Calorimetric and Structural Studies of Tetrabutylammonium Bromide

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Calorimetric and Structural Studies of Tetrabutylammonium Bromide Ionic Clathrate Hydrates Tatyana V. Rodionova, Vladislav Yu. Komarov, Galina V. Villevald, Tamara D. Karpova, Natalia V. Kuratieva, and Andrey Yu. Manakov J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp406082z • Publication Date (Web): 15 Aug 2013 Downloaded from http://pubs.acs.org on August 26, 2013

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The Journal of Physical Chemistry

Calorimetric and Structural Studies of Tetrabutylammonium Bromide Ionic Clathrate Hydrates

Tatyana V. Rodionova*, Vladislav Yu. Komarov, Galina V. Villevald, Tamara D. Karpova, Natalia V. Kuratieva, Andrey Yu. Manakov

Nikolaev Institute of Inorganic Chemistry, SB RAS 3, Acad. Lavrentieva avenue, Novosibirsk, 630090

* Tel.+7 383 316 53 46, Fax: +7 383 330 94 89; e-mail: [email protected]

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Abstract In the present work, characteristic properties of tetrabutylammonium bromide (TBAB) ionic clathrate hydrates structures were studied by single crystal X-ray structure analysis. The structures of three different tetragonal TBAB ionic clathrate hydrates that were formed in our experiments were based on the same water lattice of tetragonal structure I (TS-I) differing in the ways of including bromide anions and arranging tetrabutylammonium cations. We demonstrated that (1) Br– can be included into the water lattice replacing two water molecules, (2) butyl group of the cation can be inserted not only in large T and P cavities, but also in small D cavities of the water lattice TS-I, and (3) one of the reasons for polytypism of ionic clathrate hydrates on the basis of TS-I is the occurrence of alternative modes of arrangements of four-compartment cavities in adjacent layers of the water framework. The compositions of three TBAB ionic clathrate hydrates TBAB·38.1H2O, TBAB·32.5H2O, and TBAB·26.4H2O were determined by chemical analysis and their enthalpies of fusion were measured by differential scanning calorimetry (DSC). From the obtained results the enthalpies of the TBAB hydrates formation from TBAB and water were calculated thermodynamically.

Keywords TBAB, ionic clathrate hydrate, enthalpy of fusion, structure


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Introduction Crystal structures of ionic clathrate hydrates are related to the structures of gas hydrates. The host water lattice of gas hydrates consists of hydrogen bonded water molecules and guest gas molecules included into its voids. In the crystal structure of ionic clathrate hydrates the water molecules, together with anions/cations, by means of hydrogen bonding form a water-ion polyhedral framework. Counter ions are included in the cavities of the framework.1 The idealized water frameworks of ionic clathrate hydrates represent a set of face-sharing polyhedra. Oxygen atoms of water molecules occupy the vertices of the polyhedra while hydrogen bonds form their edges. Among the typical polyhedra are pentagonal dodecahedron, D (512); tetrakaidecahedron, T (51262); and pentakaidecahedron, P (51263). The characteristic idealized water frameworks of ionic clathrate hydrates are tetragonal structure I (TS-I), hexagonal structure I (HS-I), and cubic structure I (CS-I) with the ideal unit cell formulas 4P•16T•10D•172H2O, 2P•2T•3D•40H2O, and 6T•2D•46H2O, respectively. Within the last few years the ionic clathrate hydrates have been intensively studied, particular attention being paid to tetrabutylammonium bromide (TBAB) ionic clathrate hydrates due to their potential applicability to different practical ends. An incentive to study ionic clathrate hydrates of TBAB as a prospective energy storage/transportation medium is the property of the crystal structure of TBAB ionic clathrate hydrates to include such gases as hydrogen2-8 and methane.4, 9-16 The studies of TBAB double clathrate hydrates with carbon dioxide4, 15-21 are of particular importance in view of the global problem of carbon dioxide sequestration. Besides, nitrogen4, 10, 16, 19 and hydrogen sulfide10, 12, 22 have been shown to form double hydrates with TBAB. Additionally, the TBAB ionic clathrate hydrates can be used for gas separation. 9, 10, 17, 19, 20, 22-25 The TBAB polyhydrates and double TBAB+ THF, TBAB+CO2 polyhydrates are also studied as promising materials for cold storage and cold transportation26-35 since they have suitable latent heats and temperatures of decomposition/formation, and form slurry that can be transported directly through a pipeline. 3 ACS Paragon Plus Environment


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The first information on the hydrate (n-C4H9)4NBr·26H2O with m.p. 14.5 oC was reported by Fowler et al.36 as far back as 1940 when the authors described a series of hydrates of tetraalkylammonium salts with abnormally large hydrate numbers. Later on, a series of crystallographic investigations of polyhydrates of tetrabutyl- and tetraisoamylammonium salts showed that the TBAB hydrate forms a tetragonal crystal structure with the hydrate number 32.6 (crystal)/30.5 (chemical), and m. p. 12.5 oC.37 The study of T–X - phase diagram of the TBAB–H2O binary system by thermal analysis, and of a series of isotherms of the TBAB – H2O – X ternary systems (used as indifferent third components X were CH3COOH, CH3OH, C2H5OH, C3H7OH, C4H9OH, HBr, C4H9NH2, and (C2H5)3N, NH4Br) by the Schreinemakers' wet residue method led to the conclusion that there are four ionic clathrate hydrates: TBAB·36H2O (m. p. = 9.5oC), TBAB·32H2O (m. p. = 11.7oC), TBAB·26H2O (m. p. = 12.2oC), TBAB·24H2O (m.p. = 12.4 oC), and two hydrates with small hydrate numbers: TBAB·3H2O (m.p.~ 15 oC); TBAB·2H2O (m.p.= 23.5 oC).38-40 One hydrate, TBAB·24H2O (m.p.= 12.9 oC), and two more hydrates of unidentified compositions with incongruent melting points 9.8oC and 12.4oC have been found in studying the phase diagram of TBAB – H2O system.41 According to crystallographic characteristics of the four TBAB polyhydrates42, TBAB·26H2O and TBAB·24H2O hydrates were classified as unknown structural types, while the TBAB·36H2O and TBAB·32H2O hydrates were recognized to be isostructural to the well-known orthorhombic hydrate of tetraisoamylammonium fluoride (i-C5H11)4NF·38H2O43 (idealized water lattice HS-I), and to the tetragonal hydrate of tetrabutylammonium fluoride TBAF·32.8H2O44 (idealized water lattice TS-I), respectively. The X-ray structure analysis of the hydrate TBAB·38H2O45 revealed its isostructurality to the above hydrate (i-C5H11)4NF·38H2O. Of the other TBAB hydrates, only the structure of the lowwater hydrate TBAB.21/3H2O46 has been determined by now. Previously published TBAB hydrates structural data are summarized in Table 1. As for the thermodynamic data, the enthalpies of fusion were


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measured to be 199.59±5.28 J/g29 for TBAB·38H2O, 193.18±8.52 J/g29 and 324.3 J/g water19 for TBAB·26H2O. Table 1. Structural data on TBAB hydrates Hydrate composition

crystal system

space group unit cell dimensions, Å; Z or diffraction class

ref

TBAB·38H2Oa

orthorhombic

Pmma

45

a=21.060(5), b=12.643(4), c=12.018(8); 2

TBAB·36H2Ob

orthorhombic

TBAB·32.6H2Ob

tetragonal

TBAB·32H2Ob

tetragonal

TBAB·26H2O b TBAB·24H2Ob

a=21.3(2), b=12.9(1), c=12.1(1); 2

42

a=23.65(5), c=12.50(2); 5

37

P4/m

a=33.4(3), c=12.7(1);10

42

tetragonal

P4/mmm

a=23.9(2), c=50.8(5); 24

42

monoclinic

C2/m

a=28.5(2), b=16.9(1), c=16.5(1), β

42

Pmmm

=125°; 6 TBAB.21/3H2Oa a b

trigonal

R3c

a=16.609(1), c=38.853(2); 6

46

The crystal structure was solved. Only the crystallographic parameters were determined. However, in spite of numerous studies of the TBAB – water system there are still uncertainties

regarding the number of hydrates, their compositions and structures. The thermodynamic data on TBAB hydrates are still scarce. In this paper we report the results of measurements of the enthalpies of fusion of the TBAB ionic clathrate hydrates and of calculations of the enthalpies of their formation from TBAB and water. Single crystal X-ray diffraction studies of TBAB hydrates have carried out in order to reveal their structural peculiarities. 5 ACS Paragon Plus Environment


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Experimental Tetrabutylammonium bromide of “pure” grade was recrystallized three times from ethyl acetate and then was held in a vacuum desiccator for about two days with periodical evacuation. The product purity was controlled by measuring the concentration of tetrabutylammonium cation and bromide anion. The concentration of the cation was measured by potentiometric titration with a sodium tetraphenylborate solution using an ion-selective electrode. The amount of the anion was determined by titration with 0.03N Hg(NO3)2 solution in aqueous ethanol with diphenylcarbazone as an indicator. The results obtained agreed well within the limits of the accuracy of the analysis. The TBAB content was not less than 99.9 mass %. The hydrate stoichiometry was determined based on the tetrabutylammonium concentration as described above. To make DSC measurements, crystals of four ionic clathrate hydrates of tetrabutylammonium bromide with the compositions TBAB·38.1(3)H2O, TBAB·32.5(4)H2O, TBAB·26.4(6)H2O and TBAB·23.7(4)H2O were prepared by cooling 10–15 mas % solution to ≈ +2 oC, ≈ 30 mas % solution to ≈ +11 oC , ≈ 37 mas % solution to ≈ +10 oC , and ≈ 50 mas % solution to ≈ +11 oC, respectively. The crystal growth and sampling were conducted in a specially constructed temperature-controlled chamber in air atmosphere. The temperature was maintained constant in the range of ±1 оС. Obtaining a wellcrystallized phase required aging for several days to weeks. The crystals were separated from the mother solution and quickly dried up between two sheets of filtering paper to the air-dry state. One part of the samples was collected to determine the hydrate stoichiometry; another, for calorimetric studies. The enthalpy of fusion was measured using differential scanning calorimeter DSC-111 from Setaram. A sample of hydrate was sealed up into a 0.1 ml steel pan, and an empty 0.1 ml steel pan was used as a reference. The heating rate was 0.5 oC per minute. The observed heats of fusion were normalized by electric calibration provided by the manufacturer. Between 7 and 10 measurements were carried out for 6 ACS Paragon Plus Environment

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each of the hydrates. Single-crystal diffraction experiments were conducted at 150±1 K on a Bruker X8 APEX CCD diffractometer with MoKα radiation using φ scans. Experimental data reduction were carried out using standard Bruker APEX software.47 The structures were solved and refined using the SHELXTL software package.47, 48 To structurally characterize the TBAB ionic clathrate hydrates, 9 single-crystal diffraction experiments were carried out. Eight crystals proved to be tetragonal structures on the basis of the idealized water framework TS-I and, according to the values of the unit cell parameters, were divided into three groups: (1) with c ≈ 12.6Å (three crystals), (2) with the threefold c = 38.021(14)Å (one crystal), and (3) the fourfold c ≈ 50.5 Å (four crystals). The best atomic structure models in each group of tetragonal crystals are denoted as s1, s2, and s3. One experiment resulted in obtaining structure on the basis of the idealized water framework HS-I (s4 in Table 2) and confirmed previously determined structure TBAB·38 H2O45 (so we will not describe it hereinafter). The crystal preparation was carried out in air. The crystal s1 for the single-crystal diffraction experiment was selected from ~ 30 mas % TBAB water solution, which had been kept for a few days at ≈ +11°C, the crystal s3, from ≈ 50 mas % TBAB water solution, also kept for a few days at the same temperature. The crystal s2 was obtained from a ternary solution (45% TBAB, 43 % Н2О, 12% NH4Br) held at 0 oC for about 10 days, as it was previously showed that in these conditions the TBAB·24H2O hydrates crystallize.38

Results and discussion Structures Crystal structure data for TBAB ionic clathrate hydrates obtained in the study are listed in Table 2.



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Table 2. Crystal data of TBAB ionic clathrate hydrates obtained in the present work crystal notation s1 s1a s1ba s2 s3 s3ab s3ba,b s3cb s4

Bravais lattice tetragonal P tetragonal P tetragonal P tetragonal P orthorhombic C orthorhombic C tetragonal P orthorhombic C tetragonal P orthorhombic C tetragonal P orthorhombic P

Laue class 4/m 4/m 4/m mmm mmm 4/m

mmm 4/m mmm

Unit cell dimensions, Å a b c 23.501(2) 12.558(1) 23.386(2) 12.561(1) 23.468(3) 12.555(2) 23.492(9) 38.021(15) 33.115(4) 33.159(4) 50.499(7) 33.036(4) 33.100(4) 50.318(7) 23.409(3) 50.404(6) 33.085(4) 33.027(4) 50.384(5) 23.376(2) 50.379(6) 33.139(2) 33.116(2) 50.414(3) 23.427(1) 50.420(3) 12.0231(5) 12.6577(4) 21.071(1)

V, Å3 6935(2) 6869(2) 6915(1) 20983(24) 55450(21) 55023(19) 27621(9) 55055(9) 27529(3) 55326(9) 27672(4) 3206.7(4)

a

Only the crystallographic parameters were determined.

b

Two variants of unit cells are presented because the structural model has not been determined.

The structural models found for the crystals s1, s2 and s3 are described on the basis of the same water lattice of the tetragonal structure I (ТС-I) with the ideal unit cell formula 4P•16T•10D•172H2O.1, 49 To describe the structure, it is convenient to distinguish in the water framework the layers that are perpendicular to the c axis and consist of D-, T- (we will further denote these layered T-cavities as Tl), and P-cavities (further DTlP-layers, Figure 1a). These layers are packed one on another with a 90-degree turn, so that only a small number of pentagonal faces prove to be discontiguous (in Figure 1a they surround the hexagonal “windows”). The remaining space consists of the columns of T-cavities which are arranged along the c axis (Figure 1b) and do not participate in the formation of the layers (further these “columnar” T- cavities are denoted as Tc). The columns also directed along c axis (Tl-columns, Figure 1c) can be singled out from the Tl-cavities belonging to the DTlP layers.


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a b c Figure 1. Polyhedral view of TS-I: (a) the layer consisting of D-, Tl- and P-cavities perpendicular to c axis; (b) column of hexagon-shared Tc- cavities along c axis; (c) column of coupled Tl-cavities along c axis. D-, T-, P- cavities are marked by yellow, blue, and lilac, respectively. Symmetrical equivalent cavities have the same color intensity. Each of the three structures s1, s2, and s3 can be described as TS-I with a different degree of defectiveness. Two types of defects can be distinguished: (1) distortions arising when an anion replaces one or more nodes of the ideal water framework (hydrophilic hydration) and (2) formation of vacant water lattice nodes when several cavities are fused to include a cation (hydrophobic hydration). In the DTlP-layers of each of the structures in question there are identical parallel chains of face-shared fourcompartment Tl2TcP-cavities. Notably, Tl -and P-cavity belong to one and the same DTlP-layer (Figure 2). There can be four equivalent ways of Tl2TcP-cavities arrangement (Figure 2a-2d). In the classical

a b c d Figure 2. Four different arrangements of Tl2TcP-cavities (marked by red and blue) in DTlPlayers. 9 ACS Paragon Plus Environment


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case, in the tetragonal hydrate TBAF·32.8H2O,44 the alternation of DTlP-layers is ..acac…, and Tlcavities not involved in the formation of Tl2TcP-cavities form inseparable columns of 4Tl-cavities. Among the structures under consideration the most ordered one turned out to be s2 (the structural model has been obtained in the space group P-4). Here the alternation of the DTlP-layers is …acdbac… (the layers a-c and d-b are related by the -4 symmetry operation). The Tl-cavities nonparticipating in the formation of Tl2TcP-cavities form eight-compartment 4D4Tl-cavities, containing two Bu4N+ cations. Only in these D-cavities a substantial electron density is observed. The other D-cavities do not contain any appreciable electron density. The distinctive feature of the 4D4Tl is the occurrence of four “windows” formed by the removal of the pairs of hydrogen bonded water molecules (Figure 3), connecting these cavities with the adjacent Tl2TcP-cavities. It suggests that division into 4- and 8-compartment cavities is rather relative. One of the two pairs of oppositely positioned windows are covered by two Br― anions with the square-pyramidal coordination by water molecules (Figure 3d-3f). A less ordered structure is s1 (the structural model has been obtained in the space group P42/m). Here the alternation of the DTlP-layers …acac… is similar to that in the structures of TBAF·32.8H2O44, TBACl·30H2O and TBACl·32H2O hydrates.50 In this structure there are Br― anions with the position occupation factor of 0.2120, which replace in the water lattice a pair of water molecules and are formally coordinated by six water molecules. All D-cavities adjacent to Tl-cavities contain an electron density. The D-cavities adjacent to the Tl-columns have the most significant electron density due to the formation of multi-compartment cavities, containing Bu4N+ cations. An electron density in the other D-cavities is apparently associated with the inclusion of water or air molecules. The said structure is similar to those of ionic clathrate hydrates of TBACl·30H2O and


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a

b

c

d

e

f

Figure 3. Inclusion of TBA+ and Br¯ into 4D4Tl-cavity. (a), (d) 4D4Tl-cavity viewed in an arbitrary orientation; (b), (e) along c axis; (c) defective positions of 4D4Tl-cavity (vacant positions of the water lattice are marked by balls, broken hydrogen bonds are dashed); (f) squarepyramidal coordination of Br¯ . TBACl·32H2O.50 The observed picture appears to arise due to the positional and occupational disordering of cations in the Tl-columns. The positional disordering results from the presence of two alternative variants of the fragmentation of Tl-columns into 4Tl -cavities (Figure 4a, 4b); the occupational disordering results from the possibility of the formation of the 4D4Tl -cavity in the position of any of the 4Tl -cavities like in s2 (Figure 4c). In this case the actual coordination of Br―anions by the water molecules is also square-pyramidal. The most disordered structure is s3 (the structural model was obtained in the space group P-1, then the symmetry was enhanced to Cccm). Besides the DTlP-layers shown in Figure 2, the structure contains layers the constituents of which have failed to be determined (they are further denoted as x) 11 ACS Paragon Plus Environment


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either due to the high disordering or to the lower symmetry of the crystal (which is possible if the “averaged” symmetry is more adequately described in the noncentro-symmetrical group). In this structure the alternation of the DTlP-layers is …acaxdbdx … and the 4D4Tl-cavities in the layers ac and db are situated in two alternative positions related by symmetry operation.

a

b

c

Figure 4. (a), (b) Two alternative variants of the fragmentation of Tl-columns into 4Tl -cavities (marked by red and blue); (c) 4D4Tl-cavity (marked by lilac), which can be formed in the position of any of 4Tl -cavities.

The structural models obtained, particularly in the case s3, are not sufficiently complete to precisely determine the “structural” stoichiometry. However the range of hydrate number variations can be estimated using found modes of TBA+ and Br¯ inclusion. Assuming that (1) each Tl2TcP-cavity includes one TBA+, (2) remaining T-cages form eight-compartment 4D4Tl- cavities with two cations within, (3) all Br¯ have square-pyramidal coordination by water molecules, the minimal hydrate 12 ACS Paragon Plus Environment

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number is 24.5. The upper limit of the hydrate number is equal to 32.4 as in the classical structural model described 44. Except for the TBAB polyhydrates based on HS-I, the stoichiometry of all other known TBAB ionic clathrate hydrates fall satisfactorily within this range. However it is possible that the guest particles allocation modes are not limited to those described above. The comparison of the structural data obtained in the present study with previous data (Table 1), makes it possible to draw the following conclusions. (1) The excellent matching of the Niggli- reduced cell parameters of the TBAB·24H2O42 (a=16.5(4) Å; b=16.6(1) Å; c=16.6(1) Å; α=61(1)°; β=60(3)°; γ=60(3)°) and of the TBAB·22/3H2O46 (a=b=c=16.115(3)Å; α=β=γ=62.04°) indicates, most likely, the wrong determination of the hydrate composition in ref 42. (2) The double volume supercell of the TBAB·32H2O42, was probably determined due to the presence of a significant diffuse scattering, characteristic for the hydrates TS-I with c ≈ 12 Å (Figure 5). (3) The crystal with the unit cell parameter c≈50 Å (in ref 42 it is related to the composition of TBAB·26H2O) is stably reproduced in our research (4 samples) and is a polytype on the basis of TS-I. It is important to note that the determination of crystal structure of ionic clathrate hydrates presents objective difficulties encountered at all stages of the XRD-experiment: (i) the low electron density, the high disordering degree, and the presence of pseudosymmetry result in a low scattering intensity of crystals, a low quality of X-ray data, and a high correlation between refining parameters; (ii) due to the formation of metastable phases it is a complicated matter to grow crystals of a definite phase and a proper quality; (iii) the sensitiveness of ionic clathrate hydrates crystals even to small changes of environmental parameters complicates crystal selection process. In spite of the fact that the structural models we have obtained are incomplete, the achieved degree of refinement makes it possible to speak with confidence about the formation of several hydrate phases with the same structural type TS-I in the ТBАB – water system.



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a, (0kl)

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b, (hk4)

c, (hk5)

d, (hk6)

e, (hk7)

Figure 5. Diffuse scattering observed on the diffraction pattern of the s1 crystal: (a) the diffuse layers normal to c* occurring at the rows of Bragg reflections; (b) - (e) reconstructions of hkn sections.

Enthalpies The results of the research we have carried out to determine the compositions of hydrates, their melting points, enthalpies of fusion and enthalpies of formation from TBAB and water are listed in Table 3. Table 3: Stoichiometry, melting points, enthalpies of fusion and formation of TBAB ionic clathrate hydrates hydrate composition

m.p., oC ∆Hfus, kJ/mol hydrate (kJ/mol H2O)

∆Hform, kJ/mol hydrate

TBAB·(38.1±0.3)H2O

9.5

219.4 ±3.9 (5.76); 59.7a

–212.4

TBAB·(32.5±0.4)H2O

12.0

179.1 ± 4.0 (5.51)

–178.4

TBAB·(26.4±0.6)H2O

12.0

150.7 ± 2.6 (5.71)

–146.9

∆Hform is the enthalpy of formation of TBAB polyhydrates from TBAB and water. a The value of 219.4±3.9 kJ/mol hydrate is the measured latent heat of the TBAB·38.1H2O and corresponds to the heat of transition of the solid TBAB·38.1H2O to the same composition liquid; the value of 59.7 kJ/mol hydrate is the enthalpy of incongruent fusion of the TBAB·38.1H2O calculated 14 ACS Paragon Plus Environment

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thermodynamically (see explanation in the text). The enthalpy of fusion of TBAB·26.4H2O satisfactorily agrees with the published values.19, 29 The value of ∆Hfus, presented in Table 3 for TBAB·38.1H2O hydrate, requires some explanation. The T-X phase diagram of the TBAB – H2O binary system40 gives evidence of the incongruent melting of the TBAB·36H2O (incongruent melting point of 9.5 oC). In the present work the stoichiometry of hydrates obtained from diluted solutions is found to be TBAB·(38.1±0.3)H2O. It should be noted that on all the thermograms that we have obtained the form of the endothermic peak of the TBAB·38.1H2O (in all eight experiments) is either dramatically asymmetric or the peak splits off, which suggests the peritectic decomposition of the hydrate (Figure 6a). Thereby, in the present series of DSC experiments the measured latent heat of 219.4±3.9 kJ/mol hydrate represents in fact the heat of transition of the solid TBAB·38.1H2O to the liquid of the same composition. To estimate the enthalpy of incongruent fusion of the TBAB·38.1H2O it is necessary to know (1) the compositions of the peritectic liquid phase and of the solid phase being at equilibrium with the peritectic liquid, (2) the heat value required for heating the

Figure 6. Typical samples of DSC decomposition curves for TBAB·38H2O (a) and TBAB·24H2O (b). 15 ACS Paragon Plus Environment


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peritectic liquid and the equilibrium solid phase from 9.5 оС ( peritectic melting point) to 12 оС (the temperature of transition of the solid TBAB·38.1H2O to the liquid of the same composition), and (3) the enthalpy of fusion of the solid phase. The composition of the peritectic liquid is ≈ 20.1 mas % TBAB according to the T-X phase diagram of the TBAB – H2O binary system40. The heat value required for heating of the peritectic liquid from 9.5 оС to 12 оС has been calculated in accordance with the specific heat capacity of the TBAB51 and that of the water52 . The composition of the solid phase being in equilibrium with the peritectic liquid was assumed to be TBAB·32.5H2O. Since the heat capacity of the TBAB·32H2O is unknown, as an approximation we used the specific heat capacity cp283K = 2.605 J/g K reported for the TBAB·26H2O29. Thus the enthalpy of incongruent fusion of the TBAB·38.1H2 O was calculated to be 59.7 kJ/mol hydrate. The composition of the fourth hydrate was determined as TBAB·23.7(4) H2O by chemical analysis (see Experimental Section). However, on all the DSC thermograms of this hydrate (seven experiments were performed), before the endothermic melting peak at 12 оС, the second endothermic peak occurs around 8 оС (Figure 6b). We believe that the second peak refers to the eutectic melting. It should be noted that on the T-X phase diagram40 in the region from 43 mas.% TBAB and above there is the eutectic at 9.25о С. It can be assumed that despite the fact that the crystal growing conditions corresponded to the stability region of the hydrate TBAB·24H2O, metastable in these conditions TBAB·26H2O hydrate was formed instead. Due to its high viscosity, the equilibrium mother solution (in this case ≈ 60 mas % TBAB) could have been partially captured in sampling the crystals. This could explain why the preparative analysis yielded underestimated hydrate number and DSC curves showed the peaks corresponding to eutectic melting. Notably, our numerous observations demonstrate that in this kind of systems metastable phases can persist for a long time without turning into stable ones. Whatever the cause is, we have not succeeded in reliable determination of the enthalpy of fusion of the 16 ACS Paragon Plus Environment

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fourth hydrate in the TBAB– H2O binary system. Using the enthalpies of fusion determined in this study as well as the published data on enthalpies of solution of TBAB in water, it is possible to calculate the enthalpies of formation of the TBAB ionic clathrate hydrates from TBAB and water. The equation for calculating the intermediate enthalpies of solution of TBAB in water is reported in ref 53: ∆Hsol m - aHm1/2 = ∆Hsol0 +bm+cm 3/2 + dm2 + ..., where m is the molality of the solution; ∆Hsol m is the intermediate enthalpy of solution of TBAB in water; ∆Hsol0 is the standard enthalpy of solution; aH is the theoretical limiting slope; b, c, d are approximation coefficients. From these data we have calculated ∆Hsol m (at the melting point temperature of the hydrate with m corresponding to the hydrate composition) to be –3.3, +0.7, and +3.8 kJ/mol hydrate for (С4H9)4NBr·38.1H2O, (С4H9)4NBr·32.5H2O, and (С4H9)4NBr·26.4H2O, respectively. Using the measured enthalpies of fusion and the calculated ∆Hsol m we have estimated the enthalpies of the TBAB ionic clathrate hydrates formation from TBAB and water (∆Hform in Table 3). Let us consider briefly similarities and differences in hydrate formation in the series of the H2O – (C4H9)4NF, – (C4H9)4NCl, – (C4H9)4NBr systems. Each salt forms several clathrate hydrates. The formation of tetragonal hydrates on the basis of the idealized water lattice TS-I is characteristic for all the three salts. In the H2O – (C4H9)4NF system the hydrate (C4H9)4NF·32.8H2O of TS-I is formed and then as salt concentration increases, the cubic hydrate SCS-I (C4H9)4NF·29.7H2O on the base of CS-I appears.44, 54 In the H2O – (C4H9)4NCl system all the three ionic clathrate hydrates, (C4H9)4NCl·32.2H2O, (C4H9)4NCl·29.7H2O, and (C4H9)4NCl·24.8H2O are formed on the basis of the same TS-I host water lattice, but they differ in the ways of including chloride anions and arranging the (C4H9)4N+cations in the cavities of the host water framework.50 In the H2O – (C4H9)4NBr system the orthorhombic hydrate (С4H9)4NBr·38 H2O on the basis of the HS-I water lattice is formed in the low 17 ACS Paragon Plus Environment


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salt content region. An increase of (С4H9)4NBr concentration results in the formation of three tetragonal hydrates, and all of them, just as in the case of (C4H9)4NCl, are formed on the base of the same TS-I water framework, but with different ways of involving anions into the water lattice and of arranging cations in the water framework cavities. The melting points of the ionic clathrate hydrates in each system differ from one another quite insignificantly: for tetrabutylammonium fluoride hydrates this difference is 0.2о С; for chloride hydrates, 0.4о С; for bromide hydrates, less than 3о С.

Conclusions The existence of three different tetragonal TBAB ionic clathrate hydrates on the basis of the same water lattice TS-I have been shown by single crystal X-ray analysis. All the structures exhibit high degree of disordering in the host and guest subsystems and differ in the ways of including bromide anions and arranging tetrabutylammonium cations. It was demonstrated that not only large T and P cavities, but also small D cavities of the water framework TS-I can include the butyl group of the cation. The Br– can be included into the water lattice replacing two water molecules. The occurrence of alternative modes of arrangements of Tl2TcP-cavities in adjacent layers TS-I is an earlier unknown reason for polytypism of ionic clathrate hydrates on the basis of the water framework TS-I. The revealed new features of TBAB ionic clathrate hydrates structures provided crystallochemical explanation of their phase variety. For the TBAB·(38.1±0.3)H2O ionic clathrate hydrate the latent heat was measured to be 219.4±3.9 kJ/mol hydrate. The enthalpy of its incongruent fusion of 59.7 kJ/mol hydrate was calculated thermodynamically. The enthalpies of fusion of two other TBAB·(32.5±0.4)H2O and TBAB·(26.4±0.6)H2O hydrates were measured to be 179.1 ± 4.0 kJ/mol hydrate and 150.7 ± 2kJ/mol hydrate, respectively. Based on obtained results we calculated the enthalpies of formation of the TBAB ionic clathrate hydrates from TBAB and water: –212.4kJ/mol hydrate for TBAB·(38.1±0.3)H2O, – 18 ACS Paragon Plus Environment

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178.4kJ/mol hydrate for TBAB·(32.5±0.4)H2O, and –146.9kJ/mol hydrate for TBAB·(26.4±0.6)H2O. These thermodynamic data are important for applications in hydrate based technologies such as cold storage, cold transportation, and gas separation.

Supporting Information Available Crystallographic information is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgement This work was supported by the grant of Presidium of the Russian Academy of Sciences (Project No7 of Program No 7) and grant No. 11.G34.31.0046 of the government of Russian Federation for state support of scientific investigations carried out under guidance of leading scientists in Russian universities.

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Calorimetric and Structural Studies of Tetrabutylammonium Bromide

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