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Hydrogen bond breaking in aqueous solutions near the critical point
16 March 2001
Chemical Physics Letters 336 (2001) 212±218
www.elsevier.nl/locate/cplett
Hydrogen bond breaking in aqueous solutions near the critical point Robert A. Mayanovic a,*, Alan J. Anderson b, William A. Bassett c, I-Ming Chou d a
Department of Physics, Astronomy and Materials Science, Southwest Missouri State University, Spring®eld, MO 65804, USA b Department of Geology, St. Francis Xavier University, Antigonish, Nova Scotia, Canada B2G 2W5 c Department of Geological Sciences, Cornell University, Ithaca, NY 14853, USA d 954 National Center, US Geological Survey, Reston, VA 20192, USA Received 14 September 2000
Abstract The nature of water±anion bonding is examined using X-ray absorption ®ne structure spectroscopy on a 1m ZnBr2 =6m NaBr aqueous solution, to near critical conditions. Analyses show that upon heating the solution from 25°C to 500°C, a 63% reduction of waters occurs in the solvation shell of ZnBr24 , which is the predominant complex at all pressure±temperature conditions investigated. A similar reduction in the hydration shell of waters in the Br aqua ion was found. Our results indicate that the water±anion and water±water bond breaking mechanisms occurring at high temperatures are essentially the same. This is consistent with the hydration waters being weakly hydrogen bonded to halide anions in electrolyte solutions. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction In spite of the importance of water as an essential ingredient of life, our understanding of the nature of bonding between water molecules and between water and solute ions, and how these impact upon the physical properties of aqueous ¯uids, is rudimentary. This is especially true for the solvation of ions in water in near critical and supercritical conditions. An intriguing conjecture in the case of anion solvation is that the hydrogen bond plays a singular role in dictating the physical properties of an aqueous ¯uid. The consequences of hydrogen bonding may be re-
*
Corresponding author. Fax: +1-417-836-6226. E-mail address: [email protected] (R.A. Mayanovic).
vealed in how the structure throughout the ¯uid changes with temperature. Postorino et al. [1] concluded from their neutron diraction results that hydrogen bonding in water disappears in the critical state. Subsequently, Gorbaty and Kalinichev [2] showed from their IR absorption and X-ray scattering measurements that signi®cant hydrogen bonding exists even in supercritical water. This was supported by Raman spectroscopy [3] and NMR measurements [4] made on water and treated successfully in molecular dynamics simulations [5±7]. Structure studies of anion hydration under elevated temperatures have been made only recently. Yamaguchi and Soper [8] found from their neutron diraction measurements that the number of water molecules hydrating a Cl aqua ion is reduced substantially at 375°C and 170 megapascals
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 0 6 1 - 6
R.A. Mayanovic et al. / Chemical Physics Letters 336 (2001) 212±218
213
(MPa). Reduced hydration of Br aqua ion was observed by Wallen et al. [9] using X-ray absorption ®ne structure (XAFS) spectroscopy on rubidium bromide solutions at 475°C and 65 MPa. Based on detailed XAFS measurements on zinc bromide aqueous solutions, we herein report evidence for signi®cant reduction of waters in the hydration shells of the ZnBr24 complex and of the Br aqua ion at near critical conditions. We infer that the Br ±water bond in electrolyte solutions is nearly identical to the hydrogen bond of water. 2. Experimental A 1mZnBr2 =6m NaBr aqueous solution was prepared using 99:99 % pure powders (Aldrich) and distilled and de-ionized water. Samples were loaded into the sample chamber, de®ned by the 0.5 mm diameter hole of a rhenium gasket (0.25 or 0.5 mm thickness) sandwiched between two diamonds, of a hydrothermal diamond-anvil cell. The diamond anvils have been drilled partially through for more enhanced X-ray transmission [10±12]. The sample density was adjusted by compressing the gasket with the diamond anvils and thus controlling the sample volume. With the volume of the sample chamber ®xed, the samples remained at isochoric conditions during our experiments. Pressure values for the samples were estimated from equation of state measurements made on a 1mZnCl2 =6m NaCl solution [11]. Both Zn and Br K-edge XAFS spectra were measured in transmission mode at the Cornell High Energy Synchrotron Source (CHESS), at temperatures ranging from 25°C to 500°C and at up to 500 MPa. CHESS was operated at 220 mA ®ll current and 5.29 GeV. Analysis was performed on the averages of four or more spectra measured at each pressure±temperature (P±T) condition during both heating and cooling cycles. Diraction lines of diamond occurring in the spectra measured from samples in the diamond-anvil cell were successfully removed using techniques described previously [11]. The ®ne structure oscillations occurring above the Br K-edge (Fig. 1) were isolated using AUTOBK [13] and standard background removal techniques [14]. The isolated XAFS oscillations
Fig. 1. Isolated Br K-edge XAFS oscillations v, weighted by the square of the photoelectron wave number k measured from a 1m ZnBr2 =6m NaBr aqueous solution sample (points) and the ®t to the data (line) versus k for dierent temperature± pressure values.
were Fourier transformed and ®t in radial distance R-space using the FEFFIT2.32 code [15] against theoretical curves generated from FEFF8 code [16]. The Fourier transformed spectra and their ®ts are shown in Fig. 2. The Br K-edge spectra were ®t using a simultaneous superposition of theoretical curves based on two models. One model has a single zinc atom in one shell and three oxygen atoms in a separate shell, with a bromine atom at the center, representing the bromo-centric view of the hydrated ZnBr24 complex 1. The other model has a single shell of six oxygens surrounding a bromine atom, representing a Br
1 in the Fourier Separate ®tting of features near 4 A transforms of the Br K-edge spectra indicates that the geometry of the ZnBr24 complex is most likely tetrahedral.
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R.A. Mayanovic et al. / Chemical Physics Letters 336 (2001) 212±218
3. Results and discussion 3.1. Zinc±bromine coordination structure of the ZnBr42 complex
Fig. 2. Fourier transforms (FT) of the isolated Br K-edge XAFS oscillations shown in Figure 1, calculated in the 0.6±13 1 k-range, as a function of radial distance R in Angstroms A relative to the absorbing ion. The FT data (points) were ®t R-range. The weak structures (shown as lines) in the 1.3±3.4 A range were ®t separately and appear to stem from in the 3±4 A Na ±Br ion pairing and Br±Br coordination of the tetrahedral ZnBr24 complex.
hexa aqua ion. The primary ®tting parameters were bond length R, coordination number N, and the Debye±Waller disorder factor r2 . The secondary ®tting parameters included higher-order anharmonic coecients C3 and C4 , whose maximum values obtained for the zinc shell were 3 and 4:6 10 5 A 4 , respectively, 3:6 10 4 A from data measured at 500°C and 500 MPa. In the case of ®tting for the hydration shells, it was necessary to constrain the C3 and C4 parameters to be identical in order to retain the fewest number of free parameters. The maximum values obtained for both hydration shells 4 were 3 and C4 3:6 10 4 A from C3 4:2 10 3 A ®tting of spectra measured at 500°C and 500 MPa.
Visual inspection of Fig. 2 shows that the peak resulting from X-ray photoelectron backscattering from Zn2 ion coordinated to Br ions and cen remains constant in height, aside tered near 1.8 A from slight diminution due to thermal disorder eects, with temperature. Analysis of Zn K-edge spectra con®rmed that the predominant zinc bromo complex throughout the P±T range in this solution is ZnBr24 [10]. The most obvious trend in this ®gure is that the peaked features in the 2±3 A range, associated with the radial distribution of oxygens of water molecules coordinated to Br ions, diminish quite dramatically with temperature. Table 1 shows the results for zinc±bromine coordination from ®tting of Br K-edge spectra measured at various P±T conditions. Aside from a in the uniform discrepancy of 0:036 0:005 A bromine±zinc bond length RBr±Zn , these results are in excellent agreement with those obtained from ®tting of the Zn K-edge spectra. The discrepancy is most likely due to inadequacy on the part of FEFF8 in the calculation of the onset of either the Zn or Br K-edge absorption in terms of the models used for our ®tting. The bromine±zinc bond length RBr±Zn was found to undergo a small contraction with temperature at a rate of roughly 0.005 A/ 100°C. This is roughly half the rate that we found for chloro zinc complexes in zinc chloride aqueous solutions [11,12]. Table 1 Structure results for zinc±bromine coordination from ®tting of Br K-edge spectraa T (°C)
P (MPa)
RBr±Zn A
r NZn
r2 2 A
25 100 200 300 400 500
Vapor Vapor Vapor 130 300 500
2.365 2.358 2.351 2.346 2.344 2.338
0.68 0.66 0.67 0.66 0.68 0.66
0.0072 0.0091 0.0108 0.0121 0.0153 0.0163
a r RBr±Zn is the bromine±zinc bond length, NZn is the raw Zn coordination number, and r2 is the Debye±Waller disorder factor.
R.A. Mayanovic et al. / Chemical Physics Letters 336 (2001) 212±218
215
Table 2 r r The bromine±oxygen bond lengths RBr±O I and RBr±O II , the raw oxygen coordination numbers NO I and NO II , and their corresponding
Debye±Waller disorder factors r2 , where O(I) and O(II) refer to oxygens in the Br and ZnBr24 hydration shells, respectivelya T (°C)
P (MPa)
RBr±O I A
r NO I
r2 2 A
RBr±O II A
r NO II
r2 2 A
25 100 200 300 400 500
Vapor Vapor Vapor 130 300 500
3.17 3.17 3.16 3.16 3.05 ±
2.65 1.65 1.23 0.54 0.43 ±
0.0182 0.0194 0.0213 0.0252 0.0278 ±
3.34 3.33 3.33 3.33 3.34 3.25
3.31 2.66 1.81 1.51 1.47 1.23
0.0186 0.0205 0.0224 0.0246 0.0278 0.0304
a
The structure results for O(I) oxygens at 500°C and 500 MPa were not determinable within the extent of noise-to-signal ratio present in our data. r The raw Zn coordination number NZn is calculated by dividing the measured amplitude of XAFS oscillations stemming from Zn backscattering by the many-body eect parameter S2o for Br. A value of 0.89 was used for S2o of Br from measurements on RbBr by Frenkel et al. [17]. We r note that NZn remains constant, close to the average value of 0:67 0:07 throughout the P±T range of study. This is in agreement with ®tting of Zn K-edge spectra, which showed that the number of Br coordinated to Zn2 remains constant with an average value of 4:17 0:4 for all the P±T conditions of the study.
3.2. Hydration shell structure of the Br aqua ion and of the ZnBr42 complex Table 2 gives ®tting results for the number of oxygen atoms in the hydration-shell (O(I)) of Br and in the outer hydration-shell (O(II)) of ZnBr24 . Our values for the bromine±oxygen bond length RBr±O I and the corresponding r2 at ambient conditions are in good agreement with results obtained from previous XAFS studies [9,18]. In order to determine the true coordination numbers for O(I) and O(II), the raw coordination r r numbers NO I and NO II shown in Table 2 should be normalized against the fraction of Br ions which are singly hydrated x and the fraction which bond to Zn2 in the ZnBr24 complex 1 x, respectively. In the ideal case, given the Br:Zn ratio of 8:1, these fractions would be identical and equal to 0.5. r As re¯ected in the average value of NZn (Table 1) and from results in Table 2, our sample did not display this ideal behavior. A most likely explanation is that
a fraction of the Br aqua ions underwent chemical reactions stimulated by radiation-induced hydrolysis or similar eects, leading to formation of a precipitate which settled on the bottom of the sample chamber, out of the direct path of the X-ray r beam. Setting 1 x NZn 0:68 and normalizing r r NO I and NO II for ambient conditions, we see that both values (8.3 and 4.8, respectively) are signi®cantly larger than previously reported values [19,20] 2. As demonstrated by Lengeler from XAFS measurements on Ni and Cr hydrates, intervening hydrogen between an absorber and backscatterer atom acts to focus the photoelectron wave despite having no measurable backscattering of its own [21]. The eect magni®es the amplitude of the XAFS by 50%. The hydrogen focusing eect was also shown by Lengeler to cause a 0.5% increase in radial distances. Our bond length results shown in Table 2 are not adjusted for this. Taking the amplitude magni®cation into account, we have further r r normalized NO I and NO II by a factor of 1.5, and obtained values for fully-weighted oxygen coordination numbers (or hydration numbers) of NO I 5:5 0:6 and NO II 3:2 0:3 under ambient conditions. These values are in very good agreement with results reported in literature [19,20]. Using the full normalization procedure, we have determined NO I and NO II for all P±T conditions. Their variation with temperature is shown
2 Several groups [19] have measured a hydration number of 6 for the Br aqua ion. An outer-hydration number of 12 (3 per anion) is reported for the MoO24 and WO24 tetrahedral complexes in [20].
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R.A. Mayanovic et al. / Chemical Physics Letters 336 (2001) 212±218
in Fig. 3. As shown in the ®gure, both sets of hydration numbers decrease substantially with temperature in roughly very similar fashion. In the case of outer-shell hydration of the ZnBr24 complex, the number of waters decreases by 63% in
Fig. 3. Oxygen coordination number for the Br hydrationshell NO I and for the ZnBr24 complex outer hydration-shell NO II plotted as a function of temperature.
going from 25°C to 500°C. This is in excellent agreement with results obtained by others for both bare anion dehydration [8,9] and for hydrogen bond breaking in water [2±7]. Our con®dence level in quoting a similar ®gure in the case of inner-shell hydration of Br is signi®cantly lower, on account of considerable relative error in NO I for values at 300°C and beyond. The extent of bond breaking in the outer-shell hydration of the ZnBr24 complex, in going from ambient conditions to 500°C and 500 MPa, is illustrated schematically in Figs. 4a and b. We regard these results as evidence for substantial bond breaking occurring in hydration shells around metal complexes having ligand anions and around bare anions, upon approaching supercritical conditions in aqueous solutions. Furthermore, since the rate of bond breaking with temperature is very similar for water±water and for water±Br interactions (both for the aqua ion and for the ZnBr24 complex) in aqueous solutions, we infer that the bonds in all such cases are essentially identical in nature. This is consistent with halide anions being weakly hydrogen bonded
Fig. 4. An illustration depicting how the number of water molecules (12) in the outer hydration shell of the ZnBr24 complex becomes substantially reduced (4) in aqueous solutions in going from ambient (a) to near-critical conditions at 500°C and 500 MPa (b). Most of the evidence indicates that the complex is tetrahedral. The bond breaking mechanisms occurring at the complex site and throughout the liquid state of water are found to be nearly identical.
R.A. Mayanovic et al. / Chemical Physics Letters 336 (2001) 212±218
to water molecules in aqueous solutions. Our conclusions are in agreement with results from IR studies on water±anion bonding, such as the study authored by Bergstr om et al. [22]. Thus, the properties of anionic aqueous solutions appear to be strongly dictated by weak hydrogen bonding, a fact that may be important in elucidating on many natural processes occurring under hydrothermal conditions, including the origins of life. It remains to be seen whether our conclusions are valid for more extensive systems, such as the solid±liquid interface between an anodized surface and water. Applications of molecular dynamics simulations, using the solvent polarization [23] or a similar model, may give additional insight on the solvation of anions and metal±halide complexes in water. 4. Conclusions We measured Br and Zn K-edge XAFS spectra from a 1mZnBr2 =6m NaBr aqueous solution, at temperatures ranging from 25°C to 500°C and pressures up to 500 MPa. The local structure results derived from these data indicate that the ZnBr24 complex predominates in the solution at all pressure and temperature conditions investigated. Accounting for the hydrogen atom photoelectron focusing eect has enabled detailed analysis of our Br K-edge XAFS spectra and for more accurate determination of structure results. These analyses show that upon heating the solution from 25°C to 500°C, a 63% reduction of waters occurs in the solvation shell of the ZnBr24 complex, and a similar reduction in the hydration shell of the bare Br ion. Our results indicate that the water±anion and water±water bond breaking mechanisms occurring at high temperatures are essentially the same. This behavior is consistent with the hydration waters being weakly hydrogen bonded to halide anions in electrolytic solutions. Acknowledgements An award from Research Corporation supported RAM's research. This research was also
217
supported by NSERC research and equipment grants to A.J.A. as well as funding from National Science Foundation grant DMR97-13424. We wish to thank Kenneth Finkelstein and other sta members for their help in carrying out our experiments at the Cornell High Energy Synchrotron Source.
References [1] P. Postorino, R.H. Tromp, M.-A. Ricci, A.K. Soper, G.W. Neilson, Nature 366 (1993) 668. [2] Y.E. Gorbaty, A.G. Kalinichev, J. Phys. Chem. 99 (1995) 5336. [3] Y. Ikushima, K. Hatakeda, N. Saito, M. Arai, J. Chem. Phys. 108 (1998) 5855. [4] M.M. Homann, M.S. Conradi, J. Am. Chem. Soc. 119 (1997) 3811. [5] T.I. Mizan, P.E. Savage, R.M. Zi, J. Phys. Chem. 100 (1996) 403. [6] A.A. Chialvo, P.T. Cummings, J. Phys. Chem. 100 (1996) 1309. [7] J. Marti, J. Chem. Phys. 110 (1999) 6876. [8] T. Yamaguchi, A.K. Soper, J. Chem. Phys. 110 (1999) 3529. [9] S.L. Wallen, B.J. Palmer, D.M. Pfund, J.L. Fulton, M. Newville, Y. Ma, E.A. Stern, J. Phys. Chem. 101 (1997) 9632. [10] A.J. Anderson, R.A. Mayanovic, W.A. Bassett, I.-M. Chou, in: P.R. Tremaine, P.G. Hill, D.E. Irish, P.V. Balakrishnan (Eds.), Steam, Water, and Hydrothermal Systems: Physics and Chemistry Meeting the Needs of Industry, NRC Research Press, Ottawa, 2000, p. 599. [11] W.A. Bassett, A.J. Anderson, R.A. Mayanovic, I.-M. Chou, Chem. Geol. 167 (2000) 3. [12] R.A. Mayanovic, A.J. Anderson, W.A. Bassett, I.-M. Chou, J. Synch. Rad. 6 (1999) 195. [13] M. Newville, P.L. vis, Y. Yacoby, J.J. Rehr, E.A. Stern, Phys. Rev. B 47 (1993) 14126. [14] B. Ravel, E. Cockayne, M. Newville, K.M. Rabe, Phys. Rev. B 60 (1999) 14632. [15] E.A. Stern, M. Newville, B. Ravel, Y. Yacoby, D. Haskel, Physica B 208±209 (1995) 117. [16] A.L. Ankudinov, B. Ravel, J.J. Rehr, S.D. Conradson, Phys. Rev. B 58 (1998) 7565. [17] A.I. Frenkel, E.A. Stern, M. Qian, M. Newville, Phys. Rev. B 48 (1993) 12449. [18] H. Tanida, H. Sakane, I. Watanabe, J. Chem. Soc., Dalton Trans. (1994) 2321. [19] M. Magini, G. Licheri, G. Piccaluga, G. Paschina, G. Pinna (Eds.), X-Ray Diraction of Ions in Aqueous Solutions: Hydration and Complex Formation, CRC Press, Boca Raton 1988, p. 164. [20] G. Johansson, Adv. Inorg. Chem. 39 (1992) 159.
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[21] B. Lengler, Phys. Rev. Lett. 53 (1984) 74. [22] P.-A. Bergstr om, J. Lindgren, O. Kristiansson, J. Phys. Chem. 95 (1991) 8575.
[23] M. Sprik, M.I. Klein, K. Watanabe, J. Phys. Chem. 94 (1990) 6483.
Chemical Physics Letters 336 (2001) 212±218
www.elsevier.nl/locate/cplett
Hydrogen bond breaking in aqueous solutions near the critical point Robert A. Mayanovic a,*, Alan J. Anderson b, William A. Bassett c, I-Ming Chou d a
Department of Physics, Astronomy and Materials Science, Southwest Missouri State University, Spring®eld, MO 65804, USA b Department of Geology, St. Francis Xavier University, Antigonish, Nova Scotia, Canada B2G 2W5 c Department of Geological Sciences, Cornell University, Ithaca, NY 14853, USA d 954 National Center, US Geological Survey, Reston, VA 20192, USA Received 14 September 2000
Abstract The nature of water±anion bonding is examined using X-ray absorption ®ne structure spectroscopy on a 1m ZnBr2 =6m NaBr aqueous solution, to near critical conditions. Analyses show that upon heating the solution from 25°C to 500°C, a 63% reduction of waters occurs in the solvation shell of ZnBr24 , which is the predominant complex at all pressure±temperature conditions investigated. A similar reduction in the hydration shell of waters in the Br aqua ion was found. Our results indicate that the water±anion and water±water bond breaking mechanisms occurring at high temperatures are essentially the same. This is consistent with the hydration waters being weakly hydrogen bonded to halide anions in electrolyte solutions. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction In spite of the importance of water as an essential ingredient of life, our understanding of the nature of bonding between water molecules and between water and solute ions, and how these impact upon the physical properties of aqueous ¯uids, is rudimentary. This is especially true for the solvation of ions in water in near critical and supercritical conditions. An intriguing conjecture in the case of anion solvation is that the hydrogen bond plays a singular role in dictating the physical properties of an aqueous ¯uid. The consequences of hydrogen bonding may be re-
*
Corresponding author. Fax: +1-417-836-6226. E-mail address: [email protected] (R.A. Mayanovic).
vealed in how the structure throughout the ¯uid changes with temperature. Postorino et al. [1] concluded from their neutron diraction results that hydrogen bonding in water disappears in the critical state. Subsequently, Gorbaty and Kalinichev [2] showed from their IR absorption and X-ray scattering measurements that signi®cant hydrogen bonding exists even in supercritical water. This was supported by Raman spectroscopy [3] and NMR measurements [4] made on water and treated successfully in molecular dynamics simulations [5±7]. Structure studies of anion hydration under elevated temperatures have been made only recently. Yamaguchi and Soper [8] found from their neutron diraction measurements that the number of water molecules hydrating a Cl aqua ion is reduced substantially at 375°C and 170 megapascals
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 0 6 1 - 6
R.A. Mayanovic et al. / Chemical Physics Letters 336 (2001) 212±218
213
(MPa). Reduced hydration of Br aqua ion was observed by Wallen et al. [9] using X-ray absorption ®ne structure (XAFS) spectroscopy on rubidium bromide solutions at 475°C and 65 MPa. Based on detailed XAFS measurements on zinc bromide aqueous solutions, we herein report evidence for signi®cant reduction of waters in the hydration shells of the ZnBr24 complex and of the Br aqua ion at near critical conditions. We infer that the Br ±water bond in electrolyte solutions is nearly identical to the hydrogen bond of water. 2. Experimental A 1mZnBr2 =6m NaBr aqueous solution was prepared using 99:99 % pure powders (Aldrich) and distilled and de-ionized water. Samples were loaded into the sample chamber, de®ned by the 0.5 mm diameter hole of a rhenium gasket (0.25 or 0.5 mm thickness) sandwiched between two diamonds, of a hydrothermal diamond-anvil cell. The diamond anvils have been drilled partially through for more enhanced X-ray transmission [10±12]. The sample density was adjusted by compressing the gasket with the diamond anvils and thus controlling the sample volume. With the volume of the sample chamber ®xed, the samples remained at isochoric conditions during our experiments. Pressure values for the samples were estimated from equation of state measurements made on a 1mZnCl2 =6m NaCl solution [11]. Both Zn and Br K-edge XAFS spectra were measured in transmission mode at the Cornell High Energy Synchrotron Source (CHESS), at temperatures ranging from 25°C to 500°C and at up to 500 MPa. CHESS was operated at 220 mA ®ll current and 5.29 GeV. Analysis was performed on the averages of four or more spectra measured at each pressure±temperature (P±T) condition during both heating and cooling cycles. Diraction lines of diamond occurring in the spectra measured from samples in the diamond-anvil cell were successfully removed using techniques described previously [11]. The ®ne structure oscillations occurring above the Br K-edge (Fig. 1) were isolated using AUTOBK [13] and standard background removal techniques [14]. The isolated XAFS oscillations
Fig. 1. Isolated Br K-edge XAFS oscillations v, weighted by the square of the photoelectron wave number k measured from a 1m ZnBr2 =6m NaBr aqueous solution sample (points) and the ®t to the data (line) versus k for dierent temperature± pressure values.
were Fourier transformed and ®t in radial distance R-space using the FEFFIT2.32 code [15] against theoretical curves generated from FEFF8 code [16]. The Fourier transformed spectra and their ®ts are shown in Fig. 2. The Br K-edge spectra were ®t using a simultaneous superposition of theoretical curves based on two models. One model has a single zinc atom in one shell and three oxygen atoms in a separate shell, with a bromine atom at the center, representing the bromo-centric view of the hydrated ZnBr24 complex 1. The other model has a single shell of six oxygens surrounding a bromine atom, representing a Br
1 in the Fourier Separate ®tting of features near 4 A transforms of the Br K-edge spectra indicates that the geometry of the ZnBr24 complex is most likely tetrahedral.
214
R.A. Mayanovic et al. / Chemical Physics Letters 336 (2001) 212±218
3. Results and discussion 3.1. Zinc±bromine coordination structure of the ZnBr42 complex
Fig. 2. Fourier transforms (FT) of the isolated Br K-edge XAFS oscillations shown in Figure 1, calculated in the 0.6±13 1 k-range, as a function of radial distance R in Angstroms A relative to the absorbing ion. The FT data (points) were ®t R-range. The weak structures (shown as lines) in the 1.3±3.4 A range were ®t separately and appear to stem from in the 3±4 A Na ±Br ion pairing and Br±Br coordination of the tetrahedral ZnBr24 complex.
hexa aqua ion. The primary ®tting parameters were bond length R, coordination number N, and the Debye±Waller disorder factor r2 . The secondary ®tting parameters included higher-order anharmonic coecients C3 and C4 , whose maximum values obtained for the zinc shell were 3 and 4:6 10 5 A 4 , respectively, 3:6 10 4 A from data measured at 500°C and 500 MPa. In the case of ®tting for the hydration shells, it was necessary to constrain the C3 and C4 parameters to be identical in order to retain the fewest number of free parameters. The maximum values obtained for both hydration shells 4 were 3 and C4 3:6 10 4 A from C3 4:2 10 3 A ®tting of spectra measured at 500°C and 500 MPa.
Visual inspection of Fig. 2 shows that the peak resulting from X-ray photoelectron backscattering from Zn2 ion coordinated to Br ions and cen remains constant in height, aside tered near 1.8 A from slight diminution due to thermal disorder eects, with temperature. Analysis of Zn K-edge spectra con®rmed that the predominant zinc bromo complex throughout the P±T range in this solution is ZnBr24 [10]. The most obvious trend in this ®gure is that the peaked features in the 2±3 A range, associated with the radial distribution of oxygens of water molecules coordinated to Br ions, diminish quite dramatically with temperature. Table 1 shows the results for zinc±bromine coordination from ®tting of Br K-edge spectra measured at various P±T conditions. Aside from a in the uniform discrepancy of 0:036 0:005 A bromine±zinc bond length RBr±Zn , these results are in excellent agreement with those obtained from ®tting of the Zn K-edge spectra. The discrepancy is most likely due to inadequacy on the part of FEFF8 in the calculation of the onset of either the Zn or Br K-edge absorption in terms of the models used for our ®tting. The bromine±zinc bond length RBr±Zn was found to undergo a small contraction with temperature at a rate of roughly 0.005 A/ 100°C. This is roughly half the rate that we found for chloro zinc complexes in zinc chloride aqueous solutions [11,12]. Table 1 Structure results for zinc±bromine coordination from ®tting of Br K-edge spectraa T (°C)
P (MPa)
RBr±Zn A
r NZn
r2 2 A
25 100 200 300 400 500
Vapor Vapor Vapor 130 300 500
2.365 2.358 2.351 2.346 2.344 2.338
0.68 0.66 0.67 0.66 0.68 0.66
0.0072 0.0091 0.0108 0.0121 0.0153 0.0163
a r RBr±Zn is the bromine±zinc bond length, NZn is the raw Zn coordination number, and r2 is the Debye±Waller disorder factor.
R.A. Mayanovic et al. / Chemical Physics Letters 336 (2001) 212±218
215
Table 2 r r The bromine±oxygen bond lengths RBr±O I and RBr±O II , the raw oxygen coordination numbers NO I and NO II , and their corresponding
Debye±Waller disorder factors r2 , where O(I) and O(II) refer to oxygens in the Br and ZnBr24 hydration shells, respectivelya T (°C)
P (MPa)
RBr±O I A
r NO I
r2 2 A
RBr±O II A
r NO II
r2 2 A
25 100 200 300 400 500
Vapor Vapor Vapor 130 300 500
3.17 3.17 3.16 3.16 3.05 ±
2.65 1.65 1.23 0.54 0.43 ±
0.0182 0.0194 0.0213 0.0252 0.0278 ±
3.34 3.33 3.33 3.33 3.34 3.25
3.31 2.66 1.81 1.51 1.47 1.23
0.0186 0.0205 0.0224 0.0246 0.0278 0.0304
a
The structure results for O(I) oxygens at 500°C and 500 MPa were not determinable within the extent of noise-to-signal ratio present in our data. r The raw Zn coordination number NZn is calculated by dividing the measured amplitude of XAFS oscillations stemming from Zn backscattering by the many-body eect parameter S2o for Br. A value of 0.89 was used for S2o of Br from measurements on RbBr by Frenkel et al. [17]. We r note that NZn remains constant, close to the average value of 0:67 0:07 throughout the P±T range of study. This is in agreement with ®tting of Zn K-edge spectra, which showed that the number of Br coordinated to Zn2 remains constant with an average value of 4:17 0:4 for all the P±T conditions of the study.
3.2. Hydration shell structure of the Br aqua ion and of the ZnBr42 complex Table 2 gives ®tting results for the number of oxygen atoms in the hydration-shell (O(I)) of Br and in the outer hydration-shell (O(II)) of ZnBr24 . Our values for the bromine±oxygen bond length RBr±O I and the corresponding r2 at ambient conditions are in good agreement with results obtained from previous XAFS studies [9,18]. In order to determine the true coordination numbers for O(I) and O(II), the raw coordination r r numbers NO I and NO II shown in Table 2 should be normalized against the fraction of Br ions which are singly hydrated x and the fraction which bond to Zn2 in the ZnBr24 complex 1 x, respectively. In the ideal case, given the Br:Zn ratio of 8:1, these fractions would be identical and equal to 0.5. r As re¯ected in the average value of NZn (Table 1) and from results in Table 2, our sample did not display this ideal behavior. A most likely explanation is that
a fraction of the Br aqua ions underwent chemical reactions stimulated by radiation-induced hydrolysis or similar eects, leading to formation of a precipitate which settled on the bottom of the sample chamber, out of the direct path of the X-ray r beam. Setting 1 x NZn 0:68 and normalizing r r NO I and NO II for ambient conditions, we see that both values (8.3 and 4.8, respectively) are signi®cantly larger than previously reported values [19,20] 2. As demonstrated by Lengeler from XAFS measurements on Ni and Cr hydrates, intervening hydrogen between an absorber and backscatterer atom acts to focus the photoelectron wave despite having no measurable backscattering of its own [21]. The eect magni®es the amplitude of the XAFS by 50%. The hydrogen focusing eect was also shown by Lengeler to cause a 0.5% increase in radial distances. Our bond length results shown in Table 2 are not adjusted for this. Taking the amplitude magni®cation into account, we have further r r normalized NO I and NO II by a factor of 1.5, and obtained values for fully-weighted oxygen coordination numbers (or hydration numbers) of NO I 5:5 0:6 and NO II 3:2 0:3 under ambient conditions. These values are in very good agreement with results reported in literature [19,20]. Using the full normalization procedure, we have determined NO I and NO II for all P±T conditions. Their variation with temperature is shown
2 Several groups [19] have measured a hydration number of 6 for the Br aqua ion. An outer-hydration number of 12 (3 per anion) is reported for the MoO24 and WO24 tetrahedral complexes in [20].
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in Fig. 3. As shown in the ®gure, both sets of hydration numbers decrease substantially with temperature in roughly very similar fashion. In the case of outer-shell hydration of the ZnBr24 complex, the number of waters decreases by 63% in
Fig. 3. Oxygen coordination number for the Br hydrationshell NO I and for the ZnBr24 complex outer hydration-shell NO II plotted as a function of temperature.
going from 25°C to 500°C. This is in excellent agreement with results obtained by others for both bare anion dehydration [8,9] and for hydrogen bond breaking in water [2±7]. Our con®dence level in quoting a similar ®gure in the case of inner-shell hydration of Br is signi®cantly lower, on account of considerable relative error in NO I for values at 300°C and beyond. The extent of bond breaking in the outer-shell hydration of the ZnBr24 complex, in going from ambient conditions to 500°C and 500 MPa, is illustrated schematically in Figs. 4a and b. We regard these results as evidence for substantial bond breaking occurring in hydration shells around metal complexes having ligand anions and around bare anions, upon approaching supercritical conditions in aqueous solutions. Furthermore, since the rate of bond breaking with temperature is very similar for water±water and for water±Br interactions (both for the aqua ion and for the ZnBr24 complex) in aqueous solutions, we infer that the bonds in all such cases are essentially identical in nature. This is consistent with halide anions being weakly hydrogen bonded
Fig. 4. An illustration depicting how the number of water molecules (12) in the outer hydration shell of the ZnBr24 complex becomes substantially reduced (4) in aqueous solutions in going from ambient (a) to near-critical conditions at 500°C and 500 MPa (b). Most of the evidence indicates that the complex is tetrahedral. The bond breaking mechanisms occurring at the complex site and throughout the liquid state of water are found to be nearly identical.
R.A. Mayanovic et al. / Chemical Physics Letters 336 (2001) 212±218
to water molecules in aqueous solutions. Our conclusions are in agreement with results from IR studies on water±anion bonding, such as the study authored by Bergstr om et al. [22]. Thus, the properties of anionic aqueous solutions appear to be strongly dictated by weak hydrogen bonding, a fact that may be important in elucidating on many natural processes occurring under hydrothermal conditions, including the origins of life. It remains to be seen whether our conclusions are valid for more extensive systems, such as the solid±liquid interface between an anodized surface and water. Applications of molecular dynamics simulations, using the solvent polarization [23] or a similar model, may give additional insight on the solvation of anions and metal±halide complexes in water. 4. Conclusions We measured Br and Zn K-edge XAFS spectra from a 1mZnBr2 =6m NaBr aqueous solution, at temperatures ranging from 25°C to 500°C and pressures up to 500 MPa. The local structure results derived from these data indicate that the ZnBr24 complex predominates in the solution at all pressure and temperature conditions investigated. Accounting for the hydrogen atom photoelectron focusing eect has enabled detailed analysis of our Br K-edge XAFS spectra and for more accurate determination of structure results. These analyses show that upon heating the solution from 25°C to 500°C, a 63% reduction of waters occurs in the solvation shell of the ZnBr24 complex, and a similar reduction in the hydration shell of the bare Br ion. Our results indicate that the water±anion and water±water bond breaking mechanisms occurring at high temperatures are essentially the same. This behavior is consistent with the hydration waters being weakly hydrogen bonded to halide anions in electrolytic solutions. Acknowledgements An award from Research Corporation supported RAM's research. This research was also
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supported by NSERC research and equipment grants to A.J.A. as well as funding from National Science Foundation grant DMR97-13424. We wish to thank Kenneth Finkelstein and other sta members for their help in carrying out our experiments at the Cornell High Energy Synchrotron Source.
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