Phase Behavior of Aqueous Suspensions of Mg2Al Layered Double

Apr 17, 2007 - Birefringence observations and rheological measurements were used to monitor the phase behavior of Mg/Al (the molar ratio of Mg2+ to Al...
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Langmuir 2007, 23, 5331-5337

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Phase Behavior of Aqueous Suspensions of Mg2Al Layered Double Hydroxide: The Competition among Nematic Ordering, Sedimentation, and Gelation Jie Zhang, Lingyu Luan, Wenxia Zhu, Shangying Liu, and Dejun Sun* Key Laboratory of Colloid and Interface Chemistry, Shandong UniVersity, Ministry of Education, Jinan, 250100, Shandong, P. R. China ReceiVed August 28, 2006. In Final Form: January 29, 2007 Birefringence observations and rheological measurements were used to monitor the phase behavior of Mg/Al (the molar ratio of Mg2+ to Al3+ being 2:1) layered double hydroxide (LDH) suspensions. The suspensions of concentration lower than 16% (w/w) appear isotropic (I) between crossed polarizers. In contrast, the suspensions of concentration between 16% and 30% (w/w) showed an isotropic (I)-nematic (N) biphasic coexistence. Detailed observations led us to divide the suspensions in the gap into three groups according to their behaviors: the suspensions with concentration between 16% and 25% (w/w) experienced an I-N phase transition and particle sedimentation simultaneously, while the suspensions of 25% to 27% (w/w) showed I-N transition after particle sedimentation, and in the suspension of 30% (w/w), a critical sol-gel transition appeared with an I-N transition. Above 33% (w/w), the gel network hindered a complete I-N separation in the suspensions. Upon raising the NaCl concentration, the liquid crystalline phase transition and the sol-gel transition shifted to higher particle concentrations. The facts demonstrate that the phase behavior of aqueous LDH suspensions is controlled by the competition among liquid crystal phase transition, sedimentation, and gelation.

1. Introduction Onsager demonstrated that the thermodynamic stability of the nematic (N) phase in anisometric (rod- and platelike) particles suspensions arises from the gain of translational entropy that overrules the loss of orientational entropy.1,2 The anisotropic shape of particles is the vital factor leading to isotropic (I)nematic (N) phase transition, so a suspension of noninteracting hard rods and plates can form a nematic phase. Computer simulations3,4 and experiments5,6 have substantiated that Onsager’s theory can explain the I-N first-order transition in suspensions of anisometric particles. Besides the particle shape, the gravitational force, electrostatic interaction, diffusion force, Brownian movement, and so on are important to the phase behavior of colloidal suspensions in practice.7-12 And hence, the phase behavior of anisometric particle suspensions diverges from Onsager’s prediction. For example, the widely studied clay suspensions experience a sol-gel transition before an I-N transition, instead of a single liquid crystal phase transition.13 * E-mail: [email protected]; Tel: +86 531 88364749; Fax: +86 531 88564750. (1) Onsager, L. Ann. N. Y. Acad. Sci. 1949, 51, 627. (2) Forsyth, P. A.; Marcˇelja, S., Jr.; Mitchell, D. J.; Ninham, B. W. AdV. Colloid Interface Sci. 1978, 9, 37. (3) Frenkel, D.; Eppenga, R. Phys. ReV. Lett. 1982, 49, 1089. (4) Frenkel, D. J. Phys. Chem. 1987, 91, 4912. (5) Davidson, P.; Bourgaux, C.; Schoutteten, L.; Sergot, P.; Williams, C.; Livage, J. J. Phys. II 1995, 5, 1577. (6) Davidson, P.; Gabriel, J.-C. P. Curr. Opin. Colloid Interface Sci. 2005, 9, 377. (7) Laird, F. W. J. Phys. Chem. 1927, 31, 1034. (8) Bellier-Castella, L.; Xu, H. J. Phys.: Condens. Matter 2003, 15, 5417. (9) Manley, S.; Cipelletti, L.; Trappe, V.; Bailey, A. E.; Christianson, R. J.; Gasser, U.; Prasad, V.; Segre, P. N.; Doherty, M. P.; Sankaran, S.; Jankovsky, A. L.; Shiley, B.; Bowen, J.; Eggers, J.; Kurta, C.; Lorik, T.; Weitz, D. A. Phys. ReV. Lett. 2004, 93, 108302. (10) Gonza´lez, A. E. Phys. ReV. Lett. 2001, 86, 1243. (11) Gonza´lez, A. E.; Odriozola, G.; Leone, R. Eur. Phys. J. E 2004, 13, 165. (12) Hecht, M.; Harting, J.; Ihle, T.; Herrmann, H. J. Phys. ReV. E 2005, 72, 011408. (13) Gabriel, J.-C. P.; Sanchez, C.; Davidson, P. J. Phys. Chem. 1996, 100, 11139.

Even now, the relationship between liquid crystal phase transition and gelation in clay suspensions is still unclear. Gravity plays an important role in the phase behavior of colloidal systems.14-16 The balance between the gravitational force and the osmotic pressure gradient results in a sedimentationdiffusion equilibrium, i.e., a density profile varying with altitude.14,17-19 In this way, the colloidal suspension spans a large density range over a few centimeters, which makes a profound impact on the phase behavior of the colloid system. Whereas much work has been devoted to the effect of gravity on the phase behavior of spherical systems,20-23 many recent studies have focused on the behavior of the anisometric particle suspensions.24-26 Atomic force microscopy measurements revealed that the monodisperse β-FeOOH rod suspensions had a smectic phase in a gravitational field.27-29 And in the gibbsite suspensions, the I-N biphasic coexistence turned into isotropic (I)-nematic (N)-columnar (C) triphase on prolonged stand(14) Schmidt, M.; Dijkstra, M.; Hansen, J.-P. J. Phys.: Condens. Matter 2004, 16, S4185. (15) Royall, C. P.; van Roij, R.; van Blaaderen, A. J. Phys.: Condens. Matter 2005, 17, 2315. (16) Rasa, M.; Erne´, B. H.; Zoetekouw, B.; van Roij, R.; Philipse, A. P. J. Phys.: Condens. Matter 2005, 17, 2293. (17) Philipse, A. P. J. Phys.: Condens. Matter 2004, 16, S4051. (18) Philipse, A. P.; Koenderink, G. H. AdV. Colloid Interface Sci. 2003, 100102, 613. (19) van Roij, R. J. Phys.: Condens. Matter 2003, 15, S3569. (20) Okubo, T.; Tsuchida, A.; Okuda, T.; Fujitsuna, K.; Ishikawa, M.; Morita, T.; Tada, T. Colloids Surf., A: Physiochem. Eng. Aspects 1999, 153, 515. (21) Zhu, J.; Li, M.; Rogers, R.; Meyer, W.; Ottewill, R. H.; STS-73 Space Shuttle Crew; Russel, W. B.; Chaikin, P. M. Nature (London) 1997, 387, 883. (22) Simeonova, N. B.; Kegel, W. K. Phys. ReV. Lett. 2004, 93, 035701. (23) Martelozzo, V. C.; Schofield, A. B.; Poon, W. C. K.; Pusey, P. N. Phys. ReV. E 2002, 66, 021408. (24) Baulin, V. A. J. Chem. Phys. 2003, 119, 2874. (25) Dogic, Z.; Philipse, A. P.; Fraden, S.; Dhont, J. K. G. J. Chem. Phys. 2000, 113, 8368. (26) Baulin, V. A.; Khokhlov, A. R. Phys. ReV. E 1999, 60, 2973. (27) Maeda, H.; Maeda, Y. Langmuir 1996, 12, 1446. (28) Maeda, H.; Maeda, Y. J. Chem. Phys. 2004, 121, 12655. (29) Maeda, H.; Maeda, Y. Phys. ReV. Lett. 2003, 90, 018303.

10.1021/la0625300 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/17/2007

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ing.30-34 In addition, gibbsite suspensions of different particle sizes showed different phase behaviors in the gravity field.35 According to the particle size, the scenarios of the suspensions changed dramatically from “phase transition first, settling later” to “settling first, phase transitions later”. Computer simulations also showed that the density profile in anisometric particle systems favored the multiphase coexistence of different symmetry or composition.36,37 In this work, we will deal with suspensions of Mg/Al layered double hydroxide (LDH), which display an interesting phase behavior in the gravity field. Layered double hydroxides (LDHs) are a large class of inorganic compounds with the chemical formula II x+ nII is the divalent MIII [M1-x x (OH)2] A x/n‚mH2O, where M metal, MIII is the trivalent metal, An- is the interlayer anion, m is the number of moles of cointercalated water per formula weight of compounds, and x is the number of moles of MIII per formula weight of compounds.38-41 LDH particles are structurally related to the mineral brucite Mg(OH)2 with Mg2+ ions replaced by trivalent cations of similar size. This replacement results in a net positive charge on the layers, which is compensated by exchangeable anions placed between an interlayer. Because they have a similar layered structure to that of clay particles, LDH particles are also known as anionic clays. The surface charge density and morphology of LDH particles, which play a crucial role in the rheology and phase transition of suspensions, can be tuned by the molar ratio of MII to MIII. Now, LDHs have been used as catalysts, catalyst precursors, adsorbents, anion exchangers, and promising materials for nanocomposites.42,43 However, only a few studies have considered the colloidal domain of LDHs. In previous papers, we have reported the rheological properties44,45 and colloidal stability46 of Mg/Al LDH suspensions. And recently, an isotropic-lamellar phase transition was found in Mg2+/Al3+ ) 1:1 LDH suspensions,47 while an I-N phase transition was found in Mg2+/Al3+ ) 2:1 LDH suspensions.48 In this study, the Mg2+/Al3+ ) 2:1 LDH (Mg2Al LDH for short) suspensions were explored simultaneously in the liquid crystal phase transition, sedimentation, and gelation. After describing experimental methods in section 2, the liquid crystal phase transition, particle sedimentation in the gravity field, and sol-gel transition of Mg2Al LDH suspensions are discussed in section 3. In section 4, we present a schematic illustration of the phase transitions of Mg2Al LDH suspensions controlled by the (30) van der Kooij, F. M.; Kassapidou, K.; Lekkerkerker, H. N. W. Nature (London) 2000, 406, 868. (31) van der Beek, D.; Lekkerkerker, H. N. W. Europhys. Lett. 2003, 61, 702. (32) van der Beek, D.; Lekkerkerker, H. N. W. Langmuir 2004, 20, 8582. (33) Petukhov, A. V.; van der Beek, D.; Dullens, R. P. A.; Dolbnya, I. P.; Vroege, G. J.; Lekkerkerker, H. N. W. Phys. ReV. Lett. 2005, 95, 077801. (34) van der Beek, D.; Petukhov, A. V.; Oversteegen, S. M.; Vroege, G. J.; Lekkerkerker, H. N. W. Eur. Phys. J. E 2005, 16, 253. (35) Wijnhoven, J. E. G. J.; van’t Zand, D. D.; van der Beek, D.; Lekkerkerker, H. N. W. Langmuir 2005, 21, 10422. (36) Savenko, S. V.; Dijkstra, M. Phys. ReV. E 2004, 70, 051401. (37) Wensink, H. H.; Lekkerkerker, H. N. W. Europhys. Lett. 2004, 66, 125. (38) Rives, V. Layered Double Hydroxides: Present and Future; Nova Science Publishers, Inc.: New York, 2001. (39) Rives, V. Mater. Chem. Phys. 2002, 75, 19. (40) Benito, P.; Labajos, F. M.; Rives, V. Cryst. Growth Des. 2006, 6, 1961. (41) Xu, Z. P.; Stevenson, G. S.; Lu, C.-Q.; Lu, G. Q.; Bartlett, P. F.; Gray, P. P. J. Am. Chem. Soc. 2006, 128, 36. (42) Greenwell, H. C.; Stackhouse, S.; Coveney, P. V.; Jones, W. J. Phys. Chem. B 2003, 107, 3476. (43) Newman, S. P.; Williams, S. J.; Coveney, P. V.; Jones, W. J. Phys. Chem. B 1998, 102, 6710. (44) Guo, P.; Sun, D.; Zhang, J.; Zhang, C. Chem. Lett. 2003, 32, 250. (45) Guo, P.; Sun, D.; Zhang, J. Chin. Chem. Lett. 2003, 14, 973. (46) Wang, X.; Sun, D.; Liu, S.; Wang, R. J. Colloid Interface Sci. 2005, 289, 410. (47) Wang, N.; Liu, S.; Zhang, J.; Wu, Z.; Chen, J.; Sun, D. Soft Matter 2005, 1, 428. (48) Liu, S.; Zhang, J.; Wang, N.; Liu, W.; Zhang, C.; Sun, D. Chem. Mater. 2003, 15, 3240.

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Figure 1. Transmission electron micrograph of Mg2Al LDH particles used in this study. Table 1. Characteristics of the Samples Used in This Study, as Obtained by TEM (for the average diameter 〈D〉) and AFM (for the average thickness 〈L〉) 〈D〉/nm

σD/%

〈L〉/nm

σL/%

〈D〉/〈L〉a

102

39

7.64

14

13

a

〈D〉/〈L〉 expresses the aspect ratio of the Mg2Al LDH particles.

competition among liquid crystal phase transition, sedimentation, and gelation. 2. Materials and Methods The Mg2Al LDH sample in this study was synthesized by a nonsteady coprecipitation method.49 A mixed solution of magnesium and aluminum chlorides was prepared with a molar ratio of Mg2+/ Al3+ ) 2:1. Then, diluted ammonia solution [20% (v/v)] was slowly added to the mixed solution with vigorous stirring. The final pH value of the suspension was about 9.8. After being aged for 1 h at room temperature, the precipitate was washed thoroughly with deionized water. Peptizing the filter cake at 80 °C for at least 8 h created a positively charged LDH sol. The sol was concentrated in a vacuum until the particle concentration was up to 40% (w/w). Long-term storage in sealed vials under N2 atmosphere prevented contamination by atmospheric CO2. After dilution to different concentrations, the suspensions were tested further. The salt concentration of the suspensions was set by adding NaCl solutions. The chemical composition of the Mg2Al LDH was Mg0.68Al0.38(OH)2.32Cl0.18‚0.79H2O based on chemical analysis. The morphology and diameter of the Mg2Al LDH particles were characterized by a JEM-100CXII (JEOL, Japan) transmission electron microscope (TEM). Most of the particles are hexagonal platelets. From the micrograph (Figure 1), the average corner-to-corner diameter of the particles (〈D〉) was obtained and the polydispersity in the diameter (σD) was determined by the equation σD )

x〈D2〉-〈D〉2/〈D〉.30 The thickness (L) of the Mg2Al LDH particles

was determined by an atomic force microscope (AFM; see Figure S1 in the Supporting Information). AFM was recorded on a Digital Instrument Nanoscope IIIa Multimode system (Santa, Barbara, CA) in tapping mode. The average thickness of the particles (〈L〉) was defined as the average of L, and therefore, the polydispersity in the

thickness was σL ) x〈L2〉-〈L〉2/〈L〉. Table 1 lists the characteristics of Mg2Al LDH particles. The ζ potential of the particles in the suspension without NaCl was +39.5 mV, measured with a Malvern Zetasizer 3000 (Malvern, U.K.). The XRD pattern (see Figure S2 in the Supporting Information) provides strong evidence for the hydrotalcite-like structure of the Mg2Al LDH powder sample. The I-N phase separation process of Mg2Al LDH suspensions was assessed in two different ways. For macroscopic experiments, the suspensions were studied between crossed polarizers in 1 mm cuvettes. Meanwhile, their microscopic textures were observed with a Motic B1-PH polarization microscope (Motic, China) with samples (49) Han, S.; Hou, W.; Zhang, C.; Sun, D.; Huang, X.; Wang, G. J. Chem. Soc., Faraday Trans. 1998, 94, 915.

Phase BehaVior of LDH Suspensions

Figure 2. Phase behavior of 20% (w/w) Mg2Al LDH suspension as observed between crossed polarizers. (a) After preparation, the suspension was a completely homogeneous and translucent liquid. (b) 39 days later, a density profile formed in the suspension. (c) 8 months after preparation, the suspension separated into a nematic bottom phase, an isotropic cloudy region, and a transparent liquid upper phase (from the bottom up). sealed between a hollow glass slide and coverslip (sample thickness of about 100 µm; see Figure S3 in the Supporting Information.). As shown later, particle concentration is a subtle parameter which determines the phase behavior of Mg2Al LDH suspensions. So, preventing the samples from losing water is very important to the macroscopic observation of the suspensions. To fix the concentration of suspensions, we kept the samples under a water-vapor saturated atmosphere. Over 8 months, the Mg2Al LDH suspensions showed rich and varied behavior. Rheological measurements were performed on a RS75 rheometer (HAAKE Company, Germany) affixed with concentric cylinder Z41 Ti geometry. The running temperature was 25 ( 1 °C. The storage modulus (G′) and loss modulus (G′′) were determined from oscillatory experiments.

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Figure 3. Phase behavior of the 27% (w/w) Mg2Al LDH suspension as observed between crossed polarizers. (a) The homogeneous and translucent suspension after preparation. (b) 13 days later, sediment formed in the suspension. (c) A month after preparation, a nematic phase grew on top of sediment. (d) The suspension finally separated into 4 phases, comprising the sediment, a nematic phase, a cloudy isotropic phase, and a transparent liquid (from the bottom up).

3. Results and Discussion 3.1. Birefringence Observations and I-N Phase Transition. When examined between crossed polarizers, Mg2Al LDH suspensions showed various phase behaviors according to their concentration. The suspensions with concentration lower than 16% (w/w) were isotropic throughout. On increasing the particle concentration to above 16% (w/w), the suspensions showed I-N biphasic coexistence. However, detailed observations indicated that suspensions between 16% and 30% (w/w) showed three different scenarios, each with its own characteristic evolution. The suspensions between 16% and 25% (w/w) exhibited an I-N phase transition in the course of particle sedimentation (Figure 2). The persistence of transient birefringence after shaking indicated the initiation of a phase transition. Eight months after preparation, the suspensions separated into a nematic bottom phase, an isotropic phase in the middle, and a transparent liquid upper phase. In contrast, suspensions between 25% and 27% (w/w) showed the I-N transition after particle sedimentation (Figure 3). The suspensions finally separated into four phases, comprising sediment, a nematic phase, a cloudy isotropic phase, and a transparent liquid. The suspension of 30% (w/w) showed birefringence due to flow during sample preparation (Figure 4). The flow birefringence relaxed within 1 month. Ten days after preparation, particle sedimentation in the 30% (w/w) suspension led to coexistence between LDH-rich and LDH-poor phases (photographed on the 34th day after preparation, Figure 4b). And, 3 months after preparation, a nematic phase grew in the middle of LDH-poor phase. But the origin of this observation is not clearly understood. On increasing the concentration even

Figure 4. Macroscopic phase behavior of the 30% (w/w) Mg2Al LDH suspension as observed between crossed polarizers. (a) The suspension showed birefringence due to flow during sample preparation. The flow birefringence relaxed in 1 month. (b) 34 days after preparation, the suspension separated into LDH-rich and LDHpoor phases. (c) 3 months after preparation, a nematic phase grew in the middle of the LDH-poor phase.

further, the suspension of 33% (w/w) formed a gel (Figure 5). The gel slowly shrunk in 5 months, while the textures remained clear (Figure 5b). In order to compare our results with numerical solutions, it is useful to express the mass concentration as the dimensionless density FD3. It has been shown that the volume fraction φ of polydisperse hexagonal platelets in suspensions equals32 2 〈L〉 1 + σD 3 x φpol ) 3 F〈D3〉 8 〈D〉 1 + 3σ 2

(1)

D

this can be rewritten as

〈D〉 1 + 3σD 8 φ F〈D3〉 ) x3 9 〈L〉 1 + σ 2 pol 2

(2)

D

The I-N phase transition of Mg2Al LDH suspensions began at

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Figure 5. Macroscopic phase behavior of the 33% (w/w) Mg2Al LDH suspension as observed between crossed polarizers. (a) The birefringent gel phase of the suspension. (b) 43 days after preparation, a transparent liquid region formed above the gel.

16% (w/w), yielding the dimensionless density for the coexisting isotropic phase FisoD3 ) 1.78. But, we cannot determine the dimensionless density for the coexisting nematic phase FnemD3, because the gel structure in the concentrated suspensions hindered a complete I-N separation. Here, only number densities are needed, so we ignored the electrostatic effects. Numerical solutions of the Onsager formalism applied to disks were obtained by Forsyth and co-workers.2 They found that the dimensionless densities FisoD3 ) 6.4 and FnemD3 ) 7.4 for platelets with D/L ) 13. Obviously, the FisoD3 in this experiment was lower than that in computer simulation, and the coexistence region was wider than what simulations predicted. The deviation of the experimental FisoD3 value from the simulation could be explained by the following factors. First, Onsager’s theory neglected interaction among three (or more) particles which are important in the concentrated suspensions. If the interactions among many particles are taken into account, the theoretical value of FisoD3 should be reduced.50 Second, the morphology of particles is important to the phase transitions of colloidal suspensions.51 Many of the previous simulation studies were performed on monodisperse hard disks,52 while the Mg2Al LDH particles are polydisperse hexagonal platelets. For monodisperse systems, the I-N coexistence density for a hexagonal particle system would shift to a lower value than that for a circular particle system, because the effective diameter of the hexagonal platelet is larger than that of the circular platelet with equivalent area. Third, semigrand simulations revealed that polydispersity in the size of the particles led to the widening of the coexistence region.50 Last but not least, the gravity field also plays a crucial role in the phase behavior of Mg2Al LDH suspensions. Gravity enables the particles to span a large density range in a test tube, and increases the particle concentration at the bottom.24-32 All of these facilitate the phase transition of colloidal suspensions and broaden the region of I-N coexistence. Electrostatic interactions between the platelets can also affect the phase behavior of the Mg2Al LDH suspensions. Electrostatic repulsion among Mg2Al LDH particles increases the effective excluded-volume of each particle. So, the relative amount of the nematic phase decreased with NaCl concentration (CNaCl) and increased with particle concentration. (50) Bates, M. A.; Frenkel, D. J. Chem. Phys. 1999, 110, 6553. (51) Bates, M. A. J. Chem. Phys. 1999, 111, 1732. (52) Veerman, J. A. C.; Frenkel, D. Phys. ReV. A 1992, 45, 5632.

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In this study, the I-N phase transitions of Mg2Al LDH suspensions shifted to higher particle concentrations with increasing NaCl concentration. Similar evolution was found in the suspensions of charged gibbsite31,32 and clays (Na-montmorillonite and laponite).53-55 The suspensions of charged gibbsite particles had an I-N phase transition at ionic strengths higher than 10-3 mol/L.32 The transition line from isotropic phase to ordered phases had a positive slope. In the clay suspensions, whereas Gabriel proved that increasing NaCl concentration stabilized the nematic gel phase in the suspensions of natural bentonite and synthetic laponite B,13 the studies of Mourchid and Levitz showed that the I-N transition of the clay suspensions extended to higher concentrations as the ionic strength increased.53-55 Until now, there is no unequivocal interpretation to reconcile all the experimental observations on charged platelet suspensions. 3.2. Particle Sedimentation and Liquid Crystal Phase Transition. Now, there is one issue to be resolved. As shown in Figures 2 and 3, the Mg2Al LDH suspensions with concentrations between 16% and 25% (w/w) experienced sedimentation and a liquid crystal phase transition simultaneously, while the suspensions of 25% to 27% (w/w) concentration showed sedimentation before phase transition. One of the most striking features of these transitions is that an increase in particle concentration promoted particle sedimentation. To get some insight, we first give an extended characterization of particle sedimentation in a gravitational field. The effect of gravity on the Mg2Al LDH particles may be characterized by the gravitational length (ξ)19 and Peclet number (Pe).35 Gravitational length is defined as ξ ) kBT/m*g, where kB is Boltzmann’s constant, T is absolute temperature, m* is the effective or buoyancy mass of the colloidal particle, and g is the gravitational acceleration.19 ξ sets the height that an object must be lifted to increase its potential energy by 1kBT in a gravitational field and is on the order of 10-3-10-6 meters for colloidal particles. Considering a system of hard hexagonal platelets suspended in water (water is regarded as an incompressible structureless continuum, characterized by its mass density F˜ ) 103 kg/m3), the buoyancy mass of a particle is m* ) m0 - F˜ V ) (F0 - F˜ )V, where m0 is the bare mass of the particle, F0 is the mass density of it, and V ) [(3x3/8)〈D〉2〈L〉] is its volume. So, ξ can be converted to ξ ) kBT/(F0 - F˜ )Vg. The Peclet number (Pe) is introduced to describe the sedimentation-diffusion equilibrium of colloidal particles in a gravitational field.35 Here, Pe is defined as the ratio of the time a particle takes to diffuse a distance equal to its diameter D to the time it takes to sediment this distance Pe ) tdiff/tsed. With the introduction of ξ, the Peclet number is written Pe ) D/ξ. Table 1 shows characteristics of Mg2Al LDH particles, and the mass density of Mg2Al LDH was 2270 kg/m3. Using this information, we can work out ξ and Pe of Mg2Al LDH particles in this study. The volume of a monodisperse regular hexagonal platelet with 〈D〉 ) 102 nm and 〈L〉 ) 7.64 nm is 5.16 × 104 nm3, so ξ ) 6.37 × 10-3 m and Pe ) 1.6 × 10 - 5. That is to say, for infinitely dilute suspensions of monodisperse regular hexagonal particles with V ) 5.16 × 104 nm3, the particles have enough time to explore configurational space before the effects of gravity can be felt. Second, in concentrated Mg2Al LDH suspensions, the spatial exclusion of any particle would obstruct the conformation (53) Michot, L. J.; Bihannic, I.; Porsch, K.; Maddi, S.; Baravian, C.; Mougel, J.; Levitz, P. Langmuir 2004, 20, 10829. (54) Mourchid, A.; Delville, A.; Lambard, J.; Le´colier, E.; Levitz, P. Langmuir 1995, 11, 1942. (55) Mourchid, A.; Le´colier, E.; Van Damme, H.; Levitz, P. Langmuir 1998, 14, 4718.

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Figure 6. The flow curves of Mg2Al LDH suspensions with different particle concentrations. 9, 16% (w/w); b, 20% (w/w); 2, 23% (w/ w); 1, 25% (w/w); (, 27% (w/w); left-facing solid triange, 30% (w/w); right-facing solid triangle, 33% (w/w).

transition of other particles. And, the number of accessible conformations reduced and the degree of particle orientational ordering increased. In addition, simulations16,17 and theory19 predict that a macroscopic electric field presence in the suspension of noninteracting charged colloids and that reduces the effective colloidal mass. So, the suspensions of individual Mg2Al LDH particles will not exhibit particle sedimentation before I-N phase transition. In addition to the effect of gravity on individual particles, the dynamical stability of the colloid also depends on the particles’ Brownian motion and the electrostatic interaction among the particles. In dilute suspensions, Mg2Al LDH particles repel each other and have time to explore configurational space before settling to the bottom. On the other hand, the Brownian motion causes particles in suspensions to collide with each other. As particle concentrations rise, the number of collisions increases, which increases the probability of particles aggregating. When the distance between two particles is close enough, some more or less extended microdomains appeared in the colloidal suspensions. The transient birefringence in sols and the permanent birefringent textures in gels suggest that large anisotropic particle associations form in these systems. The diameter and volume of the associated structure is larger than of the individual Mg2Al LDH particle, so the Peclet number of the associated structure is larger. Hence, in the concentrated suspensions, these associated structures settled to the bottom before having time to explore configurational space. Sedimentation occurred in the suspensions of 25% to 27% (w/w) before I-N transition. 3.3. Rheological Characterization. Rheograms of Mg2Al LDH suspensions are shown in Figure 6. The suspensions with concentration between 16% and 27% (w/w) were pseudo-plastic fluids, while the suspensions of 30% and 33% (w/w) had distinct yield stress. The emergence of yield stress indicates the suspensions had solidlike structures with elastic properties. Oscillatory experiments are very useful for studies of sol-gel transitions.56 In oscillatory experiments, the system is exposed to an oscillating deformation, and the storage (elastic) modulus G′ and loss modulus G′′ of the system are determined. G′ and G′′ are reliable values only in the case that the structure of the system is not disrupted during the oscillating deformation. So, the first stage in our oscillatory experiments is the determination of the critical strain amplitude, above which the disruption of the (56) Abend, S.; Lagaly, G. Appl. Clay Sci. 2000, 16, 201.

Figure 7. (a) The stress dependence of the storage modulus (G′) and loss modulus (G′′) for Mg2Al LDH suspensions of 20%, 30%, and 33% (w/w) without NaCl. The suspensions of 30% and 33% (w/w) showed a viscoelastic linear region. The frequency was 1 Hz for all suspensions. (b) The frequency dependence of the G′ and G′′ for the suspensions of different concentrations. 9, 0: 20% (w/w). b, O: 30% (w/w). 2, 4: 33% (w/w). Full symbols, storage modulus G′; open symbols, loss modulus G′′.

system provokes a nonlinear viscoelastic behavior. Typical results for the dependence of G′ and G′′ on shear stress for Mg2Al LDH suspensions without NaCl are shown in Figure 7a. The suspension of 20% (w/w) was a sol, and G′′ was higher than G′. In contrast, for suspensions of 30% (w/w) and 33% (w/w), G′ was higher than G′′, and both stayed constant up to a critical shear stress. When the shear stress exceeded the critical point, the structure of the system was disrupted by the oscillating deformation. G′ and G′′ decrease drastically, and G′′ dominated over G′. The linearity limit increases gradually from the suspension of 30% (w/w) to the suspension of 33% (w/w). Hence, it indicates that the structural strength of the network also increases gradually. Figure 7b presents the frequency dependence of G′ and G′′ in the viscoelastic linear region for Mg2Al LDH suspensions without NaCl. (The curves of 20% (w/w) suspension are shown for comparison.) Mg2Al LDH suspensions underwent a sol-gel transition at 30% (w/w). G′ was almost equal to G′′ in the viscoelastic linear region. Below 30% (w/w), the suspensions were sols and G′ and G′′ were both weak, indicating a slightly viscous suspension. In contrast, above 30% (w/w), the suspensions were gels; G′ was significantly higher than G′′, and G′ and G′′ do not vary much with frequency in the viscoelastic linear region. The evolution of the G′ and G′′ with the concentrations of Mg2Al LDH and NaCl are shown in Figure 8. Increasing the particle concentration and decreasing the NaCl concentration

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Figure 9. Schematic illustration of the phase transitions of Mg2Al LDH suspensions. The solid lines indicate boundaries between the phase regions. (F) Flocculated samples, no phase separation observed in flocculated sample. (0, I) Isotropic sol samples. (9, I+N) Isotropic-nematic biphasic sol samples. (O, G) Gel samples.

ordering in repulsive gels of bentonite and laponite in the gravity field.13,64-68 Considering the nematic ordering in repulsive gels, it would be rather tempting to assume that the formation of the network hindered a clear I-N phase separation in colloid suspensions.54,55 However, Michot and co-workers argued that the sol-gel transition was not directly related to the emergence of the incomplete I-N transition in clay suspensions.53 They showed that the sol-gel transition of polydisperse Wyoming Na-montmorillonite suspensions increased linearly with particle size, while the I-N transition had a reverse evolution. Therefore, at present there is no universal interpretation to reconcile all the experimental observations in clay suspensions. Figure 8. (a) The evolution of G′ and G′′ with frequency in a 30% (w/w) suspension with different NaCl concentrations. (b) The evolution of G′ and G′′ with frequency in a 33% (w/w) suspension with different NaCl concentrations. 9, 0: without NaCl. b, O: CNaCl ) 10-5 mol/L. 2, 4: CNaCl ) 10-4 mol/L. 1, 3: CNaCl ) 10-3 mol/L. Full symbols, G′; open symbols, G′′. Suspensions showed sol-gel transitions at 30% (w/w) without NaCl and 33% (w/w) with CNaCl ) 10-5 mol/L.

stabilized the gel phase. Suspensions showed sol-gel transition at 30% (w/w) without NaCl and 33% (w/w) with CNaCl ) 10-5mol/L. The rheological behavior of the gels can be interpreted as corresponding to repulsive gels.56-58 As opposed to the sol-gel transition of clay suspensions shown by Gabriel,13 increasing the NaCl concentration shifted the solgel transition of Mg2Al LDH suspensions to higher concentration. Similar behavior was found in clay suspensions at very low ionic strength.56-59 In this case, long-range electrostatic repulsion determined the sol-gel transition. In Mg2Al LDH gels, particles fell in the “cages” formed by neighboring particles.59-63 Addition of salt reduced the extension of the diffused ionic layer, and thus the sol-gel transition shifted to higher particle concentration. Much evidence has demonstrated that there is orientational (57) Tanaka, H.; Meunier, J.; Bonn, D. Phys. ReV. E 2004, 69, 031404. (58) Levitz, P.; Lecolier, E.; Mourchid, A.; Delville, A.; Lyonnard, S. Europhys. Lett. 2000, 49, 672. (59) Levitz, P.; Delville, A.; Lecolier, E.; Mourchid, A. Prog. Colloid Polym. Sci. 2001, 118, 290. (60) Bonn, D.; Tanaka, H.; Wegdam, G.; Kellay, H.; Meunier, J. Europhys. Lett. 1998, 45, 52. (61) Abou, B.; Bonn, D.; Meunier, J. Phys. ReV. E 2001, 64, 021510. (62) Sciortino, F.; Tartaglia, P. AdV. Phys. 2005, 54, 471. (63) Cipelletti, L.; Ramos, L. Curr. Opin. Colloid Interface Sci. 2002, 7, 228.

4. Summarizing the Phase Behavior of Mg2Al LDH Suspensions The Mg2Al LDH suspensions had an I-N transition before gel formation. Below a concentration of 16% (w/w), the suspensions were isotropic sols, and the particles settled very slowly. Upon increasing the concentration, the sols with concentration between 16% and 30% (w/w) had an I-N biphase coexistence. At the same time, dense aggregates of particles formed in the suspensions. The associate structures speeded up sedimentation. So, the suspensions from 16% to 25% (w/w) experienced the I-N phase transition and particle sedimentation simultaneously, while the suspensions with concentrations between 25% and 27% (w/w) showed an I-N transition after particle sedimentation. When the concentration reached 30% (w/w), the suspension displayed a critical sol-gel transition and an I-N transition appeared in the suspension after particle sedimentation. The rigidity of the gel network increased with the Mg2Al LDH particle concentration. Upon increasing the concentration even further, in the suspensions of 33% (w/w), the gel network hindered complete I-N phase separation and particle sedimentation. Figure 9 summarizes the phase behavior of Mg2Al LDH suspensions. Increasing the NaCl concentration shifted I-N (64) 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. (65) DiMasi, E.; Fossum, J. O.; Gog, T.; Venkataraman, C. Phys. ReV. E 2001, 64, 061704. (66) Bihannic, I.; Michot, L. J.; Lartiges, B. S.; Vantelon, D.; Labille, J.; Thomas, F.; Susini, J.; Salome´, M.; Fayard, B. Langmuir 2001, 17, 4144. (67) Bhatia, S.; Barker, J.; Mourchid, A. Langmuir 2003, 19, 532. (68) Fossum, J. O.; Gudding, E.; Fonseca, D. d. M.; Meheust, Y.; DiMasi, E.; Gog, T.; Venkataraman, C. Energy 2005, 30, 873.

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transition and sol-gel transition to higher concentrations. In conclusion, the phase behavior of the Mg2Al LDH system is controlled by the competition among liquid crystal phase transition, sedimentation and gelation. Now a significant question emerges. Why did the Mg-Al LDH suspensions have an I-N phase transition before a sol-gel transition, while the clay suspensions had a reverse evolution? The main difference between clays and LDH particles was that LDH particles were difficult to exfoliate due to their high surface charge density, which contributed to the higher concentration of gel in LDH suspensions.38 In Langmuir’s experiment, where clays with calcium carbonate impurities were used, the I-N phase separation might also come from the fact that the clay particles were not fully exfoliated.66 However, Gabriel and coworkers used Wyoming bentonite and laponite B, which can fully delaminate into single layers.13 They found that I-N phase separation was prevented by gelation.

pending on the concentrations of Mg2Al LDH particles and NaCl, the Mg2Al LDH suspensions show distinct phases of isotropic sol, I-N coexistence sol, and gel. Increasing the NaCl concentration shifted the phase transition to a higher particle concentration. With the effect of gravity, the macroscopic behavior of I-N coexistence sols can be divided into two different scenarios, which either showed “I-N transition in the course of particle sedimentation” or “particle sedimentation before I-N transition”.

5. Conclusion

Supporting Information Available: AFM micrograph of Mg2Al LDH particles, XRD pattern of the Mg2Al LDH powder sample, and the microscopic textures of the suspensions. This material is available free of charge via the Internet at http://pubs.acs.org.

The phase behavior of Mg2Al LDH suspensions was studied by birefringence observations and rheological measurements. The experimental results indicate that there is competition among liquid crystalline transition, sedimentation, and gelation. De-

Acknowledgment. This work is funded by National Natural Science Foundation of China (20373036), Key Project of the Ministry of Education of China (No. 02128) and Natural Science Foundation of Shandong Province (Y2001B01). We thank Prof. Zhiwei Sun (NML, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100080, P. R. China) for helpful discussions. The authors also thank Ms. Harmony Downs and Dr. Pamela Holt for help in preparation of the manuscript.

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