J. Phys. Chem. C 2008, 112, 10573–10578
10573
Ionic Conductivity Enhancement Due to Coguest Inclusion in the Pure Ionic Clathrate Hydrates Jong-Ho Cha, Kyuchul Shin, Sukjeong Choi, and Huen Lee* Department of Chemical and Biomolecular Engineering, Korea AdVanced Institute of Science and Technology (KAIST), 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea ReceiVed: February 28, 2008; ReVised Manuscript ReceiVed: April 18, 2008
In this study, we present the unique role of the coguest when it is additionally included in a pure ionic clathrate hydrate. First, the ionic conductivities of xTHF · TPAOH · 32H2O hydrates at various coguest THF concentrations (x ) 2, 1, 0.5, 0.25, 0.13, 0) were measured in a temperature range from -40 to -10 °C and at ambient pressure. The double 2THF · TPAOH · 32H2O hydrate (σ ) 1.06 × 10-3 S · cm-1) exhibits ionic conductivity 2 orders of magnitude higher than that of THF-free TPAO · 32H2O hydrate (σ ) 6.01 × 10-6 S · cm-1) at -30 °C. This considerably different ionic conductivity behavior strongly implies that the inclusion of coguest THF induces a structural transformation via host-water lattice distortion, providing such high conductivity values for the mixed (THF + TPAOH) hydrate. We found a maximum conductivity of 0.0184 S · cm-1 at 1.49 THF mol % and -10 °C The present results provide strong evidence that THF serves as a promoter for greatly enhancing the ionic conductivity in ionic clathrate hydrates. The structure-II (sII) host lattices formed by THF inclusion can provide an effective pathway for moving the charge carriers. Furthermore, the channel pattern of sII small cages seems to contribute to a further increase in the ionic conductivity. The double tetramethylammonium hydroxide (TMAOH + TPAOH) hydrate structured with TMA+ cation in the 51264 cage and TPA+ cation in the four 512 cages was observed to maintain its solid state up to 31 °C, while the pure TPAOH and TMAOH hydrates melt below 0 °C. The physical characteristics of high ionic conductivity as well as high melting temperature of the double ionic clathrate hydrates might contribute to their use as solid proton conductors. Introduction Nonionic clathrate hydrates are stabilized by van der Waals interaction between a guest molecule and a host framework, and thus, with their property they have been employed for application for versatile energy resources, gas storage, and carbon dioxide sequestration.1–4 On the other hand, the ionic clathrate hydrates are generated by an ionic interaction between an ionic guest and a surrounding host water framework, which differs from nonionic clathrate hydrates.5,6 Most pure ionic clathrate hydrates are known to form their unique structures, largely depending on the size and valence of cations or anions as well as hydration number. We experienced that the water content in ionic clathrate hydrates plays a significant role in inducing structural transformation and thus controls the phase behavior and physical properties.7 In particular, tetraalkylammonium salt clathrate hydrates have been widely studied to understand the unique role of ionic guests in the host water framework.8–19 Both the complex polyatomic and simple monatomic ions incorporated with cage lattices might function as proton conductors.20–23 Various types of tetraalkylammonium salt hydrates exhibit quite low ionic conductivity, which hinders their practical application to ionic conductors.21–23 Pure TMAOH · 7.5H2O and TMAOH · 10H2O hydrates have high conductivity values, but their melting temperatures are considerably low at approximately -30 °C.20 Herein, we suggest an effective approach for enhancing the ionic conductivity using double ionic clathrate hydrates. TPAOH as a guest is used but known not to form an intrinsic clathrate hydrate structure on * To whom correspondence should be addressed. Phone: 82-42-869-3917. Fax: 82-42-869-3910. E-mail:
[email protected].
its own.24 The TPA+ cation in tetrapropylammonium fluoride (TPAF) occupies a supercage composed of four 512 cages as the nitrogen atom of TPA+ cation replaces the centered H2O in the supercage and four propyl groups stretch out to each 512 cage.25,26 The cage occupation of the TPA+ cation is found not to be sufficient to stabilize the clathrate structure. Thus, the introduction of THF to a pure TPAF hydrate system induces the formation of a sII clathate structure with the double guests.25,26 However, a strong bonding between the F- anion and H2O renders the ionic species immobile, contributing to lowering the ionic conductivity. In this context, using TPAOH with a mobile anion, OH-, we present enclathration of the TPA+ cation in a double (THF + TPAOH) ionic clathrate hydrate, drastically enhancing the ionic conductivity. We additionally check the difference in melting temperature between the double (THF + TPAOH) and pure TMAOH ionic clathrate hydrates, probably resulting from the degree of structure stabilization. For the double (THF + TPAOH) hydrate, the OH- anion occupying one of the vertices of the polyhedral cage framework is expected to be transported through the regularly arranged host lattices. Furthermore, the channel pattern formed by successive linkage of small cages in sII might have a further increase in ionic transportation; TPA+ cations are likely to be regularly stacked along the channel-like small cages and thereby the counterion, OH-, which occupies one of the vertices of the polyhedral framework of the small cage to compensate for the positive charge, would be able to easily migrate through the small cage. Finally, we attempted to synthesize a double (TMAOH + TPAOH) ionic clathrate hydrate and identify the structural transformation accompanying the phase transition.
10.1021/jp801748d CCC: $40.75 2008 American Chemical Society Published on Web 06/24/2008
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Figure 1. Ionic conductivities of the double (THF + TPAOH) ionic clathrate hydrates with a variation of THF concentration at -30 (circles), -20 (diamonds), and -10 °C (triangles).
concentrations were crystallized at -70 °C for at least 1 day, but 2TMAOH · TPAOH · 30H2O hydrate was synthesized at room temperature due to their favorable thermodynamic stability. The hydrate samples were ground to a fine powder (∼200 µm). The PXRD patterns were recorded on a Rigaku D/MAX2500 with low-temperature equipment. A light source was the graphite-monochromatized Cu KR1 radiation with a wavelength of 1.5406 Å at a generator voltage of 40 kV and current of 300 mA. The PXRD experiments were carried out in step mode with a fixed time of 3 s and step size of 0.03° for 2θ ) 5-55 for each hydrate sample. During the measurements, temperature was maintained to about -70 °C. Ionic conductivities were measured by a complex impedance analysis using a Solartron 1260 impedance/gain-phase analyzer and a 1287 electrochemical interface. This apparatus was connected to Teflon-coated cell containing a pair of SUS electrodes (1.1 cm × 0.6 cm), the two being 5.5 mm apart. The electrodes in the cell were immersed in the liquid state of the double hydrate sample and immediately crystallized at -70 °C. Real and imaginary parts of the complex impedance were plotted over a frequency range from 0.1 to 106 Hz, and the ionic conductivity could be calculated from the bulk resistance (Rb) found in the complex impedance diagram. Temperature-dependent phase patterns were determined in a sealed aluminum pan under helium gas at cooling and heating rates of 5 and 3 °C · min-1 using a NETZSCH DSC 204 F1, respectively. Results and Discussion
Figure 2. Temperature dependence of ionic conductivities for the double (THF + TPAOH) hydrates: 5.71 THF mol % (circles), 2.94 THF mol % (diamonds), 1.49 THF mol % (triangles), 0.75 THF mol % (rectangles), 0.39 THF mol % (dotted circles), and 0 THF mol % (dotted diamonds).
Accordingly, the key task of this study is to find double ionic clathrate hydrates that exhibit the high conductivity and high melting temperature and can be potentially used in solid conducting materials. Experimental Methods Reagents. Tetrahydrofuran with a minimum purity of 99.0% was obtained from Aldrich and used as received. TPAOH aqueous solution was used after further treatment with removing a schoichiometric amount of water in vacuo. TMAOH with 97% purity and lithium hydroxide with 98% purity were purchased by Aldrich. Water of ultrahigh purity was obtained from a Millipore purification unit. Experimental Measurements. For the PXRD measurements TPAOH aqueous solutions mixed with several different THF
xTHF · TPAOH · 32H2O hydrates at various coguest THF concentrations (x ) 2, 1, 0.5, 0.25, 0.13, 0) were prepared by crystallization of aqueous solutions at -70 °C. The ionic conductivities of the xTHF · TPAOH · 32H2O samples were measured in a temperature range from -40 to -10 °C. Figure 1 shows that the ionic conductivities of these mixed hydrates at -30, -20, and -10 °C significantly increase with THF concentration, reaching a maximum at a value of 1.84 × 10-2 S · cm-1 at nearly 1.49 THF mol % and -10 °C. Above this concentration, the ionic conductivity tends to slightly and gradually decrease until the stoichiometric THF concentration (x ) 2, 5.71 mol %) is achieved in xTHF · TPAOH · 32H2O hydrates, where the THF molecules completely occupy the sIIL. This mixed 2THF · TPAOH · 32H2O hydrate exhibits ionic conductivity that is 2 orders of magnitude higher than that of THF-free TPAOH · 32H2O hydrate (1.06 × 10-3 S · cm-1 versus 6.01 × 10-6 S · cm-1 at -30 °C). At the present stage, it must be noted that the pure THF · 17H2O hydrate free of TPAOH gives an insignificant ionic conductivity value that is almost nondetectable, even at 0 °C. This considerably different ionic conductivity behavior strongly implies that inclusion of coguest THF induces a structural transformation with host-water lattice distortion, providing the high conductivity values for the mixed (THF + TPAOH) hydrate. Additionally, we measured the ionic conductivities of xTHF · TPAOH · 32H2O hydrates at several temperatures, as presented in Figure 2, and found that Arrhenius behavior is shown over the measured temperature range. The calculated activation energies range from 0.25 to 0.49 eV, and the details are summarized in Table 1. It is interesting to see that the 0.5THF · TPAOH · 32H2O hydrate exhibits the highest
TABLE 1: Activation Energies of Double (THF + TPAOH) Hydrates
activation energy
2THF · TPAOH · 32H2O
THF · TPAOH · 32H2O
0.5THF · TPAOH · 32H2O
0.25THF · TPAOH · 32H2O
0.13THF · TPAOH · 32H2O
TPAOH · 32H2O
0.29 eV
0.45 eV
0.49 eV
0.49 eV
0.25 eV
0.25 eV
Ionic Conductivity Enhancement in the Clathrate Hydrates
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Figure 3. XRD patterns of the double (THF + TPAOH) ionic clathrate hydrates at -25 °C: (a) 2THF · TPAOH · 32H2O, (b) THF · TPAOH · 32H2O, (c) 0.5THF · TPAOH · 32H2O, (d) 0.25THF · TPAOH · 32H2O, and (e) 0.13THF · TPAOH · 32H2O hydrates.
ionic conductivity over the entire temperature range. Therefore, in order to investigate the relationship between the structural change of double (TPAOH + THF) hydrate and the tendency of the ionic conductivities, the PXRD patterns of xTHF · TPAOH · 32H2O hydrates were determined at -25 °C and several THF concentrations. The PXRD patterns at the high THF concentration region identified that the formed hydrates are much alike to an exemplary sII crystal structure, while at the low THF concentration region the peaks assigned to sII almost disappear, causing the ionic conductivity to considerably decrease (Figure 3). Accordingly, it becomes clear that the critical guest concentration exists in the THF concentration range of 0.75-1.49 mol %, which plays a key role in formation of the hydrate as we previously described.27 Creation of hydrate structure near the critical guest concentration results in the large difference of conductivity between 0.25THF · TPAOH · 32H2O and TPAOH · 32H2O hydrates. This notable ionic conductivity behavior suggests that sII host lattices formed by the dominant role of THF provide an effective pathway to easily transport the charge carriers. The ionic conductivity difference between 5.71 and 1.49 THF mol % double hydrates might be attributed to the fact that the 5.71 THF mol % sII hydrate matrix possesses the more empty small cages without TPAOH, which acts as a potential barrier to ionic motion, relative to the 1.49 THF mol % one. The explanation might be rationalized by the existence of a weak endothermic peak around -30 °C arising from TPAOH in the DSC curve of 2THF · TPAOH · 32H2O hydrate (Figure 6), indicating that a small amount of TPAOH is not enclathrated in small cages of the double hydrate. On the other hand, the hydrate with lower THF concentration seems to have less empty small cages due to excess TPAOH. The unit cell parameter of 2THF · TPAOH · 32H2O hydrate was determined to be 17.5239 ( 0.0332 Å, a little larger than that of THF · 17H2O hydrate, which agrees well with the reported result for a double (THF + tetrapropylammonium fluoride (TPAF)) clathrate hydrate.25 This expansion might arise from replacing the centered H2O in the supercage of four 512 cages with a nitrogen atom of the TPA+ cation, and thus, the adjacent H2O exists far apart from the nitrogen atom as compared to the TPAOH-free sII lattice, which leads to an increase in the unit
Figure 4. (a) Structure of the double (THF + TPAOH) ionic clathrate hydrate: Yellow, oxygen; red, nitrogen; green, carbon; blue, THF. Hydrogen atoms are omitted for clarity. (b) Channels composed of small cage stacking in sII structure.
cell parameter (Figure 4a). The cell parameter values of (THF + TPAOH) hydrates appear to be almost identical as long as the sII structure is preserved (Table 2). For a while it would be worthwhile to examine what the sII structure is and how it more strongly promotes ionic conductivity than do other structures. For a better understanding of the THF effect on the ionic conductivity, we attempted to measure the ionic conductivities of the (THF + LiOH) hydrate, at first, for
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Figure 5. Temperature dependence of ionic conductivities for 2THF · LiOH · 32H2O (circles) and LiOH · 32H2O (diamonds) hydrates.
Figure 6. DSC curves of ionic clathrate hydrates: (a) TMAOH · 16H2O, (b) 2THF · TPAOH · 32H2O, and (c) 2TMAOH · TPAOH · 30H2O hydrates.
a direct comparison with those of the (THF + TPAOH) hydrate. To more clearly identify the THF role, we tested two samples, 2THF · LiOH · 33H2O and LiOH · 33H2O hydrates, and the resulting data are presented in Figure 5. For the 2THF · LiOH · 33H2O hydrate, the ionic conductivity values vary from 4.94 × 10-6 to 2.53 × 10-5 S · cm-1 at a temperature range from -45 to -30 °C with an activation energy of 0.60 eV. These conductivity values are roughly 2 orders of magnitude smaller than those of the 2THF · TPAOH · 32H2O hydrate. This rather significant difference might be attributed to a much weaker Coulombic interaction between the TPA+ cation and the OH- anion in the host lattice when compared with that between the Li+ cation and the OH- anion. As expected, the 2THF · LiOH · 33H2O hydrate exhibits much higher ionic conductivity than the LiOH · 33H2O hydrate (conductivity range ) 1.46 × 10-7 to 2.57 × 10-7 S · cm-1, activation energy ) 0.21 eV). In both double hydrates containing LiOH and TPAOH,
THF is likely to play a key role in enhancing the ionic conductivity. Furthermore, we note that in the sII structural pattern the functional group of the OH- anion becomes part of the host lattice, providing a pathway for mobile ion transport, while the cation functions as a guest. In a specific intracrystalline clathrate hydrate structure guest molecules of a suitable size and shape may be readily accessible to the resulting patterns of channels and cages, as observed in zeolite inclusion complexes. Accordingly, such an unusual conductivity enhancement can be better understood through simultaneous consideration of channel-like as well as cage-like host framework characteristics, as shown in Figure 4b. The polygonal faces provide windows essential for creating continuous diffusion paths for guest molecules or exchangeable cations. Thus far, although the vacant channels formed by the linkage of specific cages have not received any attention in the inclusion phenomena of clathrate hydrates, it is essential to realize that these channels can play an important role in guest diffusion pathways and occupancy occurring in a complex clathrate hydrate matrix. Apparently, the presence of guest molecules in the nearly immobile host frameworks is capable of inducing the crystalline structure to transform to a more stable structure by lowering the chemical potential of the fresh host lattice formation. The structural transition and ionic conductivity pattern of double (THF + TPAOH) hydrate can also be understood by the above explanations. The regularly arranged TPA+ cations in channel-like small cages effectively promote transportation of the counteranion, OH-, through the channel composed of small cages (sII-S), whereas the OH- anions in whole host lattices do not make any comparable contribution for the 2THF · LiOH · 33H2O hydrate. Therefore, the magnitude of enhancement for 2THF · LiOH · 33H2O hydrate is smaller than that for 2THF · TPAOH · 32H2O hydrate. The present results would be suitable evidence that THF serves as a promoter for greatly enhancing the ionic conductivity in both ionic clathrate hydrates as well as nonclathrate alkali metal salt hydrates. Another notable feature that can be seen in the double (THF + TPAOH) hydrate is its relatively high melting temperature as shown in the differential scanning calorimetric (DSC) curves of TMAOH · 16H2O, 2THF · TPAOH · 32H2O, and 2TMAOH · TPAOH · 30H2O hydrates (Figure 6). The melting temperature of 2THF · TPAOH · 32H2O hydrate is observed to be -0.9 °C, whereas TMAOH · 16H2O hydrate melts at -26.5 °C. The TMAOH · 16H2O hydrate can serve as a potential solid electrolyte owing to its high ionic conductivity, but its low melting temperature might cause operating difficulties in real applications as solid proton conductors.7 In addition, we further characterized the stable hydrate phase region for the double (THF + TPAOH) hydrates at several THF concentrations (Figure 7). The hydrate stability is strongly dependent upon the THF concentration, and thus, the melting temperature is found to be the maximum at 5.71 THF mol %, where the THF molecules completely occupy the sII-L. This indicates that both excess TPAOH, which is not enclathrated at lower THF concentration, and excess THF influence the destabilization of the hydrate structures.
TABLE 2: Hydrate Crystal Cell Structures Determined by the PXRD Patterns 2THF · TPAOH · 32H2O THF · TPAOH · 32H2O 0.5THF · TPAOH · 32H2O 2TMAOH · TPAOH · 30H2O
type
crystal
sII sII sII HS-II
cubic cubic cubic hexagonal
space group Fd3m Fd3m Fd3m Pi63/mmc
a (Å)
c (Å)
17.5239 ( 0.0332 17.5243 ( 0.0046 17.4910 ( 0.0012 12.4204 ( 0.0192
53.4151 ( 0.1239
Ionic Conductivity Enhancement in the Clathrate Hydrates
Figure 7. Melting diagram of the xTHF · TPAOH · 32H2O hydrates (x ) 3, 2, 1, 0.5, 0.25, 0.13, 0).
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Figure 9. Temperature dependence of ionic conductivities for the 2TMAOH · TPAOH · 30H2O hydrate.
(THF + TPAOH) hydrates. The corresponding activation energy of conductivity is 0.49 eV. An interesting feature is that the mixed (TMAOH + TPAOH) hydrate maintains its solid state up to 31 °C, while the pure TPAOH and TMAOH hydrates are melted below 0 °C (Figure 6). Conclusion
Figure 8. XRD pattern of the 2TMAOH · TPAOH · 30H2O hydrate at -70 °C.
The TMAOH hydrates have been the subject of some research, primarily owing to their peculiar structural characteristics, which are strongly influenced by the water concentration and temperature. TMAOH · nH2O has been reported to have eight different crystalline hydrate phases according to its hydration number and temperature.28 Among these phases, the TMAOH · 7.5H2O and TMAOH · 5H2O structures are true ionic clathrate hydrates composed of an encaged TMA+ cationic guest and a water host lattice in which the anions are incorporated by hydrogen bonding. Meanwhile, certain hydrates have a clathrate-like structure in which some of the oxygen atoms are not fully connected. TPAOH is known not to form a genuine hydrate by itself,24 but with the aid of the second guest a real hydrate structure can be made. Accordingly, out of curiosity we attempted to synthesize a double (TMAOH + TPAOH) hydrate, where TMA+ cation occupies the 51264 cage, while TPA+ cation fills four 512 cages, quite similarly to the double (tetramethylammonium fluoride (TMAF) + TPAF) hydrate.29 Figure 8 shows the PXRD pattern of 2TMAOH · TPAOH · 30H2O hydrates at -70 °C. The observed reflections correspond to a HS-II (hexagonal structure II) with a space group of P63/mmc. The unit cell parameters are determined to be a ) 12.4204 Å and c ) 59.4151 Å (Table 2), which are in good agreement with the reported values for double (TMAF + TPAF) hydrate.29 Again, we measured the ionic conductivity for this sample up to 29 °C, reaching 3.92 × 10-2 S · cm-1 (Figure 9). This value is comparable with that of double
We examined the unique role of two second guests, THF and TMAOH, participating in formation of the double ionic clathrate hydrates. Of course, besides these two guests, more effective ones that can promote the physical and electronic properties via lattice rearrangement as well as electron transfer might be found. The melting temperature was drastically increased by host water lattice distortion due to a small addition of the second guest to pure ionic hydrates. This considerable increase in the melting temperature to a point near ambient temperatures implies that ionic (TMAOH + TPAOH) clathrate hydrates might have advantages for application to ionic conducting solid materials. Furthermore, the double ionic hydrate matrix can be readily synthesized at ambient pressure. In particular, we observed that the sII-S channels play a certain role in promoting the ionic conductivity, but the real phenomenon seems to be so complex, and thus the physicochemical details should be revealed through further research. Finally, we would like to emphasize that ionic hydrate materials with specific functions are designed and synthesized for their applications to various types of energy devices and storage systems. Acknowledgment. This research was supported by the Korea Science and Engineering Foundation (KOSEF) through the National Research Laboratory. This program was funded by the Ministry of Science and Technology (R0A-2005-000-100740(2007)) and also partially supported by the Brain Korea 21 Project. References and Notes (1) Park, Y.; Kim, D.-Y.; Lee, J.-w.; Huh, D.-G.; Park, K.-P.; Lee, J.; Lee, H. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 12690–12694. (2) Yeon, S.-H.; Seol, J.; Lee, H. J. Am. Chem. Soc. 2006, 128, 12388. (3) Kim, D.-Y.; Lee, J.-w.; Seo, Y.-T.; Ripmeester, J. A.; Lee, H. Angew. Chem., Int. Ed. 2005, 44, 7749–7752. (4) Hailong, L.; Seo, Y.-T.; Lee, J.-w.; Moudrakovski, I.; Ripmeester, J. A.; Chapman, N. R.; Coffin, R. B.; Gardner, G.; Pohlman, J. Nature 2007, 445, 303–306.
10578 J. Phys. Chem. C, Vol. 112, No. 28, 2008 (5) Jeffrey, G. A. Inclusion Compounds; Academic Press: London, 1984; Vol. 1. (6) Platteeuw, J. C.; van der Waals, J. H. Mol. Phys. 1958, 1, 91–96. (7) Choi, S.; Shin, K.; Lee, H. J. Phys. Chem B 2007, 111, 10224– 10230. (8) Dyadin, Yu. A.; Udachin, K. A. Zh. Strukt. Khim. 1987, 28, 75– 116. (9) Dyadin, Yu. A.; Udachin, K. A. J. Inclusion Phenom. 1984, 2, 61– 72. (10) Aladko, L. S.; Dyadin, Yu. A.; Rodionova, T. V.; Terekhova, I. S. Zh. Strukt. Khim. 2002, 43, 1068–1072. (11) Suwinska, K.; Lipkowski, J. S.; Dyadin, Yu. A.; Komarov, V. Yu.; Terekhova, I. S.; Rodionova, T. V.; Manakov, A. Yu. J. Inclusion Phenom. Macrocycl. Chem. 2006, 56, 331–335. (12) Komarov, V. Yu.; Rodionova, T. V.; Terekhova, I. S.; Kuratieva, N. V. J. Inclusion Phenom. Macrocycl. Chem. 2007, 59, 11–15. (13) Feil, D.; Jeffrey, G. A. J. Chem. Phys. 1961, 35, 1863–1873. (14) McMullan, R.; Jeffrey, G. A. J. Chem. Phys. 1959, 31, 1231–1234. (15) Bonamico, M.; Jeffrey, G. A.; McMullan, R. K. J. Chem. Phys. 1962, 37, 2219–2231. (16) Jeffrey, G. A.; McMullan, R. K. J. Chem. Phys. 1962, 37, 2231– 2239. (17) McMullan, R. K.; Bonamico, M.; Jeffrey, G. A. J. Chem. Phys. 1963, 39, 3295–3310.
Cha et al. (18) Beurskens, G.; Jeffrey, G. A.; McMullan, R. K. J. Chem. Phys. 1963, 39, 3311–3315. (19) Beurskens, P. T.; Jeffrey, G. A. J. Chem. Phys. 1964, 40, 2800– 2810. (20) Borkowska, Z.; Tymosiak, A.; Opallo, M. J. Electroanal. Chem. 1996, 406, 109–117. (21) Borkowska, Z.; Opallo, M.; Tymosiak, A.; Zoltowski, P. Colloids Surf. A: Physicochem. Eng. Aspects 1998, 134, 67–73. (22) Opallo, M.; Tymosiak-Zielinska, A.; Borkowska, Z. Solid State Ionics 1997, 97, 247–252. (23) Prokopowicz, A.; Opallo, M. Solid State Ionics 2001, 145, 407–413. (24) Fowler, D. L.; Loebenstein, W. V.; Pall, D. B.; Charles, A. K. J. Am. Chem. Soc. 1940, 62, 1140–1142. (25) Dyadin, Yu. A.; Udachin, K. A.; Bogatyryova, S. V.; Zhurko, F. V.; Mironov, Yu. I. J. Inclusion Phenom. 1988, 6, 565–575. (26) Manakov, A. Yu.; Udachin, K. A.; Dyadin, Yu. A.; Mikina, T. V. J. Inclusion Phenom. 1994, 17, 99–106. (27) Kim, D.-Y.; Park, J.; Lee, J.-w.; Reepmeester, J. A.; Lee, H. J. Am. Chem. Soc. 2006, 128, 15360–15361. (28) Mootz, D.; Seidel, R. J. Inclusion Phenom. 1990, 8, 139–157. (29) Udachin, K. A.; Lipkowski, J. Supramol. Chem. 1997, 8, 181–186.
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