Langmuir 2007, 23, 5289-5291
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Spontaneous Two-Dimensional Spherical Colloidal Structures He´ctor Gonza´lez-Ochoa† and Jose´ Luis Arauz-Lara*.†,‡ Instituto de Fı´sica “Manuel SandoVal Vallarta”, UniVersidad Auto´ noma de San Luis Potosı´, AlVaro Obrego´ n 64, 78000 San Luis Potosı´, S.L.P., Mexico, and Departamento de Fı´sica, CINVESTAV, AVenida IPN 2508, Colonia Zacatenco, 07360 Mexico D.F., Mexico ReceiVed January 17, 2007. In Final Form: March 16, 2007 Two-dimensional spherical crystalline colloidal structures are formed at the interface between water and oil as the result of spontaneous emulsification and colloidal self-assembly. When water droplets are introduced in oil containing a lipophilic surfactant, smaller water droplets of uniform size are spontaneously produced at the spherical interface. Initially of submicrometer size, the small droplets at the interface self-assemble, forming ordered structures, and grow uniformly with time until they reach a size of a few micrometers, maintaining the crystalline structure.
Water and oil, two immiscible liquids, readily phase separate when mixed. However, when surface active agents are added to the system, those liquids can form a myriad of interesting structures, depending on their relative proportions and the amounts and properties of the surfactants.1,2 A simple example is the production of emulsions, that is, the dispersion of one liquid into the other in the form of small droplets (sizes in the micrometer range). Emulsification usually requires a supply of external energy to enlarge the interfacial area between the liquids where the surfactant molecules self-assemble, increasing the lifetime of the dispersion. As it happens, much of the supplied energy is used to shear the liquids and therefore is lost by viscous dissipation. Thus, the development of efficient, energy-saving methods are of great interest in a number of applications in industries such as those of foods, pharmaceuticals, cosmetics, paints, and so forth, where the production of emulsions is an everyday process. Interestingly, emulsification can also occur spontaneously; that is, one of the liquids can readily disperse into the other in the form of small droplets when both liquids are put in contact, without the need of external forces. Spontaneous emulsification (SE) phenomena are really exciting to observe, and their understanding poses challenging questions of current interest in science and engineering. It is clear that spontaneous emulsification can have multiple interesting applications, not only as an energy saving mechanism to produce emulsions, but it can be quite useful in situations where energy is difficult to be supplied externally as it is for the case, for instance, of porous oil fields.3 Spontaneous emulsification has been observed to occur under different experimental conditions,4 always in the presence of surfactants in at least one of the phases. Accordingly, different mechanisms have been proposed to account for those observations. Here, we mention briefly a few of them to illustrate the complexity and diversity of the phenomenon and the increasing interest in it (for a review, see ref 4). For instance, the model of interfacial turbulence considers that thermal fluctuations in the diffusion of surfactant molecules moving toward the interface produce turbulence, resulting in the formation of droplets of one phase * To whom correspondence should be addressed. † Universidad Auto ´ noma de San Luis Potosı´. ‡ CINVESTAV. (1) de Gennes, P.-G.; Brochard-Wyart, F.; Que´re´, D. Capillarity and wetting phenomena: drops, bubbles, pearls, waVes; Springer: New York, 2004. (2) Sjo¨blom, J. Encyclopedic handbook of emulsion technology; Marcel Dekker: New York, 2001. (3) Schramm, L. L. Surfactants: Fundamentals and applications in the petroleum industry; Cambridge University Press: Cambridge, U.K., 2000. (4) Lo´pez-Montilla, J. C.; Herrera-Morales, P. E.; Pandey, S.; Shah, D. O. J. Dispersion Sci. Technol. 2002, 23, 219-268.
into the other. The model of diffusion and stranding considers the transport of material of one phase into the second, as carried by species dissolved in the first phase and diffusing into the second phase where they are more soluble. A third mechanism considers that transient negatiVe interfacial tensions between the two phases would increase the interfacial area, resulting in the emulsification of one phase into the other. Spontaneous emulsification can also be the result of a more elaborate process. Recent work with tertiary systems, water/dodecane/lipids (either anionic or nonionic), shows that SE occurs as the result of a complex combination of various mechanisms.5,6 There, it is shown that in those tertiary systems lipids self-assemble at the interface between both liquids, forming a thick nonuniform multilamellar film with liquid-crystalline structures. Swelling of that film leads to the spontaneous emulsification of water in oil and the appearance of multilamellar onions in the oil phase. The selfassembly of thick films of complexes of water, oil, and surfactants, on spherical interfaces between water and oil, has also been observed in double emulsions (W/O/W, water in oil in water) in the presence of both lipophilic and hydrophilic surfactants.7 In general, SE is likely to be a combination of more than one of the mechanisms mentioned above and/or other proposed mechanisms in the literature. An unrelated phenomenon to spontaneous emulsification is the self-assembly of colloids on curved surfaces. That matter is important on its own right and is of current interest, since it can be useful in addressing issues such as answering long standing unresolved questions like that of the minimum-energy configurations of particles on spherical surfaces,8,9 novel routes to colloidal stability,10 the design of strategies for the production of new materials with specific elasticity and permeability properties for the microencapsulation and release of drugs, or biological materials.11 A system of recent interest, where the self-assembly of colloidal particles on curved surfaces has been studied, is constituted by water droplets immersed in oil containing (5) Shchipunov, Y. A.; Schmiedel, P. Langmuir 1996, 12, 6443-6445. (6) Pautot, S.; Frisken, B. J.; Cheng, J.-X.; Xie, X. S.; Weitz, D. A. Langmuir 2003, 19, 10281-10287. (7) Gonza´lez-Ochoa, H.; Ibarra-Bracamontes, L.; Arauz-Lara, J. L. Langmuir 2003, 19, 7837-7840. (8) Bausch, A. R.; Bowick, M. J.; Cacciuto, A.; Dinsmore, A. D.; Hsu, M. F.; Nelson, D. R.; Nikolaides, M. G.; Travesset, A.; Weitz, D. A. Science 2003, 299, 1716-1718. (9) Lipowsky, P.; Bowick, M. J.; Meinke, J. H.; Nelson, D. R.; Bausch, A. R. Nat. Mater. 2005, 4, 407-411. (10) Tohver, V.; Smay, J. E.; Braem, A.; Braun, P. V.; Lewis, J. A. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 8950-8954. (11) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006-1009.
10.1021/la070144s CCC: $37.00 © 2007 American Chemical Society Published on Web 04/03/2007
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Figure 1. Schematic transversal view of the sample cell showing a water droplet at time t ) 0, sedimented at the bottom due to its higher density (a); top view image of the initial stage (time t ) 3 min) of a water droplet of ∼145 µm diameter immersed in oil with 0.5% (w/w) Span 80 (b).
colloidal particles dispersed in it. The self-assembly of the colloids on the water/oil interface can be induced by manipulating the interactions between the colloids or between the colloids and the two phases.8,9,11,12 For instance, the interactions can be manipulated by varying the surface tension differences between the particles and both liquids, by electrostatic repulsion or van der Waals interactions, and by other methods. The colloidal particles used in those works are usually polystyrene spheres of ∼1 µm diameter, but sterically stabilized poly(methyl methacrylate) particles can also structure around water droplets in decahydronaphthalene. The use of colloids with sizes in the micrometer range allows one the visualization and characterization in real space of the structures and dynamics of the self-assembled colloids by optical microscopy. Here, we report a novel manifestation of spontaneous emulsification, which, when combined with other spontaneous physical processes, leads to the formation of colloidal structures on spherical surfaces. We study an apparently quite simple system consisting of water droplets, oil, and a lipophilic surfactant. In marked contrast to the cases discussed in the reports mentioned above, in which the colloidal particles are added to the system, the assembling colloids in our case are spontaneously produced in situ; that is, they are small droplets of water spontaneously emulsified at the spherical interface between a larger water droplet and oil containing the lipophilic surfactant. Samples were prepared as follows. A small volume of pure deionized water was poured into oil (dodecane) containing a nonionic lipophilic surfactant (sorbitan monooleate, Span 80) at a given concentration. The system was gently sheared, by moving the vial back and forth by hand, to produce water droplets immersed in the oil phase. The samples were loaded in a glass cell of the dimensions 2 × 2 × 0.02 cm3 which was then sealed to avoid turbulence. Figure 1a shows a schematic view of the transversal section of the sample cell at time t ) 0. The water volume fraction φw was kept low (∼10-2) to have independent water droplets dispersed in the oil. Immediately after preparation, the sample cell was placed on the stage of an optical microscope for observation using long working distance, either 50× (numerical aperture 0.5) or 100× (numerical aperture 0.8), objectives. Figure 1b shows an optical microscope image, as seen from the top view, of a water droplet (dark ring) with a diameter of 145 µm immersed in dodecane with a Span 80 concentration of 0.5% (w/w). The picture was taken at t ) 3 min after sample preparation. At this early stage, one observes only the water droplet in the continuous oil phase, with a smooth interface between both liquids. However, as time goes on, simple systems such as the one pictured in Figure 1 evolve in a very interesting way. The time evolution of systems prepared with (12) Velev, O. D.; Furusawa, K.; Nagayama, K. Langmuir 1996, 12, 23742384.
Figure 2. Time evolution of the spontaneous crystalline structure formed on the spherical interface between a water droplet and oil containing a lipophilic surfactant. Images a-e were taken at t ) 2.5, 4.4, 6.1, 7.8, and 19.5 h, respectively, and image f was taken at t ) 10 days.
different amounts of surfactant was recorded using a charge coupled device (CCD camera), which is coupled to the optical microscope. The images of the samples were digitized, using a standard video frame grabber, at different times and stored for further analysis. Samples prepared using surfactant concentrations below 0.01% (w/w) did not show any appreciable unusual behavior; that is, they just remained as plain (inverted) emulsions of water in oil. However, in the samples prepared using larger concentrations of surfactant, we observed the spontaneous formation of small (submicrometer) objects at the spherical interface between the original large water droplet and oil. Figure 2 shows a set of images, taken at different times, of a water droplet of initial diameter σ ) 104 µm in oil with a Span 80 concentration of 2% (w/w). The image in Figure 2a is a picture taken at time t ) 2.5 h. The large circle is the water droplet, and the surrounding is the continuous oil phase with the dissolved surfactant. As one can see here, smaller spherical objects of ∼2 µm size, with a very narrow size distribution, appear, decorating the spherical surface of the water droplet. The objects cover practically the entire surface of the droplet, forming a two-dimensional spherical crystalline structure. The images in Figure 2b-f show the time evolution of the system. As seen here, a number of the objects remain attached to the interface and grow with time while maintaining an ordered structure, whereas the others go to the bulk of the oil phase. These observations pose different questions, and we believe the most prominent are those concerning (i) the origin and nature of the small objects, (ii) their narrow size distribution, (iii) what keeps them attached to the interface, and (iv) why they grow. Since our system consists only of water, oil, and Span 80 (a surfactant widely used to stabilize water in oil), the small spherical objects could be nothing else but small droplets of water spontaneously emulsified at the interface between the large water droplet and oil, without the supply of external energy. The small droplets could not be formed during sample preparation since they are not present at early times, as one can see in Figure 1b, which is a typical image of the initial stage of the immersed water droplets in all experimental conditions studied here. In addition, let us note that the same phenomenology is observed when a single large water droplet is directly injected in the oil phase, avoiding, in this way, the initial shearing of the sample. The small droplets, however, appear on the interface after some time, which can range from minutes to hours, depending on the
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amount of surfactant. In Figure 2a, the droplets have already reached a size of ∼2 µm, but they were smaller at earlier times. The first clear sight of the small water droplets occurs at ∼10 min after sample preparation. At that time, the total surface of the large water droplet appears already decorated with the crystalline structure of small spherical objects of ∼300 nm size. The initial size of the spontaneously emulsified small water droplets, which is likely to be in the range of tens of nanometers, is not captured in our experiment. We observe the system using an optical microscope, and we can see the droplets only when their size is within the optical resolution. Parts b-e of Figure 2 show images taken at times t ) 4.4, 6.1, 7.8, and 19.5 h, respectively, whereas Figure 2f shows an image taken 10 days after sample preparation. As the system evolves, the small water droplets attached to the interface grow, but some of them detach at different times and go to the oil phase, leaving room for those remaining attached to rearrange in a crystalline structure. The evolution process is slow, allowing the droplets to search for the configuration of minimum energy. One can also observe the continuous decrease of the large water droplet diameter. In Figure 2f, one can see a high concentration of small water droplets, with a broad size distribution, dispersed in the continuous oil phase (macroscopically, the sample at this time appears quite turbid, whereas at the initial time it appears quite transparent). At times much longer than 10 days (Figure 2f), the small water droplets reach a final maximum size and then all of them detach continuously from the mother droplet, leaving it as a simple water droplet in oil. The final size of the small water droplets depends on the initial size of the large water droplets. The amount of surfactant determines the kinetics of the process and other specific features. The main features of the process described here are reproducible under different conditions; they are observed to occur for a wide range of initial large water droplet sizes, from a few tens to a few hundreds of micrometers, and for a wide range of surfactant concentrations. We considered here surfactant concentrations up to 5% (w/w). At higher surfactant concentrations, the processes are much faster and other complexes develop in the system. The formation of mesoscopic crystalline structures of water droplets at the interface between water and oil is the result of a sequence of physical processes starting with the self-assembly of surfactant molecules in a thick (multilayer) film around the initial large water droplet. As mentioned above, such assemblies of surfactants have been observed in other experiments with similar systems.5,6,7 The film will swell at the layer of lower interfacial tension, driven by chemical potential gradients. The water accumulated in the surfactant layers will form liquid bridges, supported by a balance between wetting and surface tension. The liquid bridges will continue to form until they undergo an instability transition, breaking off in small droplets to reduce internal energy. Figure 3 shows a simplified schematic of the process. This simple picture explains in a consistent way much of the phenomenology observed in our system: the spontaneous, and simultaneous, production of a large number of nearly identical drops and their continuous swelling by being connected to the mother drop through the film of surfactant. The droplets are very likely produced collectively, probably by an instability transition of the water layer formed inside the surfactant film, driven by capillary forces, similar to the Plateau-Rayleigh instability.1 Coalescence between small water droplets, either at the interface or in the bulk, is not observed and can be ruled out due to the large amount of stabilizing surfactant available in the system. Furthermore, coalescence would lead to size polydispersity. Therefore, the growth of the small water droplets is not due to
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Figure 3. A simplified schematic of the process. (a) Right after sample preparation, a multilayer of surfactant forms at the interface between water and oil. (b) The film of surfactant swells, forming liquid bridges. (c) The liquid bridge undergoes an instability transition, breaking off in small water droplets of uniform size, reducing the internal energy, and then forming a crystalline structure.
coalescence and it cannot be due to Oswald-Ripening either, because the latter is the mechanism by which large drops grow at the expense of the smaller ones. Thus, it has to occur at the expense of the large drop, which is also suggested by the decrease of its diameter. The required connection between the large water droplet and the small water droplets at the interface would be provided by the film of surfactants which acts as a permeable membrane. Water droplets are produced in a sufficient amount to cover the spherical interface, and the interaction between them is basically that of the excluded volume, since the stabilizing surfactant is nonionic. Thus, the small droplets arrange themselves in the configuration of maximum entropy consistent with their concentration, which is the spherical crystalline structure. Although the droplets at the interface grow continuously, leading to the expulsion of some of them, the process is sufficiently slow to allow the droplets to rearrange at each step in an ordered configuration. In summary, we report here a set of interesting, and pretty provocative, experimental results carried out in a very simple system consisting only of water, oil, and a lipophilic surfactant. As we show in Figure 2 and discuss in the text, there are a number of underlying physical phenomena occurring in such a system, from the self-assembly of surfactant molecules at the interface between water and oil to the self-assembly of colloidal structures on curved surfaces. We provide here a simple model consistent with our observations. The spontaneous emulsification of water droplets of uniform size, whose kinetics is tunable by varying the amount of surfactant and/or the water content, clearly suggests the possibility of using this simple system in designing different applications: in developing new technologies, in the optimization of industrial processes, and also in basic research such as crystallographic studies in spherical surfaces (in Figure 2, one can see many features of curved crystals: 6, 5, and 7 coordination sites, scars, and so forth). The spontaneous formation of ordered mesoscopic structures, such as those reported here, starting from self-assembly processes at the molecular level, also suggests that similar mechanisms intervened in the formation of primitive organisms. Acknowledgment. We acknowledge fruitful discussions with M. Medina-Noyola. This work is partially supported by the Consejo Nacional de Ciencia y Tecnologı´a, Me´xico, Grants 46121-F and U47611-F. LA070144S
