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Interlayer water structure in Na- and Li-montmorillonite clays
ELSEVIER
Physica B 241 243 (1998} 387 389
Interlayer water structure in Na- and Li-montmorillonite clays D.H. PowelP'*, Kowut
T o n g k h a o b, S h a n e J. K e n n e d y c, P h i l l i p G . S l a d e d
lnstitut de Chimie Min~rale et Analvtique. Unicersite de Lausanne-BCH. CH-IOI 5 Lausanne, Switzerland t'Department q['ChemistJ T, Unirersity ~?fAdelaide. SA 5005. Australia Australian Nuclear Science & Technoh~v Organisation, Menai. NSW 2234. Australia d CSIRO Dirision Of Soils, Glen Osmond, SA 5064, Australia
Abstract
We measured the neutron diffraction patterns of dry Li-montmorillonite and its D20 hydrate at an incident wavelength of 1.057 A. We used a difference method to obtain the diffraction pattern due to the interlayer water. Comparison with data for liquid DzO shows that the interlayer water has a liquid-type structure, although water molecules may be associated with specific crystallographic sites. C: 1998 Elsevier Science B.V. All rights reserved. Keywords: Diffraction: Clay; Montmorillonite: Water
Montmorillonites have applications in fields including waste confinement and catalysis [1]. The clay particles are stacks of negatively charged aluminosilicate layers separated by cations. Hydration of interlayer cations and internal clay surfaces is responsible for many of the specific properties of swelling clays. We recently confirmed an earlier indication [2] that interlayer water contributes a liquid-like component to the neutron diffraction pattern of Na-montmorillonite [3]. We used an approximate difference method, with the dry clay as a reference, to isolate this liquid-type component. Here, we extend our method to Li-montmorillonite.
* Corresponding author. Tel.: +41 21 6923918;fax: +41 21 6923925: e-mail: hpowell(aiicma.unil.ch.
The experimental and correction procedures were as described previously [3]: the Li-Wyoming montmorillonite was prepared from the Naclay (SWy-2, CMS) by ion exchange. We assume a simplified unit formula Lio.75(SiT.75Alo.25) (A13.sMgo.5)Oz0(OH)4 for the dry clay. The D 2 0 content of the hydrated sample was 220_+ 5 mgg-1 corresponding to a two-layer hydrate with 10.3 D 2 0 per unit formula. The diffraction patterns at 298 K for the dry and hydrated clays and liquid D 2 0 were measured on M R P D [4] at the Lucas Heights Research Laboratories. The scattering and absorption cross sections used to correct the data were 6.55 and 0.813 b for the dry clay and 5.52 and 0.46 b for the hydrate at the incident wavelength of 1.057(2) A (1 barn, b = 100 fm21. The corrected, normalised intensities, l(k), for dry Li-montmorillonite and its D 2 0 hydrate,
0921-4526.98/$19.00 ~ 1998 Elsevier Science B.V. All rights reserved P l l S092 I -4526(97)0059 7-8
388
D.H. Powell et al. / Physica B 241 243 (1998) 387 389 0.6 I 1.4
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/~ (*') Fig. 1. Corrected and normalised neutron diffraction patterns obtained for (a) D20 hydrated and (b) vacuum dried Li-montmorillonite.
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lc (/~') Fig. 2. The difference function, A(k), corresponding to the interlayer water diffraction in (a) Li-montmorillonite and (b) Namontmorillonite compared to the diffraction pattern, l(k), of pure liquid D20 (c). The latter function is multiplied by 0.42 to take account of the lower mole fraction of water in the clays.
shown in Fig. 1, are very similar to those for Namontmorillonite [3]. Both l(k) are d o m i n a t e d by Bragg scattering from the aluminosilicate structure, but that of the hydrate also has a diffuse c o m p o n ent due to the interlayer water. If the Bragg scattering is unchanged on hydration, one can extract the interlayer water contribution by weighted subtraction of the two I(k) [3]. The difference, A(k), is a weighted sum of w a t e r - w a t e r and water-clay partial structure factors, S,~(k), plus a self-scattering
Fig. 3. The Fourier transform, G(r), of the functions in Fig. 2, after empirical correction for k-dependent inelasticit3 effects (see text).
term (0.231 b). The weighting factors are similar to those given in [3] for Na-montmorillonite. A(k) is shown in Fig. 2 c o m p a r e d to that for Namontmorillonite [33 and the I(k) of liquid D 2 0 . The cancellation of the Bragg peaks is almost perfect. We a d o p t e d an empirical approach, assuming the largest term in the inelasticity corrections to be proportional to k 2 [5], to correct the d o w n w a r d slope of the patterns (for Li-montmorillonite A(k) = A(k) . . . . + 0.00125 k 2 - 0.012 and for D 2 0 A(k) = A(k) . . . . + 0.0022 k 2 + 0.001: see [3] for the criteria used). Fourier transformation of the corrected patterns with no s m o o t h i n g yields the radial distribution function, G(r), in Fig. 3, a sum of partial radial distribution functions, g~(r) [3]. Note that the form of G(r) is rather insensitive to the exact values used in the above correction procedure, except at low r. Removing the apparent remnants of Bragg peaks in A(k) at ca. 1.5 and 2.8 A - 1 using a spline fitting procedure had no discernible effect on the form of G(r). The similarity of the k-space and r-space functions for the clays in Figs. 2 and 3 to those for D 2 0 indicates that the interlayer water is liquid-like, although water molecules m a y be associated with specific crystallographic sites [6]. The new results for Li-montmorillonite confirm the potential of our difference technique and show that a liquid-state a p p r o a c h based on radial distribution functions is appropriate for the study of interlayer water in
D.H. Powell et al.
Physica B 241 243 tl998) 387 389
clays. N e w e x p e r i m e n t s are in progress, using a larger k-range a n d isotopic substitution, that will give better definition of b o t h intra- a n d interm o l e c u l a r water structure a n d enable detailed c o m p a r i s o n with recent c o m p u t e r s i m u l a t i o n s [7].
Acknowledgements W e t h a n k the A u s t r a l i a n Research C o u n c i l (4003/96), the A u s t r a l i a n Institute of N u c l e a r Science a n d Engineering (96/205NS) a n d the Swiss N a t i o n a l Science F o u n d a t i o n (21-49280.96, 2124045100.95).
389
References [1] A.C.D. Newman, Chemistry of Clays and Clay Minerals, Longman, London, 1987. [2] R.K. Hawkins, P.A. Egelstaff, Clays Clay Miner. 28 {1980) 19. [3] |).H. Powell, K. Tongkhao, S.J. Kennedy, P.G. Slade, Clays Clay Miner. 45 (1997) 290. [4] S.J. Kennedy, Adv. X-Ray Anal. 38 [1995) 35. [5] D.G. Montague, I.P. Gibson, J.C. Dore, Molec. Phys. 44 11981) 1355. [6] P.G. Slade, P.A. Stone, E.W. Radoslovich, Clays Clay Miner. 33 [1985) 55. [7] F.-R.C. Chang, N.T. Skipper, G. Sposito, Langmuir 13 [19971 2974.
Physica B 241 243 (1998} 387 389
Interlayer water structure in Na- and Li-montmorillonite clays D.H. PowelP'*, Kowut
T o n g k h a o b, S h a n e J. K e n n e d y c, P h i l l i p G . S l a d e d
lnstitut de Chimie Min~rale et Analvtique. Unicersite de Lausanne-BCH. CH-IOI 5 Lausanne, Switzerland t'Department q['ChemistJ T, Unirersity ~?fAdelaide. SA 5005. Australia Australian Nuclear Science & Technoh~v Organisation, Menai. NSW 2234. Australia d CSIRO Dirision Of Soils, Glen Osmond, SA 5064, Australia
Abstract
We measured the neutron diffraction patterns of dry Li-montmorillonite and its D20 hydrate at an incident wavelength of 1.057 A. We used a difference method to obtain the diffraction pattern due to the interlayer water. Comparison with data for liquid DzO shows that the interlayer water has a liquid-type structure, although water molecules may be associated with specific crystallographic sites. C: 1998 Elsevier Science B.V. All rights reserved. Keywords: Diffraction: Clay; Montmorillonite: Water
Montmorillonites have applications in fields including waste confinement and catalysis [1]. The clay particles are stacks of negatively charged aluminosilicate layers separated by cations. Hydration of interlayer cations and internal clay surfaces is responsible for many of the specific properties of swelling clays. We recently confirmed an earlier indication [2] that interlayer water contributes a liquid-like component to the neutron diffraction pattern of Na-montmorillonite [3]. We used an approximate difference method, with the dry clay as a reference, to isolate this liquid-type component. Here, we extend our method to Li-montmorillonite.
* Corresponding author. Tel.: +41 21 6923918;fax: +41 21 6923925: e-mail: hpowell(aiicma.unil.ch.
The experimental and correction procedures were as described previously [3]: the Li-Wyoming montmorillonite was prepared from the Naclay (SWy-2, CMS) by ion exchange. We assume a simplified unit formula Lio.75(SiT.75Alo.25) (A13.sMgo.5)Oz0(OH)4 for the dry clay. The D 2 0 content of the hydrated sample was 220_+ 5 mgg-1 corresponding to a two-layer hydrate with 10.3 D 2 0 per unit formula. The diffraction patterns at 298 K for the dry and hydrated clays and liquid D 2 0 were measured on M R P D [4] at the Lucas Heights Research Laboratories. The scattering and absorption cross sections used to correct the data were 6.55 and 0.813 b for the dry clay and 5.52 and 0.46 b for the hydrate at the incident wavelength of 1.057(2) A (1 barn, b = 100 fm21. The corrected, normalised intensities, l(k), for dry Li-montmorillonite and its D 2 0 hydrate,
0921-4526.98/$19.00 ~ 1998 Elsevier Science B.V. All rights reserved P l l S092 I -4526(97)0059 7-8
388
D.H. Powell et al. / Physica B 241 243 (1998) 387 389 0.6 I 1.4
'T E o
0.~
i
,
i
i
I
I
i
2
4
6
8
0.4
~
0.2 2 -0.2
0.2 0
i
i
i
i
i
2
4
6
8
10
12
0
10
/~ (*') Fig. 1. Corrected and normalised neutron diffraction patterns obtained for (a) D20 hydrated and (b) vacuum dried Li-montmorillonite.
0.6 0.5 "7
o 0.4 0.3 0.2
~
0.2 0.i
0.1 0
2'
4'
6'
8~
1'0
12
lc (/~') Fig. 2. The difference function, A(k), corresponding to the interlayer water diffraction in (a) Li-montmorillonite and (b) Namontmorillonite compared to the diffraction pattern, l(k), of pure liquid D20 (c). The latter function is multiplied by 0.42 to take account of the lower mole fraction of water in the clays.
shown in Fig. 1, are very similar to those for Namontmorillonite [3]. Both l(k) are d o m i n a t e d by Bragg scattering from the aluminosilicate structure, but that of the hydrate also has a diffuse c o m p o n ent due to the interlayer water. If the Bragg scattering is unchanged on hydration, one can extract the interlayer water contribution by weighted subtraction of the two I(k) [3]. The difference, A(k), is a weighted sum of w a t e r - w a t e r and water-clay partial structure factors, S,~(k), plus a self-scattering
Fig. 3. The Fourier transform, G(r), of the functions in Fig. 2, after empirical correction for k-dependent inelasticit3 effects (see text).
term (0.231 b). The weighting factors are similar to those given in [3] for Na-montmorillonite. A(k) is shown in Fig. 2 c o m p a r e d to that for Namontmorillonite [33 and the I(k) of liquid D 2 0 . The cancellation of the Bragg peaks is almost perfect. We a d o p t e d an empirical approach, assuming the largest term in the inelasticity corrections to be proportional to k 2 [5], to correct the d o w n w a r d slope of the patterns (for Li-montmorillonite A(k) = A(k) . . . . + 0.00125 k 2 - 0.012 and for D 2 0 A(k) = A(k) . . . . + 0.0022 k 2 + 0.001: see [3] for the criteria used). Fourier transformation of the corrected patterns with no s m o o t h i n g yields the radial distribution function, G(r), in Fig. 3, a sum of partial radial distribution functions, g~(r) [3]. Note that the form of G(r) is rather insensitive to the exact values used in the above correction procedure, except at low r. Removing the apparent remnants of Bragg peaks in A(k) at ca. 1.5 and 2.8 A - 1 using a spline fitting procedure had no discernible effect on the form of G(r). The similarity of the k-space and r-space functions for the clays in Figs. 2 and 3 to those for D 2 0 indicates that the interlayer water is liquid-like, although water molecules m a y be associated with specific crystallographic sites [6]. The new results for Li-montmorillonite confirm the potential of our difference technique and show that a liquid-state a p p r o a c h based on radial distribution functions is appropriate for the study of interlayer water in
D.H. Powell et al.
Physica B 241 243 tl998) 387 389
clays. N e w e x p e r i m e n t s are in progress, using a larger k-range a n d isotopic substitution, that will give better definition of b o t h intra- a n d interm o l e c u l a r water structure a n d enable detailed c o m p a r i s o n with recent c o m p u t e r s i m u l a t i o n s [7].
Acknowledgements W e t h a n k the A u s t r a l i a n Research C o u n c i l (4003/96), the A u s t r a l i a n Institute of N u c l e a r Science a n d Engineering (96/205NS) a n d the Swiss N a t i o n a l Science F o u n d a t i o n (21-49280.96, 2124045100.95).
389
References [1] A.C.D. Newman, Chemistry of Clays and Clay Minerals, Longman, London, 1987. [2] R.K. Hawkins, P.A. Egelstaff, Clays Clay Miner. 28 {1980) 19. [3] |).H. Powell, K. Tongkhao, S.J. Kennedy, P.G. Slade, Clays Clay Miner. 45 (1997) 290. [4] S.J. Kennedy, Adv. X-Ray Anal. 38 [1995) 35. [5] D.G. Montague, I.P. Gibson, J.C. Dore, Molec. Phys. 44 11981) 1355. [6] P.G. Slade, P.A. Stone, E.W. Radoslovich, Clays Clay Miner. 33 [1985) 55. [7] F.-R.C. Chang, N.T. Skipper, G. Sposito, Langmuir 13 [19971 2974.