Phase Separation during Freezing upon Warming of Aqueous Solutions

Using differential scanning calorimetry, we show that the addition of solute(s) to emulsified water lowers the freezing temperature to <231 K, the homogeneous nucleation temperature of pure bulk water, or even completely suppresses freezing. In the latter case, freezing upon warming occurs above T X ≈ 150 K and leads to a phase separation into pure ice and a freeze-concentrated solution (FCS) which crystallizes upon further warming. We also show that emulsified 20–21.5 wt. % HCl solutions and the FCS of HCl/H 2 O solutions transform to glass at T g ≈ 127–128 K, i.e., lower than T g ≈ 136 K of water. We suggest that water nanodrops adsorbed on fumed silica resemble bulk water more than water confined in nanoscaled confinement and also more than nanoscaled water domains in aqueous solution. © 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.


INTRODUCTION
2][3][4][5] The properties of supercooled water in this domain have remained speculative, therefore.[10] Water in the NML is usually investigated by computer simulations 2,11,13 and measurements of water confined in cylindrical nanopores of silica glasses, 1,3,9,10,14,15 see Fig. 1(a).We emphasize that, strictly speaking, the NML is defined for pure, bulk water, but not for water in solutions or water in confinement.The dynamic properties of freeze-concentrated solution (FCS or "water confined within ice itself" 16 ), which was formed after freezing upon warming of quenched bulk TEMPOL/water solution, was studied with electron spin resonance spectroscopy. 17It was concluded that the mobility of TEMPOL molecules in FCS was consistent with key issues concerning the properties of supercooled water in the NML. 17It was also concluded by temperatureprogrammed desorption measurements on thin amorphous water films that they may crystallize above pure, bulk water's a) E-mail: anatoli.bogdan@uibk.ac.at crystallization temperature T X . 180][21] Strong and highly directional hydrogen bonds between SiOH and H 2 O form a viscous boundary region up to 3 monolayers (∼0.9 nm) in which hydrogen bonding is strongly perturbed.][14][15] Other ways of lowering the freezing point to <231 K are the addition of solute(s) to water with subsequent emulsification 5 or the use of water nanodrops with a large free surface, for example, by the adsorption of vapour on fumed silica, 22,23 as schematically is shown in Fig. 1(b).In the past, experiments performed on emulsified sulphuric acid solutions showed that freezing and crystallization of ice could be avoided during a slow cooling/warming cycle. 5xperiments performed on water nanodrops adsorbed on fumed silica showed that the freezing temperature may shift to below 228 K. 22 Recently, ultrafast X-ray measurements showed that some fraction of droplets cooled evaporatively remained liquid at 227 K on the time scale of milliseconds. 24hese authors employed micron-sized droplets, which are per se much closer to bulk water than the nanoscopic volumes of water studied by us here.
In this work, we show that emulsified solutions may or may not freeze at <231 K both upon cooling and warming.We also show to our best knowledge for the first time that: (i) freezing upon warming leads to a separation into pure ice and freeze-concentrated solution (FCS) and a second freezing event upon warming, (ii) the FCS of emulsified HCl/H 2 O and emulsified ∼20-21.5 wt.% HCl solutions undergo a glass transition at T g ≈ 127 and 128 K, respectively, that is below T g ≈ 136 K of water.Analyzing published data, we propose that water nanodrops adsorbed on fumed silica resemble bulk water more than water confined in nanopores or water in aqueous solutions.This is especially valid also at subzero temperatures.In aqueous solutions, some solutes such as ethanol suppress the anomalies of water in the supercooled state even at quite low mole fractions of 5%. 25

RESULTS
In Fig. 2, we present thermograms obtained from emulsified solutions.In 30 wt. % HNO 3 (thermograms (1)), the freezing of ice at T f,ice ≈ 167 K is followed by a subtle transformation of FCS to glass with the onset at T g ≈ 155 K. Upon subsequent warming the reverse glass-to-liquid transition at T g ≈ 152 K and freezing of FCS at T f,FCS ≈ 165 K are observed.Upon further warming two endothermic events are seen, which we assign to the eutectic melting of ice/NAT (nitric acid trihydrate, HNO 3 • 3H 2 O, at T e,ice/NAT ≈ 232 K) and to ice melting (small peak at T m,ice ≈ 237 K).In the ternary solution of eutectic composition 32.5/1 wt.% HNO 3 /H 2 SO 4 (thermograms (2)), no freezing occurs upon cooling but only a liquid-to-glass transition at T g ≈ 144 K. Upon warming first the glass-to-liquid transition is observed at T g ≈ 141 K, subsequently ice crystallizes at T f,ice ≈ 156 K and then the formed FCS freezes at T f,FCS ≈ 166 K to produce NAT and SAT (sulphuric acid tetrahydrate, H 2 SO 4 • 4H 2 O).Upon further warming only eutectic melting of ice/NAT/SAT occurs at T e,ice/NAT/SAT ≈ 231 K. 26,27 In 32.5/4 wt.% HNO 3 /H 2 SO 4 (thermograms (3)) also only a liquid-to-glass transition occurs at T g ≈ 145 K.However, upon warming above T g ≈ 142 K, a single prolonged freezing process between ∼195 and 217 K produces a solid mixture of ice, NAT and SAT.Upon further warming the eutectic melting of ice/NAT/SAT is shifted by H 2 SO 4 to T e,ice/NAT/SAT ≈ 229 K.The origin of the next melting double-peak is not clear.It could be produced by melting NAT and SAT.
Figure 3 displays the thermograms of emulsified HCl/ H 2 O.In 17 wt.% HCl/H 2 O (thermograms (1)), the freezing of ice at T f,ice ≈ 174 K is followed by a liquid-to-glass transition of FCS (onset is hard to determine because of the inclined baseline).In the warming thermogram, a reverse glass-toliquid transition at T g ≈ 127 K is observed.This is 9 K below water's T g ≈ 136 K.More precisely, it is 9 K below the calorimetric glass transition of low-density water, but 11 K above the calorimetric glass transition of high-density water. 28he nature of the second glass transition at T g ≈ 132 K is not clear.In the past, a T g of 126-130 K (also below water's T g ) was found for solutions of 2-3 mol.% of ethanol, ethylene glycol, and LiCl. 29he freezing of FCS at T f,FCS ≈ 162 K is followed by two eutectic melting events of ice/HAT (HCl trihydrate, HCl   2)), there are no freezing events but a liquid-to-glass transition which is also hard to determine.In contrast to 17 wt.% HCl, upon warming only one glass-to-liquid transition is seen at T g ≈ 127 K which is followed by ice freezing at T f,ice ≈ 149 K and the freezing of FCS at T f,FCS ≈ 155 K. Upon further warming, an eutectic melting occurs at T ice,HAH ≈ 200 K and ice melting at T m,ice ≈ 220 K.The melting of ice/HAT is absent.Finally, in 21.5 wt.% HCl/H 2 O (thermograms (3)), there are no freezing and melting events but only the glass transition at T g ≈ 128 K.
Figures 2 and 3 demonstrate that phase separation into pure ice and FCS occurs both when freezing occurs upon cooling 5 and when crystallization of the devitrified solution occurs upon warming above T g .As temperature decreases, the concentration inhomogeneity in liquid drops increases, i.e., solutions continuously fluctuate into pure water domains and regions of increasing concentration.In moderately concentrated solutions, the water/solute(s) ratio is relatively large and the size of pure water regions is ∼1.5 nm or larger. 5Ice nucleation in the largest region may trigger freezing of a drop both upon cooling or warming.That is, in these solutions the interaction with the solute causes the patches of water to freeze below the homogeneous nucleation temperature of pure bulk water upon cooling and to freeze above the crystallization temperature of ultraviscous pure bulk water upon heating.
It was assumed that if solutions separate into pure ice and FCS during freezing then the low-temperature behaviour of such solutions could be similar to that of water. 5If this is the case, then hydrogen bonding between H 2 O in solutions and pure water could not differ strongly and, consequently, emulsified solutions may be used for the study of supercooled water in the NML.This suggestion finds a strong confirma-tion in the above mentioned electron spin resonance measurements of FCS in frozen bulk TEMPOL/water solution. 18owever, no doubt that water hydrogen binding in solutions is perturbed, for example, by hydration shells.But how much the perturbations in solutions differ from those produced by pore walls remains an open question.
Figure 4 shows a comparison of the freezing behaviour for different types of water.Bulk water freezes rapidly at T f,ice and ice starts melting at ∼273 K, i.e., the calorimeter is well calibrated (thermograms (1)).The freezing of emulsified pure water drops occupies a large temperature region (thermograms (2)) and continues below T s ≈ 228 K.The double T f,ice is due to the bimodal size-distribution of drops.Nanodrops with a large free surface freeze at temperatures colder than emulsified drops and the freezing continues to ∼215 K (thermograms (3)). 22Ice melting occurs between ∼236 and 264 K, i.e., ice crystals are very small and have a broad size-distribution.The average diameter of ice crystals was calculated using the Gibbs-Thomson equation and was found to be 7.6 nm. 22The fact that water in pores of 3.43 nm in diameter freezes at temperatures warmer than the T f,ice of larger water samples suggests that it freezes heterogeneously (thermogram (4) vs. thermograms ((2) and ( 3)).Water nanodrops with a large free surface freeze at temperatures comparable or colder than that of water in nanopores (thermograms (3) vs. ((4) and ( 5)).It is unclear what the internal pressure of such adsorbed nanodroplets is, but according to the Young-Laplace equation it might be high, and so the droplets might resemble bulk water at higher pressures.Also, the surface to volume ratio in nanodrops is high, such that nanodrops might be representative of the surface structure of water.However, the absence of confining walls and long lifetime of nanodrops 22 makes them more suitable for a comparison with bulk water than water confined in nanoporous media.The thermograms (1) are obtained from a half-sphere drop (5.78 mg), thermograms (2) from emulsified water drops, thermograms (3) from water nanodrops with a large free surface of total weight of 1.7 mg, 22 and thermograms (( 4) and ( 5) from water loaded into cylindrical nanopores of diameter 3.43 and 3.03 nm. 30The filled arrows mark the end of freezing and open arrows the beginning and end of ice melting.The thermograms (1) were obtained using a Mettler Toledo DSC 822, (2) Perkin-Elmer DSC-7 at 5 K/min, (3) Mettler DSC-30 at 3 K/min, 22 and (( 4) and ( 5)) TA Instruments Model Q1000 DSC at 0.5 K/min. 30

CONCLUSIONS
Our calorimetric measurements show that liquid water may be studied in aqueous solution below 231 K and above 150 K. Water nanodrops adsorbed on fumed silica, emulsified aqueous solutions near the eutectic composition or water confined in nanometer-sized pores may be used to that end.Whether or not such studies may help in understanding bulk water needs to be addressed in future studies.We propose that water nanodrops adsorbed on fumed silica may show the least perturbation compared to bulk water and may be most relevant for our understanding of the properties of supercooled water.

18C533- 2 AFIG. 1 .
FIG. 1. Scheme of differently confined water.(a) Water in cylindrical nanopores, pure water regions in emulsified solutions drops, and water nanodrops obtained by the adsorption of vapour on fumed silica.(b) Fumed silica is a fractal object which forms a voluminous powder in which silica particles occupy less than 1% of total volume.

FIG. 2 .
FIG. 2. Cooling (upper blue lines) and warming thermograms obtained from emulsified HNO 3 /H 2 O and HNO 3 /H 2 SO 4 /H 2 O. Thermograms (1) were obtained from 30 wt. % HNO 3 , thermograms (2) from 32.5/1 wt.% HNO 3 /H 2 SO 4 which is the eutectic composition of 32.5 wt.% HNO 3 + 1 wt.% H 2 SO 4 , and thermograms (3) from 32.5/4 wt.% HNO 3 /H 2 SO 4 .Freezing of ice, FCS, and solid mixture of ice/NAT/SAT are marked by T f,ice , T f,FCS , and T f,mix , respectively.The melting of ice, NAT, SAT, eutectic ice/NAT, and ice/NAT/SAT are marked by T m,ice , T m,NAT , T m,SAT , T e,ice/NAT , and T e,ice/NAT/SAT , respectively.T g marks the onset of a liquid-to-glass and reverse glass-to-liquid transitions which are usually separated by ∼3K.Horizontal arrows mark the direction of temperature change.The scale bars denote heat flow through samples.