Combined SAXS−Rheological Studies of Liquid-Crystalline Colloidal

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Combined SAXS-Rheological Studies of Liquid-Crystalline Colloidal Dispersions of Mineral Particles F. Camerel,*,† J. C. P. Gabriel,†,‡ P. Batail,† P. Panine,§ and P. Davidson*,| Laboratoire Chimie Inorganique, Mate´ riaux et Interfaces, FRE 2447 CNRS, Baˆ t. K, Universite´ d’Angers, 2, Boulevard Lavoisier, 49045 Angers, France, Nanomix Incorporated, 5980 Horton Street, Sixth Floor, Emeryville, California 94608, European Synchrotron Radiation Facility, BP 220, 38043 Grenoble, France, and Laboratoire de Physique des Solides, UMR 8502 CNRS, Baˆ t. 510, Universite´ Paris Sud, 91405 Orsay, France Received April 14, 2003. In Final Form: September 9, 2003 This article describes simultaneous rheological and small-angle X-ray scattering (SAXS) studies of complex fluids, using a modified rheometer that allows in situ synchrotron SAXS measurements. We investigated the behavior under shear stress of lamellar liquid-crystalline suspensions, recently reported, comprised of covalent mineral sheets of H3Sb3P2O14 in water, that form sols and gels. Two original nematic mineral suspensions of HSbP2O8 disklike and H4Nb6O17 rodlike nanoparticles were also examined. We correlate both the existence of a yield stress and a strong decrease in viscosity (shear-thinning behavior) with textural changes easily detected by SAXS. The exact nature of the phase (nematic or lamellar) does not seem to affect such phenomena as long as there is orientational order. Moreover, strongly flow-birefringent and shear-thinning isotropic suspensions of anisotropic nanoparticles displayed very anisotropic SAXS patterns under shear.

1. Introduction Correlating the microstructure and the rheological behavior of colloidal suspensions is needed to better tailor them in view of specific applications for use both in industry and in everyday life (oil, photographic, and food industries, paints, cosmetics, etc.). In the past decades, the advent and the progress of X-ray synchrotron diffraction techniques have provided scientists with a very efficient tool to probe colloidal organizations on length scales ranging from angstroms to microns. Moreover, several kinds of flow cells have been built and set in the synchrotron X-ray beam in order to investigate the structures of colloids submitted to deformations and flows.1 The most popular of these devices is the Couette shear cell that is also used in combination with neutron and light scattering. However, these flow cells do not allow one to measure the stresses applied to the samples and do not give access to the rheological parameters such as the viscosity and complex dynamic modulus while performing scattering experiments. Then, relating rheological properties with changes in microstructure relies on the comparison of data obtained in different experimental conditions, which can be very misleading. In fact, there have been so far very few reports of studies where the * To whom correspondence should be addressed. E-mail: [email protected]; [email protected]. † Universite ´ d’Angers. ‡ Nanomix Inc. § European Synchrotron Radiation Facility. | Universite ´ Paris Sud. (1) See, for instance: (a) Picken, S. J.; Aerts, J.; Visser, R.; Northolt, M. G. Macromolecules 1990, 23, 3849. (b) Plano, R. J.; Safinya, C. R.; Sirota, E. B.; Wenzel, L. J. Rev. Sci. Instrum. 1993, 64, 1309. (c) Keates, P.; Mitchell, G. R.; Peuvrel-Disdier, E.; Navard, P. Polymer 1993, 34, 1317. (d) Diat, O.; Roux, D.; Nallet, F. Phys. Rev. E 1995, 51, 3296. (e) Hongladarom, K.; Ugaz, V. M.; Cinader, D. K.; Burghardt, W. R.; Quintana, J. P.; Hsiao, B. S.; Dadmun, M. D.; Hamilton, W. A.; Butler, P. D. Macromolecules 1996, 29, 5346. (f) Romo-Uribe, A.; Windle, A. H. Macromolecules 1996, 29, 6246. (g) Andresen, E. M.; Mitchell, G. R. Europhys. Lett. 1998, 43, 296. (h) Villetti, M.; Borsali, R.; Diat, O.; Soldi, V.; Fukada, K. Macromolecules 2000, 33, 9418. (i) Idziak, S. H. J.; et al. Eur. Phys. J. E 2001, 6, 139.

small-angle X-ray scattering (SAXS) and the rheological behavior of complex fluids have been simultaneously measured.2 Although the flow mechanisms of spherical colloids have recently been the focus of intense studies, comparatively less attention has been so far devoted to colloidal suspensions of anisotropic particles. However, the latter systems have the additional potential interest of forming liquidcrystalline phases in a given range of concentration.3 Liquid crystals display complex rheological behaviors that depend on their state of alignment with respect to the flow geometry. Therefore, their texture (i.e., the spatial and orientational distribution of microscopic liquidcrystalline domains) must also be considered in order to understand their flow properties. The new field of mineral liquid crystals (MLCs) provides many colloidal systems suitable for combined SAXS/rheometry studies.4 Mineral moieties are good candidates in this respect because of their high electronic contrast with the solvent (mostly water) and because they are robust, even in a strong shear flow, compared to organic colloids that are often obtained by self-assembly. For instance, colloidal suspensions of clays, Ni(OH)2 and V2O5 nanoparticles have already been conveniently aligned and studied in situ by X-ray (or neutron) diffraction in a Couette shear cell.5 Moreover, a large variety of mineral mesophases, comprised of both (2) (a) Pople, J. A.; Hamley, I. W.; Diakun, G. P. Rev. Sci. Instrum. 1998, 69, 3015. (b) Hamley, I. W.; Pople, J. A.; Gleeson, A. J.; Komanschek, B. U.; Towns-Andrews, E. J. Appl. Crystallogr. 1998, 31, 881. (c) Panine, P.; Narayanan, T.; Vermant, J.; Mewis, J. Phys. Rev. E 2002, 66, 022401. (3) De Gennes, P. G.; Prost, J. The Physics of Liquid Crystals; Clarendon Press: Oxford, 1993. (4) (a) Davidson, P.; Batail, P.; Gabriel, J. C. P.; Livage, J.; Sanchez, C.; Bourgaux, C. Prog. Polym. Sci. 1997, 22, 913. (b) Gabriel, J. C. P.; Davidson, P. Adv. Mater. 2000, 12, 9. (c) Gabriel, J. C. P.; Davidson, P. Top. Curr. Chem. 2003, 226, 119. (5) (a) Davidson, P.; et al. J. Phys. II 1995, 5, 1577. (b) Clarke, S. M.; Rennie, A. R.; Convert, P. Europhys. Lett. 1996, 35, 233. (c) Pelletier, O.; Bourgaux, C.; Diat, O.; Davidson, P.; Livage, J. Eur. Phys. J. B 1999, 12, 541. (d) Brown, A. B. D.; Rennie, A. R. Phys. Rev. E 2000, 62, 851. (e) Ramsay, J. D. F.; Lindner, P. J. Chem. Soc., Faraday Trans. 1993, 89, 4207.

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rodlike and platelike particles, have already been reported.4 Some of these liquid-crystalline phases also sometimes form gels, which makes them very valuable systems to understand the microscopic changes occurring when a gel starts to flow. In this article, we describe a series of combined SAXS/rheometry experiments that aim at better understanding the flow properties of colloidal gels and sols comprised of anisotropic mineral moieties. 2. Materials and Methods 2.1. Synthesis and Exfoliation of K3Sb3P2O14, KSbP2O8, and K4Nb6O17. A homogeneous mixture of stoichiometric proportions of NH4H2PO4 (Alfa, 23 mmol, 2.66 g), Sb2O3 (Merck, 17 mmol, 5.06 g), and KNO3 (Prolabo, 34 mmol, 3.51 g) was placed in a platinum crucible and heated in air, first to 300 °C for 10 h to decompose NH4H2PO4, and then heated to and kept at 1000 °C for 24 h to yield high-purity K3Sb3P2O14 (9 g, 12 mmol, checked by X-ray powder diffraction).6 The total amount of crude powder was thoroughly ground and stirred in a 1 L solution of 8.0 N nitric acid at 50 °C for 24 h.7 During this process, the solid is filtered off and the acid solution is renewed three times to ensure complete exchange of the alkali metal cations for protons to yield H3Sb3P2O14 (7.44 g, 11 mmol) (over 98% potassium to proton exchange). This product is then rinsed in deionized water (resistivity > 18 MΩ) and retrieved by centrifugation (13 000 rpm). This process is repeated three times. Usually, after the third rinse, the powder starts to swell significantly. After the last centrifugation step, only the gel phase is collected; it is placed in a regenerated cellulose tubular membrane (Cellu Sep; width, 46 mm; thickness, 28 µm; pore size, 10 Å) and subjected to dialysis in deionized water. The water bath is replaced every 24 h, until the nitrate ion concentration has decreased to less than 1 ppm (JBL, nitrate test).8 During the progress of the dialysis, the lamellar, protonated phosphatoantimony acid H3Sb3P2O14 continues to swell. The pH of the suspensions obtained varied from 1.5 to 2.5 depending upon their volume fraction. The suspension volume fraction was determined by thermogravimetric analysis (TGA) under N2 (Perkin-Elmer TGS2) by heating to 500 °C. Note that the material collected after TGA remained white, indicating that the dialysis procedure did not lead to contamination of the gel with cellulose from the dialysis membranes. A similar procedure, which extends that published earlier,7,8 is now described for the preparation of the suspensions of the monoacid HSbP2O8. The starting material KSbP2O8 was synthesized from a homogeneous mixture of stoichiometric proportions of NH4H2PO4 (Alfa, 44 mmol, 5.00 g), Sb2O3 (Merck, 11 mmol, 3.17 g), and KNO3 (Prolabo, 22 mmol, 2.20 g) placed in a platinum crucible.9 The mixture was first kept at 300 °C in air for 4 h to decompose NH4H2PO4 and then at 950 °C for 24 h to yield high-purity KSbP2O8 (5.2 g, 16.5 mmol). Solutions of H3Sb3P2O14 and HSbP2O8 at various concentrations were prepared by dilution with deionized water from two starting gels, of volume fractions φ ) 2.77% and 1.77%. K4Nb6O17 was synthesized from a mixture of Nb2O5 (Fluka, 15 mmol, 4.05 g) and K2CO3‚1.5H2O (Merck, Suprapur, 13 mmol, 1.84 g) placed in a platinum crucible. K2CO3 was used in slight excess (10 mol %), in regard to stoichiometric quantities, to replace the potassium carbonate that can be removed by evaporation during the heating stage.10 The mixture was heated in air, first to 300 °C for 2 h to remove the water and then to 900 °C to melt the potassium carbonate, and finally was kept at 1050 °C for 24 h to yield high-purity K4Nb6O17‚3H2O (4.32 g, 4 mmol). Proton exchange was performed on 4 g of ground K4Nb6O17‚3H2O (6) Piffard, Y.; Lachgar, A.; Tournoux, M. J. Solid State Chem. 1985, 58, 253. (7) (a) Piffard, Y.; Verbaere, A.; Lachgar, A.; Deniard-Courant, S.; Tournoux, M. Rev. Chim. Miner. 1986, 23, 766. (b) Piffard, Y.; Verbaere, A.; Oyetola, S.; Deniard-Courant, S.; Tournoux, M. Eur. J. Solid State Inorg. Chem. 1989, 26, 113. (8) Gabriel, J. C. P.; Camerel, F.; Batail, P.; Davidson, P.; Lemaire, B.; Desvaux, H. Nature 2001, 413, 504. (9) Piffard, Y.; Oyetola, S.; Courant, S.; Lachgar, A. J. Solid State Chem. 1985, 60, 209. (10) Reisman, A.; Holtzberg, F. J. Am. Chem. Soc. 1955, 77, 2115.

Figure 1. (a) Schematic view of the rheometer Couette shear cell illustrating the radial and tangential geometries with respect to the X-ray beam (flat arrows). We also depict, within the cell, the three different types of orientations (a, b, and c) of the lamellar phase, as seen by the X-rays in both geometries (the layers are sketched as circular disks). (b) Cross-sectional view of the shear setup and its associated thermal environment. powder in HCl aqueous solution at 40 °C for 1 week. During this process, the solid was filtered and the acid solution was renewed four times, and 3.6 g of product, in which 3.3 potassium cations have been exchanged by protons, was obtained.11 Exfoliation of 0.5 g of the protonated compound was performed in 5 mL of water with tetra-(n-butyl)-ammonium hydroxide at pH ) 12. The solution was stirred for 12 h. Insoluble materials were removed by centrifugation at 5000 rpm during 5 h. The supernatant pH was adjusted to 7 with 0.1 M HCl. A final concentration of 5.6% w/w of [Nb6O174-]n tubules was determined by TGA analysis. A milky white solution was obtained, but optical microscopy observations revealed no particles in suspension. Suspensions at various concentrations were prepared by dilution with deionized water. 2.2. Combined SAXS/Rheometry Experiments. The combination of a rheometer and SAXS in situ, implemented on a 3rd generation synchrotron source, is a very powerful technique to study transient phenomena and control the mechanical history of a shear- or stress-sensitive material. Two typical scattering configurations (called radial and tangential geometries) were used for rheo-SAXS measurements: the incident X-ray beam passes either through the center of the cell along the radial direction (perpendicular to the velocity/vorticity plane) or through the center of the gap along the tangential direction (perpendicular to the vorticity/shear gradient plane) (Figure 1a). The combined setup is described in detail elsewhere.12 A. On-line Rheometry. The rheological measurements presented here were performed using a true stress-controlled RT20 Rotovisco (Haake) apparatus. The rheometer shaft drives the (11) Saupe, G. B.; Waraksa, C. C.; Kim, H.-N.; Han, Y. J.; Kaschak, D. M.; Skinner, D. M.; Mallouk, T. E. Chem. Mater. 2000, 12, 1556. (12) Panine, P.; Gradzielski, M.; Narayanan, T. Rev. Sci. Instrum. 2003, 74, 2451.

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Figure 3. Series of SAXS patterns of a H3Sb3P2O14 gel of volume fraction φ ) 2.34% submitted to increasing shear stresses. Figure 2. (a) Dependence on frequency of the storage modulus G′ and the loss modulus G′′ of a H3Sb3P2O14 gel of volume fraction φ ) 2.34%. (b) Flow curve showing the evolutions of the shear stress and the dynamic viscosity versus the shear rate for a H3Sb3P2O14 gel of volume fraction φ ) 2.34%. inner cylinder of the coaxial cylindrical geometry (Searle’s type cell). This rheometer was operated in continuous or oscillatory motion with imposed stress or controlled shear. The inner rotor of the shear cell has an outer diameter of 20 mm, and the outer stator has an inner diameter of 22 mm corresponding to a shear gap of 1 mm. A maximum shear rate γ˘ of 1200 s-1 and maximum shear stress τ of 5300 Pa were thus obtained. Measurements, in imposed stress or in controlled shear rate, were acquired with a 10 s delay. Measurements of the complex modulus were performed between 0.01 and 100 Hz, and due care was paid to stay in the linear regime that was previously determined. The cell was made from polycarbonate (PC) that gives little X-ray scattering intensity, with a wall thickness of only about 100 µm at the beam position. The inner rotor has a window where the beam passes through, and the combined thickness of the rotor and the outer stator does not exceed 0.5 mm along the beam path. The typical sample volume required is about 2 mL. To avoid undesired shearing and associated structural changes, extreme care has to be taken while lowering the inner rotor into the stator. The cross-sectional view of the shear setup and its associated thermal environment is shown in Figure 1b. The availability of synchrotron beamtime being severely limited, rheology experiments performed under such constraints cannot be extensive. In particular, experiments at very low frequencies or shear rates could not be carried out because they require equilibration times that are too long. B. SAXS Measurements. The SAXS measurements were performed at the ID2 beamline of the European Synchrotron Radiation Facility in Grenoble, France.13 To minimize the air path, the X-ray beam is incident through a thin evacuated telescopic tube close to the cell; then it travels through the shear cell surrounded by the thermostatic environment. The transmitted and scattered beams enter the evacuated detector tube through a cone on the other side. The typical beam size at the sample position is 0.1 mm × 0.1 mm (full width at half-maximum, (13) Boesecke, P.; Diat, O.; Rasmussen, B. Rev. Sci. Instrum. 1995, 66, 1636.

fwhm). The experiments were performed with an incident wavelength (λ) of 0.099 51 nm, and the sample-to-detector distance was varied from 1 to 10 m. Two-dimensional SAXS patterns were recorded using an X-ray image intensifier (Thomson) lens coupled to a fast read-out CCD (FreLoN) camera, and appropriate corrections were carried out.14 For the sampleto-detector distance of 10 m, a useful scattering wave vector (q) range of 0.02 nm-1 e q e 1 nm-1 was obtained, where q is given by q ) (4π/λ) sin θ and 2θ is the scattering angle. The low-q limit is mainly determined by the parasitic scattering from the cell, which strongly depends on the material as well as the quality of machining (wall thickness, homogeneity, roughness, etc.). As a result, the cell has been selected from a given batch based on the transmitted intensity and parasitic scattering to ensure good reliability of background subtraction. Furthermore, the high incident flux (∼1013 photons/s) allows time-resolved experiments down to the millisecond range. Therefore, transient structural and rheological behaviors can be probed simultaneously.

3. Results and Discussion 3.1. Lamellar Gels of H3Sb3P2O14 Suspensions. Aqueous suspensions of Sb3P2O143- sheets, at volume fractions φ > 1.8% and at [NaCl] ) 10-4 M, are known to form liquid-crystalline phases of lamellar symmetry and gel-like mechanical properties.8 (Here, we define gels as viscoelastic dispersions that display a yield stress below which they do not flow and that are more elastic than viscous, that is, G′ . G′′ in the frequency range of interest.15) Indeed, the elastic modulus G′ (≈500 Pa) of these gels is about 1 order of magnitude larger than their loss modulus G′′ (≈50 Pa) (Figure 2) over the whole range of frequencies probed. Moreover, the gels show a yield stress (≈50 Pa) below which they do not flow. Mechanical shearing could easily produce aligned samples of this lamellar phase, which were examined by SAXS in both radial and tangential geometries. As expected, very anisotropic diffraction patterns were observed in the tangential geometry (Figure 3). This means that among (14) Narayanan, T.; Diat, O.; Boesecke, P. Nucl. Instrum. Methods Phys. Res. 2001, A467-468, 1005. (15) Warriner, H. E.; et al. Science 1996, 271, 969.

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Figure 4. SAXS patterns of a H3Sb3P2O14 sol of volume fraction φ ) 0.75%, in radial and tangential geometries, first at rest, then submitted to a shear stress of τ ) 23 Pa, and after the shearing was stopped.

the three possible different types of orientation of the layers in the shear cell (Figure 1a), the rather intuitive orientation c, in which the layers orient parallel to the shearing surfaces, is very strongly predominant.16 The stability of the aligned domain is proved by the fact that no relaxation was observed when the shearing was stopped. The simultaneous use of rheology and SAXS reveals the textural change that takes place around the yield point (Figure 3). In the absence of shear stress, the SAXS pattern, recorded in the tangential geometry, demonstrates the lamellar structure of the gel and shows that it was partially aligned in a more or less random direction when the shear cell was filled. As the stress increases, the SAXS pattern remains unchanged until the yield stress is reached, between 35 and 40 Pa, at which point the gel starts to flow. Then, the pattern suddenly changes as the Sb3P2O143- sheets align parallel to the shearing cylinders. This is only a textural change because the lamellar period does not vary; the material structure is actually not affected by the shear flow. However, in terms of rheology, this textural evolution is quite significant because the liquid-crystalline gel is now aligned with respect to the shearing geometry. The determination of the yield stress by rheology is often made difficult when the material is strongly shearthinning. The extrapolation at zero shear rate of the stress then becomes inaccurate. For instance, a fit of the rheological data in Figure 2b by the Herschel-Bulkley equation (τ ) τy + Kγ˘ n where τy is the yield stress, K is an effective viscosity, and n is an exponent that describes the nonideal behavior)17 gives yield stresses that vary between 10 and 40 Pa, depending on the mode of data acquisition and treatment. However, on-line rheometry combined with SAXS studies allowed us to determine more precise values of the yield stress on such model systems. (16) Safinya, C. R.; Sirota, E. B.; Bruisma, R. F.; Jeppesen, C.; Plano, R. J.; Wenzel, L. J. Science 1993, 261, 588. (17) Barnes, H. A.; Hutton, J. F.; Walters, K. An Introduction to Rheology; Elsevier: Amsterdam, 1989.

Figure 5. (a) Dependence on frequency of the storage modulus G′ and the loss modulus G′′ of a H3Sb3P2O14 sol of volume fraction φ ) 1.74%. (b) Flow curve showing the evolutions of the shear stress and the dynamic viscosity versus the shear rate for a H3Sb3P2O14 sol of volume fraction φ ) 1.74%.

More generally, this series of experiments clearly illustrate the fact that as soon as the yield stress is reached, the texture of the gel is severely modified, which should in turn deeply influence the mechanical properties of the material, possibly even resulting in an avalanche behavior. 3.2. Fluid Lamellar Phases of H3Sb3P2O14 Suspensions in Water. At volume fractions smaller than 1.8% and at [NaCl] ) 10-4 M, the suspensions form fluid lamellar liquid crystals.8 Very dilute lamellar phases (φ ) 0.75%) already align fairly well when they are inserted between the shearing cylinders at rest. This may be due either to the flow of the sample during this operation or to the homeotropic anchoring of the liquid crystal on the polycarbonate surface. This tendency is actually so strong that we found it impossible to fill the rheometer cell without achieving this strong alignment (Figure 4). Shearing the dilute phases improved their alignment, but an even better alignment was obtained when the shear was stopped and the sample was left to rest for a few minutes (Figure 4). This could be due either to a slight wobbling of the cell or to small flow instabilities that take place at high shear rates. The alignment can be quantified by the mosaic spread of the lamellar reflections, which is only about 10° fwhm. Applying a shear stress thus provides a better alignment of the phase than that observed by action of a magnetic field (about 20° fwhm).8 Actually, the mosaic spread obtained here by shear compares very well with those reported in the literature for thermotropic smectic phases aligned by shear or by a magnetic field.18 More concentrated lamellar phases (φ ≈ 1.7%) show a small yield stress (such suspensions nevertheless flow (18) (a) Hamley, I. W.; Davidson, P.; Gleeson, A. J. Polymer 1999, 40, 3599. (b) Kaganer, V. M.; Ostrovskii, B. I.; de Jeu, W. H. Phys. Rev. A 1991, 44, 8158.

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Figure 6. Series of SAXS patterns of a H3Sb3P2O14 sol of volume fraction φ ) 1.74% submitted to increasing shear stresses.

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Figure 8. Series of SAXS patterns of a H3Sb3P2O14 dilute suspension of volume fraction φ ) 0.12% submitted to increasing shear stresses.

Figure 7. Flow curve showing the evolutions of the shear stress and the dynamic viscosity versus the shear rate for a H3Sb3P2O14 dilute suspension of volume fraction φ ) 0.12%.

when the test tubes are held upside-down), but they cannot be called gels because their viscoelastic moduli are roughly equal over the whole range of frequencies probed (Figure 5a). These results and those obtained on suspensions at volume fractions φ > 1.8% confirm the presence of a sol/gel transition in the phase diagram of H3Sb3P2O14/ water at [NaCl] ) 10-4 M.8 The fluid lamellar phases (φ ≈ 1.7%) are also very shear-thinning since their viscosity drops from about 1 Pa‚s at shear rates around 30 s-1 to about 0.1 Pa‚s around 500 s-1 (Figure 5b) at φ ) 1.7%. Meanwhile, SAXS experiments showed that the phase aligns with respect to the flow upon increasing shear rate (Figure 6). This explains the shear-thinning behavior of these suspensions: Owing to their intrinsic anisotropy, lamellar liquid crystals have different viscosity coefficients according to the direction of the flow with respect to the lamellar periodicity axis.3 When the samples are not aligned, each of these viscosities is involved in the flow, depending on the proportion of domains aligned in a given geometry. As the sample aligns, one (or a linear combination) of the viscosities becomes more and more relevant and it is often the weakest one in order to minimize dissipation. Once the sample is aligned, its texture does not change any more, even if the shear stress is suppressed. This is the reason the texture must be very

Figure 9. (a) Dependence on frequency of the storage modulus G′ and the loss modulus G′′ of a HSbP2O8 gel of volume fraction φ ) 1.77%. (b) Flow curve showing the evolutions of the shear stress and the dynamic viscosity versus the shear rate for a HSbP2O8 gel of volume fraction φ ) 1.77%.

carefully controlled during rheology experiments on liquid crystals in order to obtain reproducible results.19 3.3. Dilute Suspensions of Sb3P2O143- Sheets in Water. At volume fractions lower than 0.75%, H3Sb3P2O14 suspensions spontaneously demix into the fluid lamellar liquid crystal and a supernatant that is essentially water.8 (19) See for instance: (a) Bartolino, R.; Durand, G. Phys. Rev. Lett. 1977, 39, 1346. (b) Cagnon, M.; Durand, G. Phys. Rev. Lett. 1980, 45, 1418. (c) Oswald, P. J. Phys. 1985, 46, 1255.

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Figure 10. Series of SAXS patterns of a HSbP2O8 gel of volume fraction φ ) 1.77% submitted to increasing shear stresses.

This corresponds to the lamellar phase maximum swelling beyond which any excess water is expelled. At volume fractions as low as 0.12%, freshly mixed suspensions are isotropic but strongly flow birefringent (if these suspensions were kept at rest for a long enough time, they would demix into a very small amount of lamellar phase and a very large amount of supernatant). They are also shearthinning (Figure 7). Such a behavior suggests that the very anisotropic particles tend to align in the flow even though their concentration is small. This phenomenon can be followed as well by SAXS with the rheometer in the tangential geometry (Figure 8). Under shear, quite anisotropic SAXS patterns are observed from which an effective nematic order parameter can be extracted by use of an already published analysis.20 The nematic order parameter at 990 s-1 is S ≈ 0.6, which is a rather large value for an isotropic phase when at rest. Even though the anisotropy appears here gradually, this phenomenon is somewhat reminiscent of the flow-induced nematic transitions that have been recently observed in several lyotropic suspensions and thermotropic liquid-crystalline polymers.21 Such a behavior also supports the considerable spatial extension determined by SAXS (at least 300 nm) and the rather large rigidity of these mineral sheets, even in the dilute regime. 3.4. Nematic Gels of HSbP2O8 Suspensions. Combined rheology/SAXS experiments have also been performed with aqueous suspensions of HSbP2O8. The parent monoacid HSbP2O8 was indeed found to fully exfoliate in (20) Lemaire, B. J.; Panine, P.; Gabriel, J. C. P.; Davidson, P. Europhys. Lett. 2002, 59, 55. (21) (a) Berret, J. F.; Roux, D. C.; Porte, G.; Lindner, P. Europhys. Lett. 1994, 25, 521. (b) Schmitt, V.; Lequeux, F.; Pousse, A.; Roux, D. Langmuir 1994, 10, 955. (c) Pujolle-Robic, C.; Noirez, L. Nature 2001, 409, 167.

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Figure 11. (a) Dependence on frequency of the storage modulus G′ and the loss modulus G′′ of a HSbP2O8 sol of volume fraction φ ) 0.44%. (b) Flow curve showing the evolutions of the shear stress and the dynamic viscosity versus the shear rate for a HSbP2O8 sol of volume fraction φ ) 0.44%.

water to yield a phase diagram very similar to that of H3Sb3P2O14 suspensions.22 Observation with the naked eye of a series of test tubes (not shown) containing suspensions of decreasing φ between crossed polarizers shows that (i) samples of high volume fraction (φ > 0.59%) form birefringent gels; (ii) for 0.19% < φ < 0.59%, fluid birefringent suspensions are observed; and (iii) for φ < 0.19%, suspensions are biphasic with a well-defined interface between a denser birefringent fluid phase and an isotropic phase. However, the birefringent phases, either fluid or gel, have SAXS patterns that never display the sharp reflections typical of a lamellar phase but show only diffuse rings instead. This means that HSbP2O8 suspensions show nematic phases rather than lamellar ones. At this moment, we are still unable to precisely identify the molecular parameters (electric charge density, thickness, flexibility, etc.) that control this behavior. The size of the SbP2O8- sheets (≈160 nm, extracted from the form factor of a very dilute suspension) is smaller than that of Sb3P2O143- sheets (>300 nm). Actually, HSbP2O8 suspensions are very similar to those of montmorillonite clays that also form nematic gels and that have been extensively studied by neutron scattering under shear.23 A crucial difference however lies in the fact that HSbP2O8 suspensions form nematic fluids at thermodynamic equilibrium as well as gels. Gels of SbP2O8- sheets behave in a way quite similar to the H3Sb3P2O14 gels already described. They show a yield stress of about 250 Pa at a volume fraction φ ) 1.77% (22) Camerel, F. Ph.D. Thesis, Universite´ de Nantes, Nantes, France, 2001. (23) Ramsay, J. D. F.; Lindner, P. J. Chem. Soc., Faraday Trans. 1993, 89, 4207.

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Figure 12. Series of SAXS patterns of a HSbP2O8 sol of volume fraction φ ) 0.44% submitted to increasing shear stresses.

Figure 13. Flow curve showing the evolutions of the shear stress and the dynamic viscosity versus the shear rate for a dilute suspension of Nb6O174- nanotubules of mass fraction 5.6%.

and G′ ≈ 2500 Pa, G′′ ≈ 250 Pa (Figure 9). The SAXS patterns in the tangential geometry reveal a partially aligned sample at rest that remains unaffected by the shear stress until the yield stress is reached. Then, the sample suddenly reorients at τ ) 220 Pa with the mineral sheets parallel to the shearing surfaces (Figure 10). The nematic order parameter at 230 Pa is S ) 0.45. Thus, the existence of a long-range positional order, absent in the nematic phase, does not seem to be relevant to the rheological behavior of these liquid-crystalline gels at the yield point. The important feature is that the anisotropic moieties be able to cooperatively realign with respect to the shear geometry. 3.5. Fluid Nematic Phases of HSbP2O8 Suspensions in Water. At small volume fractions (φ < 0.59%), suspensions of SbP2O8- sheets form fluid nematic phases (G′ ≈ G′′, Figure 11) with rheological properties quite

Camerel et al.

Figure 14. Series of SAXS patterns, in the tangential and radial geometries, of a dilute suspension of Nb6O174- nanotubules of mass fraction 5.6% submitted to increasing shear stresses.

comparable to those of Sb3P2O143- sheets. A random distribution of domains could not be achieved as the nematic phase always aligned in some preferred orientation upon filling the rheometer cell. However, shearing the suspensions resulted in the alignment of the nematic phase with the director perpendicular to the shearing cylinders (Figure 12). This alignment also explains the shear-thinning behavior of these suspensions. 3.6. Suspensions of Nb6O174- Nanotubules in Water. In contrast with the previously described suspensions that are comprised of sheets, the exfoliation of the lamellar compound K4Nb6O17, at pH 7, leads to nanotubules produced by scrolling of the mineral sheets.11 (At higher pH, flow uncoils the structure, and at lower pH, a flocculation of the colloids is observed.11 The pH of the supernatant was adjusted to 7 in this study to avoid these problems. A transmission electron microscopy analysis confirmed the well-defined tubular structure at pH 7 (results not shown). Besides, a liquid-crystalline phase of sheets has also been recently reported by exfoliation with propylamine hydrochloride.24) These nanotubules have outer diameters of about 25 nm and can be microns long. They were recently used to produce mesoporous composite materials by direct assembly of anisotropic hollow objects.25 At moderately low concentrations, suspensions of Nb6O174- nanotubules are optically isotropic but they show a very strong flow birefringence that is quite comparable to that of the dilute suspensions of H3Sb3P2O14. This suggests the existence of a liquid-crystalline phase at (24) Miyamoto, N.; Nakato, T. Adv. Mater. 2002, 14 (18), 1267. (25) Camerel, F.; Gabriel, J. C. P.; Batail, P. Chem. Commun. 2002, 17, 1926.

Colloidal Dispersions of Mineral Particles

higher concentrations. However, the suspensions flocculate when concentrated, probably because the particles are not charged enough at this neutral pH to overcome the attractions that appear when the ionic double layers are compressed. At rest, as expected, the SAXS patterns of homogeneous suspensions, of mass fraction 5.6%, of Nb6O174- nanotubules are isotropic. (The mass fraction is easier to use here than the volume fraction because of the presence of tetra-(n-butyl)-ammonium ions.) Several broad and diffuse peaks are observed, and their positions do not seem to depend on concentration. Then, these peaks are probably due to intraparticle interferences (i.e., form factor) and should be maximum in the plane perpendicular to the nanotubule axis. In rheology experiments, the suspensions show shear-thinning behavior (Figure 13). Meanwhile, the SAXS patterns (Figure 14) start displaying some anisotropy that increases with the shear rate. Although the orientation degree reached at the highest shear rate (≈1000 s-1, τ ) 10 Pa) was rather large (S ≈ 0.40) and comparable to that of a nematic phase, this evolution was gradual and no hint of a real isotropic/nematic phase transition under shear was found so far. This point is presently being investigated in more detail in our laboratories. 4. Conclusion This study has shown that the yield point of liquidcrystalline gels is clearly related to a strong textural change. At this stage, it is hard to determine if the gel

Langmuir, Vol. 19, No. 24, 2003 10035

flows, in an avalanche effect, because the particles reorient under the stress or if this reorientation is a mere consequence of the flow. A very different type of experiment that would strictly focus on the onset of the flow, at very low deformations, needs to be devised to elucidate this point. Besides, the shear-thinning behavior of liquidcrystalline or isotropic sols is a more gradual phenomenon that is indisputably related to the orientation of the platelike or rodlike particles. The situation here is quite similar to that of thermotropic liquid crystals for which the role of phase anisotropy was recognized long ago. Such studies will be helpful when macroscopic single domains, produced by shearing, are used to align and investigate anisotropic molecules under confinement and/or to synthesize highly organized composite hybrid materials. More generally, we hope to have illustrated that combined SAXS/rheometry can be a valuable technique to investigate the molecular origin of the flow behavior of complex fluids. In the future, it could also be used to examine in more detail shear-induced phase transitions in suspensions of anisotropic moieties. Acknowledgment. We are deeply indebted to B. J. Lemaire and T. Narayanan for useful discussions and for help during the synchrotron experiments. We also gratefully acknowledge ESRF for the award of beamtime SC728. In addition, special thanks are due to L. Goirand for his accurate mechanical design of the shear cell and to J. Gorini for very useful technical support. LA034626P