J. Phys. Chem. 1995,99, 14201-14204
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Direct Measurement of the Electric Double-Layer Structure in Hydrated Lithium Vermiculite Clays by Neutron Diffraction N. T. Skipper,* M. V. Smalley, and G. D. Williams Department of Physics and Astronomy, University College, Gower Street, London WCIE 6BT, UK
A. K. Soper Neutron Science Division, Ruthelford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 OQX, UK
C. H. Thompson Department of Chemistry, Lensfield Road, Cambridge CB2 IEW, UK Received: June 14, 1 9 9 p
Neutron diffraction, in conjunction with isotopic substitution of 6Li for 7Li and 2H for 'H, has been used to detennine the structure of the electric double-layer region close to hydrated lithium vermiculite clay platelets. We have measured the intensities of the first 27 (001) Bragg reflections for samples in which the clay platelet spacing is 14.67 A. Difference analysis has then allowed us to establish the interlayer counterion and water distributions. We find that the lithium counterions are located midway between the clay platelets and form octahedral hydration complexes with six water molecules. The behavior of lithium is therefore rather different from that of the larger alkali-metal ions; sodium, potassium, and cesium. These ions prefer to bind directly to vermiculite clay surfaces, rather than fully solvate. Since only lithium-substituted vermiculites will swell macroscopically when soaked in water, we conclude that interlayer cations must detach themselves from the clay surfaces if the particles are to expand colloidally.
match real behavior at much larger particle separation^."-'^ There are currently two explanations as to why this might be The interface between a charged surface and an aqueous the case. electrolyte solution is known as the electric double layer. A First, Sogami et al. have derived a new expression for the detailed knowledge of the structure within this region is essential electrostatic interaction between two isolated charged plates.I6.l7 if we are to understand many important natural and industrial This differs qualitatively from the DLVO theory. Second, it processes. These include colloidal interactions, electrode reachas been postulated that the majority of counterions lie in the tions, and membrane activity and stability. Stem layer and that the diffuse double layer central to DLVO Our present picture of the electric double layer is based largely probably accounts for less than 2% of the interlayer ion^.'^.'^,'* on theoretical studies of primitive model systems. These studies Colloidal interactions are then attributed to the long range of identify two regions within the aqueous medium. First, there the solid-solution interface, that is to hydration force^.'^,*^ is a Stem layer of immobile ions attached to the salid surfaces.' To resolve these different mechanisms of colloidal interaction, The charge density in this layer is treated as an empirical it is essential that we have access to detailed structural data. In parameter. Second, there is a diffuse atmosphere of ions outside this letter we present results from a neutron diffraction study the Stem layer. The detailed characteristics of the diffuse layer, of hydrated lithium vermiculite clay. In this series of experisuch as the screening length which describes its thickness, are ments we have used isotopic substitution of 6Li for 'Li and 2H determined by solution of the Poisson-Boltzmann e q ~ a t i o n . ~ - ~ for 'H, in conjunction with difference analysis, to establish the Within the primitive model interactions between dispersed interlayer distributions of both counterions and water molecules. macroions are generally treated in terms of the DLVO t h e ~ r y . ~ , ~ To our knowledge these experiments are the first direct in situ This is an analysis of the one-dimensional problem of parallel measurement of both ionic and aqueous structure within an plates immersed in solution, a geometry chosen because it lends electric double layer. itself to mathematical solution of the Poisson-Boltzmann equation. The DLVO theory is used widely to model interacExperimental Methods and Results tions between charged plates immersed in aqueous solutions, and is a comerstone of modem physical chemistry (see ref 7, Our experiments were conducted using the time-of-flight for example). There is, however, evidence that if a full range liquids and amorphous materials diffractometer (LAD) on the of interparticle separations is considered, then the DLVO theory ISIS pulsed neutron source at the Rutherford Appleton Laboracannot match real colloidal swelling behavior. tory.2' The method we use to obtain diffraction data has been As would be expected, DLVO theory cannot reproduce real covered in detail in previous p a p e r ~and ~ ~will , ~ be ~ described swelling behavior at small particle separations, since the details only briefly here. of solvent structure are then important.8-'o More unexpectedly, We have studied a macroscopic crystal of lithium substituted there is some recent evidence that the DLVO theory does not vermiculite from Llano, TX. This is special clay VTx- 1 of the Clay Minerals Society's Source Clays Repository, which has Abstract published in Advance ACS Abstracts, September 15, 1995. the structural formula24
Introduction
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0022-365419512099-14201$09.0010
0 1995 American Chemical Society
14202
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J. Phys. Chem., Vol. 99, No. 39, 1995 0.025 0.0025
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Order of (OOl) Bragg reflection Figure 1. Neutron-scattering intensities of the (Ool) Bragg reflections for lithium vermiculite, showing the effect of isotopic substitution as follows: 6Liand 'H,circles connected by solid line; 'Li and 'H,stars connected by dotted line; 'Li and 2H, triangles connected by dashed line. The insert shows peaks on a scale 10 times smaller than the main figure. The density profiles fitted to these data are shown in Figure 2, the total R factor of this fit being 4.18.
Our crystals had approximate dimensions 15 x 10 x 2 mm. In its natural form Mg2+ is the interlayer cation. For our experiments they were prepared with interlayer *Li+by repeated soaking in 1 M Li(N03) at about 50 "C, over a period of 1 year. Isotopically enriched crystals were then prepared by further soaking over a period of 1 month. Samples were secured between two 0.3 mm thick vanadium sheets and placed in a vacuum-tight cylindrical aluminum container. This container had a thin window through which the incident and scattered neutron beams passed.22 The relative humidity of the sample environment was maintained at 100% by placing a dish of either H20 or D20 within the sealed sample container. The sample was oriented so that the c* axis was parallel to the scattering vector, Q. Diffraction data were gathered at scattering angles of 20" and 150", allowing the intensities of the basal plane (001) reflections to be measured up to 1 = 27. Two complete sets of data were obtained at a temperature of 27 "C. The first were gathered when the samples had been deuterated by soaking in NMR standard D20 at 30 "C, over a period of 2 weeks. The second were collected after the same samples had been hydrogenated by repeated soaking in H20, over a period of 24 h. This rather short time scale was dictated by the availability of the neutron diffractometer. The raw diffraction pattems were corrected for background, absorption, and multiple scattering25and normalized by reference to the scattering from vanadium. The integrated (001) Bragg intensities, Z(Q), were then obtained from these corrected pattems. Examples of the normalized intensities are shown in Figure 1. Such intensities are related to the neutron scattering density along the c* axis, p(z), via the structure factor, F(Q):
where c is the layer spacing. M ( Q ) is a Q-dependent form factor that takes into account the effects of mosaic spread, the finite size of the sample, and the Debye-Waller factor. The detailed behavior of this form factor was derived in our previous paper,22 and the parameters that describe it are treated as fitting parameters along with the atomic coordinates. Neutronscattering density profiles, p(z), were obtained by Monte Carlo simulation of the integrated intensities.26 In these simulations 200 movable particles, each giving rise to a Gaussian neutron scattering distribution,were used to represent the density profile: 2 0
(4) i= I
where zi is the position of the ith particle and (T = 0.1 A, to correspond to the theoretical resolution of the instrument. C is a normalization constant to put the data into units of barns str-' nucleus-'. It is determined by equating the area under the peak at 1.04 8, with the theoretical value due to the apical plane oxygen atoms of the clay layer, namely 1.74 barns sr-' nucleus-' (Table 2). This method was chosen because the peak at 1.04 8, is isolated from all others, and its composition is known from chemical analysis of the dry clay.24 The simulations were started from a uniform background density with clay peaks placed in the positions measured by X-ray diffra~tion.~'The entire structure, including the clay layers, was then refined. Because of the 6LiPLi and 2W'H substitutions we are able to separate the lithium and hydrogen
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J. Phys. Chem., Vol. 99, No. 39, I995 14203
experiments (6Li/lLi and 2W'H). Subsequent difference analysis has therefore enabled us to obtain all three density profiles describing the double-layer structure, namely, oxygen, hydrogen, and lithium. However, before we focus on the interlayer region we discuss the structure of the clay layers themselves; that is, the region from -1 to 4 A. To guide the reader through this discussion, we draw attention to the molecular model of the clay layers and interlayer complexes, above the density profiles in Figure 2. The structure of the clay layer is deduced from the solid line in Figure 2: this density profile contains all constituents of the sample that have not been subjected to isotopic substitution. The peak at 0.0 A is immediately attributed to the octahedral cations of the clay sheet; we discuss the area of this peak shortly, in the context of our lithium profile. Apical oxygen atoms of the Si04 or A104- tetrahedra are located at 1.04 A. The area of this oxygen peak is known from chemical analysis of the dry clay, and we therefore-use it to normalize our data. The tetrahedral Si and A1 cations themselves are located at 2.75 A, and the corresponding basal plane oxygen atoms at 3.28 A. We find that the area under the peak at 3.28 8, is equivalent to about 3.12 atoms. Since we would expect a value of 3.0 from chemical analysis we may assign the small additional area to adsorbed water molecules. These water molecules interact strongly with the hydroxyl groups of the clay surface and have z (4 been observed in nickel, sodium and calcium vermiculite^.^^,^^ Figure 2. Neutron scattering density profiles, g(z), for lithiumThis interpretation of the data is consistent with the hydrogen substituted vermiculite. Oxygen plus clay layer is the solid line, hydrogen is the dashed line, and lithium is the stars. The molecular density profile, which is shown as the dotted line in Figure 2. model above shows two sections of clay surface and an undistorted We find that the area under this profile up to 4.4 8, is equivalent octahedral Li+(H20)6'complex. In this model all six water molecules to 1.12 hydrogen atoms. Of this total, 1.0 is due to the hydrogen are hydrogen bonded directly to the clay surface; in practice we find atoms of the surface hydroxyl groups. We assign the remaining that, on average, two of the six molecules are less strongly oriented area to adsorbed water molecules. We now turn to the interlayer towards the surface. structure, in the region 4.0-10.3 A. TABLE 1: Relevant Neutron-Scattering Lengths Within the double-layer region, we first draw attention to the density profile for lithium (crosses in Figure 2). To our neutron-scattering neutron-scattering knowledge this curve represents the first direct experimental species length (fm) species length (fm) measurement of the ionic distribution within an electric double 1H -3.74 *Li --2.03 layer. Our data show clearly that the ions are located midway 6.67 Mg 5.38 2H (D) 0 between the clay sheets, giving rise to a peak at 7.335 A. 5.8 1 Si 4.15 6Li 2.0 A1 3.45 If we integrate under the lithium density profile, we find that 7Li -2.2 V 0.38 the area is equivalent to 0.68Li. This value should be compared with the figure of 0.93Li obtained from chemical analysis.24 distributions from the oxygen plus clay d i s t r i b ~ t i o n . ~ ~We ,~**~~ We offer two simple explanations for the difference between show the density profiles obtained in this way in Figure 2. The these numbers. First, it is possible that our lithium solutions total R factor for this fit to the corrected intensities is 4.1% themselves were not isotopically pure. Second, we may not (see also Figure 1). For clarity, lithium and hydrogen have been have achieved complete isotopic exchange of lithium within our assigned the same scattering length as oxygen (5.8 fm) in all samples. In this context, it has been suggested that lithium ions the plotted density profiles. The number densities, rather than are able to migrate into the clay layer itself, to occupy the (small the neutron scattering densities, of these species are therefore number of) octahedral vacancies.30 If this were the case, we on the same scale in Figure 2. We now discuss these three would expect to record a small decrease in scattering density density profiles in detail. within the octahedral layer, relative to the predicted value (remember that natural lithium has a negative scattering length, Discussion Table 1). We can see from Table 2 that this is indeed the case. Interpretation of our data is greatly simplified by our use of We therefore favor the second explanation and on this basis two independent sets of isotopic substitutions during our proceed to interpret the interlayer water structure.
TABLE 2: Analysis of Peaks in the Simulated Density Profiles of 14.67 di Li Vermiculite (Figure 2) unnormalized normalized area Ped position (A) assignment chemical equivalent area (barns str-' nucleus-') 0.0 Octahedral cations 60.50 1S O 0.95(Mg*.81Al0.08Feo.o7) vermiculite clay layer 1.04 apical oxygen 70.20 1.74 3.00 0 2.75 tetrahedral cations 31.87 0.79 Si2.8di.iI 3.28 basal plane oxygen 73.21 3.12 0 1.81 1.O-4.4 hydrogen 1.19 H 27.75 0.65 7.34 lithium 0.64 Li+ interlayer cation-water 14.96 0.37 6.13 oxygen complexes 3.74 0 87.62 2.17 5.16-5.88 hydrogen 174.10 4.32 7.43 H
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The interlayer oxygen density profile has peaks at 6.13 and 8.54 A. The area under these peaks is equivalent to 3.74 oxygens; that is, 5.86 oxygen atomsflithium ion. At this point we remind the reader that in aqueous solution lithium forms an octahedral hydration complex, with an average lithium-oxygen separation of 1.9 As3' We conclude that within our vermiculite this octahedral lithium-water complex is somewhat squashed. If we now turn to the hydrogen density profile (dashed line), we first note that our measured ratio of O:H is 1:1.99; we would of course expect 1:2.0. Peaks in our hydrogen density occur at 5.16 and 5.88 A with the latter peak having about twice the area of the former. We therefore find that of the six water molecules in each interlayer complex, an average of about four are able to hydrogen bond directly to the clay surface. We complete our discussion with some general observations concerning the swelling mechanisms of vermiculites. First, we note that in vermiculites the behavior of lithium seems to be rather different from that of sodium (or potassium or cesium). In particular, there is evidence from previous neutron diffraction22 and computer modeling studies32that sodium ions are captured by the clay surfaces, rather than fully solvated. Next, we recall that among the alkali-metal ions, only lithium will enable vermiculite crystals to swell colloidally when immersed in ~ a t e r . We ~ ~therefore . ~ ~ attribute colloidal swelling of lithium vermiculites to the formation of fully hydrated ion-water complexes. These complexes enable lithium ions to become detached from the clay surfaces and subsequently to form a diffuse ionic double layer.
Conclusions Neutron diffraction in conjunction with isotope substitution has been used to determine the ionic and aqueous structure within the electric double-layer region close to lithium vermiculite clay platelets. We have measured the first 27 (001) Bragg reflections, for samples in which the clay platelet separations is 14.67 A. Isotopic substitutions of 2H for 'H and 6Li for 'Li followed by difference analysis has allowed us to establish the interlayer water and counterion distributions, respectively. We find that each lithium ion is fully solvated by six water molecules. These interlayer cations lie midway between adjacent clay layers. The behavior of lithium may therefore be contrasted with that of the other alkali-metal ions, which bind directly to the clay surfaces. Since only lithiumsubstituted vermiculites are found to swell macroscopically when soaked in water, we conclude that the full solvation of the interlayer cations is a necessary step if the clay particles are to swell colloidally.
Acknowledgment. N.T.S. and C.H.T. would like to thank British Gas plc for their support of this research through the
award of the Sir Henry Jones Research Fellowship and a CASE Studentship respectively. M.V.S. would like to thank EPSRC for the award of an Advanced Research Fellowship.
References and Notes Stem 0. Z. Elektrochem. 1924, 30, 508. Gouy G. Ann. Phys. (Paris) Ser. 4, 1910, 9, 457. Chapman D. L. Philos. Mag. 1913, 25, 475. Langmuir I. J. Chem. Phys. 1938, 6 , 873. Dejaguin B.; Landau L. D. Acta Phys. Chim. URSS 1941,14,633. (6) Verwey E. J. W.; Overbeek J. T. G. Theory of the stability of lyophobic colloids; Elsevier: Amsterdam, 1948. (7) Israelachvili J. N. Intermolecular and surface forces; Academic Press: London, 1985. (8) Nomsh K. Trans. Faraday Soc. 1954, 18, 120. (9) Pashley R. M. J. Colloid Inte$ace Sci. 1981, 80, 153. (10) Israelachvili J. N.; Pashley R. M. Nature 1983, 306, 249. (11) Viani B. E.; Low P. F.; Roth C. B. J. Colloid Interface Sci. 1983, 96, 229. (12) Low P. F. Langmuir 1987, 3, 18. (13) Braganza L. F.; Crawford R. J.; Smalley M. V.; Thomas R. K. Prog. Colloid Polym. Sci. 1990, 81, 232. (14) Miller S. E.; Low P. F. Langmuir 1990, 6 , 572. (15) Smalley M. V. Mol. Phys. 1990, 71, 1251. (16) Sogami I. S . ; Shinohara T.; Smalley M. V. Mol. Phys. 1991, 74, 599. (17) Sogami I. S.; Shinohara T.; Smalley M. V. Mol. Phys. 1992, 76, 1. (18) Skipper N. T.; McConnell J. D. C.; Refson K. Recent Deuelopments in the Physics of Fluids, International Symposium, Oxford, F269; Institute of Physics: London, 1991. (19) Pashley R. M.; Israelachvili J. N. J. Colloid Interface Sci. 1984, 101, 511. (20) Rand R. P.; Parsegian V. A. Biochim. Biophys. Acta 1989, 988, 351. (21) Howells W. S. Internal Report RL-80-017; Rutherford Appleton Laboratory: Chilton, Didcot, Oxon OX11 OQX, U.K., 1980. (22) Skipper N. T.; Soper A. K.; McConnell J. D. C. J. Chem. Phys. 1991, 94, 5751. (23) Skipper N. T.; Smalley M. V.; Soper A. K. J. Phys. Chem. 1994, 98, 942. (24) Newman A. C. D. Chemistry of Clays and Clay Minerals; Mineralogical Society: London, England, 1987. (25) Soper A. K.; Howells W. S.; Hannon A. C. Intemal Report RL89-046; Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX1 1 OQX, UK, 1989. (26) Semenovskaya S. V.; Khachaturyan K. A,; Khachaturyan A. G. Acta Crystallogr. 1985, A41, 268. (27) Mathieson A. M.; Walker G. F. Am. Miner. 1954, 39, 231. (28) Hawkins R. K.; Egelstaff P. A. Clays Clay Miner. 1980, 28, 19. (29) Skipper N. T.; Soper A. K.; Refson K.; McConnell J. D. C. Chem. Phys. Lett. 1990, 166, 141. (30) Alvero R.; Alba M. D.; Castro M. A.; Trillo J. M. J. Phys. Chem. 1994, 98, 7848. (31) Neilson G. W.; Enderby J. E. R. SOC. Chem. Ann. Rep. C 1979, 185. (32) Chang F.-R.; Skipper N. T.; Sposito G. Langmuir, in press. (33) Brindley G. W.; Brown G. Crystal Structures of Clay Minerals and Their X-ray Identification; Mineralogical Society: London, England, 1980. (1) (2) (3) (4) (5)
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