Phase Behavior of Mixtures of Positively Charged Colloidal Platelets

We investigated the effect of nonadsorbing polymer on the phase behavior of suspensions of positively charged Mg2Al layered double hydroxide (LDH) pla...
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Phase Behavior of Mixtures of Positively Charged Colloidal Platelets and Nonadsorbing Polymer Lingyu Luan, Wei Li, Shangying Liu, and Dejun Sun* Key Laboratory for Colloid and Interface Chemistry of the Education Ministry, Shandong University, Jinan, Shandong 250100, People’s Republic of China Received December 5, 2008. Revised Manuscript Received February 1, 2009 We investigated the effect of nonadsorbing polymer on the phase behavior of suspensions of positively charged Mg2Al layered double hydroxide (LDH) platelets by birefringence observations and rheological measurements. We show that the depletion attraction, induced by the addition of a high-molecular-weight polyvinyl pyrrolidone (PVP), enriches the phase behavior of these electrostatically stabilized suspensions. At intermediate LDH and polymer concentration, two isotropic phases (I1-I2) coexist, nematic-nematic (N1-N2) demixing occurs, and a sediment phase is observed, with the appearance of two-, three-, four-, and even six-phase coexistence. Upon increasing the polymer concentration, the I-N phase transition and the sol-gel transition shift to lower LDH concentrations; meanwhile, the I-N coexistent samples enter the purely nematic phase. We explain the richness of the phase behavior in such LDH-PVP mixtures by discussing the interactions among PVP-induced depletion attraction, particle polydispersity, and particle sedimentation.

1. Introduction Nonadsorbing-polymer-induced depletion attraction between colloidal particles plays an important role in industrial and biological applications, such as the phase separation of colloid-polymer mixtures, colloidal suspension flocculation, and protein crystallization.1,2 The nature of the depletion interaction depends on the polymer to colloid size ratio, and the range and depth of this effect can be controlled by tuning the size and concentration of the polymer. The depletion interaction between two colloidal spheres has been investigated extensively.3-6 However, cases of colloidal suspensions containing anisotropic (rodor platelike) particles may be more interesting because of the possibility of forming liquid-crystal phases. From a theoretical viewpoint, Bates and Frenkel have studied a colloidal model system containing infinitely thin disks and polymer by computer simulations and perturbation theory. They found isotropic fluidfluid coexistence for a polymer to platelet size ratio of Æd æ/ÆDæ > 0.3. They also found nematic-nematic demixing at very high platelet density and predicted that this may occur in experimental systems if a nonspherical depleting agent is used.7 But for a realistic description of the experiments, it is necessary to study platelets with a finite thickness. Zhang et al. adopted a similar method to investigate the phase behavior of colloidal platelets with nonadsorbing polymer and showed that for a platelet aspect ratio of ÆLæ/ÆDæ = 0.1 and a polymer to platelet size ratio of Æd æ/ ÆDæ g 0.2 two isotropic phases were observed.8 Namely, the introduction of depletion attraction gives rise to an additional isotropic phase in these plate-polymer mixtures. Furthermore, an external gravitational field also plays an important role *Corresponding author. E-mail: [email protected]. (1) Tuinier, R.; Rieger, J.; de Kruif, C. G. Adv. Colloid Interface Sci. 2003, 103, 1. (2) Poon, W. C. K. J. Phys.: Condens. Matter 2002, 14, 859. (3) de Hek, H.; Vrij, A. J. Colloid Interface Sci. 1979, 70, 592. (4) Sperry, P. R. J. Colloid Interface Sci. 1984, 99, 97. (5) van Duijneveldt, J. S.; Heinen, A. W.; Lekkerkerker, H. N. W. Europhys. Lett. 1993, 21, 369. (6) Dinsmore, A. D.; Yodh, A. G.; Pine, D. J. Phys. Rev. E. 1995, 52, 4045. (7) Bates, M. A.; Frenkel, D. Phys. Rev. E. 2000, 62, 5225. (8) Zhang, S. D.; Reynolds, P. A.; van Duijneveldt, J. S. J. Chem. Phys. 2002, 117, 9947.

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because particle sedimentation leads to a density gradient within a single cuvette and makes the larger particles initially settle down to the bottom, which may give rise to the simultaneous presence of multiple phases.9 However, these theoretical investigations are focused on monodisperse colloidal platelets because of their simplicity. The effect of platelet polydispersity should also be taken into account, especially in practical colloidal suspensions. Lekkerkerker et al. have studied the influence of particle polydispersity on the phase behavior of sterically stabilized colloidal gibbsite platelets.10 They found that the density gaps of the I-N biphasic region depended strongly on the diameter polydispersity, consistent with computer simulations for polydisperse thin disks.11 They also showed that size fractionation reduced the polydispersity of the ordered phase and thus made it stable in these systems. Moreover, when nonadsorbing polymer was added, an additional isotropic phase was observed.12 Compared to neutral or sterically stabilized colloidal platelets, in many practical colloid-polymer mixtures charges are usually present on the surfaces of colloidal particles. Therefore, attention should be given to depletion attraction in suspensions of charged colloidal platelets. Some recent research has shown that the addition of low-molecular-weight polyethylene oxide (PEO) melts Laponite glass because of a depletion force caused by excess PEO chains in solution. In such a system, PEO adsorbs onto Laponite particles and provides a steric barrier to aggregate formation, which is quite different from the mechanism of depletion interaction in other suspensions of charged colloid platelets and nonadsorbing polymer.13-15 In addition, gelation inhibits phase (9) Wensink, H. H.; Lekkerkerker, H. N. W. Europhys. Lett. 2004, 66, 125. (10) van der Kooij, F. M.; Kassapidou, K.; Lekkerkerker, H. N. W. Nature (London) 2000, 406, 868. (11) Bates, M. A.; Frenkel, D. J. Chem. Phys. 1999, 110, 6553. (12) van der Kooij, F. M.; Vogel, M.; Lekkerkerker, H. N. W. Phys. Rev. E 2000, 62, 5397. (13) Baghdadi, H. A.; Sardinha, H.; Bhatia, S. R. J. Polym Sci., Part B: Polym. Phys. 2005, 43, 233. (14) Baghdadi, H. A.; Jensen, E. C.; Easwar, N.; Bhatia, S. R. Rheol. Acta 2008, 47, 121. (15) De Lisi, R.; Gradzielski, M.; Lazzara, G.; Milioto, S.; Muratore, N.; Prevost, S. J. Phys. Chem. B 2008, 112, 9328.

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separation so that clay suspensions are not an obvious model system for this study. Our group has reported that the addition of nonadsorbing polyethylene glycol (PEG) to Mg2Al layered double hydroxide (LDH) suspensions results in multiphase coexistence rather than the usual I-N phase transition; however, the induced interaction between the platelets is an effective attraction but not a depletion attraction because of the much lower PEG to platelet size ratio.16 To gain a better understanding of the depletion interaction induced between charged platelets, in this work high-molecularweight polyvinyl pyrrolidone (PVP) is added to Mg2Al LDH suspensions, and PVP is indeed shown to act as a depleting agent (section 3.1). In the case where charges on particles are involved, the influence of the ionic strength on the stability of such colloidal suspensions should be taken into account. Some theoretical research has shown that at higher ionic strength, like charges on polymer and the colloids do not seem to feel the depletion attraction between colloids induced by polymer, whereas at low ionic strength the situation becomes quite complicated and is not yet completely understood.17-20 Nevertheless, what is identifiable is that with increasing ionic strength the depletion layer thickness of the charged particles is decreased along with the depth of the depletion potential. Therefore, in this study, we consider only LDH-PVP mixtures with a fixed ionic strength of I = 10-4 M and discuss the effects of depletion attraction, particle polydispersity, and particle sedimentation on the phase behavior of LDH-PVP mixtures.

2. Experimental Section 2.1. Preparation of Materials. Colloidal Mg2Al LDH (2:1 Mg/Al molar ratio) suspensions were prepared by a nonsteady coprecipitation method. First, dilute NH3 3 H2O solution was quickly added to a mixed aqueous solution of MgCl2 3 6H2O and AlCl3 3 6H2O (total metal concentration 0.5 M) under vigorous stirring. The obtained precipitate was aged at room temperature for 45 min. After filtration, the filter cake was washed thoroughly with deionized water and enclosed in a glass bottle for peptization at 80 C for 24 h. To decrease the ionic strength and purify the suspensions, we added ion-exchange resins (AG 501-X8 and Bio-Rex MSZ 501(D) mixed bed resin) directly to the resulting sol and stirred for 1 h. Then we separated the resins from the colloidal suspensions. The deionized sol was concentrated by vacuum distillation until the particle concentration reached 28 wt %, and then the concentrated sample was kept in a closed flask under a nitrogen atmosphere to prevent acidification by dissolved CO2. The polymer used in this article was polyvinyl pyrrolidone (PVP) with a weight-average molar mass of Mw = 630 000 g/mol. First, an aqueous solution of PVP was prepared and added to a solution of the colloidal LDH suspensions and stirred to homogeneity. The resulting LDH-PVP mixtures with different Mg2Al LDH (cLDH) and PVP (cpol) concentrations were put into flat cuvettes with 1 mm slit widths and observed without shaking. The salt concentration of all of the mixtures was set to 10-4 M by adding NaCl solutions. The powders of LDH-PVP mixtures used in XRD and FTIR measurements were pretreated by thorough centrifugation and washing with distilled water, and then the white sediment obtained at the bottom of the test tube was dried under vacuum at 80 C for 12 h. (16) Zhu, W. X.; Sun, D. J.; Liu, S. Y.; Wang, N.; Zhang, J.; Luan, L. Y. Colloids Surf., A 2007, 301, 106. (17) Bohmer, M. R.; Evers, O. A.; Scheutjens, J. M. H. M. Macromolecules. 1990, 23, 2288. (18) Walz, J. Y.; Sharma, A. J. Colloid Interface Sci. 1994, 168, 485. (19) Odijk, T. Langmuir. 1997, 13, 3579. (20) Ferreira, P. G.; Dymitrowska, M.; Belloni, L. J. Chem. Phys. 2000, 113, 9849.

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Figure 1. XRD patterns of dried powders of pure Mg2Al LDH (a) and the LDH-PVP mixture after thorough centrifugation and washing with distilled water (b).

2.2. Characterization of Materials. Atomic force microscopy (AFM), which was recorded on a Digital Instrument Nanoscope IIIA Multimode system (Santa Barbara, CA) in tapping mode was used to characterize LDH particles because particle diameter and thickness can be obtained simultaneously and also because of its high resolution and ability to image individual particles. Powder X-ray diffraction patterns (XRD) were collected using a Ringku D/MAX-γA X-ray diffractometer with Cu KR radiation (λ = 1.54184 A˚). Infrared spectra were collected on a VERTEX-70 FTIR after 40 scans from 4000 to 370 cm-1 at a resolution of 4 cm-1 by measuring the IR absorbance of a KBr disk containing 1 to 2 wt % of the Mg2Al LDH sample. The rheology measurements were performed on a Hakke RS75 rheometer with a coaxial cylinder sensor system (Z41 Ti). The macroscopic phase separation of LDH-PVP mixtures was observed between crossed polarizers. The chemical composition of the Mg2Al LDH was Mg0.68 Al0.38(OH)2.32Cl0.18 3 0.79H2O on the basis of chemical analysis. The zeta potential of the particles in the suspension without NaCl was +40 mV, which was measured with a Malvern Zetasizer 3000 (Malvern, U.K.).

3. Results and Discussions 3.1. Characterization of the Interaction between Mg2Al LDH Particles and PVP. The XRD patterns of pure Mg2Al LDH and the LDH-PVP mixture (Figure 1) indicate that both samples exhibit a hydrotalcite-like layered structure due to the relation d(003) = 2d(006) = 3d(009), and the basal spacing is 0.77 nm. Taking into account the thickness of 3.2 nm obtained from AFM (Figure 2, Table 1), it can be inferred that one Mg2Al LDH particle is a stack of four brucite-like nanosheets. Obviously, the added PVP does not intercalate into interlayers nor affect the crystal structure of Mg2Al LDH. Pure Mg2Al LDH and the LDH-PVP mixture form hexagonal platelets with an average corner-to-corner diameter of 120 nm and a thickness of 3.2 nm, and the polydispersity values in particle diameter and thickness are approximately unchanged (Figure 2, Table 1). FTIR spectra of pure Mg2Al LDH and the LDH-PVP mixture (Figure 3) both show typical Mg2Al-Cl LDH peaks including a broad band at 3450 cm-1, a rather weak peak at 1365 cm-1 (due to CO3-2 converted from CO2 captured from air during drying), a peak at 1625 cm-1 (δH2O), a band at 680 cm-1 Langmuir 2009, 25(11), 6349–6356

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Figure 2. AFM images of particles in pure Mg2Al LDH (a) and the LDH-PVP mixture (b). Table 1. Characteristics of Mg2Al LDH Platelets before and after the Addition of PVP sample

ÆDæ (nm)

ÆLæ (nm)

σD (%)a

σL (%)a

ÆLæ/ÆDæb

LDH 120 3.2 23 20 0.026 LDH + PVP 120 3.2 24 19 0.026 a The polydispersity in average diameter ÆDæ and thickness ÆLæ is given √ 2 √ by σL = [ÆL æ - ÆLæ2]/ÆLæ and σD = [ÆD2æ - ÆDæ2]/ÆDæ, respectively. b ÆLæ/ÆDæ expresses the aspect ratio of the Mg2Al LDH particles.

(M-O vibration), and a sharp peak at 447 cm-1 (typical evidence for Mg2Al LDH hydroxides).21-23 It is noticeable that there are no characteristic peaks of PVP in the spectra. In terms of the AFM and FTIR analyses, we can conclude that PVP does not adsorb onto the Mg2Al LDH particle surface and thus is free in suspension. As referred to in section 1, the depletion attraction can be induced between colloidal particles by the addition of nonadsorbing polymer, and it depends on the size ratio of the polymer coil over the diameter of the colloidal particle.24-26 In this study, the radius of gyration and the effect diameter of the PVP coil can be 27 calculated by the equations Rg  M3/5 w (nm) and Æd æ = 2.25Rg (nm),27 which give Rg ≈ 42 nm and Æd æ ≈ 94 nm. The size ratio of the polymer coil diameter over the platelet diameter (Æd æ/ÆDæ) (21) Hernandez-Moreno, M. J.; Ulibarri, M. A.; Rendon, J. L.; Serna, C. J. Phys. Chem. Miner. 1985, 12, 34. (22) Xu, Z. P.; Stevenson, G.; Lu, C. Q.; Lu, G. Q. J. Phys. Chem. B. 2006, 110, 16923. (23) Xu, Z. P.; Zeng, H. C. J. Phys. Chem. B. 2001, 105, 1743. (24) Lekkerkerker, H. N. W.; Poon, W. C. K.; Pusey, P. N.; Stroobants, A.; Warren, P. B. Europhys. Lett. 1992, 20, 559. (25) Sperry, P. R. J. Colloid Interface Sci. 1984, 99, 97. (26) Poon, W. C. K.; Pusey, P. N. Observation, Prediction and Simulation of Phase Transitions in Complex Fluids; Baus, M.; Rull, L. F., Ryckaert, J.-P., Eds.; Kluwer Academic: Dordrecht, The Netherlands, 1995. (27) de Hek, H.; Vrij, A. J. Colloid Interface Sci. 1981, 84, 409.

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is 0.78, and according to the literature,7-9,28 the addition of nonadsorbing PVP can induce a depletion attraction effect between the Mg2Al LDH particles. The mechanism of the depletion attraction induced by the nonadsorbing polymer between Mg2Al LDH particles can be illustrated on the level of a pair of platelets by the schematic picture in Figure 4.29-32 Mg2Al LDH particles are positively charged, so in aqueous suspensions they are surrounded by a diffuse electrical double layer (depletion layer, as presented by the dashed lines in Figure 4a) and stabilized by electrostatic repulsion. When the polymer is added to the colloidal suspensions (Figure 4b), because the depletion layers are impenetrable to the polymer, the polymer concentration varies from zero (around the depletion layer) to the value of the bulk polymer, so there is a polymer concentration gradient resulting in an osmotic pressure gradient around the particles. For a single platelet, the osmotic pressure is balanced. However, when two platelets approach each other and the depletion layers start to overlap, the osmotic pressure becomes unbalanced and there is a net osmotic pressure around the platelets, as indicated by the arrows in Figure 4b. The net osmotic pressure pushes the two platelets together and gives rise to the so-called depletion attraction effect. The range and strength of this depletion attraction can be controlled by tuning the size and concentration of the polymer. Also, this effect is related to the solvent quality, the properties of the suspensions, and particle size. (28) Sear, R. P. Phys. Rev. Lett. 2001, 86, 4696. (29) Asakura, S.; Oosawa, F. J. Chem. Phys. 1954, 22, 1255. (30) Koenderink, G. H.; Vliegenthart, G. A.; Kluijtmans, S. G. J. M.; van Blaaderen, A.; Philipse, A. P.; Lekkerkerker, H. N. W. Langmuir 1999, 15, 4693. (31) Tuinier, R.; Rieger, J.; de Kruif, C. G. Adv. Colloid Interface Sci. 2003, 103, 1. (32) Yang, S.; Yan, D. D. Phys. Rev. E. 2006, 74, 041808.

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Figure 5. Phase behavior of the LDH (20 wt %)-PVP (0.3 wt %) mixture as observed between crossed polarizers: (a) just prepared, (b) 2 days later (c) after 30 days, and (d) after 55 days. (e) Schematic of the multiphase coexistence.

Figure 3. FTIR spectra of dried powders of pure Mg2Al LDH (a) and the LDH-PVP mixture after thorough centrifugation and washing with distilled water (b).

Figure 6. Magnification of the liquid-crystal droplets (a) and the phase boundaries (b) from Figure 5d.

Figure 4. Schematic of the depletion attraction induced between two positively charged Mg2Al LDH platelets (ellipses in solid lines with “+” on the surfaces). The diffuse electrical double layer (depletion layer) thickness is κ-1 and is indicated by dashed lines. When there is no polymer (a), the particles are stabilized by electrostatic repulsion. After polymer is added (b), the osmotic pressure around the platelet is unbalanced when the depletion layers overlap. The net osmotic pressure is indicated by the arrows.

3.2. Phase Behavior of LDH-PVP Mixtures. In pure Mg2Al LDH suspensions, as the particle concentrations increase we observe an isotropic sol phase, isotropic-nematic coexistence, a nematic phase, and a nematic gel phase, but not any other phases. The I-N and sol-gel transitions are located at particle concentrations of 25 and 30 wt %, respectively,33 but the addition of nonadsorbing PVP enriches the phase behavior of such suspensions. Figure 5 shows the phase behavior of 20 wt % Mg2Al LDH suspensions with 0.3 wt % PVP. The LDH-PVP mixture shows birefringence due to flow during sample preparation (Figure 5a). The flow birefringence relaxes within 2 days, and a liquid phase rich in polymer but poor in LDH separates from the turbid sample volume (Figure 5b). After a resting time of 30 days, a birefringent nematic phase (N2) appears in the middle of the cuvette (Figure 5c). After a resting time of 55 days, remarkable six-phase coexistence is observed (Figure 5d). As the volume of the N2 phase increased with the sedimentation of liquid-crystal droplets (Figure 6a), the phase above the N2 phase appeared as a faint nematic phase (N1). Besides, a diluted isotropic (I1) and a more concentrated isotropic (I2) phase are also observed on the top of the N1 phase. Figure 5e shows the schematic representation of six-phase coexistence comprising a sediment phase (S), a nematic phase (N2), a faint birefringent phase (N1), a concentrated (33) Liu, S. Y.; Zhang, J.; Wang, N.; Liu, W. R.; Zhang, C. G.; Sun, D. J. Chem. Mater. 2003, 15, 3240.

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isotropic phase (I2), a diluted isotropic phase (I1), and a transparent liquid (L) (from the bottom up). A recent computer simulation using cut spheres to represent colloidal platelets with nonadsorbing polymer showed that to induce an additional isotropic phase the polymer should not be too small.8 For a platelet aspect ratio of ÆLæ/ÆDæ = 0.1, the size ratio required for I1 and I2 phase coexistence was Æd æ/ÆDæ g 0.2, whereas for ÆLæ/ÆDæ = 0.05, Æd æ/ÆDæ = 0.2 is not large enough.8 That is to say, the I1-I2 coexistence is driven by depletion attraction induced by the addition of nonadsorbing polymer, and the lower the platelet aspect ratio, the higher the size ratio that is required for the two isotropic phases to coexist.8 I1 and I2 phase coexistence occurred in a system of gibbsite platelets and nonadsorbing polymer with ÆLæ/ÆDæ = 0.067 when Æd æ/ÆDæ ≈ 0.35.12 In mixtures of Mg2Al LDH and PVP, where ÆLæ/ÆDæ = 0.026, the I1 and I2 phase are observed for Æd æ/ÆDæ = 0.78, albeit in coexistence with other phases. The coexistence of N1 and N2 phases may be interpreted by nematic-nematic (N-N) demixing, which originates from a competition among translational entropy, mixing entropy, and orientational entropy (all favoring the mixed state) and excluded volume entropy (favoring demixing). At a sufficiently high density gradient, the excluded volume entropy will become dominant, and demixing occurs.34 A similar mechanism was also given by Roij and Mulder, who suggested that the N-N demixing driving force lies in the excess excluded volume entropy at higher concentrations.35 For pure colloidal suspensions, N-N demixing has been observed experimentally in suspensions of lengthpolydisperse rods but not in diameter-polydisperse rods.36-39 (34) Wensink, H. H.; Vroege, G. J.; Lekkerkerker, H. N. W. J. Phys. Chem. B. 2001, 105, 10610. (35) van Roij, R.; Mulder, B. J. Phys. II 1994, 4, 1763. (36) Vroege, G. J.; Lekkerkerker, H. N. W. J. Phys. Chem. 1993, 97, 3601. (37) van Roij, R.; Mulder, B. J. Chem. Phys. 1996, 105, 11237. (38) Itou, T.; Teramoto, A. Macromolecules 1984, 17, 1419. (39) Kajiwara, K.; Donkai, N.; Hiragi, Y.; Inagaki, H. Makromol. Chem. 1986, 187, 2883.

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Furthermore, N-N demixing was also found in rod-plate mixtures, where an isotropic phase coexisted with a separate rod-dominated (N+) and plate-dominated uniaxial nematic phase (N-).40 The polydispersity in length and diameter for rods is equal to that in diameter and thickness for platelets, respectively. Theoretically, N-N demixing has been found in polydisperse platelets at sufficiently high density gradients; however, this did not occur in practical suspensions of polydisperse gibbsite platelets because of their much higher polydispersity in thickness (σL = 50%).41 Compared to suspensions of gibbsite platelets, the thickness polydispersity of Mg2Al LDH particles is much lower (σL = 19%) whereas the gap in diameter polydispersity of both systems is not wide. For colloid-polymer mixtures, computer simulation shows that an infinitely thin plate-rod mixture (L = 0.2D) undergoes N-N demixing as a result of the introduction of a nonspherical depleting polymer.7 The short and infinitely thin rods can be regarded as low aspect ratio platelets so that the model system can be considered to be a bidisperse colloidal suspension containing low- and high-aspect ratio platelets, which is analogous to our polydisperse experimental systems. Furthermore, in the process of particle sedimentation, there is a particle density gradient over the whole nematic region and the number of Mg2Al particles in the N1 phase gradually decreases, so the effective excluded volume of each particle is increased. Therefore, the observation of N-N demixing in LDH-PVP mixtures is possible, which is a result of interactions between the gaining of excluded volume entropy and the particle polydispersity. N1 is a liquidlike phase containing smaller liquid-crystal droplets, and N2 is a close-packed solidlike phase with obvious birefringence The N1 and N2 phases are both nematic in nature. Although the sample has been macroscopically observed for about 2 months, the phase behavior of the LDHPVP mixtures continuously changes. The phase boundaries are not very sharp (Figure 6b) and do not reach a steady state. These observations indicate that the phase separation has not reached thermodynamic equilibrium, and it may require a long enough resting time. Given the present results, one may question that the appearance of multiple phase conflicts with the Gibbs phase rule, which implies that at a given temperature the number of coexisting phases is limited to three for a binary mixture (Mg2Al LDH particles and PVP molecules). One possible explanation for the multiphase coexistence in LDH-PVP mixtures is the gravitational field, where we observed the sedimentation of the liquidcrystal droplets and a sediment phase on the bottom of the cuvette after a long resting time. Furthermore, the effect of gravity can be characterized by the sedimentation Peclet number (Pe) and the gravitational length (ξ).42,43 The Peclet number is the ratio of the time a particle takes to diffuse and sediment a distance equal to its diameter, Pe = tdiff/tsed = m*gD/kBT, where kB is the Boltzmann constant, T is absolute temperature, g is the gravitational acceleration, and m* is the buoyancy mass of the colloidal particle. The gravitational length is the height that an object must be lifted to increase its potential energy by 1kBT in a gravitational field, ξ = kBT /m*g, and hence Pe can be written as Pe = D/ξ, which changes the form of the ratio from a time scale to a length scale and simplifies the calculation. Because of the much smaller (40) van der Kooij, F. M.; Lekkerkerker, H. N. W. Langmuir 2000, 16, 10144. (41) van der Kooij, F. M.; van der Beek, D.; Lekkerkerker, H. N. W. J. Phys. Chem. B 2001, 105, 1696. (42) Wijnhoven, J. E. G. J.; van’t Zand, D. D.; van der Beek, D.; Lekkerkerker, H. N. W. Langmuir 2005, 21, 10422. (43) Zhang, J.; Luan, L. Y.; Zhu, W. X.; Liu, S. Y.; Sun, D. J. Langmuir 2007, 23, 5331.

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Figure 7. Phase behavior of the LDH-PVP mixtures with fixed cLDH (20 wt %) and varied cpol after a resting time of 55 days: (a) 0.1, (b) 0.3, (c) 0.6, and (d) 1.0 wt %.

buoyant mass of the polymer, the gravitational length of the polymer is much larger than that of the Mg2Al LDH particles, opposite to the Peclet number, so the effect of gravity on the polymer is negligible and it will separate from the volume in the initial stages of phase separation (the L phase). For an Mg2Al LDH platelet with ÆDæ = 120 nm and ÆLæ = 3.2 nm, m* can be expressed by (F - F0 ) ν, where F = 1000 kg/m3 and F0 = 2270 kg/m3, the mass density of the solvent and √ the particle, respectively, ν is the particle volume, and v = [(3( 3)/8)ÆDæ2ÆLæ = 3.0  104 nm3, so the Peclet number is Pe = 1.2  10-5. Under this condition, the particles in dilute suspensions have enough time to explore configurational space by Brownian motion and depletion attraction before the effect of gravity can be felt. However, for concentrated suspensions, the size and volume of particle aggregates is larger than that of a single platelet, as is the Peclet number, and hence gravity will act on the phase behavior of such suspensions. The gravitational length worked out from Pe = D/ξ is ξ = 1.0 mm, so the ratio of the sample height (H = 4.0 cm) to the gravitational length is H/ξ = 40. In contrast, the theoretical value for four-phase coexistence with fixed H = 1.5 cm is H/ξ g 11.70,9 so multiphase coexistence may appear in our present LDH-PVP mixtures. Another possibility to reconcile the multiphase coexistence with Gibbs phase rule is the particle polydispersity. In fact, the Mg2Al LDH suspensions with added PVP are polydisperse in both diameter and thickness (σD = 24%, σL = 19%), so the LDH-PVP mixtures can be regarded as suspensions containing infinitely monodisperse colloidal components with different diameter and thickness. The presence of many components in principle allows for the coexistence of an arbitrary number of phases.9,12 That is to say, particle polydispersity and gravity provide extra degrees of freedom for the Gibbs phase rule. The other possible explanation is the emergence of the additional isotropic phase, which leads to the appearance of multiphase coexistence. Therefore, the richness of phase behavior in LDHPVP mixtures is a consequence of interactions between PVPinduced depletion attraction, particle polydispersity, and particle sedimentation. 3.3. Effect of Polymer Concentrations. The effect of the nonadsorbing polymer on the phase behavior is the introduction of a depletion attraction, whereas the range and depth of this depletion attraction can be controlled by varying the size and concentration of the polymer, and thus at different cpol we can observe different phase transitions. The phase behavior of the LDH-PVP mixtures with fixed cLDH (20 wt %) and varied cpol after a resting time of 55 days is shown in Figure 7. At cpol of DOI: 10.1021/la804023b

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Figure 8. (a) Flow curves of the LDH-PVP mixtures with fixed cLDH (20 wt %) and varied cpol (I = 10-4 M): (1) 0.1 wt %; (solid, leftpointing triangle) 0.3 wt %; (solid, right-pointing triangle) 0.6 wt %; (9) 1.0 wt %; (2) 1.5 wt %; (b) pure LDH. (b). Stress dependence of the storage modulus (G0 ) and loss modulus (G00 ) of the mixtures containing 1.0 and 1.5 wt % PVP, respectively: solid symbols, storage modulus G0 ; open symbols, loss modulus G00 .

0.1 wt % (Figure 7a), the mixture shows four-phase coexistence. When cpol is up to 0.3 wt % (Figure 7b), a remarkable six-phase coexistence is observed; however, at higher cpol of 0.6 wt % (Figure 7c), the system collapses to two-phase coexistence, and when cpol is above 1.0 wt % (Figure 7d), only a cloudy isotropic phase appears. Besides, the volume of the sediment phase is reduced as cpol increases. The different phase behavior may be attributed to the variation in viscosity of the LDH-PVP mixtures because of the addition of the polymer, so we made rheological measurements on the same samples used in Figure 7. Figure 8a shows the flow curves of the LDH-PVP mixtures. Although they are all shear-thinning compared to the pure Mg2Al LDH suspension, the shear viscosity of the mixture is slightly reduced and then markedly increased with increasing cpol, and the critical polymer concentration is cpol = 0.3 wt %. The polymer with much lower cpol (cpol e 0.3 wt %) acts as a diluent so that the shear viscosity is reduced, whereas at higher cpol (cpol g 0.6 wt %), the polymers themselves have a higher viscosity and may be entangled with each other so that the shear viscosity is increased. The emergence of shear viscosity at higher polymer concentrations may relate to the solidlike structures with elastic properties in LDH-PVP mixtures. Because the pure Mg2Al LDH suspension of 20 wt % is a sol, we performed oscillatory experiments on the LDH-PVP mixtures with higher cpol, and the storage (elastic) modulus G0 and loss modulus G00 of the systems were determined. As shown in Figure 8b, for the mixture containing 1.0 wt % PVP, G00 is higher than G0 , and the mixture is slightly viscous. The opposite is observed in the mixture containing 1.5 wt % PVP, where G0 is higher than G00 and both stay constant up to a critical shear stress. This limited stress dependence indicates a nonlinear viscoelastic behavior typical of a gel. Another explanation for the difference in phase behavior is the variation in interactions between the Mg2Al LDH particles. At much lower cpol, in addition to the polymerinduced weak depletion attractions between the particles, particle sedimentation also affects the phase separation of the LDH-PVP mixtures. At moderate polymer concentrations (0.1 wt % e cpol e 0.3 wt %), the interactions among the particles are dominated by depletion attraction, which gives rise to multiphase coexistence. For 0.3 wt % e cpol e 1.0 wt %, polymers with higher viscosity may be entangled with each other and slow down the particle sedimentation. The phase separation is controlled by the competition among depletion attraction, particle sedimentation, and gelation. For cpol > 1.0 wt %, the entanglement of polymer may form an LDH-PVP network, which prevents particle sedimentation and the phase separation of the LDH-PVP mixtures. 6354

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Figure 9. Phase behavior of the LDH-PVP mixtures with fixed cpol (0.3 wt %) and varied cLDH: (a) 14 and (b) 23 wt %.

Therefore, polymer concentrations play an important role in controlling the depth of depletion attraction as well as the phase behavior of LDH-PVP mixtures. 3.4. Effect of Mg2Al LDH Concentrations. A striking effect of the addition of nonadsorbing polymer is the introduction of a nematic phase into the 14 wt % Mg2Al LDH suspension (Figure 9a), where only a single isotropic phase appears in the absence of depletion attraction. This indicates that I-N biphasic coexistence shifts to lower particle concentrations, even to a cLDH of 10 wt %. Besides, up to a higher cLDH of 23 wt % (Figure 9b), no matter what cpol is (represented by cpol = 0.3 wt %), the LDH-PVP mixtures show permanent birefringence and the shear textures do not relax over time, typical of a nematic gel. Namely, the I-N phase coexistent samples enter a purely nematic phase because of the addition of the nonadsorbing polymer. Therefore, the actual effect of the depletion attraction on the phase behavior depends on the range and depth of the depletion attraction as well as the properties of the Mg2Al LDH suspensions.

4. Summary of the Phase Behavior of LDH-PVP Mixtures All of the observations of the LDH-PVP mixtures are depicted in the phase diagram in Figure 10. Because high concentrations of the polymer may form an LDH-PVP entanglement network and prevent phase separation, we present only the phase behavior of cpol lower than 0.6 wt %. Apparently, the addition of nonadsorbing polymer remarkably enriches the phase behavior of Mg2Al Langmuir 2009, 25(11), 6349–6356

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Figure 10. Schematic of phase transitions in LDH-PVP mixtures. The solid lines represent boundaries between different phase regions. The dashed line represents the critical polymer concentration of 0.3 wt %. (L) Liquid phase, (I1) dilute isotropic phase, (I2) concentrated isotropic phase, (N1) faint birefringent phase, (N2) nematic phase, (S) sediment phase, and (b) experimental data points.

LDH suspensions. Because of the much smaller buoyant mass of the polymer, the effect of gravity on the polymer is negligible, and they will separate from the volume in the initial stages of phase separation. Up to a cLDH of 10 wt %, the LDH-PVP mixtures are isotropic sols and exhibit two-phase coexistence, comprising a liquid upper phase (rich in polymer but poor in LDH) and a dilute isotropic bottom phase (rich in LDH but poor in polymer). In such dilute suspensions, the Brownian motion causes particles to collide with each other, and the depletion attraction makes particles approach more closely. Both effects increase the probability of particle aggregation, so when cpol exceeds the critical value of 0.3 wt %, a more concentrated I2 phase takes the place of the dilute I1 phase. Now gravity is not strong enough to affect the phase separation. At intermediate particle concentrations (10 wt % e cLDH e 14 wt %), the formation of dense particle aggregates speeds up the sedimentation, a nematic phase coexists with a liquid phase, and an isotropic phase is observed in the LDH-PVP mixtures, where only isotropic sols appear in the absence of depletion attraction. Therefore, the I-N phase transition shifts to lower LDH concentration with increasing of polymer concentration, and the I2 phase replaces the I1 phase when cpol exceeds 0.3 wt %. Because particle sedimentation enables the particles to span a large density range in a cuvette and makes the larger particles settle down to the bottom,42-45 the LDH-PVP mixtures with cLDH from 14 to 23 wt % show surprising features. I1-I2 coexistence, N1-N2 demixing, and a sediment phase are also observed, which leads to the appearance of two-, three-, four-, and even six-phase coexistence. Here, the phase behavior is controlled by depletion attraction, particle sedimentation, and particle polydispersity simultaneously. However, above cLDH = 23 wt %, the gelation thoroughly hinders particle sedimentation, so no matter what cpol is, the LDH-PVP mixtures show permanent birefringence. That is to say, the I-N biphasic coexistent samples enter a purely nematic phase and they are nematic gels. Because of the polydispersity in particle diameter and the depletion

(44) van der Beek, D.; Lekkerkerker, H. N. W. Europhys. Lett. 2003, 61, 702. (45) van der Beek, D.; Lekkerkerker, H. N. W. Langmuir 2004, 20, 8582.

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attraction, the density gap of I-N phase coexistence is broadened from cLDH between 16 and 25 wt % to a range of 10 to 23 wt %. Compared to a previous report on pure Mg2Al LDH suspensons,43 the sol-gel transition also takes place at lower LDH concentration. Although there is not yet a clear and convincing explanation for gel formation in charged colloidal platelet suspensions, our results can be explained by the following factors. First, the average particle diameter in the present experiment is larger than that (ÆDæ = 102 nm, ÆLæ = 7.64 nm) in our previous reports. Michot et al. showed that larger particles underwent a sol-gel transition at much lower particle concentrations than the smaller ones in takovite suspensions.46 Similar behavior was also found in clay suspensions at higher ionic strength (I g 10-4 M), where the interactions between colloids are dominated by electrostatic attraction, such as the depletion attraction between Mg2Al LDH particles in gels of LDH-PVP mixtures.47,48 Second, the addition of nonadsorbing polymer to Mg2Al LDH suspensions with particle concentrations close to the sol-gel transition may form an LDH-PVP network by polymer entanglement and promote gelation. Most notably, the maximum number of phases that can appear simultaneously in the cuvette is governed only by the effective interactions between the colloidal particles. In pure Mg2Al LDH suspensions, electrostatic interactions and particle sedimentation lead to two-, three-, and four-phase coexistence,43 whereas an external PEG-induced effective attraction between colloids gives rise to five-phase coexistence.16 In our present LDH-PVP mixtures, the nonadsorbing-polymer-induced depletion attraction leads to I1-I2 coexistence, the gaining of excluded volume entropy and particle polydispersity give rise to N1-N2 demixing, and particle sedimentation leads to a density gradient within a single cuvette and makes the larger particles initially settle down to the bottom, so the appearance of six-phase coexistence is a result of interactions among PVP-induced depletion attraction, particle polydispersity, and particle sedimentation.

5. Conclusions The effect of nonadsorbing polymer on the phase behavior of suspensions of positively charged Mg2Al LDH platelets was investigated by birefringence observations and rheological measurements. Depletion attraction leads to the appearance of multiphase coexistence, and I1-I2 coexistence, N1-N2 demixing, and the sediment phase are observed at intermediate LDH and polymer concentration. At the same time, increasing the polymer concentration shifts the I-N biphasic coexistence and the sol-gel transition to lower LDH concentration, with the I-N phase coexistent samples entering a purely nematic phase. The richness of the phase behavior in LDH-PVP mixtures can be understood by the combined influences of depletion attraction, particle polydispersity and particle sedimentation. This study reveals the potential of colloidal Mg2Al LDH suspensions to serve as good model systems for investigating the effect of nonadsorbing polymer on the phase behavior of electrostatically stablized colloidal platelets. Further study should combine the computer simulation and theoretical work to address to what extent particle polydispersity and gravity affect the phase behavior of mixed suspensions of Mg2Al LDH and PVP. We also plan to investigate the different influences of the depletion attraction induced in (46) Michot, L. J.; Ghanbaja, J.; Tirtaatmadja, V.; Scales, P. J. Langmuir 2001, 17, 2100. (47) Gabriel, J-C. P.; Sanchez, C.; Davidson, P. J. Phys. Chem. 1996, 100, 11139. (48) Mourchid, A.; Lecolier, E.; Van Damme, H.; Levitz, P. Langmuir 1998, 14, 4718.

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sterically stabilized and electrostatically stabilized colloidpolymer suspensions. Acknowledgment. We acknowledge the financial support from the National Nature Science Foundation of China

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(20603020) and the Doctoral Programs Foundation of the Ministry of Education of China (20060422021). We thank Professor Xusheng Feng for helpful discussions and Professor Pamela Holt (Shandong University) for assistance in editing the manuscript.

Langmuir 2009, 25(11), 6349–6356