aldrich) with the corresponding amount of ultrapure deionized water. Emulsions were prepared according to a well known and widely used emulsification technique described elsewhere [5,9,17,18]. Briefly, the oil phase for the preparation of emulsions was prepared by mixing 80 wt% Halocarbon 0.8 oil (Halocarbon Products Corp.) and 20 wt% lanolin (Sigma Aldrich).…”

“…The probability of the occurrence of T res in different temperature regions is schematically shown in figure 3. It should be noticed that the T res s of (NH 4 ) 2 SO 4 /H 2 O droplets placed on hydrophobic halocarbon grease substrate [17] also tend to fall into the ∼210-225 K region. The fact that the freezing of the residual (NH 4 ) 2 SO 4 /H 2 O solution tends to occur in a narrow temperature region is not unique.…”

“…T ice − T res > 0 K. The large frequency of the occurrence of T res in the temperature region of ∼210-225 K may be accounted for by the approximately similar concentration of a residual solution. Assuming that all NH + 4 and SO 2− 4 ions are expelled from the ice lattice during the first freezing event, the concentration of the residual solution [17] may be determined by using the onset of the temperature of the cold melting event and the equilibrium phase diagram of (NH 4 ) 2 SO 4 /H 2 O. Figure 1 shows that in all thermograms the onset of the cold melting event is ∼254.5 K. According to the phase diagram of (NH 4 ) 2 SO 4 /H 2 O this temperature is the onset of eutectic melting.…”