Effect of Adding Sodium Chloride on Tetra-n-butylammonium Chloride

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Effect of Adding Sodium Chloride on Tetra‑n‑butylammonium Chloride Semiclathrate Thermal Stability at Atmospheric Pressure Miad Siddiq* and Rodney Wigent

J. Chem. Eng. Data Downloaded from pubs.acs.org by UNIV OF GOTHENBURG on 12/07/18. For personal use only.

Department of Chemistry and Biochemistry, University of the Sciences in Philadelphia, 600 S 43rd Street, Philadelphia, Pennsylvania 19104, United States ABSTRACT: Tetra-n-butyl ammonium chloride (TBAC) is known to form semiclathrate cages when dissolved in water. Melting points determined using differential scanning calorimetry (DSC) were used to create a phase diagram for the binary TBAC−H2O semiclathrate system from 0 to 8 molal TBAC at atmospheric pressure. This phase diagram shows the existence of TBAC semiclathrate cages with a congruent melting point at about 286.8 K at approximately 2 molal TBAC as well as the existence of a TBAC−H2O eutectic mixture of unknown composition but with fewer associated water molecules at higher concentrations of TBAC. Further, NaCl was added to this binary system to create ternary solutions of various ionic strength fractions of TBAC (YTBAC). The results of DSC scans were used to create a phase diagram which showed that there was no effect on the system if there was sufficient bulk water to dissolve the NaCl. However, if there was insufficient bulk water to dissolve the NaCl, then the water in the clathrate cages of the TBAC was disrupted creating both lower melting semiclathrates, with fewer water molecules within the cage, and a new TBAC−NaCl−H2O peritectic structure of unknown composition with an invariant melting point. Further, if NaCl was added to the binary system with no free, bulk water present (i.e., ≥ 2 molal TBAC), then this system additionally showed a new eutectic TBAC−NaCl−H2O structure of unknown composition with an invariant melting point.

1. INTRODUCTION Previous studies have shown that clathrates or gas hydrates are “ice-like cages” of water molecules that host non- or slightly polar gas molecules, such as methane.1−3 These solid structures are formed only at high pressures and very low temperatures. Gas molecules are hosted inside the cavities of the network cages of water molecules that are hydrogen bonded to each other.2,4,5 Because of these clathrates’ ability to entrap gases, they have attracted extensive attention in recent years. These structures have been utilized in the thermal storage, storage and transportation of natural gas, and can be used to sequester greenhouse gases, such as carbon dioxide.2,4−6 Quaternary ammonium salts, such as the tetra-n-butyl ammonium halides (TBAX), have been shown to form clathrate-like structures, known as semiclathrates, that are more stable at lower pressures and higher temperatures than are the clathrate structures.7,8 This extra stability is due to the halide ion replacing one or more water molecules within the water cage forming ion−dipole bonds with the water molecules that are stronger than the dipole−dipole water bonds within the clathrate cages.9,10 Thermodynamic characterizations of tetra-n-alkyl ammonium salts, including the tetra-n-butyl ammonium halides, such as tetra-n-butyl ammonium bromide (TBAB), tetra-n-butyl ammonium chloride (TBAC), and less frequently, tetra-n-butyl ammonium fluoride (TBAF), have been studied.7,11−17 The thermal stability, as indicated by the © XXXX American Chemical Society

melting points of the semiclathrates, indicates that the melting points increase with the strength of the anion−water dipole bond, that is, melting points TBAF > TBAC > TBAB.18 Shimada et al., studied crystal growth of TBAB semiclathrates and determined their latent heats of crystallization, specific heats, and hydration numbers as a function of TBAC/H2O composition.19,20 The hydration numbers have been reported to be from 26 to 38 water molecules per molecule of TBAB and the melting point of the semiclathrate to be from 282.9 to 285 K, respectively, at different compositions. The phase diagram of TBAB semiclathrates with entrapped CO2, CH4, H2, and H2S also have been reported.7,8,13,21−24 TBAC semiclathrates are more stable than those formed from TBAB as the smaller chloride anion has a larger surface-charge density than does the bromide anion causing the chloride to form stronger ion−dipole bonds with water. However, only a few studies have reported thermodynamic characterizations of TBAC semiclathrates.9,25−27 TBAC is known to be a very hygroscopic molecule and is well accepted as having the ability to organize water in cagelike structures about the tetra-n-butyl ion (TBA+) that is similar to, but is somewhat different than, the water structures found in bulk water.28 While water structures in both bulk Received: September 10, 2018 Accepted: November 23, 2018

A

DOI: 10.1021/acs.jced.8b00810 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Concentrations of the Binary TBAC−H2O and the Ionic Strength Fraction of TBAC (YTBAC) in the Ternary TBAC−NaCl−H2O Prepared from the Binary Solutionsa binary TBAC molality (m)

standard uncertainty u(m)TBAC

0.000 0.10117 0.24071 0.43638 0.74324 1.01984 1.49849 1.74917 1.99560 3.00568 4.0009 4.9976 5.9852 7.0008 8.102c

±0 ±0.00002 ±0.00005 ±0.00009 ±0.00015 ±0.00021 ±0.00032 ±0.00039 ±0.00045 ±0.00078 ±0.0012 ±0.0017 ±0.0023 ±0.0031 ±0.017

TBAC ionic strength fraction in ternary TBAC-NaCl-H2O (YTBAC)a

0.90120 0.89769

0.74753 0.75544

0.50127 0.49917

0.24749 0.25032

0.90002 0.89879 0.90125

0.74992 0.75109 0.75421

0.50127 0.49831

0.35214 0.45236

Binary NaCl Molality (m)b 0.000 0.50675 1.46969 3.49838 5.01935 5.91285

Estimated standard uncertainty is u(YTBAC) = ±0.00006. bEstimated standard uncertainty is u(mNaCl) = ±0.00011 m. cDenotes the TBAC stock solution.

a

water and in the cage-like structures surrounding the TBA+ are thought to be largely driven by the hydrogen bonding between water molecules, the vicinal water next to the hydrophobic butyl groups of the TBA+ and the inclusion of chloride ions within this semiclathrate water cage causes the formation of more stable water structures compared to the structures found in bulk water.29 However, during industrial processes used to form these semiclathrates, it is likely that the TBAC−H2O complexes may be exposed to electrolytes, such as NaCl and KCl, which may affect the thermodynamic properties of TBAC−H2O complexes. Despite this, only few studies have examined the effect of these salts on formation and stability of these semiclathrates.8 Sangwai et al., have investigated the effect of presence of NaCl on the sequestering of CH4 by TBAB semiclathrates over the TBAB mass fraction range of 0.05 and 0.20 and the mass fraction of 0.03 and 0.10 for NaCl.8 It was reported that NaCl enhances the entrapment of CH4 within the TBAB semiclathrate at low concentrations of both TBAB and NaCl and inhibits it at high concentrations of either TBAB or NaCl. In this paper, we report the effect of the presence of NaCl on the thermodynamic properties of forming and melting of TBAC semiclathrates over a large range of concentrations of both the TBAC and NaCl that may potentially provide a more comprehensive insight in the optimization of semiclathrate formation and its potential applications, such as a gas sequestering agent or in thermal energy storage (e.g., thermal storage for air conditioning systems). The melting points of the semiclathrates and other complexes formed in binary tetra-nbutylammonium chloride and water (TBAC−H2O) and ternary TBAC−NaCl−H2O systems were investigated at atmospheric pressure. These were measured using differential scanning calorimetry (DSC) over solution concentrations ranging from 0 to ∼8 molal TBAC for binary TBAC−H2O solutions and over the range of ionic strength fractions from 0 to 1 of TBAC (YTBAC) for ternary TBAC−NaCl−H2O solutions containing approximately 1, 1.5, 2, 3, and 4 molal TBAC. Phase diagrams were created to show the influence of adding NaCl to TBAC semiclathrate systems.

2. EXPERIMENTAL METHODOLOGY 2.1. Materials. Tetra-n-butylammonium chloride (TBAC; CAS no. 1112-67-0) (>99.31%; 277.921 g/mol) was purchased from Chem-Impex International, Inc., sodium chloride (NaCl; CAS no. 7847-14-5) (>99.5%; 58.44 g/mol) was obtained from Fisher Science Education and SigmaAldrich, and silver nitrate (AgNO3; CAS no. 7761-88-8) (>99.0%) was purchased from Sigma-Aldrich. Distilled− deionized water (18.015 g/mol), with a resistivity of at least 18.3 MΩ × cm, was produced using Barnstead Nanopure II purification system. Gravimetric halide analysis showed the TBAC to be about (99.90 ± 0.04%) pure, so no additional purification was performed. 2.2. Equipment. A Mettler-Toledo analytical balance (MS205DU), with readability of 0.01 mg, was used for all weighings. To minimize the hydroscopic effects of TBAC, weighing burets were used to determine the precise mass of TBAC in all procedures. A Mettler Toledo differential scanning calorimetry (DSC822e) was used in this work. 2.3. Gravimetric Halide Analyses. TBAC purity was verified using a slightly modified gravimetric halide analysis.30 Previously, precisely weighed, empty weighing burets were filled with TBAC and then placed in a vacuum oven to dry for 48 h at 50 °C before being reweighed to precisely determine the mass of TBAC. The TBAC was dissolved in about 50 mL of distilled−deionized water containing 1 mL of reagent grade, concentrated nitric acid. The chloride in the resulting solution was precipitated as silver chloride by adding a slight excess of a 1 molal silver nitrate solution. The precipitated silver chloride was quantitatively collected and washed with the nitric acid solution, and rinsed with copious amounts of distilled− deionized water using a clean porcelain-filter crucible whose empty mass was precisely known. A drop of the silver nitrate solution was added to the filtrate to ensure all of the chloride had been precipitated. The filter crucible containing the precipitated silver chloride was dried in an oven at 140 °C overnight and cooled within a desiccator. The mass of silver chloride was determined by reweighing the crucible containing B



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the silver chloride and subtracting the mass of the empty crucible. The equivalent mass of TBAC was calculated from the calculated mass of chloride in the silver chloride (143.32 g/ mol) and then compared to the original TBAC mass to determine its purity. This analysis was repeated two more times to obtain mean and standard deviation values. A similar procedure was performed to verify the TBAC stock solution concentration (8.102 ± 0.017 molal TBAC). 2.4. Solutions Preparation. A series of binary TBAC− H2O solutions, ranging from 0 to 8.102 molal TBAC, were gravimetrically prepared from the stock solution. Binary NaCl−H2O solutions from 0 to 5.913 molal NaCl were gravimetrically prepared as well. Precise amounts of dried, solid NaCl were dissolved in known quantities of various concentrations of the binary TBAC−H2O solutions to produce ternary TBAC−NaCl−H2O solutions of various ionic strength fractions of TBAC, YTBAC, (i.e., ITBAC/(ITBAC + INaCl) where ITBAC and INaCl are the ionic strengths of TBAC and NaCl, respectively which are equal to their molalities in this case for these electrolytes with mono-cations and -anions). The lower limit of YTBAC (i.e., high NaCl concentrations) depended on the NaCl solubility in each of the binary solutions. The compositions of binary and ternary solutions are shown in Table 1. 2.5. Differential Scanning Calorimetry (DSC). Aluminum 40 μL DSC pans with a lid were used to hold the samples. An empty pan and lid served as the reference. The sample pans were filled with a precisely determined mass of about 22 to 25 mg of the liquid sample material to be studied and were sealed with a lid. All heating and cooling scans were performed at 1 K/min. The instrument was calibrated with indium and checked with ultrapure water. After loading the liquid sample solution at room temperature, a cooling scan was performed to a specified temperature, which ranged from 248 to 213 K, followed by a resting time of 10 to 15 min. Then a heating scan was performed until the sample reached 308 K. After the completion of this initial cooling and heating cycle, additional cooling and heating cycles of the same sample were performed as a check of the results.

Figure 1. Cooling thermograms of binary solutions of TBAC−H2O at concentrations from 0 to ∼2 molal TBAC at 1 K/min.

Figure 2. Cooling thermograms of binary solutions of TBAC−H2O at concentrations from 2 to ∼8 molal TBAC at 1 K/min.

3. RESULTS AND DISCUSSION 3.1. Binary Solutions of TBAC−H2O. Figures 1 and 2 show the DSC cooling thermograms for binary solutions of TBAC−H2O at concentrations from 0 to ∼2 and from ∼2 to ∼8 molal TBAC, respectively, obtained by cooling at 1 K/min. Similarly, Figures 3 and 4 show the DSC heating thermograms for binary solutions of TBAC−H2O at concentrations from 0 to ∼2 and from ∼2 to ∼8 molal TBAC, respectively, obtained by heating at 1 K/min. Normally, liquid water freezes at 273 K at atmospheric pressure. However, in Figure 1, the cooling scan for pure water does not show a normal freezing transition at 273 K. Rather, a relatively sharp exothermic transition occurs at about 251.5 K that is interpreted as the sudden and rapid freezing of the ultrapure water due to supercoolinga kinetic effect caused by the lack of nucleation sites to initiate the crystallization process for the freezing of pure, bulk water at the normal freezing point of water. It can be seen in Figure 1 that such transitions are observed for binary solutions of TBAC−H2O up until concentrations of 1.75 molal TBAC and disappear at 2.0 molal TBAC. Therefore, we believe that the lack of an observation of such a sharp transition in this temperature region indicates that there is insufficient bulk water, (i.e., water

Figure 3. Heating thermograms of binary solutions of TBAC−H2O at concentrations from 0 to ∼2 molal TBAC at 1 K/min.

that is not associated with a solute molecule) present to cause crystallization of pure water to occur. C



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The DSC heating scans seen for pure water and for solutions up through 1.5 molal TBAC in Figure 3 indicate that there are endothermic transitions with onset temperatures of about 273 K that are attributed to the melting of crystallized, bulk water, that is, ice. It should be noted that the melting temperatures for water, unlike the freezing points seen in Figure 1, are near 273 K as expected. The areas associated with these transitions decrease with increasing concentrations of TBAC. Since the areas of these melting transitions are proportional to the amounts of bulk water that are melting, then these observations indicate that increasing the concentration of the TBAC is causing the remaining amounts of free, bulk water in the solution to decrease. This is due to the need for more water to hydrate the TBAC as its concentration is increased. Also, it should be noted that none of the heating thermograms for the greater concentrations of TBAC in Figure 4 show evidence of any melting of free, bulk water near 273 K. Therefore, we believe that there is no free, bulk water present within these solutions as all of the water is tied up within the semiclathrate structures and these solutions now behave as ionic liquids at these concentrations. Examination of the DSC heating scans in Figure 3, for the same solutions that are in Figure 1, show higher-temperature, endothermic melting transitions beyond those seen for the free, bulk water with onset temperatures of about 273 K. Like that of the freezing transitions seen in Figure 1, the temperatures and areas of these melting transitions are found to increase with increasing TBAC concentration from 0.43 through 1.75 molal TBAC solutions. It is believed at 0.25 molal TBAC solution the semiclathrate melting transition is merged with the transition due to the melting of the bulk water. Similar to what was seen for this concentration in Figure 1, there is no evidence of a melting transition for the semiclathrate structures that might be occurring in the 0.1 molal TBAC solution. It should be noted that these transitions show very unsymmetrical peaks. These are very broad transitions whose breadth seem to decrease with increasing TBAC concentration. The shape of these transitions show that the melting begins to occur at lower temperatures with the degree/amount of melting increasing with temperature until the melting is completed. This suggests that these semiclathrate structures may consist of different substructures with different degrees of thermal stability. The heating of these semiclathrates may possibly cause a partial thermal degradation of these less stable substructures leaving the more stable semiclathrate structure intact to melt at a higher temperature. This may explain some of the different hydration numbers observed for these semiclathrates. It is interesting to note that the freezing temperatures observed in Figure 1 are different than the observed melting temperatures seen in Figure 3 for the same systems. It is believed that this might be partially due to a kinetic effect similar to that observed for the supercooling of pure water as the complexity of the semiclathrate structures, including substructures, would take time to become organized so that crystallization can occur, even at temperatures when it is thermodynamically favorable for freezing to occur. This would cause a delay in the freezing of these structures within the DSC sample pan. For this reason, it is believed that the use of the melting thermograms is more appropriate to analyze for these systems than are those observed in the freezing thermograms. Figure 4 shows that each of these solutions has two melting transitions. The lower temperature transition for each solution

Figure 4. Heating thermograms of binary solutions of TBAC−H2O at concentrations from 2 to ∼8 molal TBAC at 1 K/min.

Besides the freezing transitions observed for the supercooling of the bulk water in Figure 1, there is also evidence of other exothermic transitions, the temperatures and areas of which are found to increase with increasing TBAC concentration from 0.25 through the 1.75 molal TBAC solutions. These transitions are thought to be due to the crystallization/freezing of one or more of the semiclathrate structures formed from the binary TBAC solutions. The shapes of these transitions suggest that there may be two or more specific semiclathrate forms that are freezing out of the solutions though these have similar freezing points and, thus, have overlapping freezing transitions. The areas of these semiclathrate melting transitions have an inverse relationship to the area of the transition attributed to the freezing of the bulk waterthe areas due to the freezing of the semiclathrates increase with increasing amounts of TBAC requiring more water to be tied up in its hydration which, in turn, decreases the amount of free, bulk water within the solution. It is of interest to note that a freezing transition that can be attributed to a semiclathrate is not observed in the 0.1 molal TBAC solution. This may be because the TBA+ and chloride ions are too dilute to interact to form the semiclathrate structure and the solution behaves as a normal electrolyte solution, or that the amount of semiclathrate in the solution is too dilute to allow appreciable crystallization of the semiclathrate to occur upon cooling. Further, the cooling scan for 2 molal TBAC shows only a single freezing transition which is attributed to the freezing of the semiclathrate and no evidence for the freezing of free, bulk water. Unlike in Figure 1, in Figure 2 there is lack of any sharp freezing transitions in the temperature region in which free, bulk water freezes. This is interpreted as indicating that there is insufficient bulk water present to cause the crystallization of free, bulk water to occur upon cooling. Therefore, the exothermic freezing transitions seen for all of the binary TBAC solutions in Figure 2 are believed to be indicative of the freezing of different forms of the TBAC semiclathrate structures. As previously noted, the shapes of many of these transitions suggest that there are multiple forms of the semiclathrate structures, with similar freezing points, causing the freezing transitions to overlap within these thermograms. D



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has similar onset melting temperature but the area of this transition is found to increase with increasing TBAC concentration. Each of these solutions also exhibits a second, higher-temperature, broad transition. The onset melting temperature for the higher temperature transition is found to decrease with increasing TBAC concentration such that melting is completed at lower temperatures. It is of interest to note that the start of melting for this second, highertemperature transition seems to occur during the melting process observed for the lower-melting transition−this may be due to the melting of substructures within the semiclathrate as suggested above. Further, the area associated with this higher temperature transition is found to decrease with increased concentration of TBAC. It is believed that at these concentrations, where there is no evidence for the presence of any free, bulk water, there is insufficient free, bulk water available to form the optimum water to TBAC ratio in forming the most stable primary semiclathrate structure. Therefore, it is postulated that since there is no free, bulk water to hydrate the TBAC, when more TBAC is added to the solution, then the semiclathrate structures already in the solution must give up some of their water to hydrate this additional TBAC. This causes these semiclathrate structures to reorganize, possibly forcing the TBAC to share semiclathrate water cages, resulting in structures with fewer water molecules per TBA+ ions that are less thermally stable than that of the optimally hydrated TBAC. This forces the TBA+ ions to be in closer proximity to each other. The repulsive forces of the adjacent positive charges on the nitrogen of the TBA+ are thought to be partially offset by the small cohesive forces between the butyl groups of these molecules due to their hydrophobic nature and the positive entropy contributions to the free energy due to the freeing up of some of the waters of hydration in the new structure while decreasing the water/TBA+ ratio. It is believed that this new structure may be partially stabilized even further by the incorporation of additional chloride ions within the new semiclathrate cage. As previously mentioned, the chloride− water interactions are stronger than the water−water interactions within the semiclathrate structure. The driving force for a hydrated chloride ion to be incorporated within the semiclathrate structure is probably largely due to the need to balance the electrical charges of the TBA+ ions within the semiclathrate structures and will likely help to alleviate the electrostatic repulsion effect between the cationsindeed, it is likely that the chloride ions are more likely to be located within the water cage that is near the positively charged nitrogen of the TBA+. It is postulated that there is a competing effect of long-range Columbic repulsion of the chloride ions within the semiclathrate structure that decreases the stability and limits the number of chloride ions that can be present within this structure. Even then, any resulting associated semiclathrate structure is expected be less thermally stable than the optimally hydrated molecule and the melting temperature should decrease. This hypothesis should be tested through experimental methods, such as X-ray crystallography or spectroscopy, and/or computational methods. Figure 5 and Table 2 show the phase diagram for the binary TBAC−H2O system from 0 to ∼8.1 m TBAC (0 to 0.13 mol fraction of TBAC, XTBAC, that is, the moles of TBAC divided by the sum of the moles of TBAC and water) based on the onset melting temperatures of the transitions seen in Figures 3 and 4. The melting point of the semiclathrate species is found to increase until a congruent melting point is reached at 286.7

Figure 5. Phase diagram of binary TBAC−H2O. ●, ■, and ◆ are TBAC semiclathrate, H2O, and TBAC−H2O eutectic, respectively. (Note: The onset melting points for the TBAC semiclathrate at 7 and 8 molal are estimated.)

Table 2. Onset Melting Points of TBAC Semiclathrate, H2O and TBAC−H2O Eutectic Present in Binary TBAC−H2O Solutionsa

a

binary TBAC molality (m)

TBAC mole fraction (XTBAC)

TBAC semiclathrates onset melting point (K)

0 0.43638 0.74324 1.01984 1.49849 1.74917 1.99560 3.00568 4.0009 4.9976 5.9852 7.0008 8.102

0 0.0078 0.0132 0.177 0.0263 0.0305 0.0347 0.0513 0.0672 0.0825 0.0973 0.1119 0.1286

277.39 281.06 283.34 285.99 286.71 286.77 283.54 279.95 277.31 274.40 271.80 269.82

H2O onset melting point (K)

TBAC−H2O eutectic onset melting point (K)

272.50 271.70 271.63 271.60 270.99

267.92 267.91 267.84 267.74 267.58 267.48

Estimated standard uncertainty is u(T) = ±0.02 K.

K at a mole fraction of TBAC of about 0.031 (between 1.74 to 2 molal TBAC). Additionally, the melting point of water, when observed, is found to decrease (i.e., freezing point depression) but less than anticipated for mole fractions below the congruent melting point. Indeed, the degree of freezing point depression suggests that there are few free ions present in the solution, resulting in a smaller than expected van Hoff factor, which adds credence to the chloride ion being tied up in the semiclathrate cage balancing the charge of the TBA+ within the cage structure. Over this composition range, the system has excess bulk water in which to form semiclathrates with different degrees of hydration per TBA+ molecule. It is believed that the degree of hydration decreases with increased TBAC forming thermally more stable hydrates until the composition of the congruent melting point is reached. At mole fractions greater than that in which the congruent melting point occurs, the temperature of the melting point of the high-temperature semiclathrate structure decreases with increase in TBAC content. This is thought to be due to the total depletion of the free, bulk water forcing the semiclathrates E



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WSample = WTBAC + Wh(f) + Wh(b)

to begin to share water cages that are less thermally stable with increase in TBAC concentration. The onset melting peaks of TBAC semiclathrate at around 7 and 8 molal TBAC were not totally resolved in the DSC thermograms, so these onset melting points are estimated. Further, since there is insufficient water to fully hydrate the TBAC to its most stable state, a portion of the TBAC forms an eutectic hydrate with a melting point seen at about 267.9 K. Figure 6 presents a comparison of equilibrium temperatures of TBAC−H2O semiclathrate melting transitions in this work

(1)

However, this can be corrected by dividing the known heat of fusion per gram of pure water, ΔHfus(h), (i.e., −0.33355 J/mg) into the integral heat of transition, Ah, for the bulk free water reported by the DSC scan for a particular run, to give the mass of the free water in the sample, Wh(f), as shown in eq 2. Ah ΔHfus(h)

Wh(f) =

(2)

Since the total sample mass is known as is the concentration of the TBAC (mol TBAC/kg H2O), mTBAC, within the solution, then the mass of TBAC, WTBAC, and the total mass of water, Wh(t), can be determined using the molecular weight of TBAC, MwTBAC, according to eqs 3A and 3B). WTBAC = mol TBAC × MwTBAC

(3A)

Wh(t) = WSample − WTBAC

(3B)

The calculated free-water mass can be subtracted from the total mass of water to determine the mass of the bound water as shown in eq 4. Wh(b) = Wh(t) − Wh(f)

(4)

The heat of fusion for the semiclathrate, ΔHfus(c), can be determined using eq 5 ΔH fus(c) =

Figure 6. TBAC−H2O disassociation temperature comparison between current study and previous studies. ○, △, ▽, □, and ◇ are current study, Nakayama, Sun et al, Sato et al. and Ye et al, respectively.

Ac Wh(b) + WTBAC

(5)

where Ac is the peak area of the semiclathrate melting transition. The hydration number (i.e., the ratio of the number of moles of bound water, nh(b), to the number of moles to TBAC, nTBAC, is given by eq 6. nh(b) hydration number = n TBAC (6)

using the peak temperatures of the melting transition with that of others reported in the literature using various methodologies. Figure 6 shows general agreement of the melting temperatures of the semiclathrate structures from ∼0.5 through 3 m TBAC from this study with those reported by Nakayama, Sun, Sato, and Ye.10,16,31,32 However, it appears that up through the congruent melting point, Sun and Ye’s data report lower temperatures for their melting points. Beyond the congruent composition, our data closely corresponds with that of Ye, but is below that reported by Sato. We have noted, through experimentation, that the transition temperatures of this system are very sensitive to the thermal history of this system. If the system is not cooled down to a sufficiently low temperature prior to being heated, the melting temperatures can be greatly affected. It is believed that this is, in part, due to the kinetic effect mentioned above. This is often referred to as a memory effect in the literature.14 This can justify the observed deviation in equilibrium temperatures (melting transitions) for TBAC−H2O semiclathrates that use different methodologies. The DSC is programmed to calculate the heat of fusion, ΔHfus, by dividing the integral area under the transition curve, A, by the mass of entire sample (mg), WSample. However, as shown in Figure 1, when there are two different melting transitions, both of which involve water, then dividing each of the transition areas by the total sample mass, which includes the masses of both free bulk water, Wh(f), and water bound in the hydration of the semiclathrate, Wh(b), as well as the mass of TBAC, WTBAC, as shown in eq 1, this analysis would be incorrect and lead to substantial inaccuracies.

and the values of the numbers of moles of TBAC and water are determined using eqs 7A and 7B. n TBAC = nh(b) =

WTBAC MWTBAC

(7A)

Wh(b) MWh(b)

(7B)

where ΔHfus(h) and ΔHfus(c) are the heats of fusion of water (−333.55 J/g) and the semiclathrate, respectively. The calculated heat of fusion and the hydration number of the semiclathrates formed from 0.4365 to 1.996 molal TBAC are shown in Table 3. For solutions less than 2 m TBAC, the magnitude of the heat of fusion of formed semiclathrate increases through 2 m TBAC. (Note that the value for 1.749 m TBAC may be within experimental error. This gives evidence of forming the most stable structure of TBAC−H2O semiclathrate once all of the free water has been consumed. Above 2 molal TBAC, there are two melting transitions which overlap and cannot readily be separated. Therefore, the method used above to determine the heats of fusion cannot be applied for solutions above 2 molal TBAC.) 3.2. Binary Solutions of NaCl−H2O. Figure 7 reports the DSC thermogams of NaCl−H2O from 0 to approximately 5.9 molal NaCl. The transition for the melting of ice indicates a F



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Table 3. Heats of Fusion and Hydration Number of the Semiclathrates at Various Concentrations of TBACa binary TBAC molality (m) 0.43638 0.74324 1.01984 1.49849 1.74917 1.99560

XTBAC

semi-clathrates heat of fusion ΔHfus(c) (J/g)

semi-clathrates hydration number per mole TBAC

0.0078 0.0132 0.177 0.0263 0.0305 0.0347

−29 −121 −144 −191 −185 −192

38.5 35.4 32.5 32.4 30.7 27.8

a Estimated standard uncertainties are u(ΔHfus(c)) = ±1 J/g and u(hydration number) = ±0.2.

Figure 8. Heating thermograms of ternary TBAC−NaCl−H2O of 1 m TBAC at various TBAC ionic strength fractions at 1 K/min.

heating thermogram for the binary solutions containing 1 molal TBAC in Figure 8 shows a melting transition for ice (i.e., unbound, bulk water) at about 271.5 K. As the amount of NaCl is increased in these ternary solutions, the peak for this transition becomes broader and the area of the transition is found to decrease indicating that the amount of free, bulk water is decreasing with an increase in added NaCl, that is, decrease in YTBAC, until this transition is minimally observable at YTBAC = 0.25. This suggests that there may be no appreciable free, bulk water present in the solution above this concentration. Additionally, the observed temperature of this transition is found to decrease with increased ionic strength fraction of NaCl−a typical freezing point depression of the solvent. Another melting transition is observed at ∼252 K for all solutions containing NaCl. This transition is believed to be due to melting of the NaCl−2H2O eutectic mixture. Increasing the concentration of the NaCl (i.e., lower values of YTBAC) increases the area under of the curve for this transition indicating that there is more of this particular species present with increased NaCl concentration. Finally, for solutions YTBAC from 1 to 0.5, the heating scans show a melting transition occurring from about 275 to 287 K, which is attributed to the melting of the TBAC semiclathrate structures formed from these solutions. The temperature of this melting transition is found to decrease substantially at YTBAC of 0.25. The areas and temperatures of these transitions seem to remain relatively constant when the NaCl concentration is increased through YTBAC of 0.5 but decreases thereafter. It is interesting to note that the temperature range of this transition becomes very broad at YTBAC = 0.25. It is believed that there is sufficient free, bulk water to allow for the formation of the NaCl−2H2O eutectic compound for YTBAC ≥ 0.5. However, for solutions with YTBAC < 0.5, it is believed that there is insufficient free, bulk water to form the NaCl−2H2O eutectic and instead water for this purpose is obtained by disrupting the TBAC semiclathrate structure by removing some of the water making up the semiclathrate cage. This forces the formation of new semiclathrate structures, with less water per TBAC molecule, that are less thermally stable. Similar trends were observed in the ternary TBAC−NaCl− H2O solutions containing 1.5 m TBAC at various values of YTBAC (Figure 9). In this case, all of the ternary solutions were

Figure 7. Heating thermograms of binary solutions of NaCl−H2O at concentrations from 0 to ∼6 molal NaCl at 1 K/min.

freezing point depression is occurring with increase in NaCl concentration up through 3.5 molal NaCl. Further, the decreasing areas of this transition with increasing NaCl concentration indicates that the amount of free, bulk water in these solutions is decreasing. The transitions occurring at about 251 K indicate the melting of the NaCl−2H2O + ice eutectic mixture and the eutectic composition is reported to be 23.3 w/w% NaCl.33−35 Since the melting scan for the 5.0 molal NaCl solution has a NaCl composition of 22.7 w/w%, which is below the eutectic composition, it is believe that this transition is superimposed with a small transition for the melting of free, bulk water. However, the melting scan for the 5.9 molal NaCl solution has a NaCl composition of 25.7 w/w%, which is above the eutectic composition, and there is no free, bulk water to melt. Interestingly, we did not see a peritectic transition for this concentration as predicted from the published phase diagrams similar to that observed by Han et al.33 The heat of fusion of NaCl−2H2O + ice eutectic mixture is calculated to be 233.6 ±1.5 J/g eutectic mixture which is in good agreement with the value of 233.0 ± 1.6 J/g eutectic mixture reported by the same study. 3.3. Ternary Solutions of TBAC−NaCl−H2O. Figure 8 shows heating thermograms for ternary TBAC−NaCl−H2O solutions containing 1 m TBAC with added NaCl to give various ionic strength fractions of TBAC, YTBAC. The cooling curves for these solutions (data not shown) show evidence for the existence of free, bulk water as evidenced by the sharp supercooling peaks similar to those observed in Figure 1. The G



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Figure 9. Heating thermograms of ternary TBAC−NaCl−H2O of 1.5 m TBAC at various TBAC ionic strength fractions at 1 K/min.

increase in added NaCl, a new melting transition is found to occur at about 270 K (Complex 1). This may be due to the formation of the peritectic complex of TBAC−NaCl−H2O of unknown composition as observed in Figure 9. In the binary 2 m TBAC system, it is believed that all of water molecules are bound to the TBAC semiclathrate. Therefore, this suggests that the most stable semiclathrate structure is being disrupted with increasing NaCl ionic strength fraction. This disruption causes the release of water molecules, decreasing the area of TBAC semiclathrate melting transitions, and shifting the melting transitions to lower temperatures. The released water molecules were incorporated in both the NaCl−H2O eutectic and the new peritectic TBAC−NaCl−H2O complex. At YTBAC = 0.35, only one melting peak was seen between 270 to 273 K. Similar to Figure 9, this is thought to be due to the merging of melting transitions of the semiclathrates and the peritectic TBAC−NaCl−H2O complex (Complex 1). The heat of fusion of NaCl−2H2O eutectic mixture calculated from binary NaCl−H2O (Figure 7) was used to obtain the percentage of NaCl incorporated in NaCl−2H2O eutectic mixture within this ternary system. Table 4 indicates that the NaCl percentage

cooled to 233 K causing the freezing of the NaCl−2H2O eutectic mixture. As with the transitions for the eutectic mixtures in 1.0 molal NaCl, for these samples, these transitions consistently melted between 250 to 252 K. However, while the area of this transition was found to increase with increase in NaCl through YTBAC = 0.5, the area of this transition was found to decrease for YTBAC = 0.25. The transitions for the melting of the semiclathrates at 1.5 molal TBAC were found to occur at slightly higher temperatures than those found for the corresponding values of YTBAC for the 1.0 molal TBAC solutions in Figure 8. The temperature and area of each of these transitions for YTBAC < 0.75 were found to decrease with increasing NaCl concentration. This lends credence to the hypothesis that there was sufficient free, bulk water for the NaCl to form the eutectic mixture through YTBAC ≥ 0.75, but beyond this point, water is being removed from the semiclathrate structure in order to form the NaCl−2H2O eutectic mixture. At YTBAC = 0.5, a new melting transition was observed at approximately 270 K. It is believed that this transition may be similar to the melting of the peritectic mixture for NaCl−H2O, which occurs ∼273.1 K near the saturation point of this system (about 6.4 molal NaCl), and is due to the formation of a new peritectic complex of TBAC− NaCl−H2O (Complex 1) of unknown composition. Furthermore, it is believed that this transition is also formed at YTBAC = 0.25 and its melting point is merged with the semiclathrate melting peak. Figure 10 shows the thermograms of ternary TBAC−NaCl− H2O solutions of 2 m TBAC at various values of YTBAC. The NaCl−2H2O eutectic mixture melting peaks were observed for all of the ternary TBAC−NaCl−H2O solutions, except for YTBAC = 0.35, over the same temperature range seen in Figures 7 and 8 for 1 and 1.5 m TBAC, respectively. The area of this transition increased with increased NaCl concentration until YTBAC = 0.75 and then decreased thereafter. There was no evidence of any free, bulk water from the corresponding freezing thermograms (data not shown) which suggests that the water needed to form the NaCl−2H2O eutectic must have come from the TBAC semiclathrate structure. Thus, the temperature and the area of the melting transition for the original semiclathrate were found to decrease from that found for the binary solution with decrease in YTBAC. With the

Table 4. Percentage of NaCl within NaCl−2H2O Eutectic Mixture in TBAC−NaCl−H2O Ternary Solutions of 2 m TBAC at Various Ionic Strength Fraction of TBAC (YTBAC) YTBAC NaCl%

0.9 34

0.75 29

0.50 5

associated in the eutectic mixture, in the ternary solution containing 2 molal TBAC as an example, decreases with an increase in the ionic strength fraction of NaCl. This confirms that some of the NaCl must have been incorporated to the new peritectic TBAC−NaCl−H2O complex. Figure 11 shows that at 3 m TBAC, without adding any NaCl, a melting transition of the semiclathrate occurs over the temperature range of about 272 to 289 K in the absence of free, bulk water. As shown in the binary TBAC−H2O thermograms above 2 molal TBAC (Figure 4), melting transitions for the TBAC−H2O eutectic were observed from about 267 to 269 K. However, with the addition of NaCl, YTBAC = 0.9, both the TBAC semiclathrate and the TBAC eutectic melting transitions are shifted to lower temperatures H



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which is believed to be due to the formation of a new eutectic TBAC−NaCl−H2O complex (Complex 2) of unknown composition. This new, lower temperature eutectic TBAC− NaCl−H2O complex show melting transitions from 263 to 266 K for YTBAC values from 0.9 through 0.45 compared to the corresponding binary transitions. Furthermore, NaCl destabilizes the original TBAC semiclathrate structure, probably by removing some of the water from the semiclathrate cage, with the addition of NaCl. For all ternary solutions, there is no evidence for the occurrence of the NaCl eutectic transition at about 252 K. However, starting at YTBAC = 0.75, we observe an additional melting peak about 270 K. It is believed that this peak is due to the formation of the peritectic TBAC−NaCl− H2O complex, (Complex 1), as seen in Figures 9 and 10. This transition is found to merge with the semiclathrate melting peak as the NaCl concentration increases. Exothermic, entropically driven, solid−solid transitions are observed at 238 K for YTBAC = 0.75 and about 228 and 251 K for YTBAC = 0.5, and at 251 K for YTBAC = 0.45. These unknown transitions, which only occur during the heating process, are thought to be due to entropically driven solid−solid transitions where ΔH > 0 and ΔS > 0. Figure 12 shows the thermograms of ternary TBAC−NaCl− H2O solutions prepared from 4 m TBAC at various values of YTBAC. The addition of NaCl shows effects similar to those observed for 3 m TBAC in Figure 11. A new, lowertemperature eutectic TBAC−NaCl−H2O complex of unknown composition with melting transitions from 263 to 266 K is observed for YTBAC values of 0.9 and 0.75. A melting transition, thought to be due to the melting of the peritectic TBAC− NaCl−H2O complex (Complex 1) of unknown composition, and an entropically driven solid−solid transition at ∼238 K are observed at YTBAC = 0.75. Figure 13 and Table 5 present the phase diagram of combined binary TBAC−H2O from 0 to 8 molal TBAC and for the ternary TBAC−NaCl−H2O system at YTBAC = 0 to 1, formed from approximately 1, 1.5, 2, 3, and 4 molal TBAC solutions (for convenience, TBAC molalities and ionic strength fraction of TBAC, YTBAC, values have been rounded). The melting points of ice (free, bulk water) decreased within the ternary systems formed from 1 and 1.5 molal TBAC due to the increase in the concentrations of both TBAC and NaCl.

Figure 13. Phase diagram of combined binary TBAC−H2O (0 to 8 molal) and TBAC−NaCl−H2O YTBAC = 0 to 1 formed from 1, 1.5, 2, 3, and 4 molal TBAC. (Note: The onset melting points for the TBAC semiclathrate at 7 and 8 molal are estimated). ◑, ○, ⊕, ⊙, ◒, ◓, and ◐ indicate the melting point of semiclathrate at YTBAC = 1, 0.9, 0.75, 0.5, 0.45, 0.35, and 0.25 at the noted molalities, respectively. ◇ indicates the melting point of TBAC−H2O eutectic at YTBAC = 1. filled right corner square, □, ⊞, and filled left corner square indicate the melting point of H2O at YTBAC = 1, 0.9, 0.75, and 0.5 at the noted molalities, respectively. △, ▲, ▽, and ▼ indicate the melting point of complex 1 at YTBAC = 0.5 and 0.25 at 1.5 molal TBAC, YTBAC = 0.9, 0.75, 0.5, and 0.35 at 2 molal TBAC, YTBAC = 0.75, 0.5, and 0.45 at 3 molal TBAC, and YTBAC = 0.75 at 4 molal TBAC, respectively. ◆ and filled right corner diamond indicate the melting point of complex 2 at YTBAC = 0.9, 0.75, 0.5, and 0.45 at 3 molal TBAC and YTBAC = 0.9 and 0.75 at 4 molal TBAC, respectively. ■ indicates the melting point of NaCl·2H2O eutectic at YTBAC = 0.9, 0.75, 0.5, and 0.25 at the noted molalities.

However, the degree of the temperature depression for the melting of water is much greater than anticipated. While we believe that the chloride from the TBAC is associated with the TBA+ within the semiclathrate structure making the van’t Hoff factor effectively equal to 1 for the TBAC in the binary TBAC−H2O system, we also believe that the situation is more complicated for the ternary system containing NaCl. It is hypothesized that the TBAC semiclathrate structure has extensive amounts of water tied up within this structure I



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Table 5. Onset Melting Points of TBAC Semiclathrate, H2O, TBAC−H2O Eutectic, Complex 1, Complex 2, and NaCl·2H2O Eutectic Present in Ternary TBAC−NaCl−H2O Solutionsa onset melting point (K) binary TBAC molality (m) 0 0.43638 0.74324 1.01984

1.49849

1.74917 1.99560

3.00568

4.0009

4.9976 5.9852 7.0008 8.102

YTBAC

1 0.9 0.75 0.5 0.25 1 0.9 0.75 0.5 0.25 1 0.9 0.75 0.5 0.35 1 0.9 0.75 0.5 0.45 1 0.9 0.75

TBAC semiclathrate 277.39 281.06 283.34 283.3 283.52 282.05 275.40 285.99 285.67 284.37 280.13 272.09 286.71 286.77 284.86 282.79 277.13 271.98 283.54 281.78 278.24 271.03 271.02 279.95 278.04 273.86 277.31 274.40 271.80 269.82

H2O

TBAC−H2O eutectic

Complex 1

Complex 2

272.5 271.7 271.63 271.6 269.65 266.90 259.18

NaCl·2H2O eutectic

250.01 250.03 250.02 250.03

270.99 264.45 258.33 256.32

270.04 270.05

250.00 250.02 250.01 250.02

270.00 270.00 270.01 270.02

250.03 250.01 250.04 250.01

267.92 270.00 270.02 270.03

264.81 264.48 264.19 264.46

270.01

265.09 264.69

267.91

267.84 267.74 267.58 267.48

Estimated standard uncertainty is u(T) = ±0.02 K.

a

above, since there is no free, bulk water to hydrate the NaCl, the addition of any NaCl causes the partial disruption of the semiclathrate water cage immediately forming new, less stable semiclathrate structures, with fewer numbers of water per TBAC molecule, with lower melting points. The eutectic melting points observed for the binary TBAC− H2O were observed to occur at approximately 268 K from 3 to 8 molal TBAC. For the ternary systems, these melting transitions have decreased to approximately 265 K at all ionic strength fractions of TBAC studied at 3 and 4 molal TBAC. It is hypothesized that NaCl has been incorporated into the TBAC−H2O eutectic structure forming a new, less stable TBAC−NaCl−H2O eutectic complex of unknown composition (Complex 2). For all ternary solutions (YTBAC = 0.5 and 0.25) and (YTBAC = 0.9 to 0.35) prepared in 1.5 and 2 molal TBAC, respectively, a new melting transition of about 270 K is observed. This melting transition is also observed for ternary solutions at YTBAC ≤ 0.75 in 3 and 4 molal TBAC. It is proposed that this invariant transition is due to a peritectic point for a complex of TBAC−NaCl−H2O of unknown composition (Complex 1). It is believed that the addition of NaCl seems to disrupt the semiclathrate cage and releases some of the water molecules. These free water molecules interact with the NaCl forming a concentrated, near saturated NaCl−H2O solution. Near

leaving substantially less free water to dissolve the NaCl making the effective concentration of NaCl to be much greater than the reported concentration. This then causes the freezing point depression of free water to be much greater than expected. Additionally, the NaCl saturation point is found to occur at lower concentrations for these ternary solutions which have less free, bulk water available to dissolve the NaCl. At TBAC concentrations of less than 1.75 molal, except for very high values of ionic strength fraction of TBAC, that is, low relative NaCl concentrations, at constant values of YTBAC, the melting points of these transitions are found to be less than those for the corresponding binary solutions. It is believed that the addition of NaCl will tie up water through hydration of the sodium and chloride ions. At low concentrations of TBAC, with the addition of relatively small amounts of NaCl, there is sufficient free, unbound water to hydrate these ions and the semiclathrate structure is unaffected resulting in no temperature change in the melting transition of the semiclathrate structure. However, if an amount of NaCl is added, such that there is insufficient free, bulk water to dissolve and fully hydrate the NaCl, then the preferential hydration of the NaCl will cause the removal of some of the water from the semiclathrate cage of TBAC causing the formation of new, less stable semiclathrate structures with fewer water molecules per TBAC molecule. At concentrations of 2 molal TBAC and J



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(7) Oyama, H.; Shimada, W.; Ebinuma, T.; Kamata, Y.; Takeya, S.; Uchida, T.; Nagao, J.; Narita, H. Phase diagram, latent heat, and specific heat of TBAB semiclathrate hydrate crystals. Fluid Phase Equilib. 2005, 234, 131−135. (8) Sangwai, J. S.; Oellrich, L. Phase equilibrium of semiclathrate hydrates of methane in aqueous solutions of tetra-n-butyl ammonium bromide (TBAB) and TBAB−NaCl. Fluid Phase Equilib. 2014, 367, 95−102. (9) Oshima, M.; Kida, M.; Nagao, J. Thermal and crystallographic properties of tetra-n-butylammonium bromide+ tetra-n-butylammonium chloride mixed semiclathrate hydrates. J. Chem. Eng. Data 2016, 61, 3334−3340. (10) Ye, N.; Zhang, P. Phase equilibrium and morphology characteristics of hydrates formed by tetra-n-butyl ammonium chloride and tetra-n-butyl phosphonium chloride with and without CO 2. Fluid Phase Equilib. 2014, 361, 208−214. (11) Jin, Y.; Kida, M.; Nagao, J. Phase equilibrium conditions for clathrate hydrates of tetra-n-butylammonium bromide (TBAB) and xenon. J. Chem. Eng. Data 2012, 57, 1829−1833. (12) Marciacq-Rousselot, M. M.; De Trobriand, A.; Lucas, M. Nuclear magnetic resonance chemical shift of the water proton in aqueous tetraalkylammonium halide solutions at various temperatures. J. Phys. Chem. 1972, 76, 1455−1459. (13) Mohammadi, A. H.; Richon, D. Phase equilibria of semiclathrate hydrates of tetra-n-butylammonium bromide+ hydrogen sulfide and tetra-n-butylammonium bromide+ methane. J. Chem. Eng. Data 2009, 55, 982−984. (14) Oshima, M.; Shimada, W.; Hashimoto, S.; Tani, A.; Ohgaki, K. Memory effect on semi-clathrate hydrate formation: A case study of tetragonal tetra-n-butyl ammonium bromide hydrate. Chem. Eng. Sci. 2010, 65, 5442−5446. (15) Sakamoto, J.; Hashimoto, S.; Tsuda, T.; Sugahara, T.; Inoue, Y.; Ohgaki, K. Thermodynamic and Raman spectroscopic studies on hydrogen+ tetra-n-butyl ammonium fluoride semi-clathrate hydrates. Chem. Eng. Sci. 2008, 63, 5789−5794. (16) Sato, K.; Tokutomi, H.; Ohmura, R. Phase equilibrium of ionic semiclathrate hydrates formed with tetrabutylammonium bromide and tetrabutylammonium chloride. Fluid Phase Equilib. 2013, 337, 115−118. (17) Wen, W.-Y.; Miyajima, K.; Otsuka, A. Free energy changes on mixing solutions of alkali halides and symmetrical tetraalkylammonium halides. J. Phys. Chem. 1971, 75, 2148−2157. (18) Fukumoto, A.; Paricaud, P.; Dalmazzone, D.; Bouchafaa, W.; Ho, T. T.-S.; Fürst, W. Modeling the dissociation conditions of carbon dioxide+ TBAB, TBAC, TBAF, and TBPB semiclathrate hydrates. J. Chem. Eng. Data 2014, 59, 3193−3204. (19) Shimada, W.; Ebinuma, T.; Oyama, H.; Kamata, Y.; Narita, H. Free-growth forms and growth kinetics of tetra-n-butyl ammonium bromide semi-clathrate hydrate crystals. J. Cryst. Growth 2005, 274, 246−250. (20) Shimada, W.; Shiro, M.; Kondo, H.; Takeya, S.; Oyama, H.; Ebinuma, T.; Narita, H. Tetra-n-butylammonium bromide−water (1/ 38). Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 2005, 61, 65−66. (21) Hashimoto, S.; Sugahara, T.; Moritoki, M.; Sato, H.; Ohgaki, K. Thermodynamic stability of hydrogen+ tetra-n-butyl ammonium bromide mixed gas hydrate in nonstoichiometric aqueous solutions. Chem. Eng. Sci. 2008, 63, 1092−1097. (22) Hashimoto, S.; Tsuda, T.; Ogata, K.; Sugahara, T.; Inoue, Y.; Ohgaki, K. Thermodynamic Properties of Hydrogen. J. Thermodyn. 2009, 2010, 1. (23) Paricaud, P. Modeling the dissociation conditions of salt hydrates and gas semiclathrate hydrates: application to lithium bromide, hydrogen iodide, and tetra-n-butylammonium bromide+ carbon dioxide systems. J. Phys. Chem. B 2010, 115, 288−299. (24) Sun, Z.-G.; Sun, L. Equilibrium conditions of semi-clathrate hydrate dissociation for methane+ tetra-n-butyl ammonium bromide. J. Chem. Eng. Data 2010, 55, 3538−3541. (25) Mohammadi, A.; Manteghian, M.; Mohammadi, A. H. Phase equilibria of semiclathrate hydrates for methane+ tetra n-butylammo-

saturation, the NaCl−H2O phase diagram (not shown) shows a peritectic transition for NaCl−2H2O with a melting point of about 273 K. It is believed that a similar peritectic complex that incorporates TBAC, with a slightly lower melting point, occurs in the ternary system at these relatively high concentrations of NaCl. Similar to suggested structures formed in the TBAC− H2O binary, these postulated structures formed in TBAC− NaCl−H2 O ternary system should be tested through experimental methods, such as X-ray crystallography or spectroscopy, and/or computational methods.

4. CONCLUSION In this work, an experimental study was performed to determine the thermal stabilities of TBAC semiclathrates formed in binary TBAC−H2O and ternary TBAC−NaCl− H2O. In the binary systems, the melting points of TBAC semiclathrate increased with an increase in the TBAC concentration to reach 286.8 K at 2 molal TBAC and then decreased above 2 to 8 molal. In the ternary systems, it was found that there was no effect on the melting temperature of the TBAC−H2O semiclathrate upon adding sodium chloride to the system if there was sufficient free, bulk water to dissolve the sodium chloride. When sodium chloride was added to the binary system but there was insufficient bulk water to dissolve all of this salt, then the water in the clathrate cages of the TBAC was disrupted creating a new semiclathrate with a lower melting point and a new TBAC−NaCl−H2O semiclathrate peritectic structure of unknown composition with an invariant melting point. If sodium chloride was added to the binary system that had no free, bulk water present, the sodium chloride disrupted the semiclathrate structure of the binary system to give a new TBAC−H2O semiclathrate structure with a lower melting point as well as a new eutectic TBAC-NaClH2O structure of unknown composition with an invariant melting point. It should be noted that these new TBAC− NaCl−H2O structures with invariant melting points have lower melting points than the corresponding structures found in either the binary TBAC−H2O or NaCl−H2O system.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +1-215-500-2930. ORCID

Miad Siddiq: 0000-0002-4032-5462 Notes

The authors declare no competing financial interest.

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Effect of Adding Sodium Chloride on Tetra-n-butylammonium Chloride

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