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This publication was processed (scanned, OCR'ed and manually corrected) by Andrew Wong. It has not yet been proofread. The original is in Solid State Ionics, 25 (1987), 295-300.

SODIUM ION CONDUCTION IN SINGLE CRYSTAL VERMICULITE

      M. Stanley WHITTINGHAM

      Schlumberger-Doll Research, Old Quarry Road, Ridgefield, CT 06877-4108, USA

       Received 23 September 1987; accepted for publication 16 October 1987

       The diffusion of sodium in single crystals of Llano vermiculite has been studied using conductivity measurements. The crystals were maintained in the bilayer water state by using aqueous contacts. The diffusion coefficient at 25°C was found to be 1.9 X 10–8 cm2/s, with an enthalpy of motion of 11.7 kcals/mole. These values are in good agreement with sodium tracer and proton NMR studies and indicate that the sodium ions probably diffuse with their hydration spheres. The conductivity in the range 10 to 90°C is less than that in sodium beta alumina and much less than that of either surface clay cations or of aqueous sodium chloride solutions.

1. Introduction

      A number of studies have been made of the diffusion of cations and water in aluminosilicate materials [1-5], yet surprisingly there have been noreports of the conductivity in single crystal samples.These layered clay/mica compounds have the ideal composition Al3(Si4O10)(OH)2, but substitution in the tetrahedral (Si) and octahedral (Al) sites lead to incorporation of cations in the Van der Waals layer to maintain charge balance. These cations are frequently hydrated, the degree of hydration being a function of the water partial pressure as well as of the cation and the charge density in the layers. Themontmorillonite class of clays have the lowest charge density (i.e. lowest cation content) and are found only as poorly crystalline fine powders; they swell continuously as the salinity of their environment decreases [6]. At the other extreme are the micas which have the compositions M+(Si3AlO10)(Mg3)(OH)2 and M2+(Si2Al2O10)(Mg3)(OH)2; their very high charge densities preclude the incorporation of water or other solvents except in a few special cases. In between reside the vermiculite class of aluminosilicates with the idealized formula M+(Si3AlO10)(Mg3)(0H)2, yH20 where y can vary from 0 to above 5; in practice the M content is somewhat less and the aluminum content somewhat greater than unity. A schematic of the three phases

Fig. 1. Schematic of aluminosilicate structure.

of sodium vermiculite is shown in fig. 1. The vermiculites and micas are readily available from natural sources as single crystals so are ideal for studies of transport properties like conductivity. Thus in these cases the external surface area is minimal in contrast to the montmorillonites where in the presence of water the surface area can be in excess of 100 m2/gm. We therefore do not anticipate any problems with surface conduction by native or foreign species such as protons. In addition, there are no grain boundaries to hinder or enhance the transport of cations. Whereas, Calvet and Mamy [1] suggested that the major charge carriers in sodium montmorillonite

Fig. 2. Arrangement of (Si,Al)04 tetrahedra showing the ditrigonal cavities and approximate size of a hydrated ion.

are proton species, possibly due to the Bronsted acidity of the surface, one might expect that in single crystals the dominant charge carrier will in most cases be the native cation. The aluminosilicate tetrahedral building blocks facing the interlayer spaces are arranged in such a way that there are large ditrigonal cavities every 5.3 Ǻ, as indicated in fig. 2. In the micas the tetrahedra are distorted so that e.g. a potassium ion has six nearest neighbor oxygen atoms at 2.8 Ǻ and six oxygens 0.5 Ǻ further away. This bonding and burying of the cation in the structure precludes any significant ionic mobility in the mica class of material. However, in the vermiculites with their lower charge density cations such as sodium are readily hydrated separating the layers by 5 Ǻ and thus permitting some mobility. Here we report on a study of the most hydrated phase of a sodium vermiculite where one might expect the highest ionic mobility.

2. Experimental

The vermiculite single crystals used here were from Llano, Texas and were obtained from the Clays Minerals Society. They were immersed in aqueous so dium chloride solution at 600C for two months to ensure complete exchange to the sodium form. They have recently been extensively analyzed [7], and shown to have the formula:

Na0.93(Si2.86Al1.14)(Mg2.94Al0.05)O10(OH)2,yH2O  (1)

Fig. 3. Water isotherm for sodium vermiculite, after ref. [8].

where y is a function of the water vapor pressure, and for this material varies from 0 to »4.6 in a stepwise function as shown in fig. 3. This isotherm was obtained using a Cahn microbalance at 28°C [8]. The number of water molecules per sodium ion are 2.0 and 5.0 for the monolayer and bilayer hydrate respectively. All the measurements reported here were carried out with the crystals in contact with aqueous solutions where the bilayer water structure is the stable phase with y around 4.6 and an aluminosilicate basal spacing of 14.89 Ǻ (Philips diffractometer using Cu Ka radiation).

      The sodium ion transference number and conductivity were measured in a simple cell in which the single crystal of vermiculite was glued into a glass tube using a silicone rubber cement (General Electric RTV 106). This tube was then filled with a sodium chloride aqueous solution and placed in another tube also containing the aqueous solution. By this means the crystal could be maintained in the bilayer water state throughout the temperature range of study, 5-90°C; even in saturated air, water loss would be observed by 35°C. In addition a sodium chloride concentration gradient could be established across the crystal, allowing a determination of the transference number to be made. Electrical contact to the solutions was made by Ag/AgCl electrodes whose surface area exceeded that of the sample by more than a factor of a 100. This minimized potential polarization problems at the electrodes, and in fact the conductivities measured varied by less than 5-10% from 10 Hz to 100 kHz. The conductivities were measured using a General Radio 1616 Bridge in a GR 1621 Precision Capacitance System, generally with 0.02 molar solutions at a frequency of 1 kHz with frequent scans over the above range. The EMF's of the concentration cells were measured using a Hewlett Packard 3468A multimeter.

3. Results and discussion

The EMF of the concentration cell:

Ag/AgCl/NaCl(a1)/membrane/NaCl(a2)/AgCl/Ag       (2)

is given at 25°C by the expression:

E=(2t+-1)59log(a1/a2)+59log(a1/a2),               (3)

where the first term is the membrane potential with t+ the transference number of the membrane or liquid junction, and the second term the electrode potential difference. For a pure ionic conductor t+ is unity and for a simple liquid junction t+ is simply the transference number of Na+ in sodium chloride which is around 0.38. The results for sodium vermiculite are shown in fig. 4. All the data points fall on the unity transference number line and clearly indicate that this material is a pure cationic conductor. The values fell sharply from this line when any leakage occurred across the cell, e.g. from minor cracks in the seal or in the crystal. Thus the cell EMF was monitored before and after conductivity measurements to ensure that the cell still had a tight seal. The liquid junction potential line was measured using a clay free Fontainbleau sandstone, with a porosity of 20%, as the membrane.

      The conductivity of sodium vermiculite is shown in fig. 5 as a function of temperature. At 25°C the conductivity is 3.0X10-4 S/cm, and in the expression:

sT=s0exp(-H/RT)  (4)

s0 and H have the values 3.15X107 S.K/cm and 11.7 kcals/mole respectively. The linear behavior of the conductivity over the temperature range studied indicates that there is probably a single conductivity

Fig. 4. EMF of NaCl concentration cell containing sodium ver- miculite as the membrane. The reference solution is one molar NaCl.

mechanism with an enthalpy of motion of 11.7 kcals/mole (a plot of logs versus 1/T gives an activation enthalpy of 11.1 kcals/mole). This enthalpy of motion is essentially the same as that measured by sodium tracer diffusion, 11±1 kcals/mole [2], in World vermiculite (of different source and composition). This is a strong indication that the mobile cation is a sodium rather than protonic specie.

The sodium ion diffusion coefficient can be calculated from the Nernst-Einstein relation by making

Fig. 5. Ionic conductivity of sodium vermiculite.

two assumptions. These are that the concentration of charge carriers equals the concentration of sodium ions, 4.2 mmoles/cc, and that the correlation coefficient (Haven ratio) is unity. Without definitive knowledge concerning the diffusion mechanism these are the simplest assumptions to make. With these assumptions, one finds the following values for the sodium ion diffusion coefficient, D, and mobility, μ, at 25°C:

D=1.9X10-8 cm2/s,        (5)

μ =7.4X10-6 cm2/V/s.     (6)

In all probability the hydrated cations jump from the neighborhood of one ditrigonal site to a neighboring one, 7% of which are vacant. If such a vacancy mechanism is operative then the number of charge carriers is equal to the vacancy concentration, so that the above values would be higher. However, these vacant sites are only vacant in the sense that there are no sodium ions occupying them; they are believed to be occupied by "free" water molecules. In addition the generally accepted size of a hydrated sodium ion, 3.58 Ǻ radius [9], is too large for the cation to sit in a regular manner in the ditrigonal cavities, and there are insufficient water molecules per sodium ion, 5, to satisfy the normal coordination requirements of the cation so it is difficult to define the in-plane structure and hence the diffusion path. A study of conductivity in a series of vermiculites of different sodium contents would help in elucidating the mechanism; the Llano vermiculite has one of the highest sodium contents, hence lowest vacancy concentrations of all the vermiculites.

Proton NMR studies [3] of Llano vermiculite have shown two relaxation processes, with the faster being associated with the motion of free water molecules and the slower with those water molecules associated with the sodium ion's hydration sphere. For the latter it was assumed that the relaxation process involved the slower motion of the sodium ion together with its hydration sphere; a diffusion coefficient of 0.5X10-8 cm2/s was calculated [3] which is in relatively close agreement with the value calculated here considering the assumptions made about the mobile ion concentration and the correlation coefficient. We can thus assume that the diffusion mechanism operative in the bilayer hydrate of sodium vermiculite

Fig. 6. Comparison of the conductivities of sodium Llano vermiculite and the fast ion conductor sodium β-alumina.

are jumps of the sodium ion, with its hydration sphere, from site to site. Thus the mechanism is not dissimilar to that in aqueous solutions but is severely constrained by the geometry of the clay layers thus leading to a conductivity several orders of magnitude lower than that for sodium chloride solutions. In agreement with this mechanism is the high enthalpy for motion of the sodium ions, 11.7 kcals/ mole, which is a factor of 3 higher than the temperature coefficient of resistivity, »3.8 kcals/ mole, for sodium chloride solutions at 25°C

It is interesting to compare the values obtained here with those [10] of sodium beta alumina, one of the highest known solid state ionic conductors. These are shown in fig. 6 and table 1. Although it would appear that the vermiculite might have the higher conductivity at elevated temperatures if the water can be retained inside the crystal by an external pressure, the presence of water of hydration does not appear

Table 1

Sodium ion mobility in sodium vermiculite and (beta) alumina.

      Property             Vermiculite          β-alumina

      σ, S/cm 25°C          3.0X10-4             1.4X10-2

      D, cm2/s 25°C         1.9X10-8             6.7X10-7

      μ, cm2/V/s 25°C       7.4X10-6             6.8X10-5

      H, kcals/mole        11.7                 3.79

      [Na], moles/cc        4.2X 10-3            5.5 X 10-3

Fig. 7. Conductivity of Bandera sandstone as a function of salinity for some sodium chloride and tetraethyl ammonium chloride solutions, after ref. [11].

to assist in the diffusion of the sodium ions at ambient temperatures where the conductivity is a factor of 50 lower than in the sodium beta alumina; this is despite the sodium ion concentration being essentially the same in the two structures. However, preliminary results in our laboratory indicate that when the water content is reduced to give the monolayer hydrate, y»1.9, the conductivity drops by about two ordes of magnitude. Recent data [4] on the high surface area clay montmorillonite also show very low conductivities when dry. Thus it appears that there is very strong bonding between the sodium ions and the aluminosilicate matrix. This may be associated either with binding of the mobile cation in the large trigonal sites as in the mica class of clays, or with ordering of the cations toward those oxygen coordinated with the lower valent aluminum ions.

This study will be extended to include the effects of change of cation and of solvating molecule, and of the large difference in mobility between surface and interlayer cations. In the latter case we know from studies on sandstones containing aluminosilicates that the mobility of surface sodium ions approaches that in bulk aqueous solutions. An example of this behavior is shown in fig. 7, where the conductivity of Bandera sandstone [11] is plotted against that of the bulk chloride solution over a wide salinity range for two cations, sodium and tetraethyl ammonium. The tetraethyl cations are essentially immobile, so that a linear plot is observed in that case. However, for the sodium ions the overall conductivity is significantly enhanced, and the data is fitted after [12] with a sodium ion mobility corresponding to an equivalent conductivity of up to 38 S.cm2/eq. This mobility is 40% of those in 1 molar solution. This may be compared with the vermiculite interlayer cations studied here which have a mobility of much less than 1% of those in solution. This may also explain why other workers [13] have found higher conductivities for the very high surface area montmorillonites. For these materials it may be conjectured that a significant part of the conductivity is associated with surface rather than bulk hydrated cations.

4. Conclusions

The sodium ion conductivity in the layered structure of the aluminosilicate, Llano vermiculite, has been measured from 5 to 90°C and found to have the value 3X10-4 S/cm at 25°C with an enthalpy of motion of 11.7 kcals/mole. This conductivity value is significantly less than that of the fast ion conductor sodium beta alumina. Combining the conductivity data with tracer sodium and proton NMR studies it seems likely that in these single crystal specimens the dominant diffusing species are sodium ions and that they jump together with their hydration spheres.

Acknowledgement

This work was presented as a recent news paper at the 6th International Conference on Solid State Ionics in Garmisch-Partenkirchen, September 7, 1987. I would like to thank D. Hines, N. Wada and P.-z. Wong for many helpful discussions.

References

[1]   R. Calvet and J. Mamy, C.R. Acad Sci. (Paris) 273D (1971) 1251.

[2]   J. Keay and A. Wild, Soil Science 92 (1961) 54.

[3]   J. Hougardy, W.E.E. Stone and J.J. Fripiat, J. Chem. Phys. 64 (1976) 3840.

[4]   T. Kawasa, H. Yokokawa and M. Dokiya, in: 6th Int. Conf. Solid State Ionics, 9th Sept. 1987, to be published.

[5]   D.J. Cebula, R.K. Thomas and J.W. White, Clays Clay Miner. 29 (1981) 241.

[6]   K. Norrish, Disc. Faraday Soc. 18 (1954) 120.

[7]   P.G. Slade, C. Dean, P.K. Schultz and P.G. Self, Clays Clay Miner. 35 (1987) 177.

[8]   M. Susuki, N. Wada, D.R. Hines and M.S. Whittingham, Phys. Rev. B36 (1987) 2844.

[9]   E.R. Nightingale, J. Chem. Phys. 63 (1959) 1381.

[10]  M.S. Whittingham and R.A. Huggins, J. Chem. Phys. 54 (1971) 414.

[11]  P.-z. Wong, in: Physics and chemistry of porous media II, AIP Conf. Proc. # 154, ed. J.R. Banavar, J. Koplik and K.W. Winkler (1987).

[12]  H.J. Vinegar and M.H. Waxman, Geophysics 49 (1984) 1267.

[13]  Y.-Q. Fan, in: 6th Int. Conf. Solid State Ionics, 9th Sept. 1987, to be published.

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