J. Phys. Chem. 1994, 98,942-945
942
Neutron Diffraction Study of Calcium Vermiculite: Hydration of Calcium Ions in a Confined Environment N. T. Skipper' Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 IEW,UK
A. K. Soper Neutron Science Division, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 OQX,UK
M. V. Smalley Department of Physical Chemistry, University of Oxford, South Parks Road, Oxford, UK Received: July 30,1993;In Final Form: November 1, 1993"
Neutron diffraction has been used to study the hydration of calcium ions confined to the 8.5-%Lpores between flat vermiculite clay platelets. W e have measured the first 27 (001) Bragg reflections for hydrated calcium vermiculite of two isotopic compositions, the first containing D2O and the second containing HzO. W e have then used difference analysis to establish the structure of the interlayer water molecules. We find that in contrast to calcium ions in 1 m CaC12, which are coordinated to 10 water molecules, calcium ions intercalated into the vermiculite clay are coordinated to an average of only 6 water molecules. This hydration is very similar to that of calcium in more concentrated aqueous solutions and also to that of divalent cadmium and mercury ions. Our results may therefore go some way to explaining why these heavy-metal ions can act as poisons for physiological processes. Introduction Calcium ions play a central role in many important biochemical processes. For example, they act as a cofactor for a variety of extracellular enzymes, and they are used to maintain the transmembrane potentials responsible for the contraction of muscle fibers and the transmission of nerve impulses.24 To understand these biochemical reactions, which take place in an aqueous environment, it is clearly necessary to have a detailed knowledge of the hydration properties of calcium ions. The hydration of calcium ions in aqueous CaC12 solutions has been established by previous neutron diffraction experiments.' These show that the parameters of the first hydration shell are rather sensitive to the concentration of the solution. In particular, the number of water molecules present in this first shell ranges from 10.0 i 0.6 a t 1 m to 6.4 i 0.3 at 4.5 m. The corresponding average Ca-0 distances, rMO, are 2.46 A at 1 m and 2.41 A at 4.5 m. From these data we conclude that in dilute ionic solutions, such as cell fluids,zcalcium will becoordinated to about 10 water molecules. However, this leaves open the important question of how calcium ions will hydrate in more confined geometries, such as those encountered a t the surface of a protein or enzyme molecule, or within the pore of a cell membrane. This is the question we now seek to address. Due to the complex nature of polypeptides and biomembranes it would be extremely difficult to measure directly the hydration of calcium ions in situ in thesesystems. We have therefore decided to study the hydration of calcium ions intercalated into the 8.5-A pores of a calcium substitutedvermiculiteclay. Vermiculite clays occur as macroscopic crystals and are comprised of well-oriented negatively charged mica-like sheets.5-6 They have an extremely large and well-characterized internal surface area and are therefore ideal for our experiments. Moreover, the samples we have chosen to study are important in their own right, being very similar to calcium-substituted smectite clays that make up a large proportion of many soils and sedimentary rocks.5-7 We have measured the (001) Bragg reflections of 15.05-Alayer
* Abstract published in Aduunce ACS
Absfrucfs, December 15, 1993.
spacing Ca vermiculite, up to I = 27. We have used isotope substitution of D2O for HzO among the interlayer water molecules, in conjunction with Monte Carlo difference analysis, to determine the position and number of the water molecules in the interlayer region.
Experimental Methods and Results Our experiments were conducted using the time-of-flight liquids and amorphous materials diffractometer (LAD) on the ISIS pulsed neutron source at the Rutherford Appleton Laboratory.* The method we use to obtain diffraction data has been described in detail in a previous paper9 and will only be described briefly here. We have studied a macroscopic crystal of calcium substituted vermiculite from Llano, TX. This is a special clay VTx-1 of the Clay Minerals Society's Source Clays Repository, which has the structural formula6
(Mg2,,,Alo.os)(Si2.s~~~~,*,)O,0(OH)2:Ca2+0.,,,~nH2~ (1) The crystal used here had approximate dimensions 15 X 10 X 2 mm. In its natural form Mg2+is the interlayer cation. For our experiments it was prepared with interlayer Ca2+ by repeated soaking in 1 m Ca(N03)2 at about 50 OC, over two years. It was then 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 p a ~ s e d .The ~ relative humidity of the sample environment was maintained at 100%by placing a dish of either HzO 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 to 1 = 27. Two complete sets of data were obtained a t a temperature of 27 OC. The first were gathered when the sample had been deuterated by soaking in N M R standard DzO at 30 OC, over two weeks. The second were collected after the same sample had been hydrogenated by
0022-365419412098-0942%04.50/0 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 3, 1994 943
Neutron Diffraction Study of Ca Vermiculite
T$-P-
0.04
I(Q)
0.03
0.02
0.01
.-=='',
10
15
20
25
Order of (001) Bragg reflection
Figure 1. Neutron-scattering intensities of the (000 Bragg reflections of calcium-substituted vermiculite. Dashed line and star are the experimental and fitted data for the hydrogenated sample,solid line and circles are the experimental and fitted data for the deuterated sample.
repeated soaking in H20, over 24 h. This rather short time scale was dictated by the availability of the neutron diffractometer. The raw diffraction patterns were corrected for background, adsorption and multiple scattering2O and normalized by reference to the scattering from vanadium. The integrated (001) Bragg intensities, I( Q), were then obtained from these corrected patterns. The normalized intensities areshown in Figure 1. These intensities are related to the neutron-scattering density along the c* axis, p(z), via the structure factor, F(Q) z/A
Figure2. Neutron-scatteringdensityprofiles, p(z), for calcium-substituted
F(Q) = ~ ~ ~ p ( r ) e dz iQz
(3)
where cis the layer spacing. M(Q) is a Q-dependent form factor which takes into account the effects of mosaic spread and the finite size of the sample, and the DebyeWaller factor. The detailed behavior of this form factor is derived in our previous paper: and the parameters which describe it are treated as fitting parameters along with the atomic coordinates. Neutron-scattering density profiles, p(z), were obtained by Monte Carlo simulation of the integrated intensities." In these simulations 200 movable particles, each giving rise to a Gaussian neutronscattering distribution, were used to represent the density profile: (4)
where zi is the position of the ith particle and u = 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 s r l nucleus-'. It is determined by equating the area under the peak at 0.0 .&with the theoretical value due to the octahedral cations of the clay layer, namely, 1.54 barns st-l nucleus-' (Table 2). This method was chosen because the peak at 0.0 .& is isolated from all others, and its composition is known from chemical analysis of the dry clay.6 The simulations were started from a uniform background density with clay peaks placed in the positions measured by X-ray diffraction.12 The entire structure, including the clay layers, was then refined. Because of the H / D substitution, we are able to separate the hydrogen distribution from the oxygen plus clay distribution.9J3J4 We show the density profiles obtained in this way in Figure 2. The final R factor for this fit to the corrected intensities is 3.7% (see also Figure 1). For clarity, hydrogen has
vermiculite. Oxygen plus clay layer is the solid line; hydrogen is the dashed line. The model, in which the hydrogen atoms are in bold, shows the clay layers and three alternative orientations for the Ca2+(H20)6 complexes. On the basis of the density profile in Figure 2 we propose that orientation 2 is the most probable in the vermiculite studied here. been assigned the same scattering length as oxygen (5.8 fm) in all the plotted density profiles. The number densities, rather than the neutron-scattering densities, of the two atoms are therefore on the same scale in Figure 2. We now discuss these two density profiles in detail.
Discussion To help in the interpretation of the density profiles Figure 2 contains a molecular drawing of the clay layer, and three possible orientations of an interlayer cation-water complex. These are also shown in more detail in Figure 3. We begin the discussion by looking at the structure of the clay layer itself. We therefore focus on the region -1 to 4 .& in Figure 2. Dealing first with the oxygen plus clay profile (solid line), the peak at 0.0 .& is due to the octahedral cations of the clay layer. Weuse this peak to normalizeour densityprofilesto the scattering from the unit cell in eq 1 (Table 2). The peak at 1.05 .& is due to apical oxygen atoms of Si04 or A104- tetrahedra. We find that the area of this peak is equivalent to 3.03 oxygen atoms; on the basis of chemical analysis we could expect 3.0 oxygen atoms.6 The peaks at 2.73 and 3.28 .& arise from the tetrahedral cations and the basal plane oxygen atoms, respectively. We are not able to fully resolve these peaks, and we therefore assign to the peak a t 2.73 A the area that is obtained from chemical analysis of the tetrahedral cations. We then find that the remaining area is equivalent to 3.60 oxygen atoms. The clay layer itselfcontributes 3.0 oxygen atoms in this region (eq l ) , and we must therefore
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Skipper et al.
The Journal of Physical Chemistry, Vol. 98, No. 3, 1994
TABLE 1: Neutron-Scattering Lengths
0
P
species H
-3.74 6.67 5.8 1 4.9 1.8 5.38 4.15 3.45 0.38
D 0 n*tCa %a Mg Si A1 Vb 0 b = neutron-scattering length/fm. sample stage.
Vanadium was used for the
TABLE 2 Analysis of Peaks in the Simulated Density Profiles of 15.05-A Ca Vermiculite (Figure 2) ~~
r 0.0 1.05 2.73 3.28 2.0 7.53
~~
assignment
A,.,Q
octahedral cations oxygen tetrahedral cations oxygen hydrogen interlayer cations
61.83 70.54 31.64 83.70 28.91 9.11
6.05 oxygen 5.00 hydrogen 6.30 hydrogen
chem equivalent
Mgz.~iAlo.o~~ Clay 3.03 0 Si2.89All.ll layer 3.600 1.24H 0.46Ca2+ interlayer complexes 60.64 1.52 1.31 0 22.59 0.56 0.97H 48.34 1.21 2.08 H
1.54 1.76 0.79 2.09 0.72 0.23
Y T
I -+X
0 Unnormalized area. Area normalized to barns st-' nucleus-I. Peak used to normalize data.
account for the equivalent of an extra 0.6 oxygen atoms between 1.6and4.0A. Weattributethisextra intensity towater molecules adsorbed into the ditrigonal cavity sites of the clay surface, formed by SiOl and A104- tetrahedra (site 2 of Figure 3b). This is consistent with our previous studies of nickel and sodium substituted vermiculite^.^ If we now consider the hydrogen density (dotted line), we would in principle expect to find 2.2 hydrogen atoms in the region -1 to 4 A: 1 atom from the structural hydroxyl group of the clay layer (eq l), and 1.2 atoms from the 0.6 water molecules adsorbed onto the clay surface. In fact we find that the measured hydrogen intensity in this region is equivalent to only 1.24 hydrogen atoms. This shortfall in the hydrogen density is probably due to incomplete isotopic substitution among the strongly bound water molecules and the structural hydroxyl groups of the clay layer. This is not surprising, given the rather limited time available to us to achieve substitution of hydrogen for deuterium in our sample. We now address the structure of the exchangeable interlayer cation-water complexes, in the region 4-12 A. Interlayer peaks in the oxygen plus calcium profile (the solid line in Figure 2) occur at 6.05 and 9.05 A. We attribute these peaks to the oxygen atoms of the cation-water complexes (Table 2). We will assume that the calcium ions themselves lie close to the midplane of the interlayer region: unfortunately it is not possible for us to locate these calcium ions unambiguously from our current experiments: a future experiment using the isotopes n*tCa and %a would enable us to do this (Table l).I To determine the number of oxygen atoms per cation, we therefore subtract the area due to the calcium ions from the total intensity between 4 and 12 A. We then find that we are left with an average of 5.7 oxygen atoms/interlayer calcium ion (Table 2). We therefore conclude from our oxygen density profile that, within errors, all the calcium ions are octahedrally coordinated. This is similar to the hydration in concentrated, rather than dilute, aqueous solution' (Table 3). Details of the orientation of the cation-water complexes can be obtained from the hydrogen density profile. To help visualize the interlayer structure we show three possible orientations for octahedral calcium-water complexes in Figures 2 and 3a.
ii) z
T Figure 3. (a, top) Perspective view of three alternative orientations for the octahedral CaZ+(H20)6complexes within the calcium vermiculite (see also the model in Figure 2). (b, bottom) Two projections of a section of a 2:l clay surface: (i) the xy plane (showing the top half of the layer only) and (ii) the XI plane. Site 1 is above a tetrahedral base, and site 2 is above a ditrigonal cavity. The vermiculite used in the experiment has the same layer structure, except that approximately 1 in 4 tetrahedral silicon ions are replaced by aluminum ions (see eq 1). This gives rise to the overall negative clay layer charge and also results in a small distortion of the ditrigonal cavity sites.5 Hydrogen atoms are in bold, oxygen atoms are the small open circles, tetrahedral silicon or aluminum ions are the large circles enclosing a - sign, octahedral magnesion ions are large circles enclosing a sign. The clay layer is 6.28-Athick in the z direction.
+
TABLE 3: Ionic Radii for Divalent Ions ion ionic radius/@ Ca2+ Cd2+ Hg2+ Mg2+ NiZ+ Zn2+
0.94 0.92 0.93 0.65 0.68 0.69
hydrated radius0/A16
hydration no.16
2.46 (1 m),2.41 (4.6m) 10.0 (1 m),6.4 (4.6m) 2.29 (1 m ) 6.0 (1 m) 2.41 (3.5 m) 6.0 (3.5 m) 2.06 (1 m) 6.0(1 m) 2.06 (2.2m) 6.0 (2.2 m) 2.09 (1 m) 6.0 (1 m )
Definedas the first maximum in the metal-oxygen radial distribution function. Concentrations in parentheses.
The total integrated interlayer hydrogen density between 4 and 13 A is equivalent to 13.0 atoms/cation, indicative of 6.5 water molecules/calcium ion. This compares well with the figure of 5.7 water molecules obtained from our oxygen density profile. Peaksin the hydrogendensityoccurat 5.00,6.30,8.80,and 10.10 A. The first and fourth of these are due to atoms forming hydrogen bonds to the clay surface, with a hydrogen-oxygen distance of about 1.72 A. If we integrate the peak a t 5.00 A, up to the minimum a t 5.55 A, we find that its area is equivalent to 0.97 hydrogen atoms. We therefore conclude that of the six water molecules in each calcium-water complex, an average of 4.2 are
Neutron Diffraction Study of Ca Vermiculite hydrogen bonded to a clay surface. We therefore propose that orientation 2, in which 4.0 water molecules are hydrogen bonded to a clay layer, is the most probable. Simple geometric considerations, outlined below, then show us that the complexes themselves are slightly distorted octahedra. If the cation-water complexes form undistorted octahedra, with an average calcium-oxygen distance of 2.40 A,1 we would expect to obtain a separation, Az,of 3.40 A between our measured oxygen peaks. In practice we find the interlayer oxygen peaks occur at z = 6.05 8,and z = 9.05 A (Figure 2). The separation Az is therefore only 3.00 A, and we conclude that the octahedral complexes are slightly compressed in the z direction. It is interesting to contrast this behavior with that in nickelvermiculites. In nickel-vermiculite the interlayer cations are also 6-fold coordinated, but the cation-oxygen distance, rMO, is now only 2.05 A (Table 3). Thecation-water complexes are therefore able to adopt orientation 3, in which all six water molecules are hydrogen bonded to the basal plane oxygen atoms of S O 4 or A104- tetrahedra (site 1 of Figure 3b).599 However, to accommodate these hydrogen bonds the complexes form octahedra that are slightly stretched in the z direction. The model we have put forward for calciumvermiculite should also be compared with that proposed by Slade et al. on the basis of their X-ray data for the (h01) and (Okl) Bragg intensities, obtained under ambient conditions.15 They found 7.4 water molecules/interlayer cation, compared with our present value of 8.2 moiecules/cation, obtained at a higher humidity. However, in contrast to our present interpretation, Slade et al. concluded that a high proportion of the calcium ions must be 8-fold coordinated. Using this model they obtained a fit to their scattering data with an R factor in excess of 15%, compared with the R factor of 3.7% observed from our current neutron diffraction experiments. This, and the fact that we have been able to locate the hydrogen atoms unambiguously, is strong evidence in favor of our current model for calcium hydration in vermiculite, in which the cations are 6-fold coordinated and the additional water molecules are bound to the clay surface. With regard to the position of the calcium-water complexes, Slade et al. propose almost equal distribution between the ditrigonal sites and tetrahedral base sites (sites 1 and 2 respectively in Figure 3b). Our present data provide no evidence that would cast doubt on this proposal. Finally, we compare and contrast the hydration of divalent calcium, cadmium and mercury ions, and relate this behavior to their respective roles in biochemistry.24 From Table 3 we see that both the heavy metal ions are 6-fold coordinated in 1 m aqueous solutions, and that the first peaks in oxygen density, ~ M O , occur at 2.29 A (Cd2+) and 2.41 A (Hg2+).16 In contrast to this, calcium is 10-fold coordinated at the same concentration, with ~ M= O 2.46 A.1 From these data we conclude that at ionic strengths relevant to cell fluids ( ~ 0 . 1 5M2) the hydration of the heavymetal ions is probably rather different to that of calcium ions. However, our present data show that in confined environments the hydration shell of calcium ions can be reduced to six water molecules, with rMO = 2.40A. This isalso thecase inconcentrated
The Journal of Physical Chemistry, Vol. 98, No. 3, 1994 945 aqueous solutions (Table 31). With regard to biochemical processes occurring at protein surfaces or within biomembrane pores, it is therefore probable that hydrated cadmium and mercury ions will compete with hydrated calcium ions." Indeed, cadmium is known to have a toxic effect on biomembranes.2 In this context it is worth remembering that the heavier metal ions will form more stable complexes with soft ligands such as proteins,17J* and may therefore block binding sites from calcium.
Conclusions We have conducted neutron diffraction experiments on calciumsubstituted vermiculite. We find that, within errors, each calcium ion is coordinated to six water molecules. Of these six water molecules an average of about four are hydrogen bonded to a clay surface. The hydration of calcium ions in this rather restricted environment is therefore based on that in concentrated, rather than dilute, aqueous solutions. Rather interestingly, the hydration we have measured is very similar to that of cadmium and mercury ions in aqueous solutions. Acknowledgment. N.T.S. would like to thank Dr. A. V. Grimstone for useful discussions and Pembroke College, Cambridge, and British Gas plc. for their support of his research through the award of the Sir Henry Jones Research Fellowship. M.V.S. would like to thank SERC for the award of an Advanced Research Fellowship. References and Notes (1) Hewish, N. A.; Neilson, G. W.; Enderby, J. E. Nature 1982, 297, 138. ( 2 ) Ochaia, E-I. Bio-inorganic chemistry. An introduction; Allyn and Bacon: Boston, 1977. I31 Fiabane, A. M.; Williams, D. R. The principles of bio-inorganic chemistry. In Chemical Society Monographs; Chemi'cal Society: London, 1977;Vol. 31. (4) Hay, R. W. 5io-inorganic chemistry; Ellis Horwood: Chichester, England, 1984. (5) Brindley, G. W.; Brown, G. CrystalStructuresof Clay Minerals and Their X-ray Identification, Mineralogical Society: London, England, 1980. (6) Newman, A. C. D. Chemistry of Clays and Clay Minerals: Mineralogical Society: London, England, 1987. (7) Grim, R. E. Applied Clay Mineralogy, McGraw-Hill: New York,
1960. (8) Howells, W. S. Internal Report RL-80-017; Rutherford Appleton Laboratory: Chilton, Didcot, Oxon, OX1 1 OQX,UK, 1980. (9) Skipper, N. T.;Soper, A. K.; McCannell, J. D.C. J . Chem. Phys. 1991, 94, 5751. (IO) Soper,A. K.; Howells, W. S.;Hannon, A. C. Internal Report RL89-046; Rutherford Appleton Laboratory: Chilton, Didcot, Oxon,OX1 1 OQX, UK, 1989. (1 1) Semenovskaya, S. V.;Khachaturyan, K. A.; Khatachturyan, A. G. Acta Crystallogr. 1985, A41, 268. (12) Mathieson, A. McL.; Walker, G. F. Am. Miner. 1954, 39, 231. (13) Hawkins, R. K.; Egelstaff, P. A. Clays Clay Miner. 1980, 28, 19. (14) Skipper, N. T.; Soper, A. K.; Refson, K.; McConnell, J. D. C. Chem. Phys. Lett. 1990, 166, 141. (15)Slade, P.G.; Stone, P. A,; Radoslovich, E. W. Clays Cluy Miner. 1985, 33, 51. (16) Ohtaki, H.; Radnai, T. Chem. Reu. 1993, 93, 1157. (17) Grimshaw,R. W. ThechemistryandphysicsofClays,4thed.;Ernest Benn Ltd.: London, 1980. (18) Ringborn, A. Complexation in Analytical Chemistry; In Chemical Analysis; John Wiley and Sons: New York, 1963;Vol. XVI.
