Ionic Mobility and Hydration Energies in Montmorillonite Clay

J. Phys. Chem. C 2008, 112, 14001–14009

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Ionic Mobility and Hydration Energies in Montmorillonite Clay Fabrice Salles,† Sabine Devautour-Vinot,‡ Olivier Bildstein,†,* Michel Jullien,† Guillaume Maurin,‡ Jean-Charles Giuntini,‡ Jean-Marc Douillard,‡ and Henri Van Damme§ DEN/DTN/SMTM/LMTE, Commissariat a` l’Energie Atomique de Cadarache, Baˆt 307, 13108 Saint Paul Les Durance, France/Institut Charles Gerhardt Montpellier, UMR 5253 CNRS, UniVersite´ Montpellier 2, Place Euge`ne Bataillon, CC15, 34095 Montpellier Cedex 05, France/Ecole Supe´rieure de Physique et Chimie Industrielle, 10 rue Vauquelin, 75231 Paris Cedex 5, France ReceiVed: NoVember 18, 2007; ReVised Manuscript ReceiVed: April 28, 2008

The structure of swelling clays consists of a framework composed of negatively charged clay platelets and interlayer cations. Swelling clays present different behaviors as a function of the nature of the interlayer cation: the swelling capacity and the adsorption ability are strongly modified. Therefore the determination of the properties of these interlayer cations is prerequisite to understand the features of the clay swelling and the exchange properties. In this paper, we propose to study the interactions between the interlayer cations and the clay framework by determining the activation energy for the cation motion. This cation displacement is observable under the action of an external electrical field during conductivity measurements. Indeed, it has already been shown that the complex impedance spectroscopy (CIS) is a well-adapted tool to study the energy related to the motion of an electric charge. We propose to determine energy values for the cation motions by electrical conductivity measurements performed at both dry and fully hydrated states for montmorillonites clays to study the influence of the nature of the cation on these energies. Then the so-obtained values in montmorillonites, linked to the hydration energy for the cation, are compared with the clay hydration energy obtained from Van Damme method using gravimetric measurements. We find that the hydration energy decreases from Li+ to Cs+ following the same trend than the weight of the cation (as well as the polarizability) in montmorillonites. These results are also in agreement with the hydration energy of the interlayer cations and layer surfaces from calculations which compare surface energy and immersion data, published recently. 1. Introduction One of the most interesting properties of smectite clays is their capacity to adsorb large amounts of water and to swell while maintaining a very low permeability.1–5 Such features make this class of material very promising for deep geological repository of radioactive wastes. Both properties are known to depend on the nature of the interlayer cations and on the crystal properties of the smectite clay, including its crystal charge density and charge location.6–15 Generally, hydration of the interlayer cations is considered to be the driving force for the overall hydration process, but other phenomena, such as hydration of the oxygen atoms at the clay surface, have to be considered.16–18 Indeed, it has already been shown experimentally that clays without interlayer cations such as talc or kaolinite adsorb water and release similar adsorption energies than montmorillonites, which is a clear indication of the hydration of clay layer surfaces.19,20 Monte Carlo simulations have illustrated this observation by showing the specific interaction between the water molecules and the interlayer space during hydration in these clay materials.17,18 Both ion exchange and selectivity are the other key properties with respect to the migration of radionuclides.5 Hence, understanding the interaction between the interlayer cations and the clay surface in different hydration conditions is crucial, as it may be considered as a * Author to whom correspondence should be addressed: Tel: +33 (0)4 42 25 37 24; Fax: +33 (0)4 42 25 62 72; Email:[email protected] † CEA, DEN/DTN/SMTM/LMTE, 13108 Saint Paul Les Durance. ‡ ICG, Universite ´ Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 05. § ESPCI, 10 Rue Vauquelin, 75231 Paris Cedex 5.

preliminary key step to address further the swelling and ion exchange processes. The purpose of the present paper is to investigate the interlayer cation-clay layer interaction in ion-exchanged montmorillonites, using a combination of electrical conductivity and thermal analysis measurements. Several electrical conductivity studies have already been performed on swelling clays,21–28 either on dilute aqueous suspensions21–24 or on powders.25–28 In powders, the general trend is an increase of the conductivity with the adsorbed amount of water.29 It is admitted that the cations are the mobile electrical charges and that, at low relative humidity, they are the preferentially hydrated sites, though not necessarily in the interlayer space.16,6,30–32 As the relative humidity increases, water clusters or even water films are formed, enhancing the migration of the cations. Here, we propose to focus on montmorillonite, an extensively studied dioctahedral smectite with ionic substitutions in the octahedral layer,33,34 involving the series of alkali cations (Li+, Na+, K+, Cs+). The ideal formula of the montmorillonite is M+(Al, Mg)4Si8O20(OH)4 · nH2O, in which M+ is the exchangeable interlayer cation. The structure of the montmorillonite is given in Figure 1a. The strength of the cation-layer interaction will be assessed mainly from the activation energies of the cation mobility derived by electrical conductivity measurements both in the dry and in water-saturated states, as previously reported in the zeolites and clays.26,35 When these activation energies will be compared in the dry and in the wet states, the hydration energies of the cations will be estimated using appropriate approximations.

10.1021/jp710976g CCC: $40.75  2008 American Chemical Society Published on Web 08/15/2008

14002 J. Phys. Chem. C, Vol. 112, No. 36, 2008 These data will be thus compared with the hydration energies derived directly from thermal analysis in the sample-controlled thermal analysis (SCTA) mode,36 and with the theoretical values recently obtained from electrostatic calculations.37 As it will be shown, this approach can be a powerful tool to identify the different contributions to the overall hydration energy of swelling clays. 2. Material and Methods 2.1. Sample Purification and Conditioning. The montmorillonite samples used here correspond to crystallites with an average size Cs+ > K+ and Li+ ∼ Na+ >K+ for montmorillonites and beidellites, respectively. The interactions between the cation and the surface are thought to be weaker, because of the existence of the hydration shell around the cation. This is an additional reason why the influence of the hydration is not the same for all the cations (see Figure 6). The comparison of the activation energy values for the saturated state between montmorillonites and beidellites shows some differences (see Table 1 and Figure 5b). For smaller cations (Li+ and Na+), the activation energy values for montmorillonites and beidellites are the same in both clays, which means that the influence of the framework on the mobility of the hydrated cations is independent of the position of the substitution. Therefore the cation does not strongly interact with the framework at the fully-hydrated state, except for the confinement or steric effect due to the hydration shell. On the other hand, the activation energy of the cation K+ is lower in montmorillonite than in beidellites, which may indicate that the cation hydration shell is incomplete in the more confined environment of beidellite. It is noticeable that in this hydration state, the Li+ and Na+ samples present higher activation energies than samples saturated by K+ or Cs+. This effect can also be

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Salles et al. TABLE 2: Difference of the Activation Energies (∆(∆Eact)) for Cations Extracted from the Conductivity Measurements for Montmorillonitea ∆(∆E Li+

Na+ K+ Cs+

act)

0.22 0.12 0.14 0.02

(eV) ∆(∆E

act)

(kJ/mol of cation) ∆(∆E 21.5 11.6 13.5 2.0

act)

(mJ/m2)

385 302 420 24

a The conversion of the energies in mJ/m2 is obtained by using specific surface areas from Berend et al.10 for alkali interlayer cations and the following structural formula Si4(Al1.52Mg0.26Fe0.17)Na0.18Ca0.11O10(OH)2 (according to Sauzeat et al.38).

Figure 6. Comparison between the activation energy ∆Eact ((0.02 eV) for the cation motion in both dry (b) and saturated states (9) in montmorillonite.

explained by the presence of the hydration shell in solution which is larger for Li+ and Na+ than for the other cations. If this also holds for the interlayer space, then this confinement will affect the motion of the cation in the same way. This behavior was also used to explain the role of the different cations in the swelling process during the hydration.6,10,16 It is thus revealed that the difference between the dry and hydrated states reduces when increasing the cation radius. As a general rule, the smaller the cation radius, the stronger the hydration energy of the cations, as it is commonly observed in solution.51 Such a trend is also obtained for the hydration energy of alkaline interlayer cations in montmorillonites57 estimated from our previous theoretical calculations. A global picture can be constructed as follows: during the hydration, the small alkali cation (Li+ or Na+) leaves the hexagonal site and this motion is made easier when the interlayer distance increases due to the hydration process. This is due to the water molecules modifying the interaction equilibrium between the surface layer and the cation. This induces an increase of the distance cation-surface thus and allows the cation to reach the interlayer space and be more coordinated by its hydration shell. The heavier cation (Cs+) is already located in the interlayer space, which means that the hydration does not affect so much the behavior of the cation. This is why no difference is observed between activation energy at the dry and saturated states. It has already been shown using electrostatic calculations that the cohesion energy is the stronger for Cs-montmorillonite,16,37,57 which is also evidenced by the small opening of the interlayer space observed by XRD.6 The combination of the low hydration capacity for the Cs+ cation and the low swelling capacity of Cs-montmorillonite makes it difficult for water molecules to get inside the interlayer space. Therefore it is clear that the Cs+ cations are surrounded only by an incomplete hydration shell (a behavior already verified in solution51). 3.3. Differences of the Activation Energy for the Cation Motion and Hydration Energies. The differences in activation energy between the hydrated and dry states (∆(∆Eact)) are provided for each alkali cation in Table 2. The small cations exhibit the higher ∆Eact, which can be related to a strong polarization of their first hydration shell.51,37,58 In contrast, the small ∆Eact values for the large monovalent cations can be attributed to their incomplete hydration sphere. When one compares these ∆(∆Eact) energy differences with the hydration energies available in the literature for the same cations in solution,51 we observe a reasonable correlation between these two energies (Figure 7). These energy values appear smaller in

Figure 7. Comparison between energies extracted from conductivity measurements (∆(∆Eact)) and the hydration energies for cations in solution.51

Figure 8. Comparison between energies extracted from the conductivity measurements (∆(∆Eact)) and the hydration energies obtained by theoretical model for cations in montmorillonite.56

clays compared to those in solution. This result can be interpreted as (1) an incomplete hydration process for the cation inside the interlayer space,12 (2) the influence of the layer-cation interaction and (3) the differences which may exist in both the original and final state for each process. Another comparison can be made with theoretical values predicted for the hydration of interlayer cation obtained from the analysis of immersion data and electrostatic calculations.37 The values are reported in Figure 8, as a function of the ∆(∆Eact) values obtained from conductivity experiments. A good trend is observed for the alkali cations, even if the conductivity measurements give lower energies than the theoretical ones. This relative difference can be explained by the fact that the experimental samples are not exactly the same compared with the ideal ones used in the theoretical model, implying a layer charge, half of those of our sample.

Driving-Force for Hydration of Swelling Clays

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TABLE 3: Temperatures for the Departure of the Interlayer Water, SCTA Rate for the Water Loss and the Water Mass Adsorbed at the Temperature to determine the Hydration Energies for Montmorillonitea

Li+ Na+ K+ Cs+ a

temp (°C)

water loss rate (mg/K)

water mass adsorbed (mg/g of clay)

clay hydration energies (kJ/mol of water)

clay hydration energies (mJ/m2)

75 60 60 40

0.05 0.035 0.015 0.003

60 35 15 30

48 41 55 7

670 565 769 105

The conversion of the energies in mJ/m2 is obtained by using a cross-sectional area for the water molecule equal to 12 Å2/molecule of water.

TABLE 4: Decomposition of the Energies Values from SCTA Experiments for Different Cations in the Montmorillonites cation Li+ Na+ K+ Cs+

clay hydration energy from SCTA experiments (mJ/m2)

∆(∆Eact) assimilated to hydration energy for the cation (mJ/m2)

hydration energy of the surface layer (mJ/m2)

670 565 769 105

384 302 420 24

290 260 349 81

It results therefore that the ∆(∆Eact) values are linked to the hydration energies for the interlayer cations. 3.4. Comparison of the Conductivity Data with the Hydration Energies Determined from SCTA Measurements. To conclude our study, we propose to compare the hydration energy values found above with a recent interpretation of the SCTA36 using the eq 4, applied to our samples. To evaluate the different terms from the SCTA equation (eq 4), we use (1) T, the temperature of the departure of the interlayer water, (2) [dMads/dT] the water loss rate evaluated from the experimental curves, and (3) Mads the water adsorbed mass. All the data are summarized in Table 3. The temperature of departure of the interlayer water corresponds to the temperature for which the curvature is modified, after the initial departure of the free water. This region is defined by the intersection between the wellpronounced decrease of mass due to the departure of the free water and the small decrease due to the departure of the interlayer water. This temperature T depends on the nature of the cation (Table 3): varying from 40 °C for Cs+ to 75 °C for Li+. This evolution follows the same trend as the hydration of cation in solution51 and the immersion data.31 The energy necessary to allow the departure of the water in the interlayer space is higher for Li+ and Na+ than for Cs+ and therefore the temperature of the departure is higher for Li+ or Na+. The exact determination of this temperature is difficult for Li+ cation. For other cations, using the SCTA technique allows us to determine the temperature of the transition between the departure of the free water and the bounded water with a reasonable accuracy. The other terms of the equation can be determined easily from the experimental curves: the derivatives are calculated as the mean value of the curve slop between the temperature of the departure of the interlayer water and 200 °C (the temperature corresponding to the end of the departure of the interlayer water as shown previously). The adsorbed water mass is also meaningful: it is generally admitted that the Li+ and Na+ cations form an outer-sphere complex and have a greater hydration shell than Cs+.58–60 If we consider only the cation, the evolution of the adsorbed water mass is in good agreement with this observation (Table 3). The activation energy value for the desorption of the interlayer water obtained from the SCTA experiments is given in Table 3 for the different cations. The first result is that these energies exhibit the same trend as those extracted from the conductivity measurements except

for the K+: K+ > Li+ > Na+ > Cs+. SCTA energies are larger than (∆(∆Eact)) obtained from the conductivity measurements. We can conclude that, in the SCTA experiments, the water released by the interlayer space is desorbed from both the cations and the layer surfaces, whereas, from the conductivity experiments, only the energy of the cation hydration can be roughly estimated. 3.5. Comparison between Conductivity Results and SCTA Interpretation. If we assume that the ∆(∆Eact) values obtained from the conductivity experiments can be assigned to the contribution of the hydration energy for the interlayer cation, it is then possible to decompose the SCTA energies (equivalent to the global hydration energy) in two contributions: the hydration energy of the layer surface (charged but without compensating cation) and the hydration energy of the interlayer cation extracted from the conductivity measurements using approximations:

ESCTA ) Elayer surface hydration + Einterlayer cation hydration This decomposition also assumes that the mechanical energy of the swelling is negligible. The results are provided in Table 4 and the energies are given in mJ/m2 to compare the experimental values with the theoretical ones.37 Such a decomposition shows that the hydration of the cation is the predominant process in the hydration of Li-, Na- and K-MX, whereas the clay surface hydration has a lower impact. For the larger cations, such as Cs+, the hydration of the surface is predominant. This difference of behavior has already been predicted by theoretical calculations.37 However, the results obtained by using theoretical determination of the surface energy and immersion data are higher than those observed experimentally. This difference can be due to the fact that the swelling process must be taken into account in the decomposition of the energy for the experimental data. 4. Conclusion Understanding the hydration process in the swelling clays requires the ability to probe the hydration of both the cation and the clay layers. Conductivity measurement is clearly a convenient technique to assess the interaction between cations and layers in presence or absence of water. It provides insights into the hydration mechanisms depending on the nature of the interlayer cations and gives an estimation of its hydration energy. SCTA is used to determine the global energy of desorption. The combination of conductivity measurements and SCTA

14008 J. Phys. Chem. C, Vol. 112, No. 36, 2008 experiments has been revealed to be a powerful tool to distinguish the different energies in the hydration process of swelling clays. Conductivity experiments illustrate (1) the influence of the cation nature, (2) the influence of the position of the substitution and (3) the hydration state on the mobility of the cation. At the dry state, small cations (Li+ and Na+) present higher jump activation energy values than Cs+. At water-saturated states, the same trend is observed which imposes to consider the hydration shell size. The final results show that two groups of cations can be considered. On one hand, the hydration process of Li-, Naand K-clays where the cation is predominant and, on the other hand, the Cs-clays where the hydration of the layer plays a key role. Some improvements of the methodology may bring more accurate values for the different energies. The first one consists of determining the specific surface area, which depends on the nature of the cation, when swelling clays are filled up with water. A technique allowing us to distinguish the temperatures of departure of free water and bonded water would also improve the calculation the global hydration energy. The SCTA curve links the mass (equal to the measured mass clay + water) as a function of the heat temperature. The curves report the evolution of the conductivity as a function of the frequency at different temperatures. The curves for chlorites and micas have the same behavior. Acknowledgment. We thank Dr. C. Pozo from LMTE (CEA Cadarache) for TEM analysis and observations, Dr. J. Raynal from LMTE (CEA Cadarache) for the purification and the exchange of the Moro samples and Dr. P. Llewellyn and Dr. S. Bourrelly from MADIREL (Marseille) for their fruitful help to perform the SCTA experiments References and Notes (1) Fowden, L., Barrier, R. M., Thinker, P. B., Eds. Clay Minerals: their structure, behaViour and use; Royal Society: London, 1984. (2) Velde, B. Introduction to clay minerals; Chapman and Hall: London, 1992. (3) Van Damme, H. C. R. Acad. Sci. Paris 1995, 320, 665–681. (4) Cases, J. M. C. R. Geosci. 2002, 334, 585–596. (5) Jullien, M.; Raynal, J.; Kohler, E.; Bildstein, O. Oil Gas Sci. Technol. 2005, 60 (1), 107–120. (6) Ferrage, E.; Lanson, B.; Sakharov, B. A.; Drits, V. A. Am. Mineral. 2005, 90, 1358–1374. (7) Marry, V.; Turq, P.; Cartellier, T.; Levesque, D. J. Chem. Phys. 2002, 117 (7), 3454–3463. (8) Chatterjee, A.; Iwasaki, T.; Ebina, T.; Miyamoto, A. Comput. Mater. Sci. 1999, 14, 119. (9) Delville, A. Langmuir 1991, 7, 547–555. (10) Berend, I.; Cases, J. M.; Franc¸ois, M.; Uriot, J. P.; Michot, L.; Masion, A.; Thomas, F. Clays Clay Miner. 1995, 43 (3), 324–336. (11) Cases, J. M.; Be´rend, I.; Besson, G.; Franc¸ois, M.; Uriot, J. P.; Thomas, F.; Poirier, J. E. Langmuir 1992, 8, 2730–2739. (12) Lee Schwartzen-Allen, S.; Matejevic, E. Chem. ReV. 1974, 74 (3), 385–400. (13) Calvet, R. Ph.D. Thesis Universite´ de Paris VI, 1972. Calvet, R.; Prost, R. Clays Clay Miner. 1971, 19, 175–186. (14) Mamy, J. Annal. Agronom. 1968, 19, 175–246. (15) Norrish, K. Discuss. Faraday Soc. 1954, 18, 120–134. (16) Salles, F. Ph.D. Thesis de l’Universite´ Paris VI (Pierre et Marie Curie), 2006. (17) Boek, E. S.; Coveney, P. V.; Skipper, N. T. J. Am. Chem. Soc. 1995, 117, 12608–12617. (18) Chang, F-R.C.; Skipper, N. T.; Sposito, G. Langmuir 1997, 13, 2074–2082. (19) Douillard, J. M.; Zajac, J.; Malandrini, H.; Clauss, F. J. Colloid Interface Sci. 2002, 255 (2), 341–351. Malandrini, H.; Clauss, F.; Partyka, S.; Douillard, J. M. J. Colloid Interface Sci. 1997, 194 (1), 183–193.

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