Ind. Eng. Chem. Res. 1993,32,273-278 Weast, R. C., Ed. CRC Handbook of Chemistry and Physics, 70th ed.; CRC Press: Boca Raton, 1989. Yagi, S.;Kunii, D. Studies on Effective Thermal Conductivities in Packed Beds. AIChE J. 1957,3 (31,373-381. Yagi, S.;Kunii, D. Studies on Heat Transfer Near Wall Surface in
273
Packed Beds. AZChE J. 1960,6 (I), 97-104. Received for review October 30, 1991 Revised manuscript received October 29, 1992 Accepted November 3, 1992
MATERIALS AND INTERFACES Silver Ion Coordination in Membranes for Facilitated Olefin Transport Mark R. Antonio*-+and Dean T. Tsou BP Research, 4440 Warrensville Center Road, Cleveland, Ohio 44128-2837
Silver K-edge XAFS (X-ray absorption fine structure) was used to determine the coordination of silver ions in synthetic polymeric membranes of interest for facilitated olefin transport. The Ag+ ions in membranes treated with aqueous solutions of AgNO,, AgClO,, AgBF,, and AgF are threecoordinate with an average Ag-0 bond length (2.22 f 0.04 A) that is independent of the nature of the counteranions &e., nitrate, perchlorate, tetrafluoroborate, and fluoride) and significantly shorter than that (2.34 f 0.04 A) for four-coordinate Ag+ in aqueous solution. Similarly, the silver ion coordination is independent of the nature (i.e., composition, thickness, manufacturer, etc.) of the membrane. The aqueous environment in these geltype membranes is sufficiently different from bulk water, such that it causes a change of coordination of the Ag+ ions. In contrast, the coordination of the linear [NC-Ag-CN]- anion is the same in both bulk solution and membrane.
Introduction Separation techniques using carrier-mediated or facilitated transport membranes have been attracting considerable attention because of their very high selectivity and flux. The high selectivity and permeability are attained through the selective enhancement of the permeant flux induced by a reversible reaction with the chosen carrier species contained in the liquid membrane (Schultz et al., 19'774;Goddard et al., 1974; Goddard, 1977; Way et al., 1982; Noble et al., 1989). Facilitated ethylene/ethane separations have been reported using aqueous silver nitrate supported by cellulose acetate membranes (Hughes et al., 1986; Teramoto et al., 1986, 1989) and sulfonated poly(phenylene oxide) cation exchange membranes (LeBlanc et al., 1980). Styrene/ethylbenzene separation using &+-facilitated transport through perfluorosulfonate ionomer membranes has also been reported (Koval et al., 1989). These separations are all based on the well-known reversible complexation of olefins by aqueous Ag+ ion (Beverwijk et al., 1970). The detailed mechanisms of the olefin transport, however, are not clear. This paper describea the application of X-ray absorption fine structure, XAFS,to provide structural and chemical information about silver ions in several membranes of interest for the facilitated transport of olefins. XAFS has been ~ucceeefuuyemployed to determine the coordination of silver in aqueous and nonaqueous solutions (Yamaguchi et al., 1984a, 1988), vermiculite f i h (Dring et al., 1989); polymeric crown ethers (Beniere et al., 19921, fast ion conducting glasses (Dalba et al., 1986,1987),chalcogenide glaseea (Oldale et al., 1988; Steel et al., 19891, argon t Present address: Chemistry Division, Argonne National Laboratory, 9700 South Case Ave., Argonne, IL 60439.
(Montan0 et al., 1984,1989),and aluminum (Weber and Peisl, 1983). Furthermore, results obtained from XAFS studies of Ndion membranes have provided detailed atomic-scale information about the site symmetry and valence of neutralizing cations (e.g., iron and nickel) in Nafion-S0,H ionomers (Pan et al., 1983a,b, 1985). We have collected silver K-edge XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure) data for three different membranes saturated with aqueous silver salt solutions. Information about the coordination environment and chemical state of silver ions in these membranes was obtained through XAFS data analyses and compared with that obtained for the bulk aqueous solutions.
Experimental Section Materials and Sample Preparations. All silver d t s (e.g., K[Ag(CN)d, AgNO3, AgBF,, AgClO,, and AgF) were obtained from Alfa Products and used without further purification. Each salt was dissolved in distilled, deionized water to make 4.0 N solutions. A 5.0 N aqueous solution of AgN03 was also prepared. As shown in Table I, nine membranes (1-9) of three different compositions (i.e., cellulose, hydrocarbon, and fluorocarbon)were examined. Natural cellulose Spectra/Por 1 membranes (1-5), obtained from Spectrum Medical Industries, Inc., had a molecular weight cutoff of 6000-8000 and a thickness of 76 pm. Ndion 117, a perfluorinated cation exchange membrane, 6 (180 pm thick) was obtained from Aldrich Chemical Co. in the hydrogen ion form with an equivalent weight of 1100. Three strong cationic ion exchange membranes from RAI Reaearch Corp. (Raipore)were examined: (1) R-1010, sulfonated styrene grafted on a l-mil polytetrafluoroethylene preformed film, 7 (wet thickness 50
0888-5885/93/ 2632-0273$04.O0/0 0 1993 American Chemical Society
274 Ind. Eng. Chem. Res., Vol. 32, No. 2, 1993 Table I. Ion Exchange Membrane Identification and Aqueous Silver Solution Treatments as Well as Least-Squares Refined Energy Threshold Differences (AEo,?V), and DebyeWaller Interatomic Distances ( r ,A), Coordination Numbers (N), Factors (a, A) Obtained from t h e Silver K-Edge EXAFS Analysis of the A g - O W ) Interactions in Membranes 1-9 and Aqueous Solutions of AgNOS and K[Ag(CN)J" material no. solution membrane r, A N A&, eV a, A c e 11 u1ose 2.00 2.2 15.5 O.Ob 1 4.0 N K[Ag(CN)Z] Spectra/Por 1 2 4.0 N AgNO3 Spectra/Por 1 cellulose 2.22 2.7 -1.37 0.118 3 4.0 N AaBF, Spectra/Por 1 ce11u1ose 2.21 3.3 -0.72 0.099 cellulose 2.20 3.1 -1.63 0.105 4 4.0 N AgC10, Spectra jpor 1 0.124 ce11u1ose 2.20 2.9 -0.62 5 4.0 N AgF Spectra/Por 1 fluorocarbon 2.22 3.1 -1.12 0.106 6 5.0 N AKNO, Nafion 117 2.26 3.1 1.45 Raipore R-1010 fluorocarbon 0.112 7 5.0 N A ~ N O ~ 2.23 3.2 -0.23 0.111 fluorocarbon 8 5.0 N AgNO3 Raipore R-4010 2.22 3.1 -1.32 0.111 hydrocarbon 9 5.0 N AgNO3 Raipore R-5010M 5.0 N AgN03 H 2 0 solution 2.34 4.0' 9.16 0.082 2.01 2.0c 10.4 4.0 N K[Ag(CN),] H 2 0 solution O.O* Estimated standard deviations are 0.04 A and 1 for r and N. remectivelv. Multi~l scattering throuah the carbon atom in the linear [NC-Ag-CN]- anion adversely affects the determination of the 'Ag-C DebykWaller factor. EXLFS moael, parameter fixed.
pm); (2) R-4010, sulfonated styrene grafted on a FEP/TFE Teflon membrane matrix, 8 (wet thickness 100 pm); and (3) R-5010M, sulfonated styrene grafted on 6-mil lowdensity polyethylene film,9 (wet thickness 200 pm). These membranes were cut into ca. 2.54-cm-diameter disks and immersed in the silver salt solutions for 5 days at room temperature prior to the XAFS experiments. All membrane disks were kept in the dark both before and during the XAFS measurements. Each membrane was removed from the mother liquor and patted dry (under ambient conditions) just prior to the XAFS experiments. After exposure to the synchrotron X-radiation, the membranes showed evidence for photoreduction. A slight darkening was observed where the synchrotron radiation impinged upon the membranes. XAFS Measurements and Data Analysis. Silver K-edge, 25.521-keV, XAFS data were obtained on beam line X-18B at the NSLS (National Synchrotron Light Source), Brookhaven National Laboratory. The X-ray beam size on the membranes, which were mounted normal to the beam axis, was ca. 1X 11mm2. Several membrane disks (three to five) were stacked as needed to obtain an X-ray absorption edge jump of unity at the silver K-edge. All silver K-edge X-ray absorption data were collected at ambient temperature in the transmimion mode. The X-ray absorption data were normalized, background-subtracted, and weighted to obtain the EXAFS, k3x(k),which was Fourier-transform-filtered to remove high-frequency noise and to isolate individual backscattering contributions from the total signal (Antonio, 1992). For the membranes and solutions examined here, backscattering by carbon and oxygen atoms comprises the whole of the EXAFS signal, in view of the limited data ranges available for this initial structural analysis, the estimated standard deviations for the Ag-O(C) interatomic distances (f0.04 A) and O(C) coordination numbers (fl)are somewhat larger than usual (Antonio et al., 1991). Additional information about the XAFS measurements, data reduction, and curve fitting is available as supplementary material (see paragraph at end of paper regarding availability of supplementary material) and elsewhere (Teo, 1986).
Results and Discussion XANES Data. The normalized silver K-edge XANES data, in the form ln(&/IJ versus X-ray energy (electronvolts), for an aqueous solution of K[Ag(CN),] and for a cellulose membrane (1) treated with this solution are shown in Figure 1. The obvious similarity of these data indicates that the structure about the silver ions in the membrane and the aqueous solution are the same. The Ag+ cyanide complex is stable in aqueous solution and has
25400
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-
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t
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d 25580
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Figure 1. Silver K-edge XANES of (bottom) an aqueous solution of 4.0 N K[Ag(CN),]; (top) membrane 1 treated with the aqueous potassium silver cyanide solution. The vertical scale is offset for clarity. 25400
25450
25500
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1 5 N AgN03
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Figure 2. Silver K-edge XANES of an aqueous solution of 5.0 N AgNOBand membranes 2, 6, 7, and 9 treated with aqueous silver nitrate solutions. The vertical scale is offset for clarity.
the linear structure [NC-Ag-CNI- (Wells, 1975). This linear, complex anion, [Ag(CN)J, also persists within the Spectra/Por 1 membrane matrix. The silver XANES data for an aqueous solution of AgN03 and for four membranes (2, 6, 7, 9) treated with silver nitrate solutions are shown in Figure 2. These data are typical of the argentous state, Ag+, which is the normal
Ind. Eng. Chem. Res., Vol. 32, No. 2, 1993 275 and dominant oxidation state in aqueous solution. The likeness of the spectra for 2,6,7, and 9 (Figure 2) suggests that the site symmetry and valence of silver in each membrane is the same. Remarkably, neither the nature of the membrane itself (e.g., cellulose, fluorocarbon, hydrocarbon) nor the nature of the counterion (e.g., [NO3]-, [BF,]-, [C104]-, and [FI-) affects the coordination environment of silver within each membrane. That is, the silver XANES for membranes 3-5 and 8 saturated with aqueous solutions of AgN03, AgBF,, AgClO,, and AgF (supplementary material) are identical to those membrane data shown in Figure 2. Most important, the XANES for the aqueous solution of 5.0 N AgN03 (Figure 2) are similar to those data for the silver-treated membranes. For aqueous AgN03 and AgC104solutions, four water molecules are known to coordinate to silver to form a stable, hydrated cation, [Ag(OH,),]+, with tetrahedral stereochemistry (Texter et al., 1983; Yamaguchi et al., 1984a,b, 1988; Sandstrom et al., 1985; Skipper and Neilson, 1989). The XANES suggest the presence of an Ag-0 complex within the membrane matrices. The presence or absence of preedge structure(s) in X-ray absorption spectra can often be used to elucidate the site symmetry of the X-ray absorbing element. For example, high-resolution K-edge XANES of 3d transition cations reveal a preedge feature, due to a 1s to 3d electronic transition which varies in intensity with the number of d-orbital vacancies and with the local symmetry of the absorbing atom site (Lytle et al., 1988). For an atom in a coordination environment without inversion symmetry (e.g., trigonal, tetrahedral, etc.), the transition is strong and well-resolved. For an atom in a coordination environment with inversion symmetry (e.g., linear, octahedral, etc.), it is weak and poorly-resolved. The absence of preedge structure (Le., shoulders, peaks) in the silver K-edge XANES of Figure 2 prevents an assessment of the coordination stereochemistry of the Ag+ cations in membranes 2-9. In general, stereochemical information obtainable from XANES above ca.25 keV is rather limited. This is due to two effects: (1)large line width caused by core-hole lifetime broadening and (2) low instrumental energy resolution, due to the large bandwidths of conventional X-ray monochromators at high energies (Lytle, 1989). For these reasons, it is not possible to obtain high-resolution Ag K-edge (25.521 kev) XANES, which would be of use in determining the symmetry of ligands about Ag+. Nevertheless, the results of Yoshiasa et al. (1988a,b) demonstrate that silver K-edge structure is of sufficient sensitivity to distinguish between tetrahedral coordination of Ag+ (with iodine in the wurtzite-type structure of 0-AgI), on the one hand, and octahedral coordination of Ag+ (with bromine in the rock-salt type structure of AgBr), on the other. High-resolution XANES measurements at the L1-, Lz-, and L3-edges (3.806, 3.524, and 3.351 keV, respectively) of silver would facilitate a detailed analysis of the coordination site symmetry and valence of silver in membranes and solutions. In this regard, the use of silver L-edge XANES has proven to be of importance in the electronic characterization of AgzO (Czyzyk et al., 1989) and other oxidic Ag+ compounds (Behrens, 19921, and fast ion conducting glasses (Dalba et al., 1986, 1987; Bernieri et al., 1983). The data of Figure 2 exhibit a narrow peak a t the top (ca. 25.510 keV) of the absorption edge followed by two broad ones at ca. 25.545 and 25.605 keV. All three peaks are due to, in large part, photoelectron backscattering (i.e., EXAFS) from the nearby atoms about silver (Lytle et al., 1989). The similarity of the XANES of Figure 2 suggests
0.0
~
%-%I\-
; c-/i \ L/-J 0. B
2.0
r',
A
9
I
4.0
6.0
Figure 3. Fourier transforms of the silver K-edge EXAFS, k3x(k), of an aqueous solution of 5.0 N AgNO, and membranes 2,6,7, and 9 treated with aqueous silver nitrate solutions. The vertical scale is offset for clarity.
that the neighboring atoms about silver are the same for the aqueous silver nitrate solution (Le., oxygen) and for membranes 2-9. Close inspection of Figure 2, however, reveals that the normalized edge-peak intensity of the AgN03 solution spectrum is slightly more intense than those for the spectra of membranes 2-9. This reflects an alteration of the coordination sphere of Ag+ on going from bulk solution to within the membrane matrix. A rigorous assessment of the structural significance of the peak intensity differences is not possible. Nevertheless, the EXAFS analyses, vide infra, provide a quantitative, metrical description of the coordination of Ag+ in aqueous solution and membrane matrix. EXAFS Data. Analysis of silver K-edge EXAFS for aqueous silver perchlorate and silver nitrate solutions by Yamaguchi et al. (1984a, 1988) and Dring et al. (1989) reveals the presence of four oxygen atoms about the argentous ion with Ag-OH2 bond lengths of 2.31-2.36 A. These results are in agreement with those from X-ray and neutron diffraction measurements, which indicate that the coordination number for Ag+ in aqueous solution is 4, with an Ag-OH, bond length of 2.40 f 0.02 A in the hydrated ion (Skipper and Neilson, 1989; Sandstrom et al., 1985; Yamaguchi et al., 1984a,b). Further, the hydrated ion, [Ag(OHJ,]+, has a total of 18 valence electrons about silver (10 from Ag+,with a ground-state electronic configuration of [Kr]4d105s05p0,and 8 from the four water molecules). The empirical "l&electron rule" predicts that this solvated, 18-electron Ag+ complex is a stable one. The most common geometry for four-coordinate Ag+ complexes is tetrahedral. For membranes 2-9, the Fourier transform data of the sinwoidal-like oscillations of the normalized EXAFS, k 3 x ( k )versus k, reveal a single peak at ca. 1.75 A (before phase shift correction), as shown in Figure 3 for membranes 2,6,7, and 9. These intense, broad peaks are due to backscattering from the nearest-neighboring atoms (i.e., oxygen) about silver. The absence of distant peaks in 2 I J I5 A indicates that the local structure around the Ag+ ions is not ordered except for the nearest oxygen neighbors. The ranges of Ag-0 peak positions, magnitudes, and full widths at half-maximum obtained from the
276 Ind. Eng. Chem. Res., Vol. 32, No. 2, 1993
Fourier transform data of Figure 3 are, respectively, as follows: 1.73-1.78 A (before phase shift correction), 30.1-37.3, and 0.68-0.73 A. The corresponding Fourier transform data for the hydrated Ag+ ion in silver nitrate solution (Figure 3) are different from those for membranes 2-9. For Ag+ in nitrate solution, the average Ag-0 distance (1.95 A) is some 0.2 A longer and the Ag-0 peak is both more intense (42.3) and sonewhat more narrow (0.68 A) than those for Ag+ ions in the membranes. These Fourier transform data suggest that the cations in the membranes do not possess the short-range structural order of the hydrated Ag+ complex ion in aqueous solution, Le., [Ag(OH&,]+,as obtains for silver nitrate and silver perchlorate. Curve fitting analyses of the k 3 x ( k )EXAFS data facilitate a more exact comparison of the argentous ion coordination in aqueous solution and in membrane matrices. The results of such (from plane-wave, single-scattering numerical models) are summarized in Table I. The EXAFS curve fitting results indicate that the average (phase shift corrected) Ag-0 distances (2.20-2.26 A) and the oxygen coordination number (3) for membranes 2-9 are significantly different from those (2.34 8, and 4, respectively) for the silver nitrate solution sample. The contraction of the Ag-O distance from bulk solution to membrane pore is consistent with the decrease in the coordination number. In the solid state, the crystal structures of common, oxygen-coordinated argentous compounds reveal 2- (linear), 4- (tetrahedral and square planar), 5(pyramidal), and 6-fold (octahedral) coordination geometries about silver. For each coordination number, the Ag-O bond lengths fall within a limited range with average values of 2.13, 2.40, 2.48, and 2.50 A for 2-, 4-, 5-, and 6-fold coordination, respectively (Yamaguchi et al., 1984b). Two-fold oxygen coordination leads to linear structures with Ag-0 distances between 2.04 and 2.20 A. When four or more oxygen atoms are coordinated to Ag+, the Ag+ ions have regular (and distorted) tetrahedral, octahedral, and antiprism eometries with Ag-0 distances between 2.30 and 2.64 . The Ag-0 interatomic distances for the membranes (Table I) are intermediate of those for 2- and 4-fold coordination by oxygen and are consistent with the coordination of three ligands to silver, i.e., [Ag(O-.)3]+. Precedence for trigonally coordinated Ag+ ions is found in the structure of partially-hydrated, fully &+-exchanged zeolite A, which has three framework oxide ions at 2.23 (2) and 2.25 (2) A about Ag+ (Kim and Seff, 1978). The silver ion complex in each membrane has a total of 16 valence electrons about silver (10 from Ag+ and 6 from the three 0-containing ligands). Although this is not an l&eledron complex, there are, however, many known exceptions, e.g., stable, 16- and 14-electron metal ion complexes (such as [Ag(CN),]-), to the 18-electron rule. The three oxygen atoms that form the first coordination sphere of Ag+ in membranes 2-9 can come from either water molecules or the membrane matrix material, Le., hydroxy groups in cellulose or sulfonate groups in Ndion and ion exchange membranes. The oxygen atoms from the membrane matrix are attached to carbon or sulfur of the polymer backbones. Thus, in principle, further backscattering by these second-sphere atoms should provide evidence of their existence. Unfortunately, the signal strength decreases precipitously as the distance between the absorbing element and the scattering element increases. Thus the absence of the second coordination sphere scattering signal cannot negate their existence. With the available data, it is not possible to determine if the oxygen atoms are from water molecules and/or the membrane matrix itself.
B
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Figure 4. Fourier transforms of the silver K-edge E M S ,k3x(k), of (bottom) an aqueous solution of 4.0 N K[Ag(CN),];. (top) membrane 1 treated with the aqueous potassium silver cyanide solution. The vertical scale ie offset for clarity.
The Fourier transform data of the normalized silver K-edge EXAFS, k3x(k),for the potassium silver cyanide treated membrane (1) are shown in Figure 4 along with the corresponding data for the aqueous K[Ag(CN),] solution. Both Fourier transforms exhibit two peaks of nearly equal intensities a t 1.63and 2.58 A (before phase shift correction). The first is due to Ag-C backscattering, and the second is due to Ag-N backscattering though the CN group. In the solid state, the Ag-C and Ag-N interatomic distances calculated from a single-crystal X-ray diffraction study of K[Ag(CN),] are 2.135 and 3.276 A, respectively (Hoard, 1933). The curve fitting analysis of the EXAFS for membrane 1 reveal an average Ag-C distance of 2.00 A with a coordination number of 2 (Table I). These results, in combination with the XANES, confirm the presence of the linear [NC-Ag-CNI- anion in the membrane matrix. The same conclusion was obtained from gold L3-edge EXAFS of poly(3-methylthiophene) doped with K[Au(CN),] from aqueous solution, i.e., the linear anion [NC-Au-CNI- exists in the doped polymer and in the K[Au(CN),] salt (Tourillon et al., 1987). Silver(1) has a relatively low affinity for oxygen donor ligands (Cotton and Wilkinson, 1988). The bonding between water molecules and Ag+ is weak. Environmental perturbation such as going from bulk solution into membrane phase could affect the silver(I)-OH2interaction and result in a change of coordination number from 4 to 3. The electronic requirement of the silver when reducing the oxygen donors from 4 to 3 can be satisfied by pulling the remaining three oxygen donors closer to the Ag+. The cyano (CN-) ligand binds Ag+ strongly. In fact the bonds in the linear [NC-Ag-CNI- anion are considered to be predominantly covalent (Griffith, 1962). Apparently the perturbation of going from bulk solution into membrane phase is not strong enough to disturb this coordination arrangement. The results indicate that the aqueous environments in these membranes are sufficiently different from that of bulk water. The membranes used in this study are all geltype membranes; i.e., the membranes swell when wet and the water molecules are well-dispersed in a polymer matrix. High dispersion by the polymer matrix leads to restricted interactions between the water molecules
Ind. Eng. Chem. Res., Vol. 32, No. 2, 1993 277 (Toprak et al., 1979). This “water” has been shown to not freeze at temperatures as low as -60 OC (Frommer and Lancet, 1972) in cellulose acetate membranes. The microstructure of water-swollen Naiion membranes has been studied by small-angle X-ray scattering (SAXS) and is thought to consist of ionic cluster regions 40-50 A in diameter connected by 10-A-widechannels and surrounded by a fluorocarbon polymer phase (Eisenberg and Yeager, 1982). Gierke and Hsu (1982) estimated from solvent absorption studies that 6 and 14 water molecules are associated with each ionic site in potassium- and sodiumexchanged swollen Nafion membranes, respectively. These water molecules should be strongly affected by the highly electrical nature of the environment and behave differently from bulk water (Yoshida and Miura, 1992). The situation in Raipore ion exchange membranes is expected to be the same. Thus, the coordination chemistry of carrier species in facilitated transport membranes can be affected by the membrane matrix. Whether the coordination behavior changea or not depends on how strong the interactions are. The strength of the interaction, and thus the coordination behavior, could also be concentration dependent. The olefin-Ag+ interaction is usually much stronger than the 0-Ag+ interaction. Since the Ag+ ion is already “coordinately unsaturated” in these membranes, its interaction with olefin could, in principle, be enhanced by this urnembrae effect”. The use of XAFS to probe the effect of concentration and the structure of the Ag+-olefin complex in the membranes under in-situ conditions is in progress and will be reported subsequently. Conclusions Insights about the unique properties of facilitated transport membranes were obtained through X-ray absorption fine structure. The principal findings from this initial silver K-edge XANES and EXAFS investigation are summarized below. 1. The coordination environment of argentous, Ag+, ions in aqueous solutions of silver nitrate (AgNO,) and silver perchlorate (AgC104) has four water molecules around silver, with an average Ag-O interatomic distance of 2.34 (4) A, in an “18-electronn, cationic, aquo complex, i.e., [Ag(OH,)J+. 2. The coordination environment of argentous ions in eight geltype membranes treated with aqueous solutions of silver nitrate, silver perchlorate, silver tetrafluoroborate (AgBF4),and silver fluoride (AgF) is different from that for the bulk solutions of the aforementioned silver salts. Within the membrane, there are just three oxygen atoms coordinated to Ag+, with an average Ag-0 interatomic distance of 2.22 (4) A, in a “16-electron”,cationic complex, i.e., [Ag(O-.-),]+,in which the oxygen atoms are from water molecules and/or the membrane matrix. There is no evidence for nonbonding Ag-Ag interactions. 3. The coordination of argentous ions in the membranes is independent of the nature of the following counterions: nitrate, [NO3]-, tetrafluoroborate, [BF,]-, perchlorate, [ClOJ, and fluoride, [FI-. 4. Similarly, the coordination of the argentous ions in the membranes is independent of the nature (Le., composition, thickness, manufacturer, etc.) of the geltype membrane. 5. The coordination of argentous ions in an aqueous solution of potassium silver cyanide, K[Ag(CN),], has two carbon atoms around silver, with an average Ag-C interatomic distance of 2.01 (4) A, in a linear, anionic complex, i.e., [NC-Ag-CNI-. The coordination of argentous ions in
a membrane treated with this aqueous solution of potassium silver cyanide is identical to that for the solution itself. 6. The aqueous environment in these geltype membranes is sufficiently different from bulk water that it causes a change of coordination of the Ag+ ions. Acknowledgment We thank Dn. Peter Meehan, Raymond G. Teller, and Jesse S. Wainright (BP Research) for assistance with the data collection on beam line X-18B at the NSLS, which is supported by the U.S.Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. Supplementary Material Available: Detailed description (text and Figures I-IV) of the synchrotron radiation XAFS measurements and data analysis,the XANES and fmt differential XANES for colloidal silver f i b , and the XANES and first differential XANES, the background-subtracted k3x(k)EXAFS, and the correspondingFourier transform data for membranes 2-7 and 9 as well as for aqueous solutions of AgN03and K[Ag(CN),] (15 pages). Ordering information is given on any current masthead page. Literature Cited Antonio, M. R. Extended X-ray Absorption Fine Structure. In Encyclopedia of Materials Characterization: Surfaces, Interfaces, Thin Films; Brundle, C. R., Evans, Jr., C. A., Wilson, S., Eds.; Butterworth-Heinemann: Boston, 1992;pp 214-226. Antonio, M. R.;Song, I.; Yamada, H. Coordination and Valence of Niobium in Ti02.NbOzSolid Solutions through X-ray Absorption Spectroscopy. J. Solid State Chem. 1991,93,183-192. Behrens, P. Bonding in Silver-Oxygen Compounds from Ag L3 XANES spectroscopy. Solid State Commun. 1992,81,235-239. Beniere, F.; Bertru, N.; Catlow, C. R. A.; Cole, M.; Simonet, J.; Angely, L. Trapping of Ag+ and Cs+ by a Crown-Ether Polymer Studied by EXAFS. J. Phys. Chem. Solids 1992,53, 449-457. Bernieri, E.; Burattini, E.; Dalba, G.; Fornasini, P.; Rocca, F. X-ray Absorption Measurements a t the Ag L3 Edge on Silver Borate Glasses with Synchrotron Radiation. Solid State Commun. 1983, 48,421-425. Bevenvijk, C. D. M.; Van Der Kerk, G. J. M.; Leusink, A. J.; Noltes, J. G. Organosilver Chemistry. Organomet. Chem. Reo. A . 1970, 5 , 215. Cotton, F. A,; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; Wiley Interscience: New York, 1988. Czyzyk, M. T.; de Groot, R. A.; Dalba, G.; Fornasini, P.; Kisiel, A.; Rocca,F.; Burattini, E. Ag,O Band Structure and X-ray Absorption Near-Edge Spectra. Phys. Reu. B 1989,39,9831-9838. Dalba, G.; Fornasini, P.; Rocca, F.; Burattini, E. XANES in Fast Ion Conducting Glases AgI:Ag20:B20,. J. Phys., Colloq. C8, Suppl. 12 1986,47,749-752. Dalba, G.; Fornasini, P.; Rocca, F.; Bernieri, E.; Burattini, E.; Mobilio, S. EXAFS Studies of Silver Ion Coordination in Silver Borate Glasses. J. Non-Cryst. Solids 1987,91, 153-164. Dring, I. S.; Hall, D. H.; Oldman, R. J.; Casci, J. L.; Merideith, W. N. E.; Tooze, R. P. The Structural Environment of Substituted Cations in Synthetic Silicate Materials. Physica B 1989, 158, 167-169. Eisenberg, A.; Yeager, H. L., Eds. Perfluorinated Ionomer Membranes; ACS Symposium Series 180,American Chemical Society: Washington, DC, 1982. Frommer, M. A.; Lancet, D. Freezing and Nonfreezing Water in Cellulose Acetate Membranes. J. Appl. Poly. Sci. 1972, 16, 1295-1303. Gierke, T. D.; Hsu, W. Y. The Cluster-Network Model of Ion Clustering in Perfluorosulfonated Membranes. Perfluorinated Zonomer Membranes; ACS Symposium Series 180;American Chemical Society: Washington, DC, 1982;pp 283-306. Goddard, J. D. Further Applications of Carrier-Mediated Transport Theory-A Survey. Chem. Eng. Sci. 1977,32,795. Gcddard, J. D.; Schultz, J. S.; Suchdeo, S. R. Facilitated Transport via Carrier-mediated Diffusion in Membranes, Part 11. Mathe-
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