Langmuir 2007, 23, 3975-3980
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Mechanical Properties and Structure of Particle Coated Interfaces: Influence of Particle Size and Bidisperse 2D Suspensions C. Monteux,*,† E. Jung, and G. G. Fuller Department of Chemical Engineering, Stanford UniVersity, Stanford, California ReceiVed NoVember 20, 2006. In Final Form: January 10, 2007 We report surface pressure-area (Π-A) isotherms of bidisperse mixtures of anionic polystyrene latex particles at a water/n-decane interface as well as optical photographs of the interface for various compressions and mixture ratios. In the case of mixtures of 3 and 5 µm particles, we observe crystalline layers at high or low concentration ratios, where the “impurity” particles concentrate at the grain boundaries of the crystalline structure. At intermediate ratios, the layers become highly disordered. However, in both cases, we show that the shape of the isotherms remains unchanged. In the case of the mixtures of 9 µm particles with either 3 or 5 µm particles, the smaller particles aggregate around the larger particles through capillary interaction resulting in the formation of large fractal aggregates. At high compression, these layers contain holes that seem very compressible. As a result, the surface pressure isotherms show a smaller surface pressure jump than for other mixtures.
1. Introduction emulsions”,1
“Pickering or emulsions stabilized by particles, have been studied for many years.2-5 These systems can be found in a wide range of applications such as enhanced oil recovery, water purification, food, cosmetics, and controlled drug delivery. Pickering emulsions can be very stable since the particles attach irreversibly to the oil/water interface of the droplets, unlike the surfactants that are used in most formulations, and provide mechanical stabilization of the droplets against coalescence and Ostwald ripening.6 This stabilization was shown in recent studies involving single drop coalescence experiments.4,6-10 Interesting buckling instabilities have also been observed upon compression or drying of particle-coated interfaces.11-13 Predicting the onset of this instability could help determine the interaction between the particles.14 Furthermore, understanding the structures of these layers raises numerous fundamental questions. When charged latex particles of µm size are spread at an oil/water interface, they usually form a hexagonal lattice.13 This shows that longrange repulsion prevents the particles from aggregating through van der Waals and capillary attractive forces. The physical nature of this interaction is still debated,13,15-17 but one theory is that the uneven distribution of electric charges of the particle across * †
Corresponding author. E-mail:
[email protected]. Present address: PPMD-ESPCI, 10, rue Vauquelin, 75005 Paris, France.
(1) Pickering, S. U. J. Chem. Soc. 2001, 1907, 91. (2) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 8622-8631. (3) Vignati, E.; Piazza, R.; Lockhart, T. P. Langmuir 2002, 19, 6650-6656. (4) Melle, S.; Lask, M.; Fuller, G. G. Langmuir 2005, 21, 2158-2162. (5) Giermanska-Kahn, J.; Schmitt, V.; Binks, B. P.; Leal-Calderon, F. Langmuir 2002, 18 (7), 2515-2518. (6) Aveyard, R.; Clint, J.; Horozov, T. S. Phys. Chem. Chem. Phys. 2003, 5, 2398. (7) Stancik, E.; Koukhan, M.; Fuller, G. G. Langmuir 2004, 20, 90-94. (8) Stancik, E. J.; Fuller, G. G. Langmuir 2004, 20, 4805. (9) Horozov, T. S.; Aveyard, R.; Clint, J. H.; Neumann, B. Langmuir 2005, 21, 2330. (10) Ashby, N. P.; Binks, B. P.; Paunov, V. N. Chem. Comm. 2004, 4, 436437. (11) Xu, H.; Melle, S.; Golemanov, K. Fuller, G. G. Langmuir 2005, 21, 10016. (12) Vella, D.; Aussillous, P. Mahadevan, L. Europhys. Lett. 2004, 68 (2), 212. (13) Aveyard, R.; Clint, J. H.; Nees, D.; Paunov, V. N. Langmuir 2000, 16, 1969-1979. (14) Tsapis, N.; Dufresne, E. R.; Sinh, S. S.; Riera, C. S.; Hutchinson, J. W.; Mahadevan, L.; Weitz, D. A. Phys. ReV. Lett. 2005, 94 (1), 018302. (15) Hurd, A. J. J. Phys. A 1985, 18 (16), 1055. (16) Pieranski, P. Phys. ReV. Lett. 1980, 45 (7), 569.
the oil/water interface plays a role. It has been shown that this asymmetry can induce an effective dipole normal to the fluid interface.15,16 The dipole-dipole repulsion through the oil phase is of much longer range than through the water phase, where the interaction is screened because of counterion condensation. Recently, it has been shown that a Coulombic repulsion through the oil phase could be the cause of the long-range repulsion of the particles.13,17 In the case of larger particles (diameter g10 µm), the interface can be deformed close to the particle because of gravity, which causes capillary interactions between the particles.18,19 Once this occurs, a delicate balance between electrostatic repulsion and capillary attraction drives the formation of remarkable structures at the oil-water interface. Such structures can be observed in the present study for the case of 9 µm particles. Modifying the patterns formed at a particle-coated interface could also be useful for the design of new functional materials. By varying the hydrophobicity of the particles, Horozov et al.20,21 tuned the interactions between the particles at the interface. As a result, the structure of the layers as well as the surface pressure rise upon compression can be changed. Changing the size of the particles and using mixtures of particles are other ways to modify the structure of the layers. In this paper, we report surface pressure-area isotherms of µm-sized latex particles spread at the water/oil interface as well as images of the structures formed by these systems. In our study, we also present the case of bidisperse mixtures of particles. Indeed, in the case of mixtures, the long-range structures can be disturbed. Recently, numerical calculations predicted the formation of a crystalline structure for given size and concentration ratios.22,23 However, in our study, we observed only disordered structures. Overall, we find that the structures that are formed strongly depend on particle size and the fraction of particles of each size. (17) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I.; Horozov, T. S.; Neumann, B.; Paunov, V. N.; Annesley, J.; Botchway, S. W.; Nees, D.; Parker, A. W.; Ward, A. D.; Burgess, A. N. Phys. ReV. Lett. 2002, 88, 246102. (18) Kralchevsky, P. A.; Nagayama, K. AdV. Coll. Int. Sci. 2000, 85, 145. (19) Kralchevsky, P. A.; Denkov, N. D. Current Opin. Colloid Interface Sci. 2001, 6, 383. (20) Horozov, T. S.; Aveyard, R.; Clint, J. H.; Binks, B. P. Langmuir 2003, 19, 2822. (21) Horozov, T. S.; Binks, B. P.; Aveyard, R.; Clint, J. H. Colloids Surf. A 2006, 282-283., 377. (22) Rabideau, B. D.; Bonnecaze, R. T. Langmuir 2005, 21, 10856. (23) Stirner, T.; Sun, J. Langmuir 2005, 21, 14, 6636.
10.1021/la063380w CCC: $37.00 © 2007 American Chemical Society Published on Web 02/17/2007
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Monteux et al. XC-75) and both 10× (Nikon Plan Fluor WD 16.0), 2* (Nikon Plan Fluor WD 8.5) and 50x (Nikon ELWD 3.7-2.7 Ph2 DM) objectives. The Delrin barriers that were used to compress the monolayers contained holes to enable the flow of n-decane upon compression. The compression was performed by moving the barriers by hand, and the surface pressure was measured using the Wilhelmy plate method with a filter paper probe. The balance and software were provided by KSV instruments. The surface concentration of the particles and the particle fractions were determined by image analysis using the Metamorph software package (Molecular Devices, CA)
3. Results and Discussion Figure 1. Surface pressure-area isotherm, Π-A, for 5 µm particles spread at a water/n-decane interface. The letters a-e represent the areas for which panels a-e of Figure 2 were taken. The initial surface coverage before compression is 42%.
2. Experimental Section 2.1. Materials. To remove polar materials, n-decane (Sigma Aldrich purity >99%) was passed through chromatographic alumina (Sigma 3. The 3 µm particles form a hexagonal lattice and the 5 µm particles exist at the grain boundaries between the crystals. Figure 9b represents the case of F < 3, where the crystalline structure is lost.
aggregating mixtures and nonaggregating mixtures. In the case of 9/3 and 9/5 mixtures, we observed an attraction between the small particles and the 9 µm ones. In the case of 5/3 mixtures, no aggregation was observed. 3.2.1. Nonaggregating Mixtures of 5 and 3 µm Particles. Figure 5 shows the surface pressure isotherms corresponding to mixtures of 5 and 3 µm particles for 10 different ratios of surface concentrations, F ) N3/N5. From Figure 6, panels a and b, it can be seen that the organization of the particles can be very different depending on
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Figure 7. Surface pressure-area isotherms of mixtures of 9 µm and 5 µm particles for ratios F ) N5µm/N9µm equal to 0 (crosses), 0.7 (solid circles), 1.9 (solid diamonds), and 50 (open diamonds).
the ratio, F. For pure 3 or 5 µm particles, i.e., F ) ∞ and F ) 0, we observe the typical hexagonal packing of the particles (as already shown in Figure 2). In the case of the mixtures, for F > 3 (Figure 6a), the 3 µm particles form hexagonally packed crystals. In the vicinity of the 5 µm particles, the organization of the small particles is disturbed. These 5 µm particles can be viewed as impurities which concentrate at the grain boundaries of the crystalline structure of the smaller particles. For 0 < F < 3 (Figure 6b), the particles do not form a crystalline structure. Instead, they are randomly distributed at the interface. At high compressions, the large particles form strings at the interface that are similar in appearance to those observed for monodisperse layers. Recently, several numerical studies have predicted the formation of a mixed crystalline structure for given conditions of size and concentration ratios.22,23 We do not observe such a structure in these experiments. One difficulty in duplicating the conditions found in the simulations is obtaining the exact size ratios recommended by the numerical results, since the supplier of our particles provides only a limited choice of particles sizes. Additionally, it is experimentally extremely difficult to precisely control the amount of particles that cover the interface after they are injected. Indeed, a lot of particles do not attach to the interface following injection, and they fall into the aqueous phase.
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Although the organization of the particles is very different depending on the particle concentration ratio, it can be seen from Figure 5 that all of the isotherms have similar shapes. In particular, the slope of the isotherms before the plateau value at high compression remains constant regardless of this ratio. This result means that the compressibility of the layers does not depend on the ratio or on the degree of disorder of the layer. The only significant factor is the nature of the interactions between the particles, which are primarily electrostatic in nature. 3.2.2. Aggregating Mixtures of 9 µm with 5 µm and 9 µm with 3 µm Particles. Figure 7 displays the surface pressure isotherms of layers composed of mixtures of 9 and 5 µm particles. It is observed that the 5 µm particles aggregate around the 9 µm particles, forming flower-like structures (Figure 8e). When the number of these flower-like structures increases, they aggregate to form flower necklaces (Figure 8, panels a and c) that coexist with crystalline lattices of the small particles for intermediate ratios (Figure 8c). Upon compression (Figure 8, panels d and f), the lattice formed by the small particles ultimately buckles to form the in-plane strings that are characteristic of the pure 5 or 3 µm isotherms, and the aggregates become denser. At lower ratios, where all of the 5 µm particles are in the mixed aggregates, the layers becomes very disordered at high compression and contain holes (Figure 8b), similar to the case of the pure 9 µm particles. Interestingly, in the case of the hole-containing layers, the isotherms have the same shape as those measured for pure 9 µm particle systems; that is, they are characterized by a high surface pressure at low compression and slightly increasing pressure at high compression. For the higher ratios, when the number of small particles increases, the isotherms have the same shape as for pure 5 or 3 µm particles. 3.2.3. Mixtures of 9 µm with 1 µm Particles. Surprisingly, in the case of the mixtures of the 9 and 1 µm particles, the 1 µm particles do not readily attach to the 9 µm particles, whereas in the case of the 3 and 5 µm particles, this process takes only a few minutes. Instead, they were observed to deplete the surface area surrounding the larger particles (Figure 9, panels a and b). We attribute this behavior to the fact that the 1 µm particles are
Figure 8. Images of oil-water interfaces covered with mixtures of 9 and 5 µm particles at different surface coverages and surface concentrations ratios. (a) F ) 0.7, c ) 17%. Insert: sketch of the fractal aggregates composed of 9 and 5 µm particles. (b) F ) 0.7, c ) 90%. (c) F ) 6, c ) 40%. Insert: sketch of the fractal aggregates or “flower necklaces” that are composed of 9 and 5 µm particles. They coexist with a 2D-crystal of the 5 µm particles at the interface. (d) F ) 6, c ) 52%. (e) F ) 50, c ) 47%. Insert: sketch of the single “flower-like” structures that are formed when 5 µm particles decorate the 9 µm particles. These flowers coexist with a 2D-crystalline structure of 5 µm particles. (f) F ) 50, c ) 75%.
Properties and Structure of Particle Coated Interfaces
Figure 9. Images of oil-water interfaces covered with mixtures of 9 and 1 µm particles for increasing ratios F ) N1µm/N9µm. For panel a, F ) 9. For panels b and c, the ratio was not determined. In panel c, the 1 µm particles are so concentrated that they cannot be individually resolved.
Brownian and are more strongly subject to thermal fluctuations. Therefore, binding to the 9 µm particles would result in an unfavorable loss of entropy. Experimentally, we have seen that the small particles explore numerous spatial configurations around the big particles without sticking to them. From Figure 9c, it can be seen that for high values of the ratio, F ) N1µm/N9µm, the 9 µm particles do not form the fractal aggregates that can be seen in Figures 6a and 8a-d. The most likely reason is that the electrostatic repulsion between these concentrated 1 µm particles overcomes the capillary attraction between the 9 µm particles. For two 9 µm particles to stick together, a large number of 1 µm particles would have to be displaced, which is difficult in such a jammed repulsive network of 1 µm particles where viscosity is likely to diverge for such high surface coverages.24 This type of behavior can be viewed as a way to prevent fractal aggregation of large latex particles when using the patterns formed at the oil/water interface to microor nanopattern a solid surface using Langmuir deposition.
4. Conclusions We have investigated the surface pressure isotherms as well as the structure of particle-coated oil/water interfaces. We present the case of monodisperse layers of particles of diameter ranging (24) Cicuta, P.; Stancik, E. J.; Fuller, G. G. Phys. ReV. Lett. 2003, 90 (23), 236101.
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between 100 nm and 9 µm. Also for the first time we present results concerning bidisperse mixtures of particles. In most cases, the surface pressure isotherms of both monodisperse and bidisperse layers show the same features: (1) a large increase of the surface pressure upon compression and (2) a plateau in the surface pressure once buckling is macroscopically observable. The slope of the portion of the Π-A curve in the first feature, which is related to a surface compressibility of the layers, remains constant in most cases. In the case of monodisperse layers, we show that the shape of the surface pressure-area isotherms remains unchanged when the diameter of the particles increases from 100 nm to 5 µm. In particular, the slope of the isotherms between the inflection points remains equal, meaning that the compressibility of the layers does not depend on the size of the particles. One remarkable feature was observed when studying monodisperse systems of 9 µm particles. These particles form fractal aggregates at the interface. This aggregation phenomenon is due to the fact that these particles are heavier than the other particles studied, and they deform the interface due to gravity. As a result, there is a capillary attraction between the menisci surrounding the particles, and this causes the particles to bind to each other. In the case of the 9 µm particles, the surface pressure at low compressions is high in comparison to the smaller particles, probably because the particles form a percolated network across the trough. Above the surface pressure jump, the layers are composed of disordered zones, holes, and ordered patches. The surface pressure continues to increase above the surface pressure jump. During this portion of the isotherm, the holes are compressed, reducing the amount of pure oil/water interface, which causes the surface pressure to increase. In the case of bidisperse mixtures, we have observed a wide range of structures depending on the differences in particle size and concentration ratios. For nonaggregating mixtures (3 and 5 µm particles), the hexagonal packing lattice is disturbed when the ratio F increases. Al low ratios, the small particles form a crystalline structure where the large particles sit at the grain boundaries. At higher ratios, the layers become completely disordered. Mixtures of 9 µm particles with 3 or 5 µm particles are aggregating mixtures, where the small particles bind to the larger ones through capillary attraction. Depending on the ratio of small particles to large particles, two types of structures are observed. At high ratios, we observe flower-like structures, where 3, 4, or 5 small particles decorate the large particles. At low ratios, the particles form flower necklace structures, i.e., fractal aggregates of flowers. Surprisingly, there is no capillary attraction between the 9 µm and the 1 µm particles. We believe that this is due to the fact that the 1 µm particles are Brownian, and therefore, thermal fluctuations prevent them from binding to the 9 µm particles. Moreover, at high concentrations of small particles, the 9 µm particles do not aggregate, because the 1 µm particles form a jammed repulsive network of very high surface viscosity that kinetically arrests the aggregation process. Very interestingly, the shapes of the isotherms, the plateau values, and the slopes between the inflection points remain roughly constant in most of the investigated cases. This includes systems of pure particles (100 nm to 5 µm) as well as bidisperse mixtures, regardless of the degree of disorder or aggregation of the layers. The common point between all of these layers is that the particles can form in-plane strings just prior to buckling. There are only two cases studied here where the isotherms have different shapes, and they both occur when these prebuckling strings cannot be seen. This behavior is obtained for
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systems of pure 9 µm particles or mixtures of 9 with 5 µm particles at low values of F, when the particles form extended aggregates that fit in loose layers containing holes. In these cases only, the surface pressure at low compression is high, the surface pressure jump is small, and the surface pressure continues to increase at high compression. These results show that the surface pressure and the surface compressibility are not highly influenced by either the degree of disorder of the layers or the size of the particles. However, these parameters do depend on the types of interactions between the particles. It was shown by Horozov et al.20,21 that it also depends on the hydrophobicity of the particles. Hydrophobicity plays a role in the strength of attachment of the particles to the interface and also in the type of interaction between the particles. We are aware of several theoretical attempts to understand the shape of surface pressure-area isotherms of latex particles at the oil-water interface. Aveyard et al.13 are able to fit their isotherms with a model that takes into account the Coulombic
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repulsion between the charged particles through the oil phase. Fainermann et al.25 simply derive a model based on a 2D solution theory and are able to fit experimental data obtained with silanized particles. Both models are very general and could explain why the shape of the isotherms is highly robust even in the case of the particles mixtures. In order to understand the shape of the isotherms that we measure in the case of the 9 µm particles, one should take into account the fact that the particles aggregate and form holes that can be compressed even at low trough area. Acknowledgment. The authors thank Unilever for financial support of this work as well as G. Gavranovic for his careful reading of the manuscript. LA063380W (25) Fainerman, V. B.; Kovalchuk, V. I.; Lucassen-Reynders, E. H.; Grigoriev, D. O.; Ferri, J. K.; Leser, M. E.; Michel, M.; Miller, R.; Mohwald, H. Langmuir 2006, 22, 1701.
