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Probing the Morphology of Laponite Clay Colloids by Atomic Force Microscopy Eric Balnois,† Serge Durand-Vidal,† and Pierre Levitz*,‡ Liquides Ioniques et Interfaces Charge´ es (CNRS - UMR 7612), Universite´ Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France, and Physique de la Matie` re Condense´ e (CNRS - UMR 7643), Ecole Polytechnique, Route de Saclay, 91128 Palaiseau Cedex, France Received January 20, 2003. In Final Form: May 20, 2003 This paper describes a direct and quantitative study of the size and shape of Laponite clay colloids by atomic force microscopy. Atomic force microscopy (AFM) images of Laponite particles deposited on mica, obtained under ambient conditions, reveal that the particles are mainly present as individual entities. Morphological quantitative analysis of the AFM images indicates that the individual particles have an anisotropic shape with a height of 1.2 nm and mean lateral dimensions of 24.0 ( 6.9 nm and 16.8 ( 4.9 nm. Furthermore, the height histogram indicates that 20% of the particles on the mica are dimers conferring to the system a height polydispersity index of 1.18. Thereafter, AFM results were used to simulate a small-angle neutron scattering (SANS) curve and good agreement was obtained with an experimental SANS curve. This work demonstrates the efficiency of the AFM technique for the characterization of Laponite colloids and nanoparticles in general and its complementarity with neutron scattering techniques.
I. Introduction In recent years, aqueous suspensions of charged platelike colloids have been the subject of intense investigations. Laponite, a synthetic smectite clay, undergoes puzzling liquid-solid transitions at low particle concentrations.1-17 The status and the origin of these transitions (Wigner glass transition, frustrated nematic transition, gelation) are still a matter of controversy. For example, it has been proposed that Laponite particles could flocculate due to electrostatic attraction between their oppositely charged faces and edges, reinforced by van der Waals interactions, leading to a three-dimensional network (“house of cards” structure).9 It has also been proposed that Laponite colloids can form an oriented network, due to the electrostatic repulsion between the overlapping double layers and † ‡
Universite´ Pierre et Marie Curie. Ecole Polytechnique.
(1) Laponite: Structure, chemistry and relationship to natural clays; Laponite technical bulletin L104/90/A; Laporte I. Ltd.: Cheshire, U.K., 1990; pp 1-15. (2) Avery, R. G.; Ramsay, J. D. F. J. Colloid Interface Sci. 1986, 109, 448. (3) Rosta, M.; von Gunten, H. R. J. Colloid Interface Sci. 1990, 134, 397. (4) Ramsay, J. D. F.; Lindner, P. J. Chem. Soc., Faraday Trans. 1993, 89, 4207. (5) Thompson, D. W.; Butterworth, J. T. J. Colloid Interface Sci. 1992, 151, 236. (6) Morvan, M.; Espinat, D.; Lambard, J.; Zemb, T. Colloids Surf., A 1994, 82, 193. (7) Mourchid, A.; Delville, A.; Lambard, J.; Lecolier, E.; Levitz, P. Langmuir 1995, 11, 1942. (8) Gabriel, J. C. P.; Sanchez, C.; Davidson, P. J. Phys. Chem. 1996, 100, 11139. (9) Pignon, F.; Magnin, A.; Piau, J. M.; Cabane, B.; Lindner, P.; Diat, O. Phys. Rev. E 1997, 56, 3281. (10) Mourchid, A.; Levitz, P. Phys. Rev. E 1998, 57, 4887. (11) Kroon, M.; Vos, W.; Wegdam, G. H. Phys. Rev. E 1998, 57, 1962. (12) Mourchid, A.; Lecolier, E.; Van Damme, H.; Levitz, P. Langmuir 1998, 14, 4718. (13) Bonn, D.; Tanaka, H.; Wegdam, G.; Kellay, H.; Meunier, J. Europhys. Lett. 1998, 45, 52. (14) Levitz, P.; Lecolier, E.; Mourchid, A.; Delville, A.; Lyonnard, S. Europhys. Lett. 2000, 49, 672. (15) Nicolai, T.; Cocard, S. Langmuir 2000, 16, 8189. (16) Lecolier, E. Ph.D. Thesis, University of Orle´ans, Orle´ans, France, 1998. (17) Grillo, I.; Levitz, P.; Zemb, T. Eur. Phys. J. B 1999, 10, 29.
excluded volume interaction induced by the specific anisotropy of the particles [e.g., refs 4, 6, and 7]. Determination of an accurate particle morphology (size and polydispersity) is certainly a key parameter that is required in order (i) to correctly interpret interparticle interactions (dispersive versus electrostatic potential), (ii) to evaluate the role of the particle anisotropy in the observed nematic-like textures,8,12 and (iii) to estimate the contribution of particle polydispersity in a possible size segregation process. In any case, the morphological properties of these platelets should be considered, whatever will be the theoretical description of these puzzling colloidal suspensions. The morphology of Laponite colloids has been mainly deduced from small-angle scattering experiments (X-ray, SAXS; neutron, SANS), in which the scattering intensity displays a q-2 dependence followed by a plateau, typical of the form factor corresponding to randomly oriented platelike colloids with a thickness of approximately 1 nm and lateral dimensions of 25-40 nm [e.g., refs 2, 4, 6, and 7]. Nonetheless, in each of the above studies, the determination of the particle morphology is not direct and implies some assumptions about the shape and polydispersity of the particles. For example, Avery et al.2 and Ramsay et al.4 hypothesized that Laponite particles were individual disklike particles but they did not consider any polydispersity nor lateral anisotropy. More recently, Kroon et al.11 have fitted their SAXS data by assuming that the distribution of particle radii was Gaussian, with an average radius of 12.5 nm, a standard deviation of 4 nm, and a thickness of 1.0 nm. Other techniques such as static and dynamic light scattering have also been used to examine the structure and polydispersity of Laponite colloids. Results obtained by these techniques demonstrated that Laponite colloids in the diluted regime could be polydisperse. For example, Rosta and von Gunten3 showed that Laponite particles disperse in the form of individual particles and tactoids composed of two to four superposed single platelets with a lateral dimension of about 30 nm and an average molecular weight of 3000 ( 500 kg mol-1. Nicolai et al.15 demonstrated that at low concentration of 0.25 and 5 g L-1, Laponite was mainly found as individual disks with
10.1021/la0340908 CCC: $25.00 © 2003 American Chemical Society Published on Web 07/03/2003
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some dimers, trimers, and even oligomers, thus conferring to the solution a relatively large size distribution. Nonetheless, the scattering experiments mentioned above are slightly controversial since such techniques provide only an indirect way to probe the morphology of individual particles and consequently, when size polydispersity and a shape distribution are present, the results are highly model dependent. More direct techniques, such as electron transmission microscopy (TEM), have also been used.5,18-20 However, TEM micrographs found in the literature are, in many cases, hardly convincing and some authors even recognize the difficulty of unambiguously identifying these particles as discrete crystals [e.g., ref 5]. It is not surprising that TEM on such systems is rather complicated, since highresolution observations of Laponite colloids are mainly limited by the absence of suitable techniques for sample preparation (i.e., the absence of an induced restructuration and/or flocculation of the particles during the sample preparation technique) and by the low electron density of the particles (only ∼1 nm thick). Because of its high resolution and the possibility of imaging individual particles, atomic force microscopy (AFM) has become an important technique for the characterization of nano-objects, since it provides direct information with respect to local properties. This technique has been already used to characterize several clay particles such as, for example, bentonite,21 hectorite [e.g., ref 22], illite/smectite,23 and kaolinite [e.g., ref 24]. In the present work, we attempt to employ for the first time AFM on Laponite in order to complement our knowledge on the morphology and polydispersity of these particles in a dilute regime. In the first part of this paper, the size and the shape of Laponite particles that were deposited on a mica surface from diluted solutions were analyzed using AFM and the results were discussed with respect to the limitations of the technique. In the second part, smallangle neutron scattering curves were simulated using AFM structural information and quantitatively compared with experimental data. From the comparison of the calculated and experimental SANS curves, an objective description of Laponite individual particles is given. II. Experimental Section Laponite RD (Laporte Inc., U.K.) is a synthetic trioctahedric hectorite clay, composed of two tetrahedral silica sheets and a central octahedral magnesia sheet.1 Its chemical composition is Si8Mg5.45Li0.4H4O24Na0.7. These particles bear a structural negative charge that is balanced by Na+ counterions located around the particles. Positive or negative charge can appear on edge sites due to amphoteric surface groups such as -MgOH, -LiOH, or -SiOH. A 1000 mg L-1 stock solution of Laponite was prepared in demineralized water (MilliQ-Plus) at pH 10 (by adding NaOH) and equilibrated for 24 h prior to further dilution. Following equilibration for 24 h, the solution was diluted to 10 mg L-1 in water at pH 10 and filtered through a 0.2 µm Nuclepore filter. Five microliters of solution was pipetted onto a 1 cm2 piece of (18) van Olphen, H. An Introduction to Clay Colloid Chemistry; Wiley and Sons: New York, 1977. (19) Norrish, K. Discuss. Faraday Soc. 1954, 18, 20. (20) Neumann, B. S. Rheol. Acta 1965, 4, 250. (21) Plaschke, M.; Ro¨mer, J.; Kim, J. I. Environ. Sci. Technol. 2002, 36, 4483. (22) Bickmore, B. R.; Hochella, M. F., Jr.; Bosbach, D.; Charlet, L. Clays Clay Miner. 1999, 47, 573. (23) Lindgreen, H.; Garnaes, J.; Hansen, P. L.; Besenbacher, F.; Laegsgaard, E.; Stensgaard, I.; Gould, S. A. C.; Hansma, P. K. Am. Mineral. 1991, 76, 1218. (24) Vaz, C. M. P.; Herrmann, P. S. P.; Crestana, S. Powder Technol. 2002, 126, 51.
Balnois et al. freshly cleaved mica. The solution was then allowed to evaporate in an enclosed Petri dish under ambient conditions (∼60% relative humidity). Images were recorded in Tapping Mode (TM-AFM) using a Nanoscope III multimode scanning probe microscope (Digital Instruments). In Tapping Mode,25 the cantilever oscillates at its resonance frequency (typically 200-400 Hz in air), so that the tip interacts very briefly with the surface during each oscillation cycle with a small amplitude (A ∼ 10 nm). The reduction of the cantilever oscillation from its set point value, due to interactions between the AFM tip and the sample during the scan, is used to determine the topography of the surface. To minimize the forces of interaction, the ratio of the set point value to the free amplitude of the cantilever was maintained at approximately 0.9 by adjusting the vertical position of the sample. Images were recorded with a resolution of 512 × 512 pixels and a scan rate of 0.5-0.8 Hz. Height measurements were performed by using a homemade program. For the determination of the lateral dimensions of the particles, AFM images, in the amplitude mode, were digitalized using Scion image analysis software (Vbeta3b, based on the NIH image software, Scion Corp.). The major and minor length axes were then determined using the software. Height and length polydispersity of the Laponite particles were evaluated using the S(A)/N(A) ratio where S(A) and N(A) represented the weightand number-average dimensions of the particles, respectively, according to eqs 1 and 2:
∑n A i
S(A) )
2
i
i
∑
(1) niAi
i
∑n A i
N(A) )
i
i
∑n
(2) i
i
where ni is the number and Ai is the height or lengths of the particle i. SANS experiments were performed on the instrument D11 (ILL, Grenoble). A similar sample preparation was used as formally described: a diluted suspension of Laponite RD (8 g L-1) was prepared in demineralized water (H2O MilliQ-Plus) at pH 10. At this ionic strength (10-4 M), the diluted suspension is liquidlike and below the glass transition.14 Mutual particle interactions are weak, and the experimental SANS spectrum can be considered as a good measure of the average particle form factor.12,17 Experimental details can be found elsewhere.17
III. Results and Discussion A. AFM Experiments. Figure 1 shows representative AFM images of Laponite particles that have been deposited from a 10 mg L-1 diluted solution (Figure 1). The image reveals that at this concentration and low ionic strength (10-4 M), Laponite particles appear mainly as isolated particles homogeneously distributed on the surface. The very few aggregates found on mica (images not shown) were deliberately not taken into account in the size distribution calculations since the objective of this work was to study the morphology and polydispersity of individual Laponite particles. Figure 2A shows a cross section of an individual Laponite particle deposited on mica. The vertical distance (Z) between the cursors is 1.0 nm. The height distribution of the Laponite particles measured on five different zones on different mica preparations is given in Figure 3. On each zone, approximately 450 particles were measured (a total of 2167 (25) Zhong, Q.; Innis, D.; Kjoller, K.; Elings, V. B. Surf. Sci. Lett. 1993, 290, L688.
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Figure 3. Height histogram of Laponite particles (C ) 10 mg L-1, pH ) 10).
Figure 1. Typical TM-AFM images of Laponite particles (C ) 10 mg L-1, pH ) 10) deposited on mica. The scan size is 370 nm × 370 nm.
Figure 2. Profile analysis of an individual Laponite particle. (A) A height of 1.0 nm is shown by the two arrows. (B) The major and minor length distances of the Laponite particles are 35.5 and 17.4 nm, respectively.
particles measured). No significant differences were observed among heights measured both in different zones of the same preparation or between different sample preparations, indicating a good reproducibility in the AFM observation and in the sample preparation. The histogram exhibits two peaks that show maxima at 1.2 and 2.1 nm. The peaks indicate that 80% of the Laponite particles are individual particles with a typical height of 1.2 nm while the remaining particles have a typical height of 2.1 nm, presumably dimers. The presence of individual particles with heights of about 1 nm is in good agreement with neutron and X-ray scattering measurements2,7,11 and with crystallographic data.1 The small difference in heights obtained by AFM (Z ) 1.2 nm) in comparison with neutron scattering (Z ) 1 nm) is not surprising since height
anomalies have been observed when measuring heights of biomolecules26-28 or, even, of less deformable particles such as gold clusters29 by AFM. For example, Van Noort et al.30 have suggested that height anomalies, in TM-AFM, could be due to a damping of the amplitude of the cantilever due to adhesive capillary forces between the tip and the water layer present on the mica. They also concluded that this effect could be minimized when working with a small tip amplitude (A < 20 nm), as was done here. Another explanation was given by Mu¨ller and Engel31 who demonstrated that electrostatic interactions between the AFM tip, the mica, and the object, which are determinant in the tip/mica and the tip/sample distance, could be responsible for fraction of nanometer height variations. Although it is difficult to obtain an accurate height of the Laponite colloids with AFM, the values obtained in this work are consistent with the literature values; that is, the height of individual Laponite particles is about 1 nm. The AFM height measurements also suggested that 20% of the Laponite particles were present as dimers (coherent stacking of two platelike particles). This result is interesting since, according to SAXS and SANS studies, the presence of dimers was not expected in dilute Laponite solutions. Nonetheless, in a recent light scattering study, it was suggested that Laponite could exist in dilute solution as individual particles with some dimers and even larger aggregates.15 Taking into account the whole range of particle heights measured by AFM, the height polydispersity of the Laponite colloids is equal to 1.18. The AFM method also provides direct information on the lateral dimensions of objects deposited on a surface. For example, a cross-sectional analysis of an individual Laponite particle (Figure 2) gave lateral dimensions of about 35 and 17 nm. Nonetheless, lateral dimensions of small objects can be overestimated due to the finite size and geometry of the AFM tip. A simple geometrical model, which describes the distortion of the topographic signal generated by a spherical tip interacting with a rectangular (or disk-shaped) particle (such as a Laponite particle), can be used to relate its apparent width Wi to its real (26) Rippe, K.; Mu¨cke, N.; Langowski, J. Nucleic Acids Res. 1997, 25, 1736. (27) Vesenka, J.; Guthold, M.; Tang, C. L.; Keller, D.; Delain, E.; Bustamante, C. Ultramicroscopy 1992, 42, 1243. (28) McIntire, T. M.; Penner, M.; Brant, D. Macromolecules 1995, 28, 6375. (29) Ku¨hle, A.; Sorensen, A. H.; Bohr, J. J. Appl. Phys. 1997, 81, 6562. (30) van Noort, S. J. Y.; van der Werf, K. O.; De Grooth, B. G.; van Hulst, N. F.; Greve, J. Ultramicroscopy 1997, 69, 117. (31) Mu¨ller, D. J.; Engel, A. Biophys. J. 1997, 73, 1633.
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Figure 4. Distribution of the relative major length axis L of Laponite particles (C ) 10 mg L-1, pH ) 10). Continuous line: a log-normal approximation given by eq 4.
Figure 5. Dependence of the minor length distance l as a function of the major length distance L.
diameter Li, its height Zi, and the AFM tip radius R by the following equation:32
probability density function (pdf), P(L), obtained by AFM can also be approximated by a log-normal distribution of the form
Wi ) 2x2RZi - Zi2 + Li
(3)
Nonetheless, this model is imprecise since it only considers geometrical effects but does not consider the interaction distances between the tip and the substrate and the tip and the specimen, which cannot be neglected for nanometer size range objects. In this size range, many examples in the literature have shown that geometrical models cannot be systematically used. For example, on one hand, Balnois et al.33 observed a large overestimation of lateral sizes of humic substances, due to the large repulsion between the humic macromolecules and the tip, both negatively charged under their operating conditions (i.e., which would give a tip radius of ∼125 nm instead of a reasonable tip radius between 10 and 30 nm). On the other hand, Lyubchenko et al.34 measured lateral dimensions of DNA molecules equal to 2.8 nm, very close to its crystallographic diameter (2 nm). These high-resolution images in the lateral dimension were attributed to the interaction of atomic asperities at the extremity of the AFM tip with the DNA molecules. These two examples illustrate the nonsystematic broadening effect of the AFM tip on lateral dimensions of nanometer size objects and underline that it is necessary to use caution when interpreting these values. Furthermore, it is also worth mentioning the difficulty of accurately calibrating the AFM tip size since very few calibration standards in this size range (nanometer or fraction of nanometer scale) are available. Finally, the actual radius of curvature of the tip may vary during image acquisition due to its contamination or alteration. Although it is difficult to obtain accurate absolute lateral dimensions from AFM images, we can deduce two important points from these raw lateral measurements: (i) the form of the relative lateral size distribution (Figure 4) and (ii) the comparison between the major and minor length axes (Figure 5). The mean major length distance 〈L〉 and mean minor length distance 〈l〉 are 24.0 ( 6.9 nm and 16.8 ( 4.9 nm, respectively. The major length size (32) Eggleston, C. M. Scanning Probe Microscopy of Clay Minerals; Nagy, K. L., Blum, A. E., Eds.; CMS Workshop Lectures, Vol. 7; The Clay Mineral Society: Boulder, CO, 1994. (33) Balnois, E.; Wilkinson, K. J. Colloids Surf., A 2002, 207, 229. (34) Lyubchenko, Y. L.; Shlyakhtenko, L. S. Proc. Natl. Acad. Sci. U.S.A. 1997, 94, 496.
P(L) )
1
(
exp -
x2πvL
)
1 (ln(L/L0))2 2 2v
(4)
L0 and ν are the two characteristic parameters defining the log-normal distribution. The “best” fit gives the following values: L0 ) 23.5 nm and ν ) 0.225 nm. The relation between the minor length dimension, l, and the major length dimension, L, is plotted in Figure 5. The “best” linear fit gave an average slope l/L ) 0.6. These results indicated that the particles were not exactly disks but were closer to a population of ellipses with a constant eccentricity of 0.8 and a polydispersity index of 1.08. The l/L ratio also equals 0.6 for particles with heights of ∼2 nm, indicating that the dimers may have a similar geometry to that of the individual particles. This suggests that dimers probably do not result from any aggregation of the particles in solution or at the interface, but rather are the consequence of a limited exfoliation of primary particles. B. Computation of the Small-Angle Scattering. The experimental SANS spectrum of a diluted suspension of Laponite is shown in Figure 6 after subtraction of the empty cell and incoherent background.17 The total scattering intensity (in absolute scale) IT(q) was normalized according to
IN(q) ) IT(q)/((∆F)2φ)
(5)
where ∆F is the contrast diffusion length density and φ is the volume fraction of the suspension. For q < 10-1 nm-1, the scattering curve reaches a plateau. In this “low-q regime”, it is relatively straightforward to show that IN(q) can be approximated as35
IN(q) )
〈Vp2〉 〈Vp〉
(6)
where 〈Vp〉 and 〈Vp2〉 are the first and the second moments of the particle volume pdf. For q > 10-1 nm, IN(q) evolves as q-2 which is the signature of a platelike particle.12,14,17,34 In this “intermediate regime”, where 1/q is larger than (35) Guinier, A.; Fournet, G. Small Angle Scattering of X-rays; Wiley: New York, 1955.
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Figure 6. Normalized small-angle scattering spectra of a diluted suspension of Laponite. Gray circles: SANS experiment (8 g L-1, I ) 10-4 M, pH ) 10). Continuous gray line: form factor of a monodisperse disk having a diameter of 28 nm and a thickness of 1 nm (ref 14). Continuous black line: computation using AFM structural data.
the particle thickness and smaller than the average diameter and for a set of polydisperse circular particles, IN(q) can be approximated as
IN(q) )
2 2π 〈e 〉 q2 〈e〉
(7)
where 〈e〉 and 〈e2〉 are the first and the second moments of the thickness pdf. Decorrelation between thickness and lateral size is assumed in order to get eq 7. This equation provides a good approximation of the intermediate q-2 regime, which is essentially independent of the lateral size of the particles. As shown in Figure 6, the experimental SANS data (gray circles) can be correctly approximated by using a form factor corresponding to a monodisperse disk having a diameter of 28 nm and a thickness of 1 nm (continuous gray line) as already reported.14 To cross-check our AFM results with other available structural characterizations, a SANS spectrum of a very diluted suspension of Laponite colloids was computed from the AFM results. As was deduced from the AFM lateral size measurements, each particle was considered to be an ellipse with L/2 and l/2 as the major and minor axes, respectively. Particle heights were distributed according to the following statistics: 80% with a height of 1.2 nm and 20% with a height of 2.1 nm. Two numerical procedures are used to provide the lateral size distribution: First of all, lateral sizes of each discrete particle (N ) 400) were used to simulate the SANS spectrum. Second, the curve is obtained by using a log-normal distribution of the lateral size pdf P(L) (continuous line in Figure 4) with L0 ) 23.5 nm and ν ) 0.225 nm and an aspect ratio l/L ) 0.6. In the latter case, a large number of particles was chosen (∼5000) in order to probe correctly the particle size distributions. At the numerical point of view, an elliptic particle is built, a vector q is chosen along a random direction, and the contribution to the total scattering intensity is computed assuming no interference between different particles. This approximation of a very diluted
suspension permits the average form factor to be computed. An angular average was performed for each particle (400 different orientations for each q value). We can observe that the small-angle scattering spectrum, simulated using either each discrete particle’s dimensions (continuous black line in Figure 6) or the lognormal distribution P(L) (not represented for clarity but similar to the continuous black line), was in very good agreement with the experimental SANS data. These results demonstrate that the tip effect on the lateral dimension of the Laponite particles measured on our AFM images was negligible, likely due to small asperities at the end of the tip. Furthermore, AFM data obtained only on individual particles allow correct computation of the small-angle scattering curve of diluted suspensions of Laponite (with concentration lower than 10 g L-1 and at low ionic strength of 10-4 M). This last point underlines the importance of a multitechnique approach in order to get an objective description of the distribution and polydispersity of the particles. IV. Conclusion The AFM technique has been used to determine the shape, size, and polydispersity of Laponite particles. Under well-controlled sample preparation and observation conditions, Laponite particles were shown to be mainly present as individual particles and dimers. The important result deduced from AFM images is that individual Laponite particles are not disks but rather ellipses with a height of ∼1 nm and major lateral dimensions which could be described by a log-normal distribution with L0 ) 23.5 nm and ν ) 0.225 nm. However, since lateral dimensions are often overestimated in AFM, SANS curves were simulated using AFM size measurements and compared with experimental data. Very good agreement was found between the experimental and computed scattering curves, indicating a negligible tip effect. This work emphasizes the complementarity of the AFM technique with other physical techniques such SANS and SAXS in the characterization of Laponite particles and demonstrates the importance of using different structural techniques in parallel in order to gain an objective description of the studied materials. Overall, the main advantage of AFM is the possibility to observe individual objects with a subnanometer resolution in contrast with more macroscopic and averaging techniques which can at times lose local information. With respect to the particular system under investigation, we stress that these new morphological results should be kept in mind for any further theoretical analysis of these puzzling colloidal suspensions. Acknowledgment. We acknowledge I. Grillo (ILL, Grenoble, France) and F. Cousin (LLB, CEA Saclay, France) for their efficient help during SANS experiments. Helpful comments on the manuscript and fruitful discussions by K. J. Wilkinson (CABE, University of Geneva, Switzerland), P. Turq, C. Treiner, and E. Dubois (LI2C) are greatly appreciated. We also thank Y. Laudernet (LI2C) for his help in the elaboration of the AFM image analysis program. LA0340908
