10065
J. Phys. Chem. 1991, 95, 10065-10070
Electron Spin Resonance and Electron Spin-Echo Modulation Studies of Silver Ion Solvation in Sllver-Exchanged Synthetic Fluorohectorite and synthetic Beidelllte Vittorio Luca, David R. Brown,+and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: May 29, 1991; In Final Form: July 12, 1991)
Radiolytic reduction of Ag(1)-exchanged synthetic beidellite and synthetic fluorohectorite generates paramagnetic Ag(O), the aqueous solvation complex of which is probed by electron spin resonance (ESR) and electron spin-echo modulation (ESEM) spectroscopies. ESR indicates that at low water contents one Ag(0) species (A) is formed and that as the extent of hydration increases, another Ag(0) species (B) forms as well as a possible Ag2+dimer. Similar ESR results are obtained for both synthetic fluorohectorite and synthetic beidellite. Ag(0) species A has a IwAg isotropic hyperfine constant of 2032 MHz, which is comparable to the gas phase value, while Ag(0) species B has a lwAg isotropic coupling of only 1565 MHz. Two-pulse ESEM analysis of species A in AgNa-beidellite suggests that the parent Ag(1) ions remain located near AI(II1) in the tetrahedral sheet as the hydration state increases. Analysis of the three-pulse ESEM signal indicates that only two water molecules are directly coordinated to one Ag(0) species when the clay is wet and at 100%relative humidity. A model is proposed for the solvation complex of Ag(1) and Ag(0) in the beidellite interlayer in which they are coordinated to two basal surface oxygen atoms and two water molecules at all levels of hydration. ESR and ESEM analysis of species B suggests that the solvation complex of the parent Ag(I) ion contains four water molecules. The solvation complex of Ag(0) speces A in the fluorohectorite interlayers could not be established definitively, although it appears that in hydrated samples the Ag(1) is also coordinated to the basal surface oxygens. A reduction in the amount of hydration of the fluorohectorite appears to result in the movement of Ag(1) toward the basal oxygen surface, and when water is totally removed, it moves into the pseudohexagonal cavities in the basal oxygen surface.
Introduction
Metal oxide supported transition metals and transition-metal ions are an important class of catalyst. It has been amply demonstrated that the properties of such catalyst systems depend not only on the nature of both the transition-metal cation and the support but also on the interaction between them.'J Several studies have explored the solvation geometry of exchangeable cations and their interactions within clay mineral interlayers as a function of hydration ~ t a t e . ~ - ' Woessner ~ and SnowdenI3 provided evidence for the preferred orientation of water molecules on montmorillonite surfaces in montmorillonite-water systems. Recently,14 more definitive evidence of the capacity of the montmorillonite surface to induce molecular ordering has been provided and the import of such ordering in catalysis emphasized. Silver is a transition metal which has proven to be catalytically very active when adsorbed on various oxide surfaces including ~eo1ites.I~Electron spin resonance (ESR) and electron spin-echo modulation (ESEM) spectroscopies have been used to elucidate the aqueous coordination geometry of silver ions when diamagnetic Ag(1) is reduced at 77 K to paramagnetic Ag(0) by X- or y-irradiation with no change in the coordination number.I6J7 The nature of the Ag(1) solvation complex in smectite clays and interactions of the Ag(1) with the clay surfaces are investigated by ESR and ESEM using paramagnetic Ag(0) as a probe. In frozen aqueous solutions a number of different Ag species have been observed by ESR at 77 K after y-irradiation at 77 K including several Ag(0) centers and Ag(II).I8 The Ag(0) signals formed in this way are isotropic with g = 1.997 and with an isotropic hyperfine coupling constant of 536-609 G. These signals are doublets because Io7Agand lwAg have I = with about the same natural abundance. The g values for these solvated Ag(0) species are lower than for gas-phase Ag(0) (g = 2.002), and the hyperfine coupling is less than that of gas-phase Ag(0) ( A = 706 G). The coordination number of Ag(1) in water is generally 4.18*'9*21 When aqueous solutions of Ag(1) are irradiated at 4 K and the ESR spectrum is recorded at 4 K, only a single Ag(0) species is observed with g = 1.999, A = 1763 MHz for lWAg and a narrow line width. When aqueous solutions of Ag(1) are irradiated at 4 K and briefly annealed at 77 K before the ESR spectrum is recorded at 4 K, the predominant species is one with
'
Present address: Department of Applied Science, Leeds Polytechnic, Leeds LSI 3HE. England.
g = 1.995, A = 1461 MHz for lWAg and a broad line width.19 The narrow line with large A value was attributed to Ag(0) centers tetrahedrally coordinated to four water molecules. The broad line with small A value was attributed to relaxation of the solvent shell in response to the reduction of Ag(1) to Ag(0). This relaxation process, which has been termed " s o l v a t i ~ n " is , ~suggested ~ ~ ~ ~ to involve the rotation of one of the four water molecules so that one HO bond points toward the Ag(0). In later work, ESEM evidence in support of the above model was pr0vided.I' Recently we studied the aqueous complex of Ag(0) in Ag(1)-exchanged montmorillonite and suggested that Ag(0) is coordinated to four water molecules when the clay is suspended in water.22 However, the electrostatic potential experienced by exchangeable cations within the interlayer space of a particular smectite depends on the type and position of cation substitution ~
0022-365419 112095-10065S02.50/0 -~ ., . 0 1991 American Chemical Society I
,
~~~
(1) Laszlo, P.; Mathy, A. Helv. Chim. Acta 1987, 70, 577. (2) Bond, G. C.; Zurita, J. P.; Flamerz, S.Appl. Caral. 1985, 27, 3513. (3) Clementz, D. M.; Mortland, M. M.; Pinnavaia, T. J. J. Phys. Chem. 1973, 77, 196. (4) Clementz, D. M.; Mortland, M. M.; Pinnavaia, T. J. Clays Clay Miner. 1974, 22, 49. ( 5 ) McBride, M. B.; Pinnavaia, T. J.; Mortland, M. M. Am. Mineral. 1975, 60, 66. (6) McBride, M. B.; Pinnavaia, T. J.; Mortland, M. M. J . Phys. Chem. 1975, 79, 196. (7) McBride, M. B. Clays Clay Miner. 1976, 24, 88. (8) McBride, M. B. Clays Clay Miner. 1976, 24, 21 1. (9) Luca, V.; Cardile, C. M. Clay Miner. 1989, 24, 115. (10) Brown, D. R.; Kevan, L. J. Am. Chem. Soc. 1988, 110, 2743. (1 1) Lapexhe, V.; Lambert, J. F.; Prost, R.; Fripiat, J. J. J . Phys. Chem. 1990, 94, 8821. (12) Kawano, M.; Tomita, K. Clays Clay Miner. 1991, 39, 77. (13) Woessner, D. E.;Snowden, B. S . J . Chem. Phys. 1969, 50, 1561. (14) Delville, A,; Grandjean, J.; Laszlo, P. J . Phys. Chem. 1991, 95, 1383. (15) Jocobs, P. A.; Uytterhoeven, J. B.; Byer, H. K. J . Chem. Soc., Chem. Commun. 1977, 128. (16) Stevens, A. D.; Symons, M. C. R. J. Chem. Soc., Faraday Trans. I 1989,85, 1439. (17) Ichikawa, T.; Kevan, L.; Narayana, P. A. J . Chem. Phys. 1979, 9, 3792. (18) Brown, D. R.; Symons, M. C. R. J . Chem. Soc., Faraday Trans. I 1977, 73, 1490. (19) Kevan, L.; Hase, H.; Kawabata, K. J. Chem. Phys. 1977,66,3834. (20) Brown, D. R.; Symons, M. C. R. J. Chem. Soc., Dalton Trans. 1976, 426. (21) Texter, J.; Hastreiter, J. J.; Hall, J. L. J . Phys. Chem. 1983,87, 4690. (22) Brown, D. R.; Luca, V.; Kevan, L. J. Chem. Soc., Faraday Trans. I, in press.
Luca et al.
10066 The Journal of Physical Chemistry, Vol. 95, No. 24, 1991
in the smectite lattice.23 In the present study we have investigated two smectite clay minerals, beidellite and fluorohectorite, which have similar structures to montmorillonite but differ in the type of cation substitution producing the negative layer charge. The position in the layer lattice at which negative layer change originates can influence the physical and chemical properties of the smectite clay minerals. Synthetic beidellite contains essentially only Si and AI cations in the lattice. The octahedral sheet is dioctahedral, meaning that two of the three available sites are occupied by AI(II1). The negative layer charge is generated by the substitution of AI(II1) for Si(1V) in the tetrahedral sheet. A consequence of tetrahedral substitution is that there is a large electrostatic field in the region of the three basal surface oxygen atoms bonded to AI(II1). This results in strong electrostatic potential fields above the basal surface.z3 In this case, therefore, coordination of Ag(1) to the basal surface oxygen atoms might be expected. On the other hand, synthetic fluorohectorite contains Si, Mg, and Li cations and its octahedral sheet is trioctahedral, meaning that all of the octahedral sites are filled. Layer charge is generated in fluorohectorite by the substitution of Li(1) for Mg(I1) in the octahedral sheet. In addition, fluorohectorite contains structural fluorine instead of structural hydroxyl groups. In the present study we seek to establish, using ESR and ESEM, the coordination environment of Ag(0) generated in different synthetic smectite clay minerals by y-irradiating the Ag(1)-exchanged samples at 77 K. Additionally, we seek to understand what differences the nature of the clay layer charge has on the hydration of Ag(0) and how its hydration relates to that of Ag(1).
Experimental Section Synthetic beidellite was prepared by a method similar to that described r e ~ e n t l y . ~ ~A. ~gel ~ was made by the method of Hamilton and Hendersonz6 with composition corresponding to the chemical formula Nao,67A14(Si7,33Alo,67)OX1(OH)4. To make 100 g of gel, 8.03 g of NaNO, was dissolved in 50 mL of deionized water. This solution was added to a solution of 250.4 g of Al(NO,), in 550 mL of water. To this were added 230 mL of ethanol and then 218.7 g of tetraethoxysilane (Aldrich). This solution was stirred for 3 h to achieve hydrolysis of the tetraethoxysilane, and then 230 mL of 25% NHIOH was added to the continuously stirred solution to precipitate the hydroxides. The viscous gel was homogenized in a blender and subsequently dried at 80 "C for 14 h, 120 "C for 8 h, 150 OC for 14 h, and finally 400 O C for 4 h. The finely ground dry gel (4 g) and 13 mL of 0.15 M NaOH were placed in a gold tube, which was welded shut a t one end. The top end of the tube was welded shut while the tube was kept cool in an ice bath. The gold tube when sealed had a final volume of about 24 mL. The sealed tube was weighed and then placed in a 1-L Parr bomb containing water, and the bomb was heated at 330 OC for 7 days. At the end of the reaction, the gold tube was weighed to confirm that the tube had not ruptured. The product was dialyzed to remove excess ions and dried. The X-ray powder diffraction (XRD) pattern of the synthetic beidellite was recorded on a Philips PW1840 diffractometer, and good agreement was obtained with the d spacings of the hk reflections reported recently.25 The ESR spectrum of the synthetic beidellite showed no signal and confirmed the absence of paramagnetic impurities. The 27Al solid-state nuclear magnetic resonance (NMR) spectrum of the beidellite gave one signal at about 0 ppm from octahedral AI(II1) and a smaller signal at about 60 ppm due to tetrahedral AI(II1). The reference was aqueous AI(N0,)3. The lvAl(Ill)/lvAl(lll) + V'AI(III) ratio as determined from integration of the N M R spectrum was about 0.2, which is similar to that of natural beidel lite^.^' (23) Bleam, W. F. Clays Clay Miner. 1990, 38, 527. (24) Schutz, A.: Stone, W. E. E.; Poncelet, G.; Fripiat, J. J. Clays Clay Miner. 1987. 35,25 I . (25) Kloprogge, J. T.; Jansen, J. B. H.: Geuss, J. W. Clays Clay Miner. 1990. 38, 409. ( 2 6 ) Hamilton, D. L.; Henderson, C. M. B. Miner. Mag. 1968, 36, 832.
The synthetic fluorohectorite was made from a wet gel with composition corresponding to the unit cell formula, Nal,*(MgS,oLil,o)Si8020F4~ The SiMg precipitate was obtained by gradual hydrolysis with excess N H 4 0 H of a 1/1 ethanol/water solution of tetraethoxysilane (41.66 g) and MgCI2.6H20 (25.41 g) which had previously been stirred for at least 4 h at about 60 OC. The precipitate was filtered, washed several times with deionized water, and then redispersed in water. To this suspension were added 0.649 g of LiF and 3.2 g of NaF. This suspension had pH = 9.0, and and the fluorohectorite was prepared by refluxing the suspension for 7 days. The product was filtered, washed thoroughly with deionized water, and dried. The synthetic fluorohectorite had an XRD pattern corres onding to that of natural hectorite and gave d(060) = 1.512 which is characteristic of trioctahedral smectites. Samples of Ag(1)-exchanged synthetic smectites were prepared by stirring about 2.0 g of the smectites with about 200 mL of 0.5 M AgN0, solution for about 12 h. The smectite suspensions were then filtered, washed, and dialyzed to remove excess ions. The pH was kept under 7, and there seemed to be no evidence of hydrolysis since the results do not change with the degree of washing. The powdered smectites were equilibrated at 0, 48, and 100%relative humidity (RH) in sealed vessels containing P2OS, saturated KNO, solution, and water, respectively. To prepare samples for ESEM studies to observe deuterium modulation, the smectites were equilibrated with D 2 0 solutions. The smectites were first dehydrated at 200 "C under oxygen to eliminate H 2 0 before exposure to the D 2 0 solutions. Wet, fully hydrated samples were prepared by adding about 60 pL of DzO or HzO to 40 mg of the synthetic clay. Samples of the Ag-exchanged fluorohectorite and Ag-exchanged beidellite will be designated as AgNafluorohect and AgNa-beid since commercial analyses indicated some residual sodium ion. All preparations were performed in the dark. Samples for ESR and ESEM analysis were sealed in 2-mm i.d. X 3-mm 0.d. Suprasil quartz tubes, rapidly frozen to 77 K, and y-irradiated a t 77 K to a total dose of 1.4 Mrad. y-Radiolysis was carried out using a @%20 source with a dose rate of 0.54 Mrad h-l. Previous workloon Cu(I1) hydration in montmorillonite shows that the freezing is rapid enough so that collapse of the clay layers by freezing the water out of the clay layers does not occur. ESR spectra were recorded at 77 K on a Bruker ESP 300 ESR spectrometer. ESEM data were recorded at 4 K on a home-built spectrometer described e l s e ~ h e r e . ~ ~Three-pulse - ~ ~ q ~ echoes were recorded with r = 0.27 ps to maximize modulation from 2H. Two-pulse echoes were recorded to observe ,'AI modulation in the absence of D20.
R
Results
XRD Data. Equilibration of AgNa-beid samples at 0,48, and 100% R H resulted in XRD patterns in which the 001 reflections had d values of 9.8, 12.6, and 19.1 A and rationality of the higher order 001 reflections. These spacings correspond to discrete zero, one, and about four molecular layers of water within the beidellite interlayers. Rationality of the 001 reflections indicates that interstratified structures are not formed at the relative humidities used and that hydration of the AgNa-beid is homogeneous. For the AgNa-fluorohect samples, basal spacings of 9.8, 15.8, and 22.2 A were obtained after equilibration at 0,48, and 100% RH. These spacings correspond to zem, two-, and about five-layer hydrates. Though higher order 001 reflections were broad, weak, and difficult to measure precisely for this clay, these higher order reflections did seem to be rationally related to the 001 reflection and therefore hydrates formed at the chosen humidities are probably homogeneous. ESR Data. ESR spectra of y-irradiated AgNa-fluorohect equilibrated at various relative humidities are shown in Figure 1. The spectrum of the sample equilibrated at 0% R H (Figure ~
~~
(27) Woessner, D. E. Am. Miner. 1989, 74, 203. (28) Narayana, P. A.; Kevan, L. Magn. Reson. Rev. 1983, I , 234. (29) Narayana, P. A.; Kevan, L. Phorochem. Phorobiol. 1983, 37, 105.
Silver Ion Solvation
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 10067 AgNa-fluorohecl
TABLE I: Simulations of Three-Pulse ESEM Data for AgNa-Fluoroheet and AgNa-Beid Equilibrated at Various Relative Humidities of D,O simulation parameters
a
shell 1 sample
field (G) N
R(A)
shell 2 A (MHz) N R ( A ) A ( M H z )
AgNa-Fluorohect wet 100% RH 48% R H
3650" 3650 3650
4 4 2
3.1 3.1
wet
3650
4
35Mb
8
3650 3650
4 4
2.8 3.0 3.0 2.4
0.15
0.15 0.15 0.15
2.8
4
3.8 3.6
0.15
0.18 0.12
4
3.5
0.12
0.09
4
4.0
0.00
4
0.09
AgNa-Beid
100% R H 48% RH
" Field positions of the lwAg ESR line of species A. of the "Ag ESR line of species B.
d
200 G
Field positions
AgNa-fluorohect/wet D20
{r
Figure 1. ESR spectra at 77 K and 9.299 GHz of AgNa-fluorohect equilibrated at (a) 0% RH, (b) 48% RH, (c) 100% RH, and (d) wet.
la) consists of an intense absorption in the central region of the spectrum due to defects generated in the clay lattice during irradiation. The sharp intense lines labeled H are due to hydrogen atoms also generated during irradiation. The small signals at both higher and lower fields relative to the hydrogen lines are due to Ag(0) species, which will be referred to as species A. The outer pair of Ag(0) lines are due to lWAgand have g = 2.022 and an isotropic hyperfine coupling constant of Ai, = 2032 MHz, while the inner lines are due to Io7Ag and have g = 2.022 and Ai, = 1760 MHz. The hyperfine coupling constants for the Ag(0) species giving these signals are larger than those observed for Ag(0) species on zeolites30and comparable to the values obtained for Ag(0) in the gas phase. As the water content of the AgNa-fluorohect samples is increased to form a two-layer hydrate at 48% RH, a triplet signal a t g = 1.81 with a splitting of about 48 MHz appears and its intensity with respect to the high field IWAg(0) line increases as the water content increases. The triplet signal may be part of the spectrum of Ag2+ that has been identified in zeolite^.^^ The ESR spectrum of the AgNa-fluorohect sample equilibrated at 100% R H has an additional absorption at 3550 G (Figure IC). This signal becomes better resolved as the water content increases (Figure Id) and results from another Ag(0) species, which will be denoted as species B. The ESR spectra of AgNa-beid are similar to those of AgNa-fluorohect except that, under highly hydrated conditions (Figure 3a), Ag(0) species B having g = 2.017 and A = 1565 MHz for the "Ag(0) component and g = 2.017 and A = 1334 MHz for the Io7Ag(0) component gives better resolved lines. The g values and hyperfine coupling constants for this species suggest that it is similar to solvated Ag(0) in AgN03 ices which is coordinated by four water molecule^.^^ Three-Pulse ESEM Data. In order to establish the solvation environment of species A, three-pulse ESEM data of AgNafluorohect and AgNa-beid with various D 2 0 contents were recorded a t t h e high-field IWAg(0) signal at 3650 G ( F i g u r e 1). However, because of the possibility of interference with the putative Ag2+signal, ESEM data were also recorded at the high-field '07Ag(0) signal at 361 2 G. These two sets of data give similar modulated decays and are well simulated by the same model. This plus the fact that the echo decays rapidly as the field is increased (30) Brown, D. R.; Kevan, L . J . Phys. Chem. 1986, 90, 1129. ( 3 1 ) Morton, J. R.: Preston, K. F. In Electron Magnetic Resonanceofthe Solid Stale; Weil, J. A., Ed.; Canadian Society for Chemistry: Ottawa, Canada, 1987; p 2 8 5 .
t
Shell 21 41 3.8 I 0.09
.2
"
40% RH
'[A
2 12.8 ,
2
,0
I
2
3
4
I 0.15 5
T, PS
Figure 2. Experimental and simulated three-pulse ESEM (H= 3650 G) at 4 K for (a) wet AgNa-fluorohect and (b) AgNa-fluorohect equilibrated at 48% RH.
above the absorption maximum of the high-field species A "Ag(0) signal indicates that the Ag2+ species does not interfere with ESEM data taken at the high-field IwAg(0) signal. Simulations were made in terms of N equivalent nuclei a t distance R and isotropic hyperfine coupling A using the spherical approximation3* and one or two nuclear shells as required. The best fit simulation parameters of the variously hydrated AgNa-beid and AgNafluorohect samples are summarized in Table I. Figure 2a shows a typical ESEM signal recorded a t the highfield "Ag(0) line (3650 G) of species A for the AgNa-fluorohect sample which was wet with D20. The best fit simulation to the experimental data is obtained using a two-shell model with four deuterons at a distance of 3.0 A and four deuterons at a distance 3.8 A. A similar model was also found to be the most appropriate for the AgNa-fluorohect which had been equilibrated in a 100% R H D 2 0 atmosphere. Less satisfactory results were obtained for simulations in which the fitting model contained more than two water molecules coordinated to Ag(0) The AgNa-fluorohect sample which had been equilibrated at 48% R H and contained two water layers gives quite a different (32) Kevan, L.; Bowman, M. K.; Narayana. P. A,; Bceckman, R. K.; Yudanov, V. F.; Tsvetkov, Yu. D. J. Chem. Phys. 1975, 63,409.
Luca et al.
10068 The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 g=2.018 A.1334 MHz
0
-
I'
0.31ps
200 G
-
0.23pS g= 2.015 A= 1566 MHz
b
.2
0
t1
I
I
I
I
I
I
2
3
4
5
T, ps
Figure 3. (a) ESR spectrum at 77 K and 9.299 GHz of wet AgNa-beid and (b) experimental and simulated three-pulse ESEM (H= 3585 G )
of wet AgNa-beid. ESEM signal from the samples with higher water contents. For this sample, the modulation is significantly more shallow (Figure 2b) and the best simulation occurs for two deuterons at a distance of 2.8 A and A = 0.15 MHz. The poor signal-to-noise ratio for this sample is a result of the reduction in the amount of adsorbed water. The deterioration of the signal to noise was observed consistently with a reduction in water content. For the samples which had been equilibrated at 0% R H and contained essentially no water, the three-pulse electron spin-echo was so weak as to preclude the observation of a decay signal. A two-pulse echo could however be observed for the dehydrated samples. No ESEM data were recorded for species B at 3550 G in Figure Id in the case of AgNa-fluorohect because these ESR lines were not well resolved from the H atom lines. The AgNa-beid samples which are wet (fully hydrated) and equilibrated at 100%RH both give ESEM data for Ag(0) species A that are well simulated with four deuterons at a distance of 2.8-3.0 A and four deuterons at a distance of 3.5-4.0 A (Table I). The sample equilibrated at 48% RH gives ESEM data which are fit using a model consisting of four deuterons at a distance of 2.4 A. Thus, two water molecules are directly coordinated to the Ag atom in this hydrate. In the ESR spectrum of the fully hydrated AgNa-kid sample (Figure 3a) the Ag(0) lines with the smaller hyperfine coupling, species B, are sufficiently well resolved to permit recording of ESEM data. ESEM data were recorded at 3585 G, and the data appear in Figure 3b. The model best fitting this experimental data is one with eight deuterons at a distance of 3.0 A with A = 0.12 MHz. Two-Pulse ESEM Data. To monitor the interaction between the paramagnetic Ag(0) species and magnetic nuclei in the silicate layers, two-pulse ESEM data were recorded for the HzO-hydrated AgNa-fluorohect and AgNa-beid at the species A lines which give the strongest echoes. The only magnetic nuclei in the fluorohectorite layers capable of giving modulation of the electron spin-echoes are Z3Na,7Li, I9F, and possibly IH. In AgNa-beid, the only magnetic nuclei capable of giving electron spin-echo modulation are 23Na,27A1,and again possibly 'H. To interpret the data, we use the observation that only if the exchangeable Ag(1) cations approach closer than 6 8, to the magnetic nuclei
I
2
7, ps
Figure 4. Two-pulse ESEM data for AgNa-fluorohect (a) wet (H= 3650 G ) , (b) equilibrated at 100%RH (H= 3650 G ) , (c) equilibrated at 48% RH (H = 2991 G ) , and (d) equilibrated at 0% RH (H= 2991 G ) . Arrows indicate positions of modulations.
is modulation of the electron spin-echo decay from Ag(0) expe~ted.~~ The two-pulse ESEM data for HzO hydrated AgNa-fluorohect samples are shown in Figure 4. Similar results are obtained at all lines corresponding to species A. For the fully hydrated AgNa-fluorohect sample (Figure 4a) weak modulations are observed with a period of 0.31 I.CS, which may correspond to the period of the 23Na nucleus of 0.25 ps at the same magnetic field (recall that fluorohectorite does not contain z7Al). The discrepancy between the two values is perhaps due to the large quadrupole moment of the 23Na nucleus and/or uncertainties in estimating the period of the weak modulations. When the sample is equilibrated at 100% RH, the two-pulse electron spin-echo (Figure 4b) shows modulations with a period of 0.23 ps which is closer to that of 23Na(0.25 ps) than that of 'Li (0.17 ps) a t this field. Reducing the water content further to the sample equilibrated at 48% R H results in an ESE with no obvious modulations. When the interlayer water is totally removed (Le., sample equilibrated at 0% RH), distinct modulations appear with a period of 0.083 ps. Such a period could correspond to 'Hor I9F, which have periods of 0.079 and 0.084 ps, respectively, at the same magnetic fields. Two-pulse ESEM data taken at the species A ESR lines for AgNa-beid samples with various water contents are given in Figure 5. For the fully hydrated sample (Figure sa) modulations appear with a period of 0.26 ps. This corresponds to the period of the 27A1nucleus which is 0.25 ps at the same magnetic field. Modulations with this same period are observed for AgNa-beid at all water contents, and the modulation depth seems to remain unchanged as the water content is lowered. The two-pulse echo recorded on the sample equilibrated at 0% RH was very weak, and satisfactory data could not be obtained. Discussion AgNa-Beidellite. In the case of AgNa-beid, the two-pulse ESEM results obtained at the field position corresponding to the (33) Kevan. L. In Time Domain Electron Spin Resonance; Kevan, L., Schwartz, R. N.. Eds.; Wiley-Interscience: New York, 1979; Chapter 8.
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 10069
Silver Ion Solvation AgNo- beid Wet D20
Si04 tetrahedron of upper tetrahedral sheet
E
b
100%RH
A104 tetrahedron of lower tetrahedral
sheet
0
0
0
< 8
I
Ag(1) ion Water molecule
Figure 6. Model for the solvation complex of Ag(1) in hydrated (wet, 100% RH,and 48% RH)AgNa-beid.
2
r, p
Figure 5. Two-pulse ESEM data for AgNa-beid (a) wet (H = 3650 G), (b) equilibrated at 100% RH ( H = 3650 G),and (c) equilibrated at 48% RH ( H = 3650 G). Arrows indicate positions of modulations.
ESR line of species A suggest that the parent Ag(1) maintains a relatively constant position with respect to the basal surface regardless of the amount of water occupying the interlayers. This follows because the 27Al modulation has a similar modulation depth at all levels of hydration. However, if the 27Alquadrupole interaction changes significantly on dehydration it complicates the interpretation. The observation of 27Almodulation is especially pivotal for the wet AgNa-bied sample since in this sample the layers may be separated by distances of up to 40 Thus, if Ag(1) is completely solvated and positioned in the middle of the interlayers separated by such large distances, no 27Almodulation is expected. The observation of 27AImodulation in the fully hydrated sample suggests that the Ag(1) is coordinated to the basal surface oxygen atoms of the aluminate ion (AIO;) within 6 A of the AI(II1) substitution site in the tetrahedral sheet of AgNa-beid. The fact that, in the one-layer hydrate, two-pulse ESEM data are modulated by 27AIas in more hydrated samples is significant since in this hydrate the Ag(0) can only be separated from one of the basal surfaces. Similar two-pulse ESEM results for the higher hydrates indicate that the Ag(0) is also directly coordinated to the basal surface oxygens of AgNa-beid. Additional evidence for the parent Ag(1) of species A being coordinated to the aluminate oxygen atoms is obtained from the simulated three-pulse ESEM data which indicate that only two water molecules directly coordinate to Ag(0) in wet and 100% RH samples. For these samples, three-pulse ESEM indicates that the oxygens of these coordinated water molecules are at a distance of 2.1-2.5 A. Additionally, three-pulse ESEM data indicate that another two water molecules are not in direct coordination to the Ag(0) but must be part of the H-bonded water structure within the swelled interlayers. Since the ESEM results indicate that two molecules of water as in direct coordination to Ag(O), this implies that the Ag(0) is also coordinated to two oxygens of the basal oxygen surface to maintain an expected coordination number of A.34335
4.
On the basis of the above discussion, the model of Figure 6 is proposed for the solvation complex of Ag(0) species A in both wet and 100% RH samples. Three-pulse ESEM data for species A in AgNa-beid equilibrated at 48% RH indicate that two water (34) Norrish, K. J . Chem. SOC.,Faraday Trans. 1954, 18, 120. (35) Foster, W. R.;Savins, J. G.: Waite, J. M.Clays Clay Miner. 1955,
3, 296.
Al(IU) ion S i ( E ) ion Oxygen atom
molecules remain directly coordinated to Ag(0) and that only water molecules not coordinated to Ag(1) are removed on reduction of the hydration state. The coordination of Ag(1) to two of the three aluminate oxygen atoms in hydrated samples is consistent with recent electrostatic potential calculation^.^^ These indicate that there is greater electrostatic field flux above two of the three aluminate oxygen atoms of beidellite compared to montmorillonite. This high electrostatic field flux above two of three aluminate oxygen atoms confers increased Lewis base character on these two aluminate oxygen atoms and suggests that Ag(1) would bind to these oxygen atoms. The greater electrostatic field flux above two of the three aluminate oxygen atoms apparently results from tilting of the tetrahedra bordering the basal surface.23 The present model for the solvation complex of Ag(1) in hydrated AgNa-beid interlayers differs from that proposed for the solvation complex of Na(1) in hydrated montmorillonite interlayers proposed recently by Grandjean and Laszlo36 and supported by Brown et aLzz In this model, some of the water molecules in the first solvation sphere of exchangeable cations are simultaneously bound to both the anionic basal surface and the exchangeable cations. Coordination of these water molecules to the exchangeable cations is by way of a lone pair on the oxygen, and binding to the surface is through H bonding. This situation apparently prevails in clay suspensions in which the water content is very high. The present model for the binding of Ag(1) directly to the AgNa-beid surface is not in conflict with that proposed for the binding of monovalent and divalent cations in montmorillonite given that these minerals differ quite dramatically in terms of the electrostatic potential near the basal surfaces. Species B in AgNa-beid is apparently derived from Ag(1) coordinated to four water molecules and can be silver on external edge sites where the ion experiences a different electrostatic field to Ag(1) ions bound to interlayer exchange sites. Alternatively, this unbound Ag(1) may be due to the incomplete removal of excess Ag(1) during the dialysis procedure. AgNa-Fluorohectorite. The two-pulse ESEM data for the AgNa-fluorohect indicate that in the wet sample and the sample equilibrated at 100%R H the Ag(1) and Na(1) cocations are within 6 A of one another because 23Namodulation is observed. When the hydration state is reduced to a two-layer hydrate at 48% RH, 23Na modulations are no longer observed. Therefore, it seems that a change has occurred in the environment of the exchangeable cations as water is removed from the interlayer regions. The three-pulse data suggest that one water ligand is removed from the first solvation shell of Ag(1) on going from a five-layer hydrate ( 3 6 ) Grandjean, J.; Laszlo, P.CIays Clay Miner. 1989, 37, 403.
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J . Phys. Chem. 1991, 95. 10070-10076
at 100%R H to a two-layer hydrate a t 48% R H and that water in the second solvation shell of Ag(1) is also removed. These observations may be explained in the following way. In wet samples and samples equilibrated at 100%RH, the Ag(1)-water complexes are bound to the basal surfaces but undergo restricted translational diffusion. Since Na(1)-water complexes coexist in this interlayer environment, some of these Na(1)- and Ag(1)-water complexes are likely to approach each other closely enough to allow observation of *,Na modulation in the ESEM of Ag(0). The reduction in water content from a five-layer hydrate to a two-layer hydrate may be expected to limit translational diffusion of the exchangeable cations and increase the residence time of the cation-water complexes at surface exchange sites. Measurements of autodiffusion coefficients have been made for homoionic ~mectites,~’*~* and it has been found that the diffusion coefficient decreases by 1 order of magnitude on reduction of the hydration state by one water layer. Because of the small amount of substitution of Li(1) for Mg(I1) in the lattice of the fluorohectorite and the diffuse nature of the surface charge,23 Ag(1)-Na(1) distances are likely to be greater than about 10 A if the Ag(1) or Na(1) is localized near surface sites. When water is totally removed from the interlayer of AgNafluorohect, the modulations observed in the two-phase ESEM can be from ‘Hor I9F,which have similar nuclear frequencies. It is unlikely that the modulations are due to ‘H because there should be few or no structural hydroxyl groups in AgNa-fluorohect. Moreover, similar intense modulations are not observed in the hydrated samples where the ’H concentration is greater. The appearance of I9F modulations in the zero-layer hydrate suggests that the Ag(1) cations move into the pseudohexagonal cavities in the basal oxygen surfaces where they are 2-3 8, from structural fluorine atoms. The solvation of Ag(1) cations in AgNa-beid and AgNafluorohect is similar in that Ag(1) seems to be coordinated to only two water molecules both in the wet state and after equilibration of the smectites at 100% RH. In AgNa-fluorohect, however, the monovalent exchangeable cations apparently move into pseudo(37) Calvet, R. D. Ph.D. Thesis, University of Paris, 1972. (38) Keay, J.; Wild, A. Soil Sci. 1961, 92, 54.
hexagonal cavities in the basal oxygen surfaces as interlayer water is removed whereas in AgNa-beid they maintain a relatively constant position with respect to the basal oxygen surface. The coordination of one water molecule in AgNa-fluorohect at 48% R H and two water molecules in AgNa-beid at 48% R H is also consistent with movement of Ag(1) into pseudohexagonal cavities in AgNa-fluorohect. This difference between AgNa-fluorohect and AgNa-beid is a consequence of the different nature of the electrostatic potential above the basal surfaces of these clays.23 In AgNa-beid and in AgNa-fluorohect, increasing water content generates species B, which gives an ESR signal with a small hyperfine coupling constant. The ESR parameters of this signal are similar to those observed in AgNO, ices after annealing at 77 K from 4 K,” and in the case of AgNa-beid, the ESE data are best simulated with eight deuterons at a distance of 3 A. Therefore, it appears that, in both AgNa-beid and AgNafluorohect, a proportion of the Ag(1) is not in direct coordination with the surface but is fully solvated. Whether this apparently fully hydrated Ag(1) is within the clay interlayers or on external surface or edge sites cannot be ascertained. In the present work the ESEM data support a model in which only two water molecules are in direct coordination to Ag(1) for both the AgNa-beid and the AgNa-fluorohect in the wet state and at 100% RH. Additionally, in the AgNa-beid sample, two-pulse ESEM data indicate that Ag(0) is bound to the basal oxygen surface in all hydration states. This is consistent with the notion that the aluminate oxygen atoms in AgNa-beid have considerably greater Lewis base character than the basal oxygen atoms in montmorillonite. Thus, Ag(1) is more likely to coordinate to the basal surface of AgNa-beid than AgNa-montmorillonite.
Acknowledgment. This research was supported by the Robert A. Welch Foundation, the Texas Advanced Research Program, and the National Science Foundation. The assistance of Xinhua Chen in the implementation of software used for the simulations is gratefully acknowledged. We also thank M. Narayana of Shell Corp. for recording NMR spectra. V.L. is grateful for G. Poncelet for instructions on the synthesis of beidellite. We also thank H. Yamada for an early beidellite sample, which was helpful in verifying our synthesis.
Kinetlcs and Thermodynamics of Inclusion of p-Nltrophenoiate with a-Cyciodextrin Measured with Pulse Voitammetry Michael J. Nuwer: John J. O’Dea, and Janet G. Osteryoung* Department of Chemistry, University at Buffalo, SUNY, Buffalo, New York 14214 (Received: June 3, 1991)
The rate constants and formal equilibrium constant are determined for inclusion of p-nitrophenolate anion into a-cyclodextrin at pH = 13.5, where a-cyclodextrinm is 82% dissociated to the basic (anionic) form. The conditional dissociation constant is 2.99 X IO-’ M (at 20 “C), about an order of magnitude larger than the value reported for pH 1 I , where a-cyclodextrin is completely in the acid form. This observation illustrates the importance of Coulombic interactions in substrate-receptor interactions. The values of rate constants, determined from a nonlinear least-squares analysis of the shape of square-wave voltammograms, are kf = 3.5 X IO5 S-I and kb = 1.2 X lo8 s-I M-’at 20 OC. This type of analysis is typically precluded by distortion of wave shape due to adsorption of cyclodextrin on the mercury electrode. This artifact was eliminated by optimization of voltammetric parameters and by competitive adsorption of tetramethylammonium ion. Values of diffusion coefficientsof p-nitrophenolate in the complex& and uncomplexed forms are 3.28 X lod and 1.05 X lW5cm2s-I, respectively.
An inclusion reaction is one in which a host molecule (or receptor) entraps a guest molecule (or substrate). The union of these two molecules does not involve covalent bonding, but rather a combination of hydrogen bonding, hydrophobic, and dipole in-
teractions.’ The included guest, being in a new chemical environment, is capable of undergoing novel and interesting reactions. For example, these systems are used as enzyme models to understand better site-recognition reactions? as synthetic tools, and
* To whom correspondence should be addressed. ‘Present address: Ciba-Geigy Corp., P.O. Box 11, St. Gabriel, LA 70776.
( I ) Bender, M. L.; Komiyama, M. Cyclodexrrin Chemisrry; SpringerVerlag: New York, 1978; pp 2-90.
0022-3654/91/2095-l0070$02.50/0 0 1991 American Chemical Society
