Metastable eutectic melting in the NaCl-H2O system

Accepted Manuscript Title: Metastable eutectic melting in the NaCl-H2 O system Author: V.A. Drebushchak A.G. Ogienko A.S. Yunoshev PII: DOI: Reference:

S0040-6031(16)30343-4 http://dx.doi.org/doi:10.1016/j.tca.2016.12.004 TCA 77649

To appear in:

Thermochimica Acta

Received date: Revised date: Accepted date:

16-8-2016 6-12-2016 10-12-2016

Please cite this article as: V.A.Drebushchak, A.G.Ogienko, A.S.Yunoshev, Metastable eutectic melting in the NaCl-H2O system, Thermochimica Acta http://dx.doi.org/10.1016/j.tca.2016.12.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Metastable eutectic melting in the NaCl-H2O system V.A. Drebushchak1,2,*, A.G. Ogienko2,3, A.S. Yunoshev2,4 1

V.S. Sobolev Institute of Geology and Mineralogy, SB RAS, Pr. Ak. Koptyuga, 3, Novosibirsk, 630090, Russia; e-mail:[email protected] 2 Novosibirsk State University, Pirogova, 2, Novosibirsk, 630090, Russia; 3 Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Sciences, Novosibirsk 630090, Russia; 4 Lavrentiev Institute of Hydrodynamics, Siberian Branch of Russian Academy of Sciences, Novosibirsk 630090, Russia.

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Highlights ● Eutectic melting in the NaCl-H2O system was detected with DSC for the first time. ● Enthalpy of the eutectic melting changes from endo to exo with the NaCl content. ● Enthalpy of hydrohalite formation was evaluated for the first time.

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Abstract Metastable eutectic melting of solid mixture NaCl+H2O was measured in a wide range of its compositions by using a DSC. The new eutectic melting was found to occur near –27 °C, below the well know ice-NaCl∙2H2O eutectic point (–21.1 °C, 23.3 wt. %) of equilibrium phase diagram. The new low-temperature eutectic was detected for all samples, in two two-phase regions, Н2О-NaCl∙2H2O and NaCl∙2H2O-NaCl. The new eutectic temperature is close to the value of ~ –28 °C reported by Roedder (1984) for the mixture of solid H2O and NaCl, which was never repeated or reaffirmed independently since then. The heat effect of the metastable eutectic melting did allow us to estimate the enthalpy of formation of hydrohalite from salt and ice: NaCl(cr) + 2H2O(cr) = NaCl∙2H2O(cr) + H (= –0.73±0.07 kJ mol–1).

Keywords: DSC; eutectic; hydrohalite; metastable; NaCl-H2O system

Introduction The phase diagram of binary system NaCl-H2O is well known for many decades (Fig. 1). It contains three liquidus lines (from the melting point of H2O to the eutectic point of Н2ОNaCl∙2H2O mixture; from the eutectic point to the peritectic point of NaCl∙2H2O; from the peritectic point to the melting point of NaCl), one eutectic line at –21.2 °C from H2O to NaCl∙2H2O, and one peritectic line at +0.1 °C from 26.3 wt. % NaCl to pure NaCl. The phase diagram was constructed after equilibrium data on mutual solubility of solid and liquid phases and the limits for homogeneous and/or heterogeneous particular phases in their composition and temperature to (co)exist. "Equilibrium" is the key word here. Most data used for the construction of the phase diagram in Fig. 1 were measured isothermally. The list of experimental data on the equilibrium in NaCl-H2O system from 1819 to 1988 was reported in [1]. Two exceptions were mentioned in the list, one with the fluid inclusions in healed fractures in quartz [2] and the other with thermal analysis [3]. Both are about high-temperature investigations and they seem to be included into the list in order to show the agreement between these two techniques and conventional equilibrium technique. Today, the phase diagram for NaCl-H2O system is considered well-known, with the variations in its characteristic points within the limits of experimental errors. One more eutectic point in the NaCl-H2O system was reported to exist at ~ –28 °C [4]. It was detected in the experiments with fluid inclusions in crystals, and attributed to the mixture of anhydrous NaCl and H2O. The phases in coexistence were identified visually, without direct structural investigation. Those data were not included in the list of references and not mentioned 3

among "characteristic points of the phase diagrams for NaCl-H2O system" in the survey on NaCl solubility in water (see [1] Table 1, p. xx), thus leaving them aside from the physical chemistry of the NaCl-H2O binary system. In geoscience, the ice – NaCl eutectic point at ~244 K (~ –29 °C) is still mentioned as metastable, though not marked on the phase diagram, [5], probably because of high scientific reputation of its discoverer (Edwin Roedder, 1919–2006) in geoscience community. The aim of this work was to measure directly the temperature of eutectic melting in the mixture of solid ice and NaCl. The experimental technique used was recently successfully applied to the eutectic melting of three polymorphs of glycine [6].

Experimental Sample preparation Ice powder was prepared from distilled water by freezing it in a metal mortar (1) kept in a liquid nitrogen. Ultrafine NaCl powder (specific surface area: 2.9±0.1m2/g) was prepared by sprayfreeze drying aqueous solutions (~ 12 wt. %) and used as the internal X-ray standard in our investigations [7-9]. NaCl powder was ground in another similar metal mortar (2) kept in the same bath with liquid nitrogen. The mixtures of ice + NaCl were prepared by adding some ice powder from mortar 1 into mortar 2 with NaCl powder. After the completion of the measurements with every particular mixture, mortar 2 was taken out of the bath with liquid nitrogen, heated up to the room temperature, washed off with water, and dried. Then it was used for the preparation of the next mixture. Overall, twelve samples were prepared and measured, ranged according to the ascending NaCl content and denoted as N1, N2, …, N12. Standard aluminum crucible (25 mkL) was also placed on the metal support in the same bath with liquid nitrogen and filled with the sample powder. The crucible was covered with a lid, but not sealed. The crucible with the sample was placed inside the DSC at temperatures below – 70 °C. The heat flow sensor of the DSC showed strong endothermic effect at the moment, proving that the crucible is undercooled, with its temperature much lower than –70 °C. It was impossible here to measure the sample mass by conventional weighing the crucible before and after loading the sample in it because all the low-temperature phases in the NaCl-H2O system melt below 0 °C or decay in peritectic reaction at +0.1 °C. So, we have weighed empty crucible, filled it with a mixture, carried out the experiments, and then weighed it again. The salt content of the mixture was derived from the additional weighing of the crucible with dry salt, after the evaporation of water. For three samples (N7, N8, and N10), the composition was measured both from the weighing the empty, filled and dried DSC crucibles and similar measurements of the same 4

mixtures in the standard aluminum crucibles for DSC-111 SETARAM (~400 mkL). These samples were prepared from the same initial reagents as the rest samples, but 3 months later. When preparing samples N7, N8, and N10 for the measurements, we found that the starting reagent NaCl has changed slightly after its storing for 3 months, becoming some denser, less fluid. One sample of solution (43.5 mg NaCl + 792 mg H2O = 5.2 wt. % NaCl) was prepared at room temperature, frozen in liquid nitrogen, and then ground. It was measured in two subsequent runs (cycling heating-cooling) in order to determine the correction in the onset of a melting point for the samples ground under liquid nitrogen. These results are discussed in section "Temperature correction".

Calorimetric measurements Calorimetric experiments were carried out using DSC-204 Netzsch, in a flow of dry argon (30 mL min–1). Heat flux sensor was calibrated according to the optimal two-point procedure [10], with additional testing measurements of ice melting at 0 °C. Additional calibration point was necessary here because the measurements were carried out at low temperatures, far beyond the conventional interval of calibration (between 156.6 °C for In and 419.5 °C for Zn). Recommended temperature fixed points are reproduced with the accuracy of ±0.1 °C. The measurements were carried out at a heating rate of 3 K min–1 in a temperature range from –40 to +7 °C. Before the measurements, the DSC cell was cooled down to –80 °C and kept isothermally for some time. Then the cooling system was switched off, and the temperature of the DSC cell increases "naturally" up to room temperature, with the heating rate decreasing exponentially. Preliminary cooling was arranged in such a way that the natural heating rate was less than 3 K min–1 above –40 °C, and the measurements were carried out without the cooling system working. Such a procedure yields extremely low noise in the DSC signal as compared with conventional measurements when the cooling system works.

X-ray powder diffraction X-ray diffraction experiment was done with Bruker D8 Advance equipped with a lowtemperature TTK 450 Anton Paar chamber. XRD patterns were measured in a temperature range from –100 to –10 °C and 2 scans from 5 to 47 °, with a step of 0.02 °.

Results and discussion Phase changes on heating

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X-ray powder diffraction patterns (XRPD) of the solid mixture of pure ice and NaCl did reveal the formation of hydrohalite some below the equilibrium eutectic temperature. Inaccuracy in the temperature values measured with our X-ray diffraction technique is too high, and the main result of structural experiments is the detection of the phase changes on heating, rather than the exact transition points. The XRPD patterns for the sample with NaCl in excess at five temperatures are shown in Fig. 2. The experiments starts near –100 °C, where the sample contains only mixture ice+NaCl. The dashes below the XRPD pattern indicate the positions of the reflections from hexagonal ice (H2O Ih) and halite (NaCl). The reflections of NaCl are stronger than those of ice. The sample remains unchanged at –50 °C. Then the changes begin. Well seen reflections of hydrohalite appear near –25 °C. They are multiple and start from less angles. Hydrohalite has more reflections as compared with ice and especially NaCl because its symmetry (monoclinic, P21/c [11]) is lower than that of ice Ih (hexagonal, P63/mmc [12]) and especially NaCl (cubic, Fm 3 m). The first reflection of hydrohalite (100) is at 15.36 ° (d = 5.76554 Å), lower than those of ice with (100) at 22.74 ° (d = 3.90664 Å) and especially NaCl with (111) at 27.466 ° (d = 3.24471 Å). The strongest reflection of hydrohalite is (131) at 37.116 °. The reflections of hydrohalite are somewhat greater at –22 °C than at –25 °C and increase even more at –10 °C. This is because of kinetic nature of the transition from starting solid NaCl+H2O mixture to final solid NaCl∙2H2O+NaCl mixture. The experiments with the ratio ice-to-NaCl close to the hydrohalite are shown in Fig. 3. Again, the starting solid NaCl+H2O mixture is detected at –100 and –50 °C. The reflections of hydrohalite appear at about –25 °C and increase at ~–22 °C. The reflections of NaCl disappear. At –10 °C, only the reflections of hydrohalite remain in the XRPD pattern, some weaker as compared with previous temperature. The sample is in the two-phase field of ice+hydrohalite, with coexistent hydrohalite and solution. The intensity of hydrohalite reflections decreases because of partial dissolution of hydrohalite in a liquid phase. The experiments with ice in excess are shown in Fig. 4. Again, the starting solid NaCl+H2O mixture is detected at –100 and –50 °C. The reflections of hydrohalite appear at about –25 °C and increase at ~–22 °C. They are very weak because of small amount of hydrohalite formed. The strongest reflection of NaCl (2 = 31.820 °) is seen at ~–25 °C, but disappears at ~–22 °C. The coexistence of three solid phases (ice+hydrohalite+NaCl) supports the idea of the lack of equilibrium in our experiments because the whole range from pure ice to pure NaCl under equilibrium is divided into two two-phase fields: ice+hydrohalite and hydrohalite+NaCl. The equilibrium is attained near –22 °C. At –10 °C, only reflections of ice are seen in the Figure. Ice coexists with the water solution of sodium chloride. 6

The temperature of eutectic melting, both metastable and equilibrium Metastable eutectic melting in solid NaCl+H2O mixture was detected at temperatures below the stable eutectic temperature H2O - NaCl∙2H2O for all samples. All the mixtures measured and their metastable and stable eutectic temperatures are listed in Table 1, ordered according to the NaCl content. [Table 1]

The heat effects of eutectic melting were found to change with the salt content and will be discussed here separately for the samples with ice in excess and salt in excess. The DSC results for ice-rich samples N1 – N4 are shown in Fig. 5. Three types of thermal effects are detected in three different temperature ranges: 1) eutectic melting of solid NaCl+H2O mixtures near –26.6 °C; 2) eutectic melting of solid H2O+NaCl∙2H2O mixtures near –20.8 °C; 3) liquidus melting at the temperatures depending on the H2O/NaCl ratio. Types 2 and 3 of thermal effects are well known after thorough investigation of the equilibrium in the NaCl-H2O system [1] and will not be discussed here. They show that the samples with solid NaCl+H2O mixtures behave in predicted way above the conventional equilibrium eutectic (H2O-NaCl∙2H2O) temperature of –21.2 °C, except the exact temperature of the eutectic melting, which is discussed below (see Temperature correction). The DSC results for samples N5 and N6 are shown in Fig. 6. Again, like Fig. 5, we can see eutectic melting of solid NaCl+H2O mixtures near –27 °C and eutectic melting of solid H2O+NaCl∙2H2O mixtures near –20.8 °C. The salt content of these samples (24.4 wt. %) is very close to the eutectic composition (23.16±0.05 wt. % [1]). The liquidus thermal effects for these samples are negligibly small, if any. The DSC results for six NaCl-rich samples (N7-N12) are shown in Fig. 7. DSC signal is not normalized to the sample mass. Only two types of thermal effects are seen in the Figure, one near –27 °C and the other with the onset near –21 °C. These are the metastable eutectic melting (NaCl-H2O) and equilibrium eutectic melting (H2O-NaCl∙2H2O), respectively. Their onset temperatures are listed in Table 1. In comparing the values of the onset point for stable (H2O-NaCl∙2H2O) and metastable (NaCl-H2O) eutectic melting in Table 1, we see evident difference in their reproducibility. All the stable values are between –20.6 and –20.8 °C or, in other words, –20.7±0.1 °C. As the accuracy of the temperature measurements for our DSC-204 is of about ±0.1 °C, we may conclude that the equilibrium eutectic temperature is reproduced very well. Here, we discuss equilibrium data only for the 1st run. The data for the 2nd run and their difference from the 1st run 7

will be discussed some below. For metastable eutectic, the onset values range in a wider ranger, from –26.5 to –27.5 °C. It is evidently beyond the limits of equipment accuracy. The reason is probably in the sample preparation. The DSC experiments were performed in two days, with the difference of three months between them. When preparing the solid NaCl+H2O mixtures, we have found that the NaCl powder did change in its flow ability after long storage, becoming denser, more packed. The thorough investigation of the relationship between the NaCl preparation and its temperature of metastable eutectic melting in the mixture with ice is beyond the scope of this work. For pure substances, it is well known that their purity affects their melting point. The highest melting point indicates the sample of the highest purity. For multicomponent eutectic melting, it is also well known that the addition of a component usually decreases the eutectic temperature of the mixture. Thus, the highest value of the melting point is always preferable for the characterization of a substance. In analyzing the values of melting points for metastable eutectic in the solid H2O+NaCl mixtures (Table 1), we may derive the best value as –26.6±0.1 °C.

Temperature correction Systematic difference in the eutectic temperature for Н2О - NaCl∙2H2O system after the first and second runs can be explained in two ways. First, we can use the sample purity like it was explained in the previous paragraph. When the starting solid NaCl+H2O mixture is prepared from pure components H2O and NaCl, hydrohalite formed above –26 °C is in mixture with pure components, and the melting point of that mixture (1st run) is high (–20.7±0.1 °C). Then, the sample melted into the solution is frozen down to new crystallization of hydrohalite with components also crystallized from the solution, and thus less pure, and the melting point of that mixture (2nd run) is low (–21.1±0.1 °C). Second, we can consider the grinding of the sample as the factor affecting the thermal conductivity between the sample and the heat flow sensor. Bad thermal conduction corrupts the signal response to the thermal events and thus shifts it to higher temperature. The problem of choosing between these two hypotheses can be solved after the experiment with the solution frozen (like in conventional 2nd run) and then grinded (like in conventional 1st run). DSC experiment in two runs with such a sample must reveal the reason of the systematic difference in the eutectic temperature. If the reason is in the purity, the melting points are to be identical in both runs. If the reason is in the grinding sample, the melting point after the 1st run is to be greater than that after the 2nd run by about 0.4 °C, like in all other experiments.

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The water solution of NaCl (5.2 wt. %) was prepared at room temperature and then frozen and ground like other samples before loading into the crucible. The equilibrium eutectic melting for the two runs are shown in Fig. 8. The signal is evidently corrupted with bad thermal conductivity for the 1st run as compared with typical peak of melting for the 2nd run. The values of the eutectic temperatures for these two runs differ from one another by 0.2 °C: –20.9 °C for the 1st run and –21.1 °C for the 2nd run. It is less than average difference in 0.4 °C, but we can see from Table 1 that the difference between two runs ranges from 0.2 °C (N1) to 0.6 °C (N11). This allows us to correct the temperatures values derived after the 1st run by 0.4 °C not only for the equilibrium eutectic melting, but also for the metastable eutectic melting. By this reason, we accept that the metastable eutectic temperature for solid H2O+NaCl mixtures is –27.0±0.1 °C. In concluding this section about the temperature correction for the values measured after the 1st run, we should remind that this problem arises from the fact that the metastable eutectic melting can be measured solely in the 1st run. For conventional DSC measurements, for example in the DSC calibration, it is usually recommended not to use the results of the first melting because they are considered incorrect, poorly reproducible. "In general, it is preferable to discard data obtained from the first melting run of a sample." [13]. In the case of solid NaCl+H2O mixtures, we have no second chance.

Heat effects of metastable eutectic melting: Ice-rich samples The DSC peak at the metastable eutectic point is in fact the complex thermal effect, consisting of several contributions. The shape of the peak is not the same for ice-rich and salt-rich samples. We will discuss them separately, starting from ice-rich samples. The first contribution is generated when the mixture of solid H2O and NaCl particles melts, forming the water solution of the salt: NaCl(cr) + H2O(cr) → solution (NaCl + H2O) +r1H.

(1)

This process is irreversible and endothermic. The irreversibility seems to be evident, but it was also tested in two runs with sample N2. Run 1 was stopped at –23 °C, immediately after the completion of the eutectic melting at –26.6 °C. Then the sample was cooled down again to – 80 °C and run 2 was started and proceeded according to conventional program. The endothermic peak at –26.6 °C was not detected. Reaction (1) is accompanied with two heat contributions, one from the enthalpy of fusion of eutectic composition, and the other from the enthalpy of dissolution of NaCl in water. Both contributions are endothermic. The water solution of sodium chloride formed after the metastable eutectic melting (Eqn. 1) is unstable below –21.1 °C with respect to the solid mixture of ice and hydrohalite: solution (NaCl + H2O) → H2O(cr) + NaCl∙2H2O(cr) +r2H. 9

(2)

This reaction is accompanied with two heat contributions, one from the enthalpy of formation of hydrohalite and the other from the enthalpy of its crystallization. Both contributions are exothermic because hydrohalite is thermodynamically stable under the P-T conditions when it is formed in our DSC experiments. Eutectic melting of solid NaCl+H2O mixtures from Figs. 5 and 6 is shown in detail in Fig. 9. The evolution of the thermal effect from low to high concentration is well seen. Samples N1 and N2 with the least concentration of NaCl look like a conventional endothermic peak. Sample N3 shows complex thermal process, revealing a small exothermic contribution immediately after the main endothermic effect. It is indicated in the Figure with the arrow. The exothermic contribution increases significantly for sample N4. Now endo- and exothermic contributions are very similar in value, and the net heat effect is close to zero. For samples N5 and N6, the exothermic effect yields the main contribution, and the net effect is negative. The true reason why the total heat effect at the metastable eutectic melting changes from positive (samples with ice >> NaCl) to negative (samples with ice ~ NaCl) values is not quite clear. Conventional thermodynamic consideration based on the partial molar values does not predict such a change in the sign because both endo- and exothermic contributions in reactions (1) and (2) increase linearly with respect to the NaCl concentration. We may suggest that the small impurity of hydrohalite formed after the metastable eutectic melting crystallizes uniformly over the whole sample volume into fine powder with large specific surface area and high surface energy. For higher concentrations, the size of the formed particles is to be greater. Direct calorimetric measurements of fine powder with different specific surface areas, which in turn depend on the size of particles, did reveal the difference in their enthalpy of formation as compared with bulk sample [14]. Excess drop solution enthalpies due to the additional surface energy were reported as high as 12 kJ mol–1 for TiO2 (rutile) [15], 16 kJ mol–1 for ZnO (various morphology) [16], 30 kJ mol–1 for Y2O3 (both cubic and monoclinic) [17], 12 kJ mol–1 for ZrO2 (monoclinic) [18]. The surface energy is the third endothermic contribution that decreases with the increase in the NaCl concentration, and thus can explain nonlinear behavior of the total enthalpy near the metastable eutectic melting. The net enthalpy of thermal effect at the metastable eutectic melting normalized per mole of hydrohalite formed decreases from 4 kJ mol – 1

for sample N1 to –0.7 kJ mol–1 for samples N5 and N6. These values seem reasonable and

agree with those from literature.

Heat effects of metastable eutectic melting: NaCl-rich samples Metastable eutectic melting of NaCl-rich solid NaCl+H2O mixtures is shown in Fig. 10. The DSC signal is normalized to the H2O content (J g–1 of H2O g). Samples N7 – N9 are from H2O 10

NaCl∙2H2O region and samples N10 – N12 are from NaCl∙2H2O - NaCl region. Samples N9, N11, and N12 are from freshly prepared NaCl, and samples N6, N7, and N10 are from NaCl 3 months old. No difference is seen between two regions of composition, while fresh and old samples differ from one another evidently. Fresh samples show small endothermic effect at the end of relatively large exothermic process, starting from about –33 °C and finishing near –24 °C. The onset point of the endothermic effect is –26.6 °C for N9 and –26.5 °C for N11 and N12, which are equal to –27.0 after the temperature correction for the 1st run. Old samples show the peaks similar with those in Fig. 9. Endothermic peak with the onset point of –27.2 °C for N7, –27.3 °C for N8, and –27.5 °C for N10 is followed by a comparable exothermic peak, like it was with samples N3 and N4. After the temperature correction, the onset temperature for old samples turns out to be close to –28 °C. It is interesting to remember here the works by Roedder (1984, p.429). He did detect the metastable eutectic melting of solid NaCl+H2O mixtures exactly near –28 °C. The samples were the saline water inclusions in halite crystals. They were surely "old", equilibrated after long storage. Thus, our results on synthetic solid NaCl+H2O mixtures with NaCl-rich specimens do confirm the results of Roedder (1984) on NaCl-brine inclusions in NaCl crystals, namely the metastable eutectic melting temperature of –28 °C, perhaps for the first time since the original works by E. Roedder. The exothermic process in fresh samples is rather small, its net enthalpy is not greater than –300 J mol–1 (mole H2O) or –600 J mol–1 (mole NaCl∙2H2O). The enthalpy evaluated for old samples within the same temperature interval is slightly positive, not greater than +10 J mol–1 (mole H2O) or +20 J mol–1 (mole NaCl∙2H2O). We normalize here the thermal effect to the amount of hydrohalite because it is hydrohalite that is formed after the finish of the whole process near –28 °C, with all its endo- and exothermic contributions. It is evident from the fact that all samples, even those with bulk composition falling into the NaCl∙2H2O - NaCl field, show the eutectic melting of 2H2O - NaCl∙2H2O field at –21.1 °C. It means that 1) hydrohalite was formed below equilibrium eutectic temperature after the reaction between pure ice and NaCl and 2) the reaction was incomplete because some ice was kept unchanged until equilibrium melting. This proves once more that the process measured by DSC in our experiments with NaCl-rich samples is nonequilibrium. The sample consists of bulk NaCl (main phase), NaCl∙2H2O (formed near –28 °C), and H2O (remains to interact with hydrohalite at –21.1 °C). Equilibrium among three phases (H2O+NaCl∙2H2O+NaCl) in the NaCl-H2O binary system is impossible. Hydrohalite: The enthalpy of formation

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In discussing metastable eutectic melting in ice-rich samples in section Heat effects of metastable eutectic melting: Ice-rich samples, we have considered two stages in sample transformations. These two stages did allow us to explain the evolution of net heat effect of metastable eutectic melting from endothermic at low NaCl concentration to exothermic at high NaCl concentration in the sample. At low concentration, fine powder of hydrohalite is formed with high surface energy, which results in net endothermic effect. At the concentration close to eutectic, the surface energy of the hydrohalite formed is negligible and the formation is accompanied with net exothermic effect. We may use the latter case for the estimation of the enthalpy of hydrohalite formation. Instead of two stages of the hydrohalite formation expressed by Eqns. (1) and (2), one should use only one equation for this purpose: NaCl(cr) + 2H2O(cr) = NaCl∙2H2O(cr) + H.

(3)

Before the start of metastable eutectic melting, the sample contains the mixture of NaCl(cr) and H2O(cr). After the completion of all thermal effects accompanying the metastable eutectic melting and before the equilibrium eutectic melting, the sample contains the mixture of H2O(cr) and NaCl∙2H2O(cr). NaCl in the sample is depleted completely to form hydrohalite, and the net thermal effect of the whole process is in fact the heat of its formation. Having the mass of NaCl in samples N5 and N6 and net heat effects measured for their metastable eutectic melting, we have evaluated the enthalpy of hydrohalite formation. The heat effect was integrated over the temperature range of about 5 K, finished at 250 K. Thus, enthalpy H in Eqn. (3) is the enthalpy of formation of hydrohalite from ice and salt: fH(250 K) = –0.73±0.07 kJ mol–1. The confident limits were estimated from two sources of errors: the errors in the enthalpy evaluation (±5 %) and the errors in weighing (±5 %). The error in the enthalpy evaluation is mainly due to the uncertainty in the baseline. An example of the difference in the enthalpy of 7 % between heating and cooling for the same reversible transition is in [19, Table 1]. In our experiments, the error in the weighing is large enough because the uncertainty in the mass loss for a sample of ~5 mg was about ±0.02 mg that results in the uncertainty up to 5 % in the NaCl concentration. From the very start, our experiments were aimed at the detection of thermal effects in the solid NaCl+H2O mixtures. To diminish the weighing errors and improve the accuracy of derived thermodynamic values, we plan to change the scheme of future experiments. The current enthalpy of the hydrohalite formation from ice and salt should be considered as a preliminary result. Our value of the fH(250 K) cannot be compared with literature data because the enthalpy of formation of hydrohalite from ice and salt was never measured or evaluated so far. Moreover, even the heat capacity of hydrohalite was never measured, but only estimated from 12

the analogy with other hydrates of alkali metal chlorides or taken as the sum of heat capacities of ice and salt [20]. Most relevant literature data to our fH(250 K) is the enthalpy of hydrohalite formation from elements fH m at 298.15 K derived by D.G. Archer in 1992 [20]. Its value was reported as –997.24 kJ mol–1 with uncertainty of 300 to 500 J mol–1. The fH m value was derived from fitting experimental results for the NaCl-H2O system over temperature range from 250 to 600 K. The whole procedure of fitting was rather complex and can be hardly analyzed for particular contribution in its total uncertainty. In following the conventional procedure of the calculation of the enthalpy of formation of a substance from elements, one should write explicitly all the reactions from pure elements to the substance, final product of all reactions. For hydrohalite, these equations are Na(cr, 298.15) + 0.5Cl2(g, 298.15) → NaCl(cr, 298.15) + H1; H2(g, 298.15) + 0.5O2(g, 298.15) → H2O(l, 298.15) + H2; NaCl(cr, 298.15) + 2H2O(l, 298.15) → NaCl∙2H2O(cr, 298.15) + H3. fH m = H1 + H2 + H3. Alternative set of equations is Na(cr, 298.15) + 0.5Cl2(g, 298.15) → NaCl(cr, 298.15) + H1; H2(g, 298.15) + 0.5O2(g, 298.15) → H2O(g, 298.15) + H2a; NaCl(cr, 298.15) + 2H2O(g, 298.15) → NaCl∙2H2O(cr, 298.15) + H3a. fH m = H1 + H2a + H3a. These are the simplest (direct) thermodynamic ways to calculate the enthalpy of formation of hydrohalite. Two first contributions, H1 and H2 (H2a), are known. The third contribution, H3 (H3a), is unknown and it is to be positive because hydrohalite is unstable at 298.15 K. And there is no experimental data on the reaction between water and salt producing hydrohalite at 298.15 K. By this reason, D.G. Archer did proceed with evaluations of the Gibbs energy of water-salt solutions. Now, we have the data for the direct reaction between salt and ice, fH(250 K) = –0.73±0.07 kJ mol–1, and it is completely within the uncertainty of the value derived by D.G. Archer: ±0.4 kJ mol–1. Conclusions DSC measurements of solid H2O+NaCl mixtures were performed in a temperature range from – 40 to +7 °C. Heat effects of metastable H2O-NaCl eutectic melting were detected near –27 °C. The sign of the heat effect changes from "+" (endothermic) for low NaCl concentration to "–" (exothermic) for the concentration close to eutectic one. The difference in the enthalpy of the eutectic melting in the H2O+NaCl samples agrees quantitatively with the surface energy 13

contribution published in literature for other substances. The enthalpy of hydrohalite formation at 250 K was derived from the enthalpy of metastable eutectic melting (fH(250 K) = –0.73±0.07 kJ mol–1). For NaCl-rich mixtures, the effect of NaCl storage was found out. Freshly prepared ultrafine NaCl powder interacts with ice exothermally starting from about –33 °C and finishing near –24 °C, with only small endothermic effect near –27 °C. Three months old salt interacts with ice near –28 °C, with endothermic effect. This finding supports the results reported by E. Roedder (1984) for NaCl-brine inclusions in halite crystals.

Acknowledgements VAD acknowledges that his work was supported by state assignment project №0330-2016-0004.

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Captions to Figures Fig. 1. Low-temperature part of the phase diagram of the NaCl-H2O system (according to [1]). Fig. 2. X-ray powder diffraction pattern of solid NaCl-H2O mixture, NaCl-rich sample. Two lines of vertical dashes at the bottom show the reflections of pure components, ice Ih and NaCl. Fig. 3. X-ray powder diffraction pattern of solid NaCl-H2O mixture with the NaCl:H2O ratio close to 1:2 (hydrohalite). Fig. 4. X-ray powder diffraction pattern of solid NaCl-H2O mixture, ice-rich sample. Fig. 5. DSC results for ice-rich samples. Three types of thermal peaks are seen well: 1) metastable eutectic melting near –27 °C, 2) equilibrium eutectic near –21 °C, and 3) liquidus with peak temperature above –5 °C. N1-I and N1-II are for the first and second heating of sample N1. Fig. 6. DSC results for samples N5 and N6 show good repeatability and small difference between two measurements. Only two types of thermal peaks are seen: metastable and equilibrium eutectics. Fig. 7. DSC results for NaCl-rich samples. Only two types of thermal peaks are seen: metastable and equilibrium eutectics. Irregularities in the amplitude of the equilibrium eutectic peaks are because of irregularities of sample mass (see Table 1). Fig. 8. Frozen water solution of NaCl (5.2 wt. %) prepared according to the procedure for solid NaCl+H2O mixtures. Comparison between 1st and 2nd runs. The onset point for the 2nd run is evidently less than that for 1st run. Fig. 9. Metastable eutectic melting for samples N1 – N6 in details. The arrows indicate exothermic contributions. Fig. 10. Metastable eutectic melting for samples N7 – N12 in details. The arrows indicate lowtemperature exothermic effects below the eutectic point.

16

Table 1. Solid NaCl + H2O mixtures used in the DSC experiments. sample №

NaCl, % wt.

NaCl, % mol.

sample mass, mg

eutectic temperature H2O - NaCl, °С

eutectic temperature Н2О - NaCl∙2H2O, °С

as measured, uncorrected

1st run

2nd run –21.0

1

1.9

0.60

5.34

–26.6

–20.8

2

2.5

0.78

6.33

–26.5

–20.7

3

4.0

1.26

3.60

–26.6

–20.8

4

5.4

1.73

4.78

–26.7

–20.8

5

24.4

9.05

3.48

–27.3

–20.8

6

24.4

9.05

4.25

–27.2

–20.7

7

32.0

12.68

2.28

–27.2

–20.7

–21.0

8

40.3

17.03

6.46

–27.3

–20.8

–21.2

9

58.1

29.96

4.66

–26.6

–20.8

10

63.0

34.47

3.29

–27.5

–20.8

–21.2

11

72.0

44.19

6.07

–26.5

–20.6

–21.2

12

84.1

61.97

7.06

–26.5

–20.6

17

Graphical Abstract

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