Langmuir 2003, 19, 7837-7840
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Two-Stage Coalescence in Double Emulsions He´ctor Gonza´lez-Ochoa, Laura Ibarra-Bracamontes, 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 Received May 28, 2003. In Final Form: July 17, 2003 We report the observation of two-stage coalescence phenomena of water droplets, encapsulated in oil globules, toward a continuous water phase. The process is captured step by step by fast digital video microscopy. In the first stage, a transient water in water emulsion is formed: the oil enveloping the water droplet peels off, leaving the droplet immersed in the continuous water phase supported by a film of oil and surfactants. The retraction of the oil occurs in a time span of 1 ms. In the second stage, the film covering the water droplet wears and the drop breaks, releasing its contents to the continuous water phase. The second stage occurs in a time span of a few tens of milliseconds.
Introduction Coalescence, the mechanism by which small droplets in contact combine to form larger drops, plays an important role in many natural and industrial processes. Coarsening of falling water drops in clouds, spray coating, and regulation of the drop size distribution (in food emulsions, cosmetics, paints, polymer blends, etc.) determines various properties such as stability, creaming, texture, spreading, and many others. Although the underlying physics of coalescence is well understood in many interesting cases, it remains unknown in many others. This is the case, for instance, of double (and multiple) emulsions of the type water-in-oil-in-water (W/O/W), which are complex systems where coalescence is an important regulating mechanism. First described by Seifriz,1 double emulsions are currently the focus of intense research, since they could be used in many industrial applications and technology development: removal of toxic materials in wastewater and encapsulation of drugs, nutrients, flavors, enzymes, and cosmetics and their gradual release for prolonged efficiency.2-10 A W/O/W double emulsion is formed by small water droplets dispersed in oil globules, dispersed in a continuous water phase. They are usually prepared in two steps.11 First, an inverted emulsion, water-in-oil (W/O), is prepared by mixing water with a lipophilic surfactant (capable of stabilizing water in oil) and oil. In the second step the double emulsion is formed by dispersing the inverted emulsion in water with a hydrophilic surfactant (capable of stabilizing oil in water).12 These systems are metastable; the water of the inner droplets * To whom correspondence should be addressed. E-mail: arauz@ ifisica.uaslp.mx. (1) Seifriz, W. J. Phys. Chem. 1925, 29, 738. (2) Food Macromolecules and Colloids; Dickinson, E., Lorient, D., Eds.; Royal Society of Chemistry: Cambridge, 1999. (3) Tadros, T. J. Cosmet. Sci. 2001, 52, 138. (4) Raghuraman, B.; Tirmizi, N.; Wiencek, J. Environ. Sci. Technol. 1994, 28, 1090. (5) Silva Cunha, A.; Grossiord, J. L.; Puisieux, F.; Seiller, M. J. Microencapsulation 1997, 14, 321. (6) Engel, R. H.; Riggi, S. J.; Fahrenbach, M. J. Nature 1968, 219, 856. (7) Chen, C.-C.; Tu, Y.-Y.; Chang, H.-M. J. Agric. Food Chem. 1999, 47, 407. (8) Zheng, S.; Beissinger, R. L.; Wasan, D. T. J. Colloid Interface Sci. 1991, 144, 72. (9) Soppimath, K. S.; Aminabhavi, T. M.; Kulkarni, A. R.; Rudzinski, W. E. J. Controlled Release 2001, 70, 1. (10) Hirai, T.; Hariguchi, S.; Komasawa, I. Langmuir 1997, 13, 6650. (11) Matsumoto, S.; Kita, Y.; Yonezawa, D. J. Colloid Interface Sci. 1976, 57, 353. (12) Bancroft, W. D. J. Phys. Chem. 1913, 17, 501.
eventually escapes to the continuous water phase, transforming the double emulsion in a more stable direct O/W emulsion. This can happen by different mechanisms: by diffusion of the water molecules through the oil phase or by coalescence of the water droplets with the outer water phase.13,14 In both cases, the water of the inner droplets and any other substance dissolved in it is gradually released to the bulk. Coalescence of the water droplets to the continuous phase can happen in a variety of ways, depending on the formulation of samples, and can be triggered to happen in a matter of minutes or can be controlled to occur gradually during a time span of weeks.15 This and other properties have urged an extensive study of the conditions of stability and coalescence of double emulsions under various conditions. Thus, different formulations have been assayed and their bulk properties characterized in terms of stability and coalescence time scales.16,17 However, a microscopical understanding of these phenomena is still missing. Models for coalescence in double emulsions envision this process as driven by interface instabilities occurring when the excess of hydrophilic surfactant in the continuous phase penetrates the oil phase and replaces part of the lipophilic surfactant at the oil-water droplet interface. At some surfactant ratio, the bending energy favors a change of the interface curvature of a droplet in contact with the oil-continuous phase interface, producing an opening and a sudden release of the droplet content to the continuous water phase.14,18 As we show here, coalescence in double emulsions can involve interesting additional physical processes not observed before. As discussed in the Results section, we present here the first report of two-stage coalescence phenomena observed in W/O/W double emulsions. Materials and Methods The systems are prepared following a protocol similar to those used by other authors.11,14 First, we mix a 1 M NaCl water solution with a lipophilic surfactant [we use sorbitan monooleate (Span 80, hydrophilic-lipophilic balance HLB ) 4.3)] and dodecane as (13) Florence, A. T.; Whitehill, D. J. Colloid Interface Sci. 1981, 79, 243. (14) Pays, K.; Giermanska-Kahn, J.; Pouligny, B.; Bibette, J.; LealCalderon, F. Langmuir 2001, 17, 7758. (15) Ficheux, M. F.; Bonakdar, L.; Leal-Calderon, F.; Bibette, J. Langmuir 1998, 14, 2702. (16) Opawale, F. O.; Burgess, D. J. J. Pharm. Pharmacol. 1998, 50, 965. (17) Omotosho, J. A.; Whateley, T. L.; Law, T. K.; Florence, A. T. J. Pharm. Pharmacol. 1986, 38, 865. (18) Kabalnov, A.; Wennerstro¨m, H. Langmuir 1996, 12, 276.
10.1021/la0349323 CCC: $25.00 © 2003 American Chemical Society Published on Web 08/12/2003
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Figure 1. Top view of the time evolution of a single oil globule in a double emulsion prepared at 30 times the cmc of SDS in the continuous water phase and 2.5% (w/w) of Span 80 in the oil phase. Parts a-f show pictures taken at t ) 0.27, 17, 17.5, 18, 21.6, and 75.4 h, respectively, after sample preparation. the oil phase. The water solution and Span 80, in a proportion of 4:1 by weight, are mixed by gentle stirring in a mortar. The creamy product is then mixed with dodecane, 10% (w/w), by fast stirring. In this way we obtain an inverse W/O emulsion consisting of nearly size-monodisperse water droplets (hydrodynamic diameter 208 nm ( 6%, determined by dynamic light scattering) dispersed in oil. Four cycles of gentle centrifugation and resuspension of the water droplets in fresh oil with a known concentration of Span 80 (in the range 0.5-5.0% w/w) were performed to fix the initial lipophilic surfactant concentration in the oil phase. This procedure does not affect the size distribution of water droplets. In the second step, the inverse emulsion is dispersed in pure water with a hydrophilic surfactant; we use sodium dodecyl sulfate (SDS, HLB ) 40). The proportion of the inverse emulsion to water is 1:9 by volume. The concentration of SDS was varied in the range 1-30 times its critical micellar concentration (cmc, 8 × 10-3 mol/L). Nanopure water was used throughout sample preparation. Thus, a double emulsion is produced consisting of submicron water droplets, encapsulated in oil globules of diameter in the micrometer range, dispersed in a continuous water phase. The osmotic pressure mismatch between the water in the droplets and the continuous phase induces an initial swelling of the oil globules.20 This does not affect the coalescence process and helps to increase the proportion of the internal water phase. All samples were prepared and studied at the temperature 20 °C. Right after preparation, the sample is loaded in a rectangular cuvette 3 cm × 4 mm × 200 µm and placed on the stage of an optical microscope for observation. The globules can be observed from a top view (microscope standing in its normal, vertical, position) or from a side view (microscope rotated 90° to be horizontal). The coalescence process is captured, step by step, by using a fast digital video camera capable of digitizing and storing video frames (128 pixels × 80 pixels) at a rate of 5000 frames/s (fps).19 Standard video equipment with a time resolution of 30 fps is used to follow the phenomena on a longer time scale. Our results are presented below.
Results and Discussion Figure 1 shows a brief overview of the behavior of the double emulsions studied here. In this figure one can see a top view of the time evolution of a single oil globule (diameter σ ) 45 µm), filled with small water droplets, in a continuous water phase with SDS at the concentration of 30 times the cmc. The concentration of Span 80 in the oil globule is 2.5% (w/w). Figure 1a shows the system at time t ) 16 min after sample preparation. Here one can see the early state of the globule. Figure 1b shows the system at t ) 17 h. Two main changes can be observed: the decrease of the globule diameter and the formation of (19) KODAK Motion Corder Analyzer, Model SR-Ultra. (20) Florence, A. T.; Whitehill, D. Int. J. Pharm. 1982, 11, 277.
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Figure 2. Time evolution of the normalized diameter σ(t)/σ(0) of oil globules in double emulsions. Samples were prepared at 2% (w/w) of Span 80 in the oil phase and 30 times the cmc of SDS in the external water phase. The inset shows t vs σ(0).
large water droplets inside the oil globule. The former is due to the escape (coalescence) of the inner water droplets toward the continuous phase, whereas the latter is due to the coalescence of the internal water droplets between themselves.15 The texture of the globule has also changed due to the decrease of the water droplets’ concentration. Figure 1c shows the system at t ) 17.5 h. At this time the larger water droplet has escaped, and the two processess coalescence of inner water droplets between them and with the continuous phasescontinue. Parts d-f of Figure 1 are pictures taken at times t ) 18, 21.6, and 75.4 h, respectively. They show the development of the system at longer times, consisting of the processes mentioned above. Quantitative changes are observed when the amount of surfactants is varied and when oil globules of different size are considered. For instance, the amount of SDS in the external phase changes the speed of the process shown in Figure 1. It can take weeks at the cmc or hours at 30 times the cmc. On the other hand, the concentration of Span 80 regulates aspects such as the formation, size, and number of large water droplets in the oil globules. At the lowest concentration studied here (0.5% w/w), coarsening of internal water droplets occurs, but they do not grow very large; at around 2% only one large water droplet is formed with its size larger than 0.4 of the oil globule’s size, and at around 5% most of the oil globules develop more than one large water droplet. Besides these quantitative differences, the evolution of double emulsions in a macroscopic time scale follows the general characteristics described here for a wide range of lipophilic and hydrophilic surfactant concentrations and globule sizes. Figure 2 shows the time evolution of the oil globules’ diameter in double emulsions. The samples were prepared at 2% (w/w) of Span 80 in the oil phase and 30 times the cmc of SDS in the external water phase to speed up the process. In Figure 2 we plot the normalized diameter σ(t)/σ(0) versus the normalized time t/τ, for different values of σ(0). Each curve in Figure 2 represents the averaged behavior of several globules. Here τ is the time at the middle of the largest decrease of σ(t). At around t ) τ there is an appreciable higher rate of coalescence of the inner droplets with the continuous phase. As mentioned above, at this concentration of Span 80, only one large water droplet grows inside each globule. It is also around t ) τ when this large water droplet exits the globule. As one can see here, coalescence happens very similarly for globules of different size, being faster for smaller ones. The inset shows τ versus σ(0) for this formulation. An apparent linear relation is observed. It is interesting to
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Figure 3. Two-stage coalescence in double emulsions. Side view of a water droplet, in an oil globule, coalescing toward the continuous water phase. See text for discussion.
note in this figure that the collective rate of coalescence of water droplets to the continuous phase changes with time. This is probably due to a redistribution of water droplets as coalescence toward the continuous phase proceeds. The description of this behavior is important in its own right. Thus, a further discussion of this and other results will be presented elsewhere. As is shown in Figure 1, on a standard time scale, coalescence appears to happen instantaneously. However, observation with higher time resolution reveals an interesting feature of the process: it occurs in two stages. Figure 3 shows a sequence of side view pictures, taken at a frequency of 5000 fps (Figure 3a-f), of a water droplet coalescing to the continuous phase. Sample was prepared as in Figure 1, but with a 2% concentration of Span 80 instead of 2.5%. In Figure 3a one can see a single oil globule of diameter 27 µm (larger circle). One can also see here a large inner water droplet (diameter 22 µm) sitting at the bottom of the globule, due to the density difference between water and oil. The texture indicates the presence also of small water droplets in the globule. Parts a-f of Figure 3 show the first stage of coalescence: the surfactant interface keeping the droplet inside the globule breaks and the oil retracts to form a smaller oil globule in a time span of 1 ms, whereas the water droplet (shadow circle) is left immersed in the continuous water phase, held together by a complex film (initial thickness ∼ 0.5 µm) of oil and surfactants, forming a transient water in water (W/W) emulsion. One should also note that the water droplet is displaced downward by a distance on the order of its radius (∼10 µm) without deformation. The fact that we can actually see the water droplet exiting the globule and remaining later on as a droplet of water in water for some time is due to the refraction index difference between the film and water. The first stage, that is, the expulsion of the water droplet toward the continuous phase, needs the breakage of the external water phase-oil interface. This may happen as the fluid (oil and surfactants) between the water-oil and oil-water interfaces drains out and the interfaces reach a critical separation,21,22 which is likely to be around the same thickness of the film surrounding the water droplet. In the second stage, the film made of oil and surfactants thins continuously (probably by autoemulsification of the oil in the film to the continuous water phase, where there is plenty of hydrophilic surfactant) until it finally breaks, releasing the water of the droplet into the external phase. This can be seen in Figure 3c-h, where one can appreciate a progressive fading of (21) Chen, J.-D.; Hahn, P. S.; Slattery, J. C. AIChE J. 1984, 30, 622. (22) Amarouchene, Y.; Cristobal, G.; Kellay, H. Phys. Rev. Lett. 2001, 87, 206104.
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Figure 4. Coalescence of a water droplet containing fluorescein. Here one can appreciate the release of the droplet’s content toward the continuous phase. The pictures were taken at a rate of 30 fps.
the shadow. In Figure 3g, taken at t ) 2.4 ms after Figure 3a, the shadow is already very tenuous, and in Figure 3h, taken at t ) 10.4 ms, the shadow can be seen only by enhancing the contrast of the image. We followed the coalescence of large (diameter g 15 µm) water droplets, where the two stages are clearly observed, of different size and for different surfactant concentrations within the ranges mentioned above. In all the cases, the process occurs in the way pictured in Figure 3. The time span of the second step is observed to be on the order of a few tens of milliseconds for all the samples studied. Figure 4 shows a similar process to that in Figure 3 but captured at the rate of 30 fps. Sample preparation is similar to those in the previous figures, except for the incorporation of 10 mg/mL of fluorescein in the water used to prepare the initial inverted emulsion. Figure 4a shows an oil globule with a large water droplet before it is released to the continuous phase. Fluorescence and transmitted light illumination are used to see simultaneously the dyed water droplet (bright circle) and the globule (dark circle). Figure 4b shows the water droplet already outside the globule, and the oil forming a smaller spherical globule. Parts c-f of Figure 4 show the radial symmetric diffusion of the dye toward the bulk after the film supporting the droplet breaks. We have also incorporated larger probes in the internal water phase such as polystyrene spheres of size in the range 20-1000 nm. No appreciable qualitative changes in the process were observed. Conclusion Here we reported the first direct observation, on the sub-millisecond time scale, of coalescence phenomena as they happen in systems of broad interest, such as W/O/W double emulsions. This observation reveals the existence of complex routes for coalescence not accounted for in current theoretical models. We found the process to be composed of two stages: the formation of a transient W/W emulsion and its destruction. As shown in Figures 1 and 2, water droplets of different sizes escape from the oil globules at different times after sample preparation. Contact between both interfaces, external (continuous water phase-oil phase) and internal (oil phase-layer structured around the water droplet), is required for the droplet to exit. Thus, droplets close to the external interface are more likely to exit earlier than those closer to the center of the oil globule, which then have the time to grow larger by internal coalescence. This time, to some extent, also depends on the time required to drag out the fluid between the two interfaces.21,22 Once in contact, temperature driven fluctuations break the external interface and
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the oil retracts to its shape of minimal energy. Thus, the time of the first stage is determined by the properties of the oil phase such as viscosity, surface tension, initial and final curvature, and so forth. The time span of the second stage, and in fact the observed separation of coalescence in two stages, depends on the formation of a film (probably a multilayer) of oil and surfactants around the water droplets and on its properties such as thickness, structure, and molar composition. Thus, the characterization of those properties under different conditions is a required step toward a microscopical understanding of coalescence in
Letters
complex systems such as those studied here. Such a study is, however, beyond the aim of the present work. Acknowledgment. This work was partially supported by the Consejo Nacional de Ciencia y Tecnologı´a, Me´xico, Grants G29589E and ER026 Materiales Biomoleculares, and by the Instituto Mexicano del Petro´leo, Me´xico, Grant FIES-98-101-I. LA0349323
