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Sol/Gel and Isotropic/Nematic Transitions in Aqueous Suspensions of Natural Nontronite Clay. Influence of Particle Anisotropy. 1. Features of the I/N Transition Laurent J. Michot,*,† Isabelle Bihannic,† Solange Maddi,† Christophe Baravian,‡ Pierre Levitz,§ and Patrick Davidson| Laboratoire EnVironnement et Mine´ ralurgie, Nancy UniVersity, CNRS-INPL UMR 7569, BP40 54501, VandœuVre Cedex, France, Laboratoire d’Energe´ tique et de Me´ canique The´ orique et Applique´ e, Nancy UniVersity, UMR 7563 CNRS-INPL-UHP, 2, AVenue de la Foreˆ t de Haye, BP160 54504, VandœuVre Cedex, France, Laboratoire de Physique de la Matie` re Condense´ e, UMR 7643 CNRS-Polytechnique, Ecole Polytechnique, 91128 Palaiseau Cedex, France, and Laboratoire de Physique des Solides, UMR 8502 CNRS-UniVersite´ Paris-Sud, Baˆ t 510, 91405 Orsay Cedex, France ReceiVed NoVember 9, 2007. In Final Form: December 23, 2007 The phase behavior of a natural nontronite clay was studied for size-selected particles by combining osmotic pressure measurements, visual observations under polarized light, and rheological experiments. In parallel, the positional and orientational correlations of the particles were analyzed by small-angle X-ray scattering. Aqueous suspensions of nontronite exhibit a true isotropic/nematic (I/N) transition that occurs before the sol/gel transition, for ionic strengths below 10-3 M/L. In this region of the phase diagrams, the system appears to be purely repulsive. The I/N transition shifts toward lower volume fractions for increasing particle anisotropy, and its position in the phase diagram agrees well with the theoretical predictions for platelets. SAXS measurements reveal the presence of characteristic interparticular distances in the isotropic, nematic, and gel phases. The swelling law (separation distance vs swelling law) exhibits two regimes. For high volume fractions, the swelling law is one-dimensional as in layered systems and reveals the presence of isolated platelets. At lower volume fraction, distances scale as φ-1/3, indicating isotropic volumic swelling. Finally, the experimental osmotic pressure curves can be satisfactorily reproduced by considering the interparticle distances between two charged planes whose effective charge is around 10% of the structural charge.
Swelling clay minerals are layered aluminosilicate compounds formed with two tetrahedral layers (Si, Al, Fe) sandwiching one octahedral layer (Mg, Al, Fe...). The whole structure bears a negative layer charge compensated by interlayer exchangeable cations whose valence and hydration properties control both swelling and colloidal behavior. Depending on the nature of the atoms in the layer and the charge location, various pure species are defined, i.e., hectorite, montmorillonite, saponite, and beidellite. In the two first compounds, the charge results from atomic substitutions in the octahedral layer (Li for Mg in hectorite and Mg for Al in montmorillonite), whereas in the two last ones, the charge is due to substitution in the tetrahedral layer. The phase behavior of the aqueous colloidal suspensions of these minerals has been studied for almost a century since the first report by Freundlich1 that bentonite (a rock whose main component is montmorillonite) forms a thixotropic gel when dispersed in water. The structure of this gel and the physical origin for gelation have been debated since the 1930s with two conflicting views. Formation of a tridimensional network is governed by electrostatic attraction between platelets, the socalled “house of cards” model,2,3 or formation of an oriented network stabilized by repulsive forces caused by interacting double layers.4-6 Most recent work on this problem has dealt * Corresponding author. E-mail:
[email protected]. † Laboratoire Environnement et Mine ´ ralurgie, Nancy University. ‡ Laboratoire d’Energe ´ tique et de Me´canique The´orique et Applique´e, Nancy University. § Ecole Polytechnique. | Universite ´ Paris-Sud. (1) Freundlich, H. Kolloid-Z 1928, 46, 289. (2) Broughton, G.; Squires, L. J. Phys. Chem. 1936, 40, 1041. (3) Van Olphen, H. Discuss. Faraday Soc. 1951, 11, 82. (4) Hauser, E. A.; Reed, C. E. J. Phys. Chem. 1937, 41, 911.
with laponite, a synthetic hectorite sample provided by Laporte Industries whose elementary particles are polydisperse ellipses (long axis 24.0 ( 6.9 nm, short axis 16.8 ( 4.9 nm, thickness around 1 nm).7 Despite the large number of experimental and theoretical studies devoted to that system, the situation remains rather unclear, and no model is able at present to reconcile all the experimental observations obtained on laponite suspensions. There is, however, a general agreement to interpret the transition occurring at very low ionic strength as a Wigner glass transition.8-14 There is also growing evidence that, at high ionic strength, aggregation phenomena play a significant role in the gelation process.15-17 In parallel, X-ray and neutron scattering techniques18-21 and NMR experiments22,23 reveal that, at least (5) Hauser, E. A. Chem. ReV. 1945, 40, 287. (6) Norrish, K. Discuss. Faraday Soc. 1954, 18, 120. (7) Balnois, E.; Durand-Vidal, S.; Levitz, P. Langmuir 2003, 19, 6633. (8) Levitz, P.; Le´colier, E.; Mourchid, A.; Delville, A.; Lyonnard, S. Europhys. Lett. 2000, 49, 672. (9) Knaebel, A.; Bellour, M.; Munch, J-P.; Viasnoff, V.; Lequeux, F.; Harden, J. L. Europhys. Lett. 2000, 52, 73. (10) Tanaka, H.; Meunier, J.; Bonn, D. Phys. ReV. E 2004, 69, art. no. 031404. (11) Tanaka, H.; Jabbari-Farouji, S.; Meunier, J.; Bonn, D. Phys. ReV. E 2005, 71, art. no. 021402. (12) Strachan, D. R.; Kalur, G. C.; Raghavan, S. R. Phys. ReV. E 2006, 73, Art n°041509. (13) Ianni, F.; Di Leonardo, R.; Gentilini, S.; Ruocco, G. Phys. ReV. E 2007, 75, art. no. 011408. (14) Bellon, L.; Gibert, M.; Hernandez, R. Eur. Phys. J. 2007, 55, 101. (15) Martin, C.; Pignon, F.; Magnin, A.; Piau, J.-M.; Cabane, B.; Lindner, P. Phys. ReV. E 2002, 66, art. no. 021401. (16) Mongondry, P.; Tassin, J. F.; Nicolai, T. J. Colloid Interface Sci. 2005, 283, 397. (17) Ruzicka, B.; Zulian, L.; Ruocco, G. Langmuir 2006, 22, 1106. (18) Ramsay, J. D. F.; Lindner, P. J. Chem. Soc. Faraday Trans. 1993, 89, 4207. (19) Saunders, J. M.; Goodwin, J. W.; Richardson, R. M.; Vincent, B. J. Phys. Chem. B 1999, 103, 9211.
10.1021/la703506z CCC: $40.75 © 2008 American Chemical Society Published on Web 02/28/2008
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Figure 2. Aqueous suspensions of size 3 nontronite (ionic strength 10-4 M/L), observed between crossed polarizers (the isotropic phase in a-d appears dark): (a) isotropic liquid sample at a volume fraction φ ) 0.5% (The small bright line observed at the bottom of the vial is due to a reflection on the curved bottom.), (b) onset of the phase separation at φ ) 0.61%, (c) biphasic sample at φ ) 0.67%, (d) biphasic sample at φ ) 0.72%, (e) birefringent gel at φ ) 1%.
Figure 3. Aqueous suspensions of size 4 nontronite (ionic strength 10-5 M/L) observed between crossed polarizers: (a) onset of the phase separation at φ ) 0.67%, (b) biphasic sample at φ ) 0.8%, (c) biphasic sample at φ ) 0.9%, (d) birefringent gel at φ ) 1.17%.
Figure 1. TEM micrographs of nontronite platelets after size fractionation: (A) size 1, (B) size 2, and (C) size 3.
at high concentration, laponite suspensions exhibit strong orientational order. Furthermore, optical measurements of the gels24,25 reveal microscopic textures closely resembling nematic ones.24 The possible occurrence of an isotropic/nematic transition in clay suspensions has been debated since Langmuir’s pioneering work26 that reported a macroscopic phase separation in suspensions of a hectorite from California. Such a behavior was subsequently rationalized by Onsager27 on the basis of a competition between orientational entropy and the packing entropy governed by the excluded volume. The relation (if any) (20) Lemaire, B. J.; Panine, P.; Gabriel, J. C. P.; Davidson, P. Europhys. Lett. 2002, 59, 55. (21) Martin, C.; Pignon, F.; Magnin, A.; Meireles, M.; Lelie`vre, V.; Lindner, P.; Cabane, B. Langmuir 2006, 22, 4065. (22) Porion, P.; Al Mukhtar, M.; Meyer, S.; Fauge`re, A.-M.; Van der Maarel, J. R. C.; Delville, A. J. Phys. Chem. B 2001, 105, 10505. (23) Porion, P.; Al Mukhtar, M.; Fauge`re, A.-M.; Pellenq, R. J. M.; Meyer, S.; Delville, A. J. Phys. Chem. B 2003, 107, 4012. (24) Gabriel, J. C. P.; Sanchez, C.; Davidson, P. J. Phys. Chem. B 1996, 100, 11139. (25) Mourchid, A.; Le´colier, E.; Van Damme, H.; Levitz, P. Langmuir 1998, 14, 4718. (26) Langmuir, I. J. Chem. Phys. 1938, 6, 838. (27) Onsager, L. Ann. N.Y. Acad. Sci. 1949, 51, 627.
Figure 4. Aqueous suspensions of size 3 nontronite (ionic strength 10-5 M/L) in square capillaries (1 mm thick) observed under a polarizing microscope.
between the sol/gel transition and an isotropic/nematic phase transition remains ill-defined. In the case of laponite, some authors have interpreted the sol/gel transition as the signature of a frustrated isotropic/nematic transition.25,28-30 However, in their study on a size-selected natural montmorillonite from Wyoming, Michot et al.31 showed that the concentration corresponding to the sol/gel transition increased linearly with particle anisotropy. (28) Mourchid, A.; Delville, A.; Lambard, J.; Le´colier, E.; Levitz, P. Langmuir 1995, 11, 1942. (29) Kroon, M.; Vos, W. L.; Wegdam, G. H. Phys. ReV. E 1998, 57, 1962. (30) Bhatia, S.; Barker, J.; Mourchid, A. Langmuir 2003, 19, 532. (31) Michot, L. J.; Bihannic, I.; Porsch, K.; Maddi, S.; Baravian, C.; Mougel, J.; Levitz, P. Langmuir 2004, 20, 10829.
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Figure 5. Aqueous suspensions of size 4 nontronite (ionic strength 10-4 M/L) in square capillaries (1 mm thick) observed under a polarizing microscope.
Figure 6. Aqueous suspensions of size 2 nontronite (ionic strength 10-4 M/L), observed between crossed polarizers: (a) φ ) 0.67%, (b) φ ) 0.86%.
This revealed that the sol/gel transition is not directly related to an isotropic/nematic transition of individual clay particles. Indeed, an opposite evolution should be observed for an I/N transition involving the individual clay particles. It then appears, as discussed by Van der Beek and Lekkerkerker,32,33 that the two phenomena are not directly coupled and could be in fact in competition with each other. A similar conclusion was reached in a very recent study on MgAl layered double hydroxides.34 By working on a natural clay, nontronite, which is a ferruginous beidellite, we recently evidenced a true I/N transition35 occurring at a lower concentration than that of the sol/gel transition. The aim of the present series of papers is to analyze in detail for this particular nontronite clay the influence of particle anisotropy on the phase diagrams and in particular to observe the evolution of both the sol/gel and I/N transitions. The first part will concentrate on the features of the I/N transition, whereas the second part will discuss in detail the features of the gel with a detailed rheological investigation. Materials and Methods The clay mineral used in this study is a nontronite from Southern Australia36 that was purchased from the Source Clays Minerals repository at Purdue University. Nontronite is a naturally occurring, swelling, dioctahedral clay mineral related to the montmorillonite-
Figure 7. Aqueous suspensions of size 2 nontronite after elimination of the coarser particles by centrifugation: (A) ionic strength 10-5 M/L from left to right, φ ) 0.3%, 0.33%, 0.37%, and 0.47%; (B) ionic strength 10-4 M/L from left to right, φ ) 0.33%, 0.37%, 0.37%, 0.40%, 0.43%, and 0.5%; (C) ionic strength 10-3 M/L from left to right, φ ) 0.50%, 0.53%, 0.57%, 0.60%, and 0.63%. beidellite series in which most aluminum atoms are replaced by iron(III) ions. The structural formula of the nontronite used in the present study was recently refined37 as (Si7.55Al0.16Fe0.29)(Al0.34Fe3.54Mg0.05)O20(OH)4Na0.72. On the basis of unit-cell parameters, its density can be estimated to be around 3.0 g/cm3. A 40 g/L clay suspension was first exchanged three times in 1 M NaCl. The suspension was then washed by centrifugation and dialyzed repeatedly against Milli-Q water until a conductivity of less than 5 µS was reached. The suspensions were then poured in Imhoff cones and settled for 24 h. The bottom of the cone that contains various impurities (quartz, feldspar, and iron oxyhydroxide mainly) was discarded. Size fractionation procedures were then applied by centrifuging the stock suspension under different gravitational fields (7000g, 17 000g, and
Table 1. Morphological Parameters of the Various Nontronite Size Fractions Deduced from the Analysis of the TEM Micrographs
size
minimal length (nm)
maximal length (nm)
av length (nm)
polydispersity length (%)a
minimal width (nm)
maximal width (nm)
average width (nm)
polydispersity width (%)a
av anisotropy in the plane
1 2 3 4
239 112 53 47
2200 986 345 250
705 368 147 120
52 41 39 35
45 45 20 21
351 245 120 106
138 93 52 44
46 39 37 35
5.5 4.2 3.1 2.9
a
Polydispersity P )
x〈D2〉-〈D〉2/〈D〉.
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Figure 8. Evolution of G′ with the oscillatory frequency for different nontronite size fractions at an ionic strength of 10-4 M/L: (A) size 1, (B) size 2, (C) size 3, (D) size 4. 35 000 g). The supernatant obtained after centrifugation at the highest speed was concentrated by rotary evaporation. Using such a procedure, four size fractions referred to as sizes 1-4 were obtained. Mineralogical purity was checked by X-ray diffraction and infrared spectrometry, whereas sizes were determined by transmission electron microscopy (TEM) that was performed on a Philips microscope. Dilute suspensions (≈1 g/L) of nontronite were stirred, and a drop of this suspension was deposited on a copper grid for observation under the microscope. (32) Van der Beek, D.; Lekkerkerker, H. N. W. Europhys. Lett. 2003, 61, 702. (33) Van der Beek, D.; Lekkerkerker, H. N. W. Langmuir 2004, 20, 8582. (34) Zhang, J.; Luan, L.; Zhu, W.; Liu, S.; Sun, D. Langmuir 2007, 23, 5331. (35) Michot, L. J.: Bihannic, I.; Maddi, S.; Funari, S. S.; Baravian, C.; Levitz, P.; Davidson, P. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 16101. (36) Keeling, J. M.; Raven, M. D.; Gates, W. P. Clays Clay Miner. 2000, 48, 537. (37) Gates, W. P.; Slade, P. G.; Manceau, A.; Lanson, B. Clays Clay Miner. 2002, 50, 223.
Osmotic pressure measurements were carried out using Visking dialysis tubes with a cutoff of 14 000 Da. Prior to the experiment, the membranes were first rinsed twice with 10-3 M sodium azide to remove impurities and prevent bacterial contamination. The membranes were then washed twice more in MilliQ water and conditioned during one night at the ionic strength of the experiment. PEG 20000 (Fluka) solutions were prepared by dilution in sodium chloride solutions. For solutions with low osmotic pressures (Πi e 1000 Pa), 40 cm3 of clay suspensions were introduced in the dialysis tubes and placed in 250 cm3 of PEG solutions. For higher osmotic pressures, either 60 or 80 cm3 of clay suspensions were placed in 500 or 1000 cm3 of PEG solutions, respectively. In all cases, the conditions were that of an osmotic stress. In order to ensure equilibrium, the PEG solutions were changed with fresh ones after 2 weeks. The total duration of the experiment was fixed at 4 weeks, as it was shown for latexes38 and laponite28,39 suspensions that osmotic (38) Bonnet-Gonnet, C. Thesis, Universite´ Paris VI, France, 1993.
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Figure 9. Phase diagrams of aqueous suspensions of nontronite: (A) size 1, (B) size 2, (C) size 3, (D) size 4. equilibrium was reached under those conditions. At the end of the experiment, the concentrated nontronite suspensions were recovered and their solid content was determined after drying the samples. In order to determine more precisely the weight fraction, the relative humidity was measured and taken into account according to the water adsorption data determined for Na-saponite.40 Rheological measurements were carried out using an Aspect 2000 rheometer from TA instruments with a cone and plate geometry with a gap of 14 µm. The elastic and viscous moduli G′ and G′′ were determined from oscillatory stress measurements using frequencies between 0.02 and 10 Hz. The strain used in the oscillatory measurements was adjusted by depending on the gel strength in order to be in the linear regime. Flow curves were carried out under controlled shear rate from 0 to 5000 s-1. The full cycle of increasing and decreasing shear rates was performed in 10 min. The birefringent nature of the various samples was assessed in two different ways. For macroscopic experiments, the clay samples were introduced in 2 cm3 glass vials with a diameter of 5 mm and were then observed between crossed polarizers. Such a procedure can only be applied for moderately concentrated systems, as concentrated systems absorb too much light. In parallel, the samples were introduced either into 100 µm thick flat optical capillaries or (39) Lecolier, E. Thesis, Universite´ d’Orle´ans, France, 1998. (40) Michot, L. J.; Bihannic, I.; Pelletier, M.; Robert, J.-L. Am. Mineral. 2005, 90, 166.
Figure 10. Aqueous suspensions of size 3 nontronite (IS 2 × 10-3 M/L), observed between crossed polarizers: (a) φ ) 0.68%, (b) φ ) 0.70%, (c) φ ) 0.75%, (d) φ ) 0.80%. Flow birefringence due to the heat convection from the light is observed in the isotropic phase in samples a and b. in 1 mm thick square glass capillaries and observed using a Nikon optical microscope equipped with a polarizer and an analyzer. The existence of flocs was determined by visual observations on samples prepared at various ionic strengths and volume fractions in cylindrical glass vials. Small-angle X-ray scattering (SAXS) experiments were carried out on beamlines A2 and BW4 at Hasylab (Hamburg, Germany). Experiments on beamline A2 were performed using a fixed wavelength of 0.15 nm and a sample to detector distance of 3 m,
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Figure 11. Evolution of the scattering intensity for nontronite suspensions at φ ) 0.07% and an ionic strength of 10-5 M/L: (A) size 1, (B) size 2, (c) size 3, (D) size 4. Curves A and D: sample to detector distance 13 m, wavelength 1.38 Å. Curves B and C: sample to detector distance 3 m, wavelength 1.50 Å.
Figure 12. Evolution of the product q2I(q) as a function of q for nontronite suspensions at an ionic strength of 10-4 M/L: (A) size 1, φ ) 0.55%; (B) size 2, φ ) 0.48%; (c) size 3, φ ) 0.39%; (D) size 4, φ ) 0.30%. Sample to detector distance, 13 m; wavelength, 1.38 Å. whereas beamline BW4 was operated at a wavelength of 0.138 nm and a sample to detector distance of 13 m. The samples were conditioned in cylindrical glass capillaries or square glass capillaries or in cuvettes between two Kapton sheets. Two-dimensional scattering patterns were collected on a CCD camera, and the curves of scattered
intensity vs scattering vector modulus q (q ) 4π sin θ/λ, where 2θ is the scattering angle and λ the wavelength) were obtained by a radial integration of the data. When anisotropic patterns were observed, the integration was carried out in the direction of maximum intensity of the anisotropic patterns.
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Figure 13. Two-dimensional SAXS pattern of the isotropic and nematic phase of a size 3 nontronite suspension (φ ) 0.7%, IS ) 10-3 M/L) and associated curves q2I(q) vs q.
Results Size Measurements. Figure 1 presents some nontronite platelets observed by transmission electron microscopy. When the particles decrease in size, they become more and more difficult to observe by TEM and are barely visible in the smallest size fraction (not presented here). Compared to common swelling clays, such as montmorillonite,31,41 nontronite platelets exhibit a clear lath shape (a feature also displayed by hectorite42) that was also observed by AFM.43 Table 1 presents some geometrical parameters deduced from the analysis of around 150 particles for sizes 1-3 and around 90 particles for size 4. Even after size fractionation, polydispersity remains high. Still, it clearly appears that the laths are less and less anisotropic with decreasing sizes. Optical Observations. Naked eye observations in polarized light of vials filled with suspensions of increasing volume fractions of size 3 nontronite at an ionic strength of 10-4 M/L (Figure 2) and size 4 nontronite at a fixed ionic strength of 10-5 M/L (Figure 3) reveal similar features. At low volume fractions, the suspensions are isotropic liquids (Figure 2a) and exhibit flow birefringence. Then, in a given range of volume fractions, the suspensions are (41) Cade`ne. A.; Durand-Vidal, S.; Turq, P.; Brendle, J. J Colloid Interface Sci. 2005, 285, 719. (42) Bickmore, B. R.; Bosbach, D.; Hochella, M. F., Jr.; Charlet, L.; Rufe, E. Am. Mineral. 2001, 86, 411. (43) ten Brinke, A. J. W.; Bailey, L.; Lekkerkerker, H. N. W.; Maitland, G. C. Soft Matter 2007, 1145.
biphasic with a clear phase separation between a denser birefringent phase at the bottom and an isotropic one at the top (Figures 2b-d and 3a-c). The proportion of birefringent phase gradually increases with the overall clay volume fraction, which clearly indicates the occurrence of a true isotropic/nematic phase transition, and both phases exhibit a liquid behavior. At higher volume fractions, the suspensions are gels exhibiting strong birefringence (Figures 2e and 3d). The phase separation is also readily observed by polarized light microscopy (Figures 4 and 5). As reported previously,35 a few weeks after sample preparation, birefringent droplets are clearly visible in the top isotropic phase; they slowly sediment and coalesce to form the nematic phase. The phase separation actually takes several months. For higher volume fractions the suspensions are gellified, they are strongly birefringent, and clearly exhibit nematic textures. For these two size fractions, no phase separation was ever observed for an ionic strength of 5 × 10-3 M/L For higher size fractions, sedimentation starts playing a role and the phase transition is then much more difficult to observe unambiguously (Figure 6). In addition, due to the large particle size, multiple light scattering hinders the observation. Still, as shown in Figure 6, a phase transition might be present and, as observed for lower size fractions, gel samples exhibit a strong birefringence.
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Figure 16. Evolution with size of the average interparticular distances as a function of volume fraction (ionic strength 10-5 M/L).
nematic phase transition, as revealed in Figure 7. As in the case of the lower size fractions, the I/N transition does not appear for an ionic strength of 5.10-3 M/L.
Figure 14. Two-dimensional SAXS pattern of the gel phase of a size 3 nontronite suspension (φ ) 1.3%, ionic strength 10-3 M/L) and associated curves q2I(q) vs q.
Figure 15. Evolution of the average interparticular distances as a function of volume fraction for nontronite size 3 (ionic strength 10-5 M/L).
In order to better detect the phase transitions, size 2 nontronite was recentrifuged at 11 000g in order to get rid of the coarser particles. The sample thus obtained, whose average length and width are 250 and 70 nm, respectively, exhibits a clear isotropic/
Figure 17. Evolution of the volume fraction corresponding to the isotropic/nematic transition as a function of the effective size for nontronite aqueous suspensions at various ionic strengths: (A) isotropic binodal of the biphasic region and (B) nematic binodal of the biphasic region.
Aqueous Suspensions of Natural Nontronite Clay
Rheological Characterization. Figure 8 presents for the four size fractions at an ionic strength of 10-4 M/L the evolution of the elastic modulus G′ as a function of frequency. For relatively high volume fractions, G′ is much higher than G′′ (not shown) and exhibits a limited frequency dependence, typical of a gel behavior, whereas at lower volume fractions, the two moduli are on the same order of magnitude and the suspensions are then slightly viscous. A more detailed rheological analysis was performed and will be discussed in part 2 of the present series of papers. The aim of the basic rheological characterization presented here is to provide a determination of the sol/gel transition on the basis of the evolution of G′ and G′′ with volume fraction. Phase Diagrams. The information derived from all the previous experiments can be used to plot the phase diagrams of the four size fractions of nontronite (Figure 9). In the high ionic strength region, i.e., > 10-3 M/L, whatever the size fraction, the sol/gel transition line exhibits a negative slope, as observed for numerous clay systems.16,24,25,28,44-46 This line tends to join the flocculation line at low volume fraction and may then be assigned to microaggregation processes. Furthermore, smaller particles exhibit a stronger negative slope, which in the framework of aggregation appears logical, smaller particles aggregating more easily. For ionic strength e 10-3 M/L, the sol/gel transition line exhibits a positive slope, revealing a system dominated by repulsions. For the two smaller fractions, a clear biphasic region is observed. Such a region should be present as well for sizes 1 and 2, but because of sedimentation phenomena, it is rather difficult to precisely define a biphasic domain. For the two smaller size fractions (Figure 9C,D) the start of the I/N biphasic domain is slightly tilted toward larger volume fractions upon increasing ionic strength.47 For an ionic strength of 10-5 M/L, the gel line seems to be located just before the end of the nematic region. This, though, did not prevent the observation of the biphasic region. The shape of the sol/gel transition line appears to explain why no phase separation is observed for an ionic strength of 5 × 10-3 M/L. Indeed, by extrapolating the biphasic domain toward higher ionic strength, the sol/gel transition should be crossed before the I/N transition upon increasing concentration. In order to prove this point, in the case of size 3 nontronite, a full set of samples was prepared for an ionic strength of 2 × 10-3 M/L. It appears that, in this case, a I/N transition is observed (Figure 10 and arrows in Figure 9C), thus confirming the phase diagrams proposed. Structure of the Suspensions. The structure of the suspensions in the whole concentration domain was analyzed by small-angle X-ray scattering. In the present paper, we will concentrate on the strucure of the suspensions up to the sol/gel transition. Very dilute isotropic suspensions (Figure 11) exhibit a scattering intensity, I(q), monotonously decreasing with increasing scattering vector modulus, q, as I(q) ∼ q-2, proving the bidimensional nature of the scattering objects. At higher volume fractions, still in the isotropic region of the phase diagram, isotropic SAXS patterns are obtained that display oscillations of the scattered intensity due to the short-range positional (i.e., “liquid-like”) order of the clay platelets (Figure 12). Once the biphasic domain is reached, the two-dimensional patterns of the nematic phase display a clear anisotropy, as shown (44) Abend, S.; Lagaly, G. Appl. Clay Sci. 2000, 16, 201. (45) Michot, L. J.; Ghanbajaa, J.; Tirtaamadja, V.; Scales, P. J. Langmuir 2001, 17, 2100. (46) Shalkevich, A.; Stradner, A.; Kumar Bhat, S.; Muller, F.; Schurtenberger, P. Langmuir 2007, 23, 3570. (47) Fraden, S.; Maret, G.; Caspar, D. L. D.; Meyer, R. B. Phys. ReV. Lett. 1989, 63, 2068.
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Figure 18. Experimental and theoretical biphasic domains for aqueous nontronite suspensions: (A) size 2 after elimination of the coarse particles, (B) size 3, (C) size 4.
in Figure 13,which presents the patterns obtained on the isotropic and nematic phase of size 3 nontronite at an ionic strength of 10-3 M/L in a single capillary. The pattern of the nematic phase does not correspond to a monodomain but is not either indicative of a fully random distribution of nematic microdomains. It must be emphasized that the anisotropy of the pattern is strongly enhanced by applying a magnetic field to the sample, thereby aligning the nematic phase.35
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Figure 19. Osmotic pressure curves obtained for the different nontronite size fractions: (A) size 1, (B) size 2, (C) size 3, (D) size 4.
In the gel region, SAXS patterns are strongly anisotropic, mainly because of the shear-alignment resulting from the introduction of the gel in the capillaries. Clear oscillations are observed, indicative of characteristic distances between objects (Figure 14).
Discussion On the basis of the information gathered by SAXS, it is possible to plot the evolution of the correlation distances as a function of volume fraction. Figure 15 displays such an evolution for size 3 nontronite at an ionic strength of 10-5 M/L. Various regimes can be distinguished on that curve. For high volume fraction, the curve scales as φ-1. Such an evolution was already observed for montmorillonite48,49 and laponite.21,48,49 It indicates a local lamellar order of the clay platelets. At low volume fraction, the slope of the curve is -1/3, indicating isotropic volumic swelling. Such a transition was already observed by SANS on both montmorillonite and hectorite.48,49 In the case presented here, the transition between these two swelling regimes is located at the biphasic region denoted by dotted vertical lines in Figure 15. In the transition region, the swelling curve may scale as φ-0.5, as observed for nematic phases of vanadium pentoxide ribbons.50
Figure 20. Evolution with ionic strength of the osmotic pressure of nontronite particles of different sizes (volume fraction 0.7%).
(48) Ramsay, J. D. F.; Swanton, S. W.; Bunce, J. J. Chem. Soc. Faraday Trans. 1990, 86, 3919. (49) Ramsay, J. D. F.; Lindner, P. J. Chem. Soc. Faraday Trans. 1993, 89, 4207. (50) Livage, J.; Pelletier, O.; Davidson, P. J. Sol-Gel Sci. Technol. 2000, 19, 275.
Similar curves are obtained for all sizes and ionic strengths except for an ionic strength of 5 × 10-3 M/L, where there is no transition region between the two swelling behaviors. The influence of particle size on distances is illustrated in Figure 16,which corresponds to an ionic strength of 10-5 M/L. The two
Aqueous Suspensions of Natural Nontronite Clay
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Figure 21. Calculated and experimental osmotic pressure for an ionic strength of 10-4 M/L: (A) size 1, (B) size 2, (C) size 3, (D) size 4.
smaller size fractions clearly exhibit the two above-mentioned swelling regimes, whereas for the two higher size fractions, no φ-1/3 regime is observed in the range of distance investigated, but a φ-0.5 dependence could be present. The evolution of correlation distances with ionic strength and particle size will be discussed in more detail in the second part of the present series of papers. Still, the local lamellar order observed provides some crucial information regarding the I/N transition. Indeed, using the lamellar swelling law d ) t/φ, where t is the layer thickness, it is possible to obtain the thickness of the individual objects from the slope of the swelling curves at high volume fractions. The average values obtained for the two smaller size fractions are 0.52 and 0.66 nm, values that are very close to the thickness of a single clay sheet (0.654 nm),51,52 proving that, in this case, the layers are perfectly exfoliated in suspension. For size fraction 2, one obtains a value of 0.76 nm, whereas size fraction 1 leads to a value of 1.05 nm. This suggests that in these last two cases, some of the layers are not fully exfoliated but are present as doublets in suspension. Such a situation was often observed by AFM7,53 or XPEEM54 on dilute suspensions of clay minerals. (51) Moore, D. M.; Reynolds, R. C., Jr. X-ray Diffraction and the Identification and Analysis of Clay Minerals; Oxford University Press: New York, 1997; 322 pp. (52) Ferrage, E.; Kirk, C. A.; Cressey, G.; Cuadros, J. Am. Mineral. 2007, 92, 994. (53) Tournassat, C.; Neaman, A.; Villieras, F.; Bosbach, D.; Charlet, L. Am. Mineral. 2003, 88, 1989.
Height histograms obtained by either technique revealed indeed the presence of doublets. Combining results from the phase diagrams (Figure 9), from size measurements, and from SAXS, it is possible to represent the evolution of the volume fraction corresponding to the isotropic (Figure 17A) and nematic (Figure 17B) binodal of the biphasic region as a function of the length of the particles. In both cases and for the three ionic strengths investigated, the position of the transition lines decrease with increasing particle anisotropy, a feature typical of isotropic/nematic transitions. Recent theoretical calculations have shown that, for the I/N transition of infinitely thin platelets transition, the densities of the coexisting phases are written as55,56
nID3 ≈ 3.7 nND3 ≈ 4.0
(1)
where nI and nN are the number densities of platelets in the isotropic and nematic phase, respectively, and D is the particle diameter. Figure 18 presents for the three size fractions of (54) Vantelon D.; Belkhou, R.; Bihannic, I.; Michot, L. J.; Montarge`s-Pelletier, E.; Robert, J-L.; Aballe, L.; Mentes, T. O.; Locatelli, A.; Orti, M. A. Manuscript in preparation. (55) Bates, M. A.; Frenkel, D. J. Chem. Phys. 1999, 110, 6553. (56) Van der Kooij, F. M.; Van der Beek, D.; Lekkerkerker, H. N. W. J. Phys. Chem. B 2001, 105, 1696.
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Michot et al.
Figure 22. Calculated and experimental osmotic pressure for an ionic strength of 10-4 M/L and γ ) 0.75: (A) size 1, (B) size 2, (C) size 3, (D) size 4.
nontronite that display a clear-cut I/N transition a comparison between experiments and theory. In the calculations, the conversion from number densities to volume fractions was carried out by taking into account the lath shape of the particle and taking as the particle thickness the size of an isolated clay layer, i.e., 0.65 nm. Considering the fact that nontronite particles are not infinitely thin and that they display a significant polydispersity, the agreement between experimental and theoretical values is quite reasonable. In the theoretical calculations, no ionic strength dependence is observed, in contrast with the experimental data. However, attempts to renormalize the particle size using the Debye length did not yield any better agreement. Indeed, in that case, the ionic strength dependency of the theoretical calculations is much more marked than that displayed in the experimental data. The larger width of the biphasic regions in the experimental data compared to the theoretical calculations could be linked to polydispersity. In any case, the semiquantitative agreement between experiments and theory proves unambiguously that the transition observed is a true I/N transition and that attractive forces do not play a role in clay suspensions at low ionic strength. It would certainly be possible to obtain a better agreement between
theory and experiment by using anisotropic potentials adapted to clay minerals.57,58 Finally, in the framework of an I/N transition, according to the theoretical predictions,55,56 the reduced osmotic pressure, Πn ) ΠD3/kT, is around 15 at the I/N transition. According to such calculations, the osmotic pressures at the transition should be equal to 1, 5, 60, and 120 Pa for sizes 1, 2, 3, and 4, respectively, taking into account the bare size of the particles. As shown in Figure 19, for all size fractions and all ionic strength, the experimental osmotic pressures close to the transition (φ around 0.5-1%) are significantly higher than these values. Furthermore, as shown in Figure 20, which displays for a volume fraction of 0.7%, i.e., before gel formation, the evolution of osmotic pressure with ionic strength for the four size fractions of nontronite displays a logarithmic dependence for ionic strengths between 10-5 and 10-3 M/L, revealing a system dominated by electrostatic repulsions. A crossover is observed for ionic strengths higher than 10-3 M/L, which reinforces the assumption discussed in the section on phase diagrams that microflocculation starts appearing for ionic strengths g 10-3 M/L. (57) Agra, R.; Trizac, E.; Boquet, L. Eur. Phys. J. E 2004, 15, 345. (58) Agra, R.; Van Wijland, F.; Trizac, E. Phys. ReV. Lett. 2004, art. no. 018304.
Aqueous Suspensions of Natural Nontronite Clay
In the context of a purely repulsive system, it is interesting to compare the experimental osmotic pressures with those obtained using the classical formula59 that applies in the weak overlap approximation for parallel plates separated by a distance D
Πosm ) 1.59 × 108[NaCl]γ2e-κD with γ ) tanh(ψ0*ze/4kT) (2) where Πosm is the osmotic pressure in Pascals and [NaCl] the concentration in mol/L. Figure 21 presents for the four size fractions of nontronite and for a ionic strength of 10-4 M/L the comparison between experimental and calculated osmotic pressures. In that case, considering the nominal charge of the layer of 0.12 C/m2, γ is equal to 1. It appears that the calculated osmotic pressures are on the same order of magnitude as the experimental ones, although they are systematically higher. Besides, in the case of clay minerals, ionic condensation is very important, and Meyer et al.60 showed that the variation of the free energy of two laponite particles in the presence of salt could be reproduced by a Yukawa potential with a residual charge equal to 7% of the nominal electrical charge of the colloids. Taking into account such a value of electrical charge, for an ionic strength of 10-4 M/L, the value of γ in eq 2 is equal to 0.75. Figure 22 displays the calculated and experimental osmotic pressures under such conditions. Except for size 4, where experimental pressures remain below the calculated ones, the agreement between calculated and experimental values is very satisfactory. However, a systematic deviation is observed for the highest osmotic pressures. This overestimation of the experimental values for the most concentrated systems used in the present study could be tentatively assigned to the strong mechanical resistance of the gels formed under those conditions. The system could then be blocked before reaching the equilibrium volume fraction. In any case, the (59) Israelachvili, J. Intermolecular and surface forces with application to colloidal and biological systems, 2nd ed; Academic Press: New York, 1991; p 246. (60) Meyer, S.; Levitz, P.; Delville, A. J. Phys. Chem. B 2001, 105, 10684.
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agreement between experimental and calculated values confirms the purely repulsive nature of the system.
Summary and Conclusions We have shown in this paper that nontronite, a natural clay mineral, exhibits a true I/N transition. The use of size-selected samples allowed us to examine in detail the features of this transition. Its evolution with size is perfectly logical, and its position in the phase diagram agrees well with the theoretical predictions for platelets. We also showed that, for ionic strengths lower than 10-3 M/L, the system is purely repulsive. For higher salt contents, microflocculation processes seem to appear, but their origin remains partially unclear and clearly deserves further attention. A remaining issue is how clay particle properties (dimensions, polydispersity, electric charge, flexibility...) control gelation or liquid-crystalline formation. In the case of the nontronite used in this study, upon increasing volume fraction, the isotropic/ nematic transition is observed before gelation, whereas suspensions of other clay minerals such as montmorillonite or laponite exhibit gelation before any phase transition takes place. Such a behavior could be assigned either to the lath shape of nontronite or to the tetrahedral location of the charge, which, through subtle modification of the electrostatics of the system, may shift the sol/gel transition toward higher volume fractions, thus revealing the I/N transition. Experiments and simulations are currently underway to test those hypotheses. Finally, the exact nature of the sol/gel transition, the structure of the gel formed, and its mechanical properties must be studied in detail. These problems will be addressed in the second part of this series of papers. Acknowledgment. Support of beamtime at Hasylab by the European Community-Research Infrastructure Action under the FP6 “Structuring the European Research Area” Programme (through the Integrated Infrastructure Initiative “Integrating Activity on Synchrotron and Free Electron Laser Science”) Contract RII3-CT-2004-506008 is gratefully acknowledged. We would like to thank Sergio S. Funari and Stefan Roth for their help during the SAXS experiments on beamlines A2 and BW4. The help of Jaafar Ghanbaja in obtaining the TEM pictures is gratefully acknowledged. LA703506Z
