Anisotropic Water Diffusion in Nematic Self-Assemblies of Clay

Nov 8, 2006 - de Pernambuco, 50670-901 Recife, Pernambuco, Brazil, and Department of Physics, The Norwegian. UniVersity of Science and Technology, ...
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Langmuir 2007, 23, 5100-5105

Anisotropic Water Diffusion in Nematic Self-Assemblies of Clay Nanoplatelets Suspended in Water E. N. de Azevedo,† M. Engelsberg,*,‡ J. O. Fossum,§ and R. E. de Souza‡ Programa de Po´ s-Graduac¸ a˜ o em Cieˆ ncia de Materiais and Departamento de Fı´sica, UniVersidade Federal de Pernambuco, 50670-901 Recife, Pernambuco, Brazil, and Department of Physics, The Norwegian UniVersity of Science and Technology, N-7491 Trondheim, Norway ReceiVed NoVember 8, 2006. In Final Form: January 8, 2007 Diffusion-weighted magnetic resonance imaging provides a vivid description of the little understood role played by interfacial interactions with macroscopic bodies in the cooperative self-assembly of clay nanoplatelets suspended in water. The interfacial interaction between hydrophilic glass walls and clay platelets in a Na-fluorhectorite gel can produce, for dilute gels, a face-to-wall anchoring of the platelets that leads to a uniaxial nematic order with platelet faces parallel to the walls but with randomly distributed normals of the faces. The application of a magnetic field perpendicular to the walls transforms this uniaxial order to an extended biaxial nematic order with orthogonal alignment between normals and the field. Moreover, for apolar walls, this face-to-wall anchoring is considerably hindered, and the uniaxial nematic order can be substantially disrupted.

Introduction Colloidal suspensions of clay platelets exhibit a large variety of phenomena with far-reaching consequences1,2 that have attracted considerable attention, both from a scientific3-6 as well as a practical point of view.7 Considerable insight has been achieved through the study of clays of the hectorite family. Fluorhectorite (Fht) has a unit cell composition of Qx-(Mg6 - xLix)Si8O20F4 (x ) 1.2) where Q denotes a solvated monovalent exchangeable counterion such as Na+ or Li+. In Na-Fht (Q ) Na+), so-called 2:1 layers8 are formed by two inverted silicate sheets, sharing their apical oxygens with one octahedral sheet. These layers, separated by exchangeable ions and also water molecules, are stacked, yielding platelets that are typically 100 nm thick9 with effective diameters in the range of 1-10 µm, as revealed by atomic force microscopy measurements.10 This large diameter/thickness ratio makes diffusion-weighted MRI of water a very sensitive tool for monitoring the self-assembly of nanoplatelets in aqueous suspensions. Because for a nematic phase diffusion paths along directions parallel to the platelet faces should be considerably less tortuous than along perpendicular directions, diffusion-weighted contrast can be quite revealing. * To whom correspondence should be addressed. E-mail: mario@ df.ufpe.br. Phone: 55-81-2126-7626. Fax: 55-81-3271-0359. † Programa de Po ´ s-Graduac¸ a˜o em Cieˆncia de Materiais, Universidade Federal de Pernambuco. ‡ Departamento de Fı´sica, Universidade Federal de Pernambuco. § The Norwegian University of Science and Technology. (1) Hanczyc, M. M.; Fujikawa, S. M.; Szostak, J. W. Science 2003, 302, 618-622. (2) Morton, O. Science 2005, 309, 1320-1321. (3) Langmuir, I. J. J. Chem. Phys. 1938, 6, 873-896. (4) Forsyth, P. A., Jr.; Marcˇelia, S.; Mitchell, D. J.; Ninham, B. W. AdV. Colloid Interface Sci. 1978, 9, 37-60. (5) Mourchid, A.; Delville, A.; Lambard, J.; Le´colier, E.; Levitz, P. Langmuir 1995, 11, 1942-1950. (6) Mourchid, A.; Le´colier, E.; Van Damme, H.; Levitz, P. Langmuir 1998, 14, 4718-4723. (7) Bergaya, F., Theng, B. K. G., Lagaly, G., Eds. Handbook of Clay Science; Elsevier: London, 2006. (8) Kaviratna, P. D.; Pinnavaia, T. J.; Schroederer, P. A. J. Phys. Chem. Solids 1996, 57, 1897-1906. (9) Fossum, J. O.; Gudding, E.; Fonseca, D. d. M.; Meheust, Y.; DiMasi, E.; Gog, T.; Venkataraman, C. Energy 2005, 30, 873-883. (10) Gmira, A.; Fossum, J. O. Preprint (submitted to Nanotechnology), 2006.

An extensively studied member of the Hectorite family, which in some ways may be been considered to be a model clay, is Na-Laponite with a unit cell composition of Nax-(Mg6 - xLix)Si8O24H4. Unlike Na-Fht, each platelet of Na-Laponite is a mesoscopic monocrystalline disk consisting of a single 2:1 layer unit. Typically, each platelet is 1 nm thick with a diameter of approximately5 30 nm. Moreover, Na-Laponite has a relatively small internal layer charge of 0.4 e-/cell compared to Na-Fht where the value of this charge8 is 1.2 e-/cell. Since Onsager’s pioneering work,11 it has been known that colloidal suspensions of anisotropic charged particles, such as needles or disks, interacting via a hard core potential are expected to undergo a transition from an isotropic phase to a nematic phase characterized by long-range orientational order and shortrange positional order. In the particular case of clay platelets, it is well known that colloidal suspensions may also undergo a transition from a fluidlike sol to a solidlike gel. Evidence of a degree of ordering in clay gels was first reported by Langmuir,3 and later observations in gels of smectite clays confirmed these results, suggesting that the nematic order occurred only on a small length scale12 or in the form of threaded textures.6 Although earlier experiments were able to detect only an incomplete nematic transition in the gel phase of Laponite, more recent NMR pulsed field gradient (PFG) experiments13,14 as well as SAXS measurements15 have indicated that an extended nematic gel could be attained, although at considerably higher clay concentrations than in the phase diagram proposed by Mourchid et al.6 Such large concentrations in a Laponite gel could be achieved only after applying substantial uniaxial compression13,14 or by slow evaporation.15 In this work, we demonstrate that for rather dilute aqueous clay suspensions (approximately 3% w/w) extended face-toface biaxial nematic ordering can be achieved in a gel of Na-Fht (11) Onsager, L. Ann. N.Y Acad. Sci. 1949, 51, 627-659. (12) Gabriel, J.-C. P.; Sanchez, C.; Davidson, P. J. Phys. Chem. 1996, 100, 11139-11143. (13) Porion, P.; Rodts, S.; Al-Mukhtar, M.; Fuge`re, A. M.; Delville, A. Phys. ReV. Lett. 2001, 87, 2083021. (14) Porion, P.; Rodts, S.; Al-Mukhtar, M.; Fuge`re, A. M.; Delville, A. Eur. Phys. J. E 2003, 12, S18-S20. (15) Lemaire, B. J.; Panine, P.; Gabriel, J. C. P.; Davidson, P. Europhys. Lett. 2002, 59, 55-6.

10.1021/la0632629 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/22/2007

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Figure 1. (A) Gravity-induced separation observed in a relatively dilute (3% w/w) aqueous suspension of Na-Fht containing 10-310-4 M NaCl. Three distinct strata are schematically shown. (B) Reference frame, unit vectors, and rotation angles employed in the definition of the diffusivity tensor components. B B0 denotes the applied magnetic field.

platelets under the action of a magnetic field and an interfacial wall potential that promotes face-to-wall anchoring of the platelets. No uniaxial compression or other external forces are needed. Not only is this self-assembly mode in Na-Fht quite different from that of Na-Laponite, but also, as a result of the large layer charge of Na-Fht, the behavior is strikingly different from what would be expected from short-range repulsive forces alone.16 The role of interfacial interactions in a polar medium17 upon the self-assembly process of Na-Fht platelets is further tested by examining the effect of apolar walls instead of hydrogen-bondforming hydrophilic glass walls. Given the current interest in the self-assembly of nanoparticles, the present results, obtained using diffusion-weighted MRI, are especially appealing because they clarify in a graphic and direct way new aspects of the complex cooperative assembly process in a suspension of polydisperse and relatively large clay platelets, thus suggesting ways of controlling it. Experimental Details An MRI system (Varian Inova), including a 2.0 T magnet with a 30 cm bore, was employed along with magnetic field gradients G B with amplitudes of up to 0.2 T/m that could be applied in any desired spatial direction. The diffusion-weighted images were obtained with a gradient amplitude of G ) 0.15 T/m and a field of view of 50 mm × 50 mm. A 64 × 64 matrix was used in all images except for the image in Figure 6A, where the matrix size was 128 × 128 and slice selection was employed with a slice width of 4 mm. The sample preparation procedure is described in refs 9 and 18. Synthetic fluorhectorite in powder form (Corning Inc., NY) was first ion-exchanged to produce Na-Fht. Aqueous suspensions having 3% (w/w) clay and NaCl concentrations ranging from 10-4 to 10-3 M were prepared and sealed in 10-mm-diameter common glass tubes, with 0.2 to 0.3 mm walls, typically containing 8 µmol/m2 OH-. After vigorous shaking and subsequent settling for a period of at least 1 week, the samples were ready for MRI measurements.

Results and Discussion Diffusion-weighted contrast was achieved using a StejskalTanner19 pulse sequence in conjunction with a standard spin-echo imaging sequence. The normalized decay of the spin-echo amplitude Ee(q, ∆) when two magnetic field (16) Harnau, L.; Dietrich, S. Phys. ReV. E 2002, 65, 021505. (17) van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Chem. ReV. 1988, 88, 927-941. (18) Fonseca, D. M.; Me´heust, Y.; Fossum, J. O.; Knudsen, K. D.; Måløy, K. J.; Parmar, K. P. S. Preprint (submitted to J. Appl. Crystallogr.), 2006. (19) Stejskal, E. O.; Tanner, J. E. J. Chem. Phys. 1965, 42, 288-292.

Figure 2. Semilogarithmic plots of the decay of the spin-echo amplitude as a function of the square of the applied magnetic field gradient amplitude G2 in a dilute Na-Fht suspension with negligible alignment by the magnetic field B B0. G B is applied along (100) (b), (010) (9), (001) (4), (110) (3), (011) ([), and (101) (0) for the three phases of Figure 1: (A) isotropic, (B) nematic, and (C) sediment.

gradient pulses separated by a time interval of ∆ are applied is given by19,20

(

(

Ee(q, ∆) ) exp -(2πq)2 eˆ ‚D 6 ‚eˆ ∆ -

δ 3

))

(1)

where the gradient G B ) Geˆ is assumed to be oriented along the direction specified by the unit vector eˆ and D 6 represents a Cartesian diffusivity tensor. The duration of the gradient pulses is denoted by δ, and the wavevector b q is defined by b q ) γG B δ/2π, where γ is the proton gyromagnetic ratio. In a suspension containing 3% (w/w) Na-Fht clay and 10-310-4 M NaCl, gravity is able to sort out various particle sizes stabilizing well-defined strata9 within a single container. These include an isotropic phase, a nematic gel, and partially flocculated sediment. Such gravity-induced phase separation is not readily observable in nearly monodisperse Na-Laponite. Moreover, earlier synchrotron X-ray diffraction measurements9 from gravitydispersed aqueous suspensions of Na-Fht such as those considered here had indicated nematic ordering in the gel phase with platelets aligned with their faces parallel to the tube walls. Figure 1 shows schematically the orthogonal axes, relative to the sample container and the magnetic field B B0, employed to (20) Callaghan, P. T. Principles of Nuclear Magnetic Resonance Microscopy; Clarendon Press: Oxford, England, 1991.

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Figure 3. (A) Polar plots of the contracted diffusivity tensor eˆ R‚(D 6 /D0)‚ eˆ R ) D(eR)/D0 obtained from the data of Figure 2B as a function of angles R ) φ (O), ψ (]), and θ (0) defined in Figure 1. D0 ) 2.45 × 10-5 cm2/s denotes the diffusivity of water at 25 °C. (B) Schematic representation of the self-assembly mode prevailing in the nematic gel phase of a dilute Na-Fht suspension contained within glass walls assuming negligible magnetic field-induced alignment. In the highlighted region near the wall, the alignment is predominantly face-to-wall. (C-E) Diffusion-weighted images with negligible alignment by the magnetic field B B0 showing projections on the (x, y) plane of the three phases of Figure 1. Magnetic field gradients are along the directions z (C), x (D), and y (E).

define the components of the diffusivity tensor D 6 . During a time interval of δ ) 7 ms in which a magnetic field gradient G B ) Geˆ is applied, the z component of the magnetic field varies as Bz ) B0 + Geˆ ‚r b, where b r is the position vector of the observation point. The six independent components of the symmetrical tensor D 6 can be determined from measurements of Ee(q, ∆) for a fixed value of ∆ ) 40 ms by varying the gradient amplitudes along six noncollinear directions. To that end, unit vectors eˆ are chosen along the three axes (001), (010), and (001) of Figure 1 and also along the three diagonals (110), (101), and (011). The spin-echo decays as a function of G2 shown in Figure 2 were obtained from diffusion-weighted images of a suspension with a NaCl ionic strength of 3.0 × 10-4 M, where proton densities from the three regions shown in Figure 1A were projected onto the (x, y) plane. Results for other salt concentrations in the range of 10-3-10-4 M did not show major differences. After preparation and settling, the suspension was placed in a 2.0 T magnetic field, and the measurements were performed without any significant time delay. It is apparent from Figure 2 that only one of these regions exhibits appreciable anisotropy in the diffusivity of water protons. For G B along (010), the diffusivity appears to have its largest value whereas for G B in the (x, z) plane it is the smallest. Furthermore, there is practically no difference among (100), (001), and (101) decays. This confirms that extended nematic ordering exists for this Na-Fht sample with the platelets oriented with their faces parallel to the tube walls.18 However, because the normals of the platelet faces are isotropically oriented in the (x, z) plane, this type of self-assembly is still uniaxial. The above conclusion is further confirmed by the polar plots in Figure 3A, obtained from the data of Figure 2B through eq 1 with unit vectors eˆ θ in the (x, y) plane, eˆ φ in the (x, z) plane, and eˆ ψ in the (y, z) plane. The polar plot corresponding to a magnetic field gradient along unit vector eˆ φ can be seen to be a circle indicating that D(eφ)/D0, where D0 ) 2.45 × 10-5 cm2/s denotes the self-diffusion coefficient of water at 25 °C, is isotropic

on the (x, z) plane. However, on the (y, z) and (x, y) planes D(eθ)/D0 and D(eψ)/D0 have their maximum values for θ ) 0 and ψ ) 0, respectively. In Figure 3C-E, typical diffusion-weighted images of the same sample are presented showing projections onto the (x, y) plane for gradients G B x, G B y, and G B z along the three orthogonal directions of Figure 1B. The larger diffusivity in the nematic gel region for the G B y gradient leads to a reduced signal amplitude in Figure 3E. Moreover, when the gradient is applied along the x or z direction, the reduced diffusivity leads to larger signal amplitudes that, as expected from the isotropy of Figure 2A, have approximately the same value in part D of Figure 3 as in part C. It is interesting that the signal amplitude in the dense but isotropic sediment region of Figure 3D,C is lower than in the corresponding nematic phase regions. This is consistent with the measured diffusivities along the x and z directions of Figure 2B and the results of Figure 2C. It indicates that the tortuosity of the diffusion path along the x and z directions in the nematic phase is more effective in reducing the diffusivity than the hindered motion in the more dense sediment. Figure 3B shows a schematic representation of the selfassembly mode prevailing in the nematic gel phase of a dilute Na-Fht suspension contained within the glass walls, assuming negligible magnetic field-induced alignment. In contrast to Figure 2, the spin-echo decays in Figure 4 correspond to a clay suspension, with NaCl ionic strength of 3.0 × 10-4 M, which was allowed to stay in a 2.0 T magnetic field perpendicular to the glass walls for a period of 36 h before performing the measurements. This was sufficient to permit the alignment effect of the magnetic field to be fully effective, as suggested by Figure 5. In Figure 5C,E, the diffusivities along the y and z directions are now comparable and both are appreciably larger than the diffusivity along the x direction. The images in Figure 5 indicate that the difference in diamagnetic susceptibility between directions

Water Diffusion in Nematic Self-Assemblies

Figure 4. Semilogarithmic plots of the decay of the spin-echo amplitude as a function of the square of the applied magnetic field B0-induced alignment in a 2.0 T field gradient amplitude G2. Full B has been achieved in this dilute Na-Fht suspension. G B is applied along (100) (b), (010) (9), (001) (4), (110) (3), (011) ([), and (101) (0) for the three phases of Figure 1: (A) isotropic, (B) nematic, and (C) sediment.

parallel and perpendicular to the platelet face21 is, as in other clays,22,23 sufficiently large and positive to promote the cooperative orientation of the Na-Fht platelets with the normals of the faces perpendicular to the magnetic field. Moreover, because of the size selection due to clay sedimentation, this cooperative effect appears not to be effective in the isotropic phase; consequently, no anisotropy in the diffusivity is observed for that phase in Figures 4A and 5C-E. The data demonstrate that extended biaxial nematic face-to-face self-assembly has been obtained as depicted schematically in Figure 5B. In contrast, a 7.0 T magnetic field applied along the cylindrical axis had no effect on the self-assembly mode. A different and subtler effect also emerges from the diffusionweighted images. When a gradient G B y is applied, in images showing projections onto the (x, z) plane, two faint, narrow (∼1 mm) regions of transition between the gel phase and the two other phases are always observed, as shown in Figures 3E and 5E. Moreover, the transition regions are not observed at all when a gradient G B z is applied for suspensions with biaxial nematic (21) Takeuchi, T.; Nakaoka, Y.; Emura, R.; Higashi, T. J. Phys. Soc. Jpn. 2002, 71, 363-368. (22) Takahashi, T.; Ohkubo, T.; Ikeda, Y. J. Colloid Interface Sci. 2006, 299, 198-203. (23) Delville, A.; Grandjean, J.; Laszlo, P. J. Phys. Chem. 1991, 95, 13831392.

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order, as shown in Figure 5C. This suggests that, at the interfaces with the nematic gel phase, the orientation of the platelets in the two isotropic phases both above and below the nematic phase is largely of the face-to-edge type.24 The polar plot of Figure 5A, obtained from the components of the diffusivity tensor determined from the data of Figure 4B, corroborates the formation of an extended face-to-face biaxial nematic self-assembly. For gradients in the (y, z) plane, where the direction is determined by eˆ ψ, the polar plot is approximately circular. For gradients in the (x, z) or (x, y) plane, directed respectively along eˆ φ and eˆ θ, the plots reveal two orthogonal lobes with substantial anisotropy of comparable size. Diffusion-weighted magnetic resonance imaging can provide, within resolution limits determined by the prevailing signal-tonoise ratio, relaxation rates and magnetic field gradient strengths,20 which provide a vivid description of the self-assembly mode of colloidal platelets. In this article, the role played by interfacial interactions between mesoscopic clay platelets and macroscopic walls17 in the cooperative self-assembly process has been examined in considerable detail. For purely repulsive forces between hard colloidal platelets and hard walls, density-functional theory calculations16 predict that the platelet faces must adopt, very close to the walls, a fully parallel alignment. However, at distances of approximately only one platelet radius a change in the order parameter to perpendicular alignment is expected. Although more recently synthesized hard colloidal platelets, such as gibbsite,25 suspended in nonpolar solvents may approach the conditions of short-range repulsive forces described in Onsager’s theory,9 the behavior of Na-Fht platelets is strikingly different, emphasizing the dominant role of polar interactions. It should be pointed out that precisely these interactions are frequently responsible for some of the most interesting properties of clays. The effect of interfacial interactions in a dilute Na-Fht nematic gel is to anchor the platelets with their faces parallel to glass tube walls, as expected for hard colloidal platelets with hard walls. However, unlike hard colloidal platelets, the parallel face to wall alignment in Na-Fht can persist, in the absence of an applied magnetic field, at large distances from the wall. This behavior is likely to be caused by the large internal layer charge in Na-Fht. The effect of electric charges in the platelets and in the ionic medium upon the structure and gelation properties of clay suspensions has been examined by a Monte Carlo simulation24 where the co-ions and counterions as well as the electric double layer around the charged platelets are substituted by a rigid point quadrupole. Within the framework of such an approximation, it is conceivable that additional multipoles and realistic boundary conditions at the walls would have to be included in order to explain the observed behavior in Na-FHt. The competition between face-to-face alignment between platelets and face-to-wall alignment eventually leads, at comparatively large distances from the walls, to frustration and randomness but still preserves the uniaxial nematic order. Figure 6A shows axial images obtained with slice selection in the (x, z) plane in a suspension where the 2.0 T magnetic field has been applied for too short a period of time to cause significant alignment. The two images in Figure 6A correspond to the same situation in Figure 3C,D but with transverse slice selection through the nematic gel region only rather than longitudinal slice selection through the three regions as in Figure 3. The almost identical signal amplitudes in Figure 6A for B z indicate that for this uniaxial nematic ordering gradients G B x and G (24) Dijkstra, M.; Hansen, J.-P.; Madden, P. A. Phys. ReV. E 1997, 55, 30443053. (25) van der Kooij, F. M.; Lekkerkerker, H. N. W. J. Phys. Chem. B 1998, 102, 7829-7832.

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Figure 5. (A) Polar plots of the contracted relative diffusivity tensor eˆ R‚(D 6 /D0)‚ eˆ R ) D(eR)/D0 as a function of angles R ) φ (O), ψ ()), and θ (0) in the nematic gel phase of Na-Fht exhibiting biaxial nematic order after full B B0-induced alignment. (B) Schematic representation of the resulting self-assembly mode in Na-Fht showing face-to-face alignment of platelets with normals of the faces perpendicular to the magnetic field. (C-E) Diffusion-weighted images after full B B0-induced alignment with magnetic field gradients applied along the directions z (C), x (D), and y (E).

Figure 6. (A) Transverse diffusion-weighted images showing a 4 mm slice selected in the (x, z) plane of the nematic phase region of Figure 1A. The alignment caused by the magnetic field in this NaFht suspension was negligible, and the magnetic field gradients were applied along x and z. (B) Simulated images assuming a selfassembly mode such as that depicted schematically in Figure 3B.

the normals of the faces in the bulk are randomly distributed in the (x, z) plane. However, a different situation prevails near the tube walls, where for a gradient along the z axis, for example, only a narrow arc where the angle Φ between G B z and the radial vector (see Figure 3B) is relatively small exhibits substantially reduced diffusivity compared to that of the bulk. Similar behavior can be observed when the gradient is applied along the x axis.

This suggests that at unexpectedly large distances from the glass walls the Na-Fht platelets must be aligned with their faces predominantly parallel to the tube walls. Because for a faceto-wall alignment a more tortuous path would be expected for water molecules diffusing in directions perpendicular to the walls than in parallel ones, the brightness of the arc should be greatly reduced at places where the angle between the gradient direction and the normal to the walls increases. The above interpretation is reinforced by the simulations of Figure 6B, showing the effect of anchoring by the glass wall on the diffusion-weighted MR images of Figure 6A. The simulated images were calculated from eq 1 using parameters δ, ∆, and G employed in the actual measurement. For the contracted diffusivity tensor eˆ ‚(D 6 /D0)‚eˆ ) D(eφ)/D0, it was assumed that the effect upon diffusivity of face to wall alignment could be described by the same diffusivity tensor prevailing when the alignment of the platelets is caused by the magnetic field. To that end, the experimental data of Figure 5A with the angular variable Φ (Figure 3B) substituted for the angle π/2 - φ in Figures 1B and 5A was used. Furthermore, a gradual transition of eˆ ‚(D/D0)‚ eˆ from its actual value of D(eφ)/D0 near the wall, to its angular average value of 〈D(eφ)/D0〉φ ≈ 0.6 at larger distances was introduced. To that end, a weighted average of these two extreme values was adopted using, for simplicity, Gaussian weighting functions. Denoting by r0 the radius of the cylindrical container and by 0 e r e r0 the radial distance of an arbitrary point, a weighting function of form W(r) ) exp((-1/2σ2)[(1/r0) - (1/r)]2) was chosen for D(eφ)/D0 with a corresponding weighting function of 1 - W(r) for the isotropic part 〈D(eφ)/D0〉φ. In the simulations of Figure 6B, the best fit to the data was obtained with a standard deviation of σ ) 1/10r0, indicating that the width of the region near the wall where substantial (weight factor 61%) face-to-wall alignment exists is approximately r0/11. With apolar Teflon walls instead of glass walls, the transition region from parallel alignment near the walls to random alignment

Water Diffusion in Nematic Self-Assemblies

is sharply reduced and becomes barely observable in images such as those of Figure 6A. Moreover the uniaxial nematic ordering, as revealed by images such as those in Figure 3C-E, is substantially disrupted. This further emphasizes the role of polar interfacial interactions in the self-assembly mode of clay platelets.

Conclusions We have demonstrated, using diffusion-weighted MRI, that for dilute aqueous clay suspensions (approximately 3% w/w), extended, face-to-face, biaxial nematic ordering can be achieved in a gel of Na-Fht platelets under the action of a magnetic field and an interfacial wall potential that promotes face-to-wall anchoring of the platelets. This self-assembly mode is quite different from that prevailing in Na-Laponite. Moreover, as a result of the comparatively large internal layer present in Na-

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Fht, its behavior is strikingly different from what would be expected from short-range repulsive forces alone. The important role of interfacial interactions upon the self-assembly process of Na-Fht platelets is further confirmed by MRI experiments in colloidal suspensions in contact with apolar walls. Acknowledgment. This work was supported by Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico CNPQ (Brazilian agency) and by the Research Council of Norway (RCN) through the SUP, NANOMAT, and FRINAT programs. Supporting Information Available: Diffusion-weighted images of the nematic-phase region (transverse) and of the three phases (longitudinal) of a Na-Fht suspension within a cylindrical container with Teflon walls and no applied magnetic field. This material is available free of charge via the Internet at http://pubs.acs.org. LA0632629