Langmuir 1998, 14, 3129-3132
3129
Ordered Phase of Platelike Particles in Concentrated Dispersions A. B. D. Brown,† S. M. Clarke,† and A. R. Rennie*,‡ Polymers and Colloids, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, U.K., and Industrial Materials Group, Department of Crystallography, Birkbeck College, Malet Street, London WC1E 7HX, U.K. Received November 25, 1997. In Final Form: March 9, 1998 The coexistence of two phases in concentrated dispersions of monodispersed platelike particles is reported. Small-angle neutron diffraction has been used to identify a highly ordered phase which coexists at equilibrium with a less ordered, less dense phase. The interparticle interactions were varied by adjusting the salt concentration, and this influenced the concentration at which the phase transition occurred. Columnar or layer structures which could account for the observed diffraction pattern from the ordered phase are discussed.
Introduction Crystal structures formed by colloidal particles have attracted considerable attention as a means of testing statistical physical models of simple materials and as a consequence of their importance in a variety of applications.1-6 Experimental studies with spherical particles, sterically stabilized with grafted polymers, have shown that large particles may approximate the physics of hard spheres.7,8 In such systems crystallization occurs at high volume fractions7 whereas dispersions with longrange electrostatic repulsions will crystallize at low volume fractions as either fcc or bcc crystals.9-11 The influence of walls12 and even gravity4,5 has been shown to affect the crystal structure, morphology, or orientation as a consequence of the delicate balance of energy in these materials. There have also been some carefully controlled experiments on systems of rods, such as the work of Langmuir13 with vanadium pentoxide, and more recent studies, such as that done by van Bruggen et al.14 with Boehmite rods, where isotropic-nematic phase transitions have been observed. Some studies have been made with plates, and order has been observed in gelled systems of bentonite,13 * Please address correspondence to this author. † Cavendish Laboratory. ‡ Birkbeck College. (1) van Blaaderen, A.; Ruel, R.; Wiltzius, P. Nature 1997, 385, 321. (2) Woodcock, L. V. Nature 1997, 385, 141. (3) Bollhuis, P. G.; Frenkel, D.; Mau, S.-C.; Huse, D. A. Nature 1997, 388, 235. Woodcock, L. V. Nature 1997, 388, 236. (4) Russel, W. B.; Chaikin, P. M.; Zhu, J.; Meyer, W. V.; Rogers, R. Langmuir 1997, 13, 3871. (5) Zhu, J. X.; Li, M.; Rogers, R.; Meyer, W.; Ottewill, R. H.; Russel, W. B.; Chaikin, P. M. Nature 1997, 387, 883. (6) Velev, O. D.; Jede, T. A.; Lobo, R. F.; Lenhoff, A. M. Nature 1997, 389, 447. (7) Lekkerkerker, H. N. W. In Structure and Dynamics of Strongly Interacting Colloids and Supramolecular Aggregates in Solution; Chen, S.-H., Huang, J. S., Tartaglia, P., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1992; p 97. (8) Pusey, P. N.; van Megen, W.; Bartlett, P.; Ackerson, B. J.; Rarity, J. G.; Underwood, J. M. Phys. Rev. Lett. 1989, 63, 2753. (9) Ottewill, R. H. Langmuir 1989, 5, 4. (10) Monovoukas, Y.; Gast, A. P. J. Colloid Interface Sci. 1989, 128, 533. (11) Clarke, S. M.; Rennie, A. R.; Ottewill, R. H. Langmuir 1997, 13, 1964. (12) Clarke, S. M.; Rennie, A. R. Faraday Discuss. 1997, 104, 49. (13) Langmuir, I. J. Chem. Phys. 1938, 6, 873. (14) van Bruggen, M. P. B.; van der Kooij, F. M.; Lekkerkerker, H. N. W. J. Phys: Condens. Matter 1996, 8, 9451
montmorillonite, and laponite,15 where a complex combination of attractive and repulsive forces are present. Ordered structures have also been observed in sediments of tungstic acid and iron oxides,16,17 where the formation of the ordered phase is driven by sedimentation under gravity. In contrast to these two cases, this paper presents a system of particles with controlled short-range interactions and a size that is sufficiently small that gravitational forces are small compared to thermal motion. Such a system can provide a comparison with liquid crystalline systems of discotic molecules which have displayed a rich variety and complexity of structures.18 Columnar mesophases may have a wide range of stability in systems of discotic particles19 whereas a nematic phase is likely to exist only over a small region of concentration. Experimental Section Platelike particles of nickel(II) hydroxide were prepared according to the method of Durand-Keklikian et al.20 using controlled precipitation from dilute aqueous solution of 0.020 M nickel(II) nitrate and 0.010 M ammonium hydroxide which were aged at 90 °C for 2 h and gave 0.9 g dm-3 of particles. To stabilize the particles, which as formed are easily flocculated, 2.6 g dm-3 of polyacrylate, N40 Allied Colloids, with a molecular mass about 3500 g mol-1, was added.21 The sample was then stable and could be centrifuged to the required concentration for the experiments. Samples were characterized with photon correlation spectroscopy and transmission electron microscopy. The nickel hydroxide was observed (see Figure 1) as thin, hexagonal plates with a diameter of approximately 85 nm and an aspect ratio of approximately 9:1 and had a polydispersity of 14% in diameter. No flocculation was observed by dynamic light scattering; reproducible results were obtained for samples (15) Gabriel, J. P.; Sanchez, C.; Davidson, P. J. Phys. Chem. 1996, 100, 11139. (16) Furusawa, K.; Hachisu, S. Sci. Light 1966, 15, 115. (17) Maeda, Y.; Hachisu, S. Colloids Surf. 1983, 6, 1. (18) Chandrasekhar, S.; Sadashiva, B. K.; Suresh, K. A. Pramana 1977, 7, 471. (19) Chandrasekhar, S. Liquid Crystals; Cambridge University Press: Cambridge, U.K., 1992. (20) Durand-Keklikian, L.; Haq, I.; Matijievic, E. Colloids Surf., A 1994, 92, 267. (21) Above pH 6 polyacrylate is known to be disassociated (communication from Allied Colloids). Adding different concentrations of polyacrylate to the sample at the concentration of the preparation shows that the charge on the particles has been reversed when 2.6 g dm-3 is added, suggesting adsorbtion of the polyacrylate. Further work is in progress to investigate this system.
S0743-7463(97)01294-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/08/1998
3130 Langmuir, Vol. 14, No. 11, 1998
Brown et al.
Figure 2. Fraction of the cell occupied by the more dense, ordered phase for a series of samples with different, overall weight fractions with 0.05 M added NaCl.
Figure 1. Transmission electron micrograph of nickel hydroxide particles. The scale bar is 100 nm. The diameter is readily observed, but only rarely (see box) can edges of the plates be seen. The uncertain tilt makes precise measurement of this dimension difficult. immediately after preparation and after redispersion to a concentration suitable for dynamic light scattering subsequent to observations. The interactions between the particles with an adsorbed layer of polyacrylate at low electrolyte concentration are principally electrostatic. A characteristic length scale for the charge interaction is given by the Debye-Hu¨ckel reciprocal screening length22 κ
∑n z
κ2 ) e2
2
i i
/kBT
(1)
which depends upon the electronic charge e, the valency of ions zi, their number concentration ni, the permittivity of the medium , and the temperature T. In these experiments sodium chloride was added to vary the screening of interparticle interactions; 0.05 M NaCl in water gives 1/κ as about 1.5 nm. Calculation of the amount of adsorbed polymer suggests that the layer is approximately 8 nm thick, which corresponds roughly to the fully extended length of the polyacrylate. Thus at 0.05 M NaCl the total range of interactions is only a few nanometers and is dominated by the polyacrylate. The large amount of stabilizer on these particles makes it difficult to provide a precise estimate of volume fraction which will depend on the hydrated density of the polyacrylate adsorbed at the interface. The aspect ratio is likely to be considerably reduced to about 5:1, but this may depend on the concentration of electrolyte. Small-angle neutron diffraction measurements were made with the D22 instrument at the ILL HFR in Grenoble, France, which is equipped with a 1 m × 1 m two-dimensional detector with 0.75 cm resolution. Two combinations of incident neutron wavelength, λ and sample-to-detector distance were used to cover a range of scattering space or momentum transfer Q between 0.01 and 2 nm-1. It is convenient to consider the circular average of the two-dimensional data, which provides intensity as a function of the magnitude of the momentum transfer Q, equal to (4π/λ) sin(θ/2), where θ is the scattering angle. A series of samples were prepared with different weight fractions of particles (generally accurate to about (0.003) at two different concentrations of added electrolyte. For both electrolyte concentrations it was found that some dispersions would separate (22) Hunter, R. J. Foundations of Colloid Science; Oxford University Press: Oxford, U.K., 1986; Vol. 1.
Figure 3. Scattering from dispersions at a range of weight fractions with 0.01 M added NaCl. (a) 0.577; (b) 0.561; (c) 0.550; (d) 0.531; (e) 0.516; (f) 0.494; (g) 0.471; (h) 0.441. For clarity each data set has been offset vertically. as droplets which then sedimented, forming a dense lower phase with a more dilute upper phase. This could be observed readily, and the droplets and lower phase displayed some birefringence, whereas no birefringence was observed in the upper phase. Further detailed optical observations could not be made due to the opacity of the sample. The fraction of the sample occupied by the more dense phase as a function of total weight fraction of particles for samples with 0.05 M added electrolyte is shown in Figure 2. This shows that there is a region of coexistence of phases with weight fractions 0.601 ( 0.003 and 0.68 ( 0.01. The volume of the concentrated phase varies linearly with weight fraction, as would be expected in an equilibrium coexistence. At 0.01 M added electrolyte a similar effect is observed but with phase separation between weight fractions of 0.556 ( 0.004 and 0.64 ( 0.01. The scattering from a number of dispersions at weight fractions between 0.441 and 0.577 at this salt concentration is shown in Figure 3. At low particle fractions there is a broad peak and no phase separation is observed, but at weight fractions above 0.550 a sharp peak is seen with phase separation. The scattering includes contributions from both phases when they coexist but is dominated by the sharp features from the more ordered phase.
Platelike Particles in Concentrated Dispersions
Langmuir, Vol. 14, No. 11, 1998 3131
Figure 4. Scattered intensity from the lower phase in sample A, which had a weight fraction 0.635 and 0.01 M NaCl, and sample B, with weight fraction 0.648 and 0.05 M NaCl. The numbers of the peaks correspond to those given in Table 1. Data for sample B have been multiplied by 5. Table 1 sample A
sample B
peak
Q/nm-1
d/nm
Q/nm-1
d/nm
1 2 3 4 5
0.071 0.124 0.26 0.52 0.77
88 51 24 12 8.2
0.074 0.134 0.32 0.66 0.99
85 47 20 9.5 6.3
The dilute, upper phase and the ordered, lower phase were found to have different scattering patterns indicating different structures. The scattering from the lower phase of a dispersion with an overall particle weight fraction 0.577 in 0.01 M NaCl solution, referred to as sample A, over a wider range of Q is shown in Figure 4. The scattering pattern of the upper, less ordered phase of sample A can be compared with the data in Figure 3 and suggests a weight fraction between 0.550 and 0.561, which is in agreement with its density. Data from a second sample, designated B, prepared with a higher weight fraction of 0.677 in 0.05 M NaCl, which showed no phase separation after 24 h, are also shown in Figure 4. This sample was extremely viscous and could only be inserted in the cell by gentle centrifugation. It is possible that it had not reached a state of homogeneity or had not separated to equilibrium phases by the time the neutron measurements were made. A series of diffraction peaks are present in the scattering from the sediment both in sample A and sample B. The positions and d-spacings of these peaks are given in Table 1. The two-dimensional intensity data for sample B indicated a preferential orientation of peaks 1 and 3, showing that they arise from orthogonal planes within the sample. The width of the peaks is dominated by instrument resolution, which varies in the range 10% < ∆Q/Q < 25% over the range of data shown.
Discussion and Conclusions The positions of reflections 3, 4, and 5 are consistent with their being first, second, and third orders of a single d-spacing associated with a length scale a few times larger than the thickness of the plates. The position of peak 1 suggests that it is associated with a feature of the order of the diameter of the particles. The diffraction for the ordered phase could result from several structures which cannot be unambiguously distinguished at present: two possible models are shown in Figure 5. In a columnar phase peaks 3, 4, and 5 would correspond to a regular spacing of particles along columns stacked face-to-face. Peaks 1 and 2 would arise from the arrangement of the columns. A simple hexagonal packing would be in reasonable agreement with the observed patterns. An alternative structure with layers of hexagonally arranged
Figure 5. Two possible schematic structures of the dense, more ordered phase: (A) a columnar structure; (B) a layered structure. Distance a corresponds to peak 1, and distance c corresponds to peak 3 in Figure 3.
particles would give peaks 1 and 2. Reflections 3, 4, and 5 would then correspond to the stacking of these layers at a regular spacing but with no lateral correlation of adjacent layers. At present we are unable to distinguish between these alternatives or structures that might include some three-dimensional order with diffraction peaks that were too weak to observe. To determine such detail, it would be necessary to obtain relative intensities of Bragg peaks from either a perfect powder or a single crystal. All the samples prepared showed some preferential orientation that arose either as a consequence of shear on inserting in the cell or from the influence of the cell walls. The principal difference between the ordered structures in samples A and B is the position of the third peak. There is only a small change in the position of peak 1 (4%) compared to the change in the position of peak 3 (23%) between the two samples, which differed by about 6% in composition by weight. The change to larger Q in the sample at higher weight fraction suggests that the packing becomes more dense along the normals to the faces of the particles. There is little evidence of change in the lateral packing of the particles side-to-side. These structural variations are not unreasonable, as the long-range repulsion between the particles faces has been reduced by the addition of more salt and the spacing suggests that the particles are almost close-packed laterally. The high electrolyte concentration may also reduce the lateral extent of the stabilizer. The lack of birefringence in the upper phase suggests that it is isotropic and not nematic as might be expected. These experiments have shown that platelike particles can separate at equilibrium to an ordered and less-ordered phase. The precise structure depends sensitively on the interparticle interactions, but a reasonable approach to hard plates can be achieved. Order in perpendicular directions has clearly indicated a structure of higher order than a nematic liquid crystal. The phase behavior with initial separation into dispersed, small drops and the development of ordered structures offer several new prospects. For example, self-assembly of anisotropic, colloidal particles in ordered arrays may have applications in nanofabrication. Use of these monodisperse materials may allow preparation and study of model mesophases such as liquid crystals. The phase behavior may be significantly different from that for polydisperse particles.
3132 Langmuir, Vol. 14, No. 11, 1998
Acknowledgment. We are grateful to the ILL for the use of neutron facilities and Dr. Roland May for his assistance. We also thank the School of Veterinary Medicine, Cambridge, for use of preparative centrifuges and Allied Colloids for providing the stabilizer. The U.K.
Brown et al.
EPSRC provided a research studentship for A.B.D.B. We are grateful to a reviewer for drawing our attention to ref 13. LA971294D
