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Materials based on solid-stabilized emulsions
Journal of Colloid and Interface Science 275 (2004) 659–664 www.elsevier.com/locate/jcis
Materials based on solid-stabilized emulsions Stéphane Arditty,a Véronique Schmitt,a Joanna Giermanska-Kahn,a and Fernando Leal-Calderon b,∗ a Centre de Recherche Paul Pascal, CNRS, Avenue A. Schweitzer, 33600 Pessac, France b Laboratoire des Milieux Dispersés Alimentaires, ISTAB, Université Bordeaux 1, Avenue des Facultés, 33405 Talence, France
Received 1 December 2003; accepted 4 March 2004 Available online 6 May 2004
Abstract Solid-stabilized emulsions are obtained by shearing a mixture of oil, water, and solid colloidal particles. In this article, we present a large variety of materials, resulting from a limited coalescence process. Direct (o/w), inverse (w/o), and multiple (w/o/w) emulsions that are surfactant-free and monodisperse were produced in a very wide droplet size range, from micrometers to centimeters. These materials exhibit original properties compared with surfactant-stabilized emulsions: outstanding stability with respect to coalescence and unusual rheological behavior. For such systems, the elastic storage modulus G is not controlled by interfacial tension but by the interfacial elasticity resulting from the strong adhesion between the solid particles adsorbed at the oil–water interface. Due to the wide accessible droplet size range, concentrated emulsions can be extremely fluid while emulsions with low droplet volume fraction can behave as solids. 2004 Published by Elsevier Inc. Keywords: Pickering emulsions; Solid particles; Biliquid foams; Monodispersity; Multiple emulsions; Outstanding stability; Rheological behavior
1. Introduction Emulsions are used in many different fields such as road surfacing, food industry, paints, coatings, cosmetics, and pharmaceutics. It is now well established that solid particles of colloidal size may be employed to kinetically stabilize emulsions. There is growing interest in the so-called Pickering or solid-stabilized emulsions [1–3], as they may advantageously replace conventional emulsions containing organic surfactants [4,5]. In Pickering emulsions, the stabilizing film in between the droplets comprises very rigid layers that provide a mechanical barrier against coalescence. The solid particles are irreversibly anchored at the oil–water interface and develop strong lateral interactions [2]. A rule similar to that in the case of surfactant molecules holds: the continuous phase of the preferred emulsion is normally the one in which the particles are preferentially dispersed. Indeed, the continuous phase of an emulsion stabilized by particles having a high degree of hydrophilicity will be the water phase, whereas hydrophobic particles will preferentially stabilize water drops dispersed in an oil phase.
The aim of this article is to exploit the solid particles as stabilizing agents to easily obtain a large variety of new materials such as direct (o/w), inverse (w/o), and multiple (w/o/w) emulsions that are surfactant-free, monodisperse, and all in a very wide droplet size range: from micrometers to centimeters. These materials exhibit original properties in terms of rheology and kinetic stability. The remainder of this article is as follows. Section 2 is devoted to the description of the preparation method, which is based on the so-called limited coalescence process initially described by Whitesides et al. [6] and recently extended by Arditty et al. [7]. We then describe, in Section 3, the different types of materials that can be obtained following this process. In Section 4, we focus on the main physical properties of the materials obtained, especially the outstanding stability toward coalescence. To probe the properties of the adsorbed interfacial layer, we measure the bulk elastic moduli (G ) of concentrated emulsions. We deduce the elastic coefficient characterizing the interfacial layer and we discuss its impact on kinetic stability. 2. Preparation method
* Corresponding author. Fax: +33-05-56370336.
E-mail address: [email protected] (F. Leal-Calderon). 0021-9797/$ – see front matter 2004 Published by Elsevier Inc. doi:10.1016/j.jcis.2004.03.001
The emulsions were fabricated following the basic principles described in Refs. [6,7]. Only solid colloidal parti-
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cles were used as stabilizing agents. In the present study, two different types of particles, termed S1 and S2, were used. Both types are monodisperse spherical silica particles that have been chemically modified to promote their adsorption at the oil–water interface. The contact angles of these particles could not be measured because of their small diameter. S1 particles, used to stabilize direct oilin-water (o/w) emulsions, were obtained from an aqueous dispersion of hydrophilic precipitated silica particles with a mean diameter of 25 nm (Klébosol 30R25 from Clariant). These particles were partially hydrophobized by grafting an n-octyltriethoxysilane with an average surface density of 5 octyl chains nm−2 . The particles were transferred from the synthesis medium (ethanol) into pure water by dialysis. The dispersions obtained were considerably flocculated, with a floc size of about 0.2 µm. S2 particles were used to stabilize inverse water-in-oil (w/o) emulsions. They consist of a hydrophobic fumed silica powder purchased from Degussa (R972), the hydrophobic coating agent being dichlorodimethylsilane. The average primary particle diameter is 16 nm, corresponding to a specific surface area of 110 m2 g−1 . They were dispersed in the oil phase under agitation and formed large aggregates of 1 µm. As the oil phase, we used polydimethylsiloxane (PDMS) with variable viscosity—350 cP (Rhodorsil 47V350, Rhodia silicone), 100 cP, or 10 cP (both from Fluka)—or alkanes such as octane and dodecane. In all the experiments, the water was passed through a Millipore purifier (Milli-Q Reagent Water System). The method employed to obtain solid-stabilized monodisperse emulsions is based on a limited coalescence process [6,7]. It consists of producing a large excess of oil– water interface compared with the amount that can be covered by the solid particles. For this process to occur, the systems were always formulated in the presence of a very small amount of solid particles (approximately 20–200 mg for 100 g of dispersed phase). When the agitation is stopped, the partially unprotected droplets coalesce, thus reducing the total amount of oil–water interface. Since the particles are irreversibly adsorbed, the coalescence process stops as soon as the oil–water interface is sufficiently covered [7]. The resulting emulsions are stable over months and remarkably monodisperse. Although simple, this preparation process is reproducible and allows the stabilization of droplets with an average diameter ranging from 1 µm to 1 cm.
3. Different types of materials 3.1. Simple emulsions Direct (o/w) emulsions were prepared by manually shaking equal volumes of oil and aqueous dispersion of S1 particles for a few seconds. The oil mass fraction could be increased up to 90% by progressive addition of oil, and the concentrated materials remained stable. The image in Fig. 1
Fig. 1. Image of an o/w emulsion containing 5 g of water, 45 g of 100-cP PDMS, and 24.4 mg of S1 particles. On the scale bar, the separation between two consecutive tips is 1 mm. (Some droplets contain visible small air bubbles.)
shows a typical o/w emulsion obtained following the partial coalescence process. The average droplet size increases and saturates 2–3 min after the agitation is stopped [7], ultimately leading to the system exhibited in Fig. 1. For those emulsions in which the drops were visible by eye, images were recorded using a Nikon digital camera (Coolpix 950). By measuring the dimensions of about 50 droplets, we evaluated the surface-weighted average diameter D as well as the droplet size distribution. We verified that 50 droplets were enough to obtain the size distribution with sufficient accuracy: larger populations lead exactly to the same size distribution. For micron-sized emulsions the droplet distribution was determined by using a commercial static light scattering laser granulometer (Malvern, Mastersizer S). The polydispersity was characterized by means of the parameter U defined as Vi i Di |D − Di | , U= Vi D iD i where Vi is the relative volume and Di is the mean diameter of droplets of size class i. One can observe in Fig. 2 that the droplet size distribution is narrow, as revealed by the value of the polydispersity parameter, U = 11%. Such a low polydispersity is quite unusual in materials deriving from either fragmentation or coalescence processes. In a similar way, inverse (w/o) emulsions could be obtained by manually shaking equal volumes of oil dispersion of S2 particles and pure water. The water fraction can be increased up to 75 wt% and stable inverse monodisperse emulsions are obtained, with an average drop diameter ranging from several tenths micrometers to 1 cm. The process of limited coalescence can also be exploited to produce droplets of colloidal size. For example, direct (o/w) emulsions were produced using a jet homogenizer (Microfluidics M110S) with an inlet air pressure of 7 bars, corresponding to a pressure of 1600 bars in the homogenizing chamber. The very important energy input enables
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Fig. 2. Droplet size distribution of an o/w emulsion containing 5 g of water, 45 g of 100-cP PDMS, and 24.4 mg of S1 particles. The value of the uniformity index U and the average drop diameter D are given.
Fig. 4. Multiple (w/o/w) emulsion. Dodecane droplets stabilized by S1 particles contain sedimented water drops stabilized by S2 particles. Bar = 4 mm.
Fig. 3. Direct (o/w) emulsion of 350-cP PDMS stabilized by S1 particles, with an average drop diameter of 5 µm, viewed by optical microscopy.
the stabilization of monodisperse emulsions with a much reduced mean diameter: from 1 to 10 µm (Fig. 3). 3.2. Double emulsions
double emulsions can encapsulate a hydrophilic species on the scale of at least 1 year. This is a quite surprising behavior considering that the process of release in double emulsions stabilized with surfactants generally occurs within hours or days [8]. The origin of this unexpected property is discussed in Section 4.2. In general, any practical application of double emulsions would require release of the entrapped species under controlled conditions. In the case of solid-stabilized double emulsions, the release can be produced mechanically on application of a shear.
4. Characterization and physical properties 4.1. Control of droplet size
Multiple (w/o/w) macroscopic drops were obtained following a two-step procedure. We first prepared an inverse (w/o) emulsion in the presence of S2 particles. Then, this primary emulsion was dispersed in a water phase containing S1 particles. In Fig. 4, we can clearly distinguish the oil globules, each of them containing sedimented internal droplets. Due to their double (or multiple) compartment structure, interest in double emulsions has been increasing, as they can be considered as reservoirs of encapsulated substances to be released under variable conditions. Copper sulfate (CuSO4 ) was used as a tracer to probe the potentiality of macroscopic double emulsions to maintain an encapsulated species inside the internal water droplets. The initial concentration of CuSO4 (0.1 mol L−1 ) provides an intense bluish color to the internal droplets. Even after 1 year, the bluish color persisted with the same apparent intensity, while the external water phase remained uncolored. It can be concluded that such
For all the materials presented, one can note the remarkable monodispersity despite the use of uncontrolled flow for the preparation. As was already underlined, this monodispersity is surprising considering that the droplet growth is driven by coalescence. Whitesides et al. [6] have proposed a theoretical analysis for either diffusional or turbulencedriven droplet collisions. Assuming that the coalescence probability between two drops is simply proportional to their individual uncovered surface fraction, Monte-Carlo simulations predict droplet size distributions that are much narrower than those resulting from unlimited coalescence. For all the previously mentioned materials, the final mean droplet diameter can be perfectly controlled by adjusting the amount of particles. Because the solid particles are totally and irreversibly adsorbed (no free particles in the continuous phase), the inverse average droplet diameter must vary
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Fig. 5. Evolution of the inverse diameter, 1/D, with mp for direct (o/w) emulsions containing 75 wt% 350-cP PDMS and stabilized by S1 particles. The emulsions were shaken manually. From the experimental slope of the curve, we deduce sf = 8.7 m2 g−1 .
linearly with the amount of particles [7], 1/D = (sf mp )/6Vd , where mp is the mass of particles, Vd is the volume of dispersed phase, sf is the specific surface area, i.e., the droplet surface covered per unit gram of silica. This latter quantity depends on the intensity of the agitation [7]. Fig. 5 provides an example confirming the validity of the previous relation. For this particular system obtained by manual shaking, sf = 8.7 m2 g−1 . A densely packed layer of adsorbed particles (2-D compacity of 0.9) would provide an sf value of 30–35 m2 g−1 . Therefore, the particles do not form a hexagonal 2-D array, but rather adsorb as large aggregates with low covering capacity [7]. 4.2. Outstanding stability of the thin liquid films The highly concentrated emulsions reported in Section 3.1 are analogous to foams, although they comprise two liquids. We should like to stress that biliquid foams of comparable diameter undergo rapid destruction through coalescence when stabilized by standard molecular surfactants, their lifetime being generally shorter than 1 h. Besides the outstanding stability, biliquid foams stabilized by solid particles exhibit a relatively low viscosity (<0.1 Pa s) despite the high droplet volume fraction. This is due to the extremely large diameter of the dispersed objects which provides a very low surface area-to-volume ratio. The inherent fluidity of these biliquid foams is an appreciable property for all applications where emulsions have to be spread or poured. Once the coalescence process is achieved, the emulsions are very stable due to the rigid barrier created by the solid particles anchored at the droplet surface. The micron-sized emulsions can even be dried without being destroyed until almost all the water is evaporated, as can be observed in Fig. 6. The original shape of the sample is preserved during the drying process, with absolutely no oil leakage, revealing the existence of a strong protective layer around the oil droplets. After removal of the oil phase, it is even possible to obtain macroporous materials composed of air compartments separated by silica aggregates. The unusual long-term encapsulation of the macroscopic double emulsions described in Section 3.2 can also be corre-
Fig. 6. Direct (o/w) emulsion of 350-cP PDMS stabilized by S1 particles, with an average drop diameter of 2 µm, dried in an oven at 40 ◦ C during 3 days.
lated to the outstanding stability of the solid-stabilized films. It is now well established that two mechanisms are responsible for the release of chemical substances in w/o/w double emulsions [8]. (i) One is due to the coalescence of the thin liquid film separating the internal droplets and the globule surfaces. (ii) The other mechanism, termed compositional ripening, occurs by diffusion and/or permeation of the chemical substance across the oil phase. The first mechanism, coalescence, did not take place in our double emulsions since we did not visualize any variation either in the number or in the size of the droplets inside the globules (Fig. 4). Concerning compositional ripening, several models have been proposed, all of them are in agreement with Fick’s law [8]. The permeation could be due to reversible and thermally activated holes that are continuously formed in the thin liquid films [8]. The characteristic hole size and lifetime may allow the passage of small hydrophilic substances. Among the many holes that are permanently formed, the smaller ones are evanescent and contribute to the permeation process, while the larger are irreversible and grow, producing coalescence events. It is unlikely that such holes are formed in our systems since we did not observe any sign of coalescence on the scale of 1 year. Another possible mechanism for the permeation could result from the solubilization of the salt ions in the oil phase followed by molecular diffusion across the globule. Since the solubility of the encapsulated species (CuSO4 ) in dodecane is negligible, the permeation following this latter mechanism occurs at extremely low rate. Another reason for the longterm encapsulation capacity lies in the fact that our double emulsions comprise droplets and globules of macroscopic size with a very low surface area-to-volume ratio. Under such conditions, the exchange capacity at the water/oil interfaces is quite reduced [8] and only application of a shear can induce the release of the entrapped species. 4.3. Elasticity and rheological properties With the aim of understanding the origin of the unusual resistance to coalescence in solid-stabilized emulsions, we
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performed rheological measurements. In concentrated emulsions, the drops are partially deformed and the rheological G and G are determined mainly by the properties of the interfacial layers. 4.3.1. Experimental procedures The rheological experiments were carried out with a controlled strain RFSII rheometer using a parallel plate geometry with a gap of 1 mm. Despite the fact that the applied stress is not constant over the whole sheared volume, this type of geometry was preferred due to the very high rigidity of the materials and to avoid problems related to confinement. The frequency of the applied strain was 0.1 Hz. We checked that, for all the measurements, the stress always varied linearly with the strain (linear regime). The interfacial tension γ between PDMS oil and pure water was determined at equilibrium using a Wilhelmy balance (T = 20 ◦ C). 4.3.2. Results Despite being composed of fluids, emulsions consisting of micron-sized concentrated droplets can possess a striking shear rigidity that is characteristic of a solid. Mason et al. [9] have studied the rheological properties of monodisperse emulsions, stabilized by sodium dodecyl sulfate. The normalized data for the elastic storage modulus, G , all fall onto a single curve as shown by the continuous line in Fig. 7. The scaling with γ /R (R is the droplet radius) confirms that the elasticity results from the droplet interfaces. The scaling indicates that the elasticity of these compressed monodisperse emulsions is universal. Shear elasticity exists because the repulsive droplets have been compressed and thus concentrated up to a sufficiently large volume fraction, φ, which permits the storage of interfacial energy [9,10]. The droplet system minimizes its total free energy by reducing the repulsion at the expense of creating some additional surface area by deforming the droplet interfaces.
Fig. 7. Storage modulus G (normalized by γ /R) of emulsions stabilized by surfactants (continuous line) and solid particles (symbols). Solid-stabilized o/w emulsions were obtained using a jet homogenizer in presence of S1 particles.
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In the following, we describe the rheological properties of micron-sized PDMS-in-water emulsions obtained using a jet homogenizer in the presence of S1 particles. From the slope of the curve 1/D versus mp , we deduce sf = 40 m2 g−1 . Such a value is consistent with a monolayer of solid particles anchored at the oil/water interface with a compacity close to 0.75 (the maximum 2-D compacity of monodisperse spheres is 0.9). In Fig. 7, we report only the values obtained for the elastic storage modulus G . G was at least 10 times larger than the dissipation modulus G , the difference increasing with the droplet volume fraction. This reflects the essentially elastic nature of the materials. The G moduli normalized by γ /R (γ = 0.03 N m−1 for the silicone oil/water interface) do not fall onto the master curve of Mason et al. [9]. Therefore, the energy storage is not controlled by the Laplace pressure of the droplets. Let us mention here that the probed emulsions were substantially flocculated, which explains the significant elasticity measured at volume fractions well below the random close packing (φ ∗ = 64%). At large volume fractions (φ > φ ∗ ), the normalized elasticity is more than one decade above Mason’s curve. This behavior is reminiscent of that already reported for proteinstabilized emulsions [11,12]. It can be argued that strong lateral interactions between the adsorbed solid particles renders the surface extremely rigid so that the deformation is not controlled by γ /R but rather by ε/R, ε being an effective elastic coefficient characterizing the droplet surface. Our rheological data fall on the master curve of Mason et al. [9] if we take ε = 0.3 N m−1 . The surface can be regarded as a compact 2-D network of solid particles with strong lateral attractive interactions: van der Waals forces, capillary forces [13], and the attractive interactions arising from the interpenetration of the octyl chains grafted on the solid particles [14]. Assuming that the spacing between the particles is equal to the length of an extended octyl chain (0.6 nm), the van der Waals attraction can be evaluated as 4–10kT . Considering the characteristic size of the solid particles (25 nm), we deduce that the capillary interactions are not larger than 10kT [13]. We therefore believe that the strongest interaction arises from the interpenetration of the octyl chains. Indeed, the octyl chains are in bad solvent in the water continuous phase and tend to overlap. The resulting attractive interaction can be as large as 104 kT [14] for particles of 25 nm densely covered by octyl chains. It is likely that this interaction is also responsible for the aggregation of the solid particles in the water phase (prior to the emulsification) and of the oil drops once the emulsions are fabricated. The very high value of the effective elastic coefficient ε can be correlated to the outstanding stability of the droplets against coalescence. Let us first remember that there are no free particles in the continuous phase and that their adsorption is irreversible [7]. The occurrence of coalescence events should imply the formation of “patches” at the oil/water interface that are not covered by the solid particles. It is clear that thermal fluctuations are unable to disrupt the lateral
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links between the adsorbed particles, and, consequently, the spontaneous formation of uncovered patches is completely inhibited. The only way to form unprotected zones is by deforming the droplets to a sufficient degree so that their total surface area is extended. This may be achieved on application of either a stress or a compression of the order of ε/R, i.e., 3 × 105 Pa for micron-sized drops. To test this idea, an emulsion with R = 1 µm was introduced into a tube and submitted to a centrifugational field (Optima TLX Ultracentrifuge). After 1 h, the drops were strongly compressed at the bottom of the tube (the average density of the droplets covered by the silica particles was larger than 1). From the oil volume fraction and the height of the sediment, we deduced the maximum osmotic pressure exerted at the bottom of the sample [11]. We observed that coalescence (release of macroscopic oil) occurs only when the osmotic pressure reaches a threshold value close to 2.4 × 105 Pa, in excellent agreement with the previous prediction.
emulsion to coalescence is strongly dependent on particles attraction. This conclusion is in agreement with the general phenomenology reported in the field of solid-stabilized emulsions: the most stable emulsions are very often obtained with solid particles that are weakly or strongly aggregated in the bulk continuous phase. Finally, we stress that monodisperse solid-stabilized emulsions are certainly appropriate precursors for the fabrication of elaborated materials like templates, solid foams, and macroporous aerogels.
Acknowledgments This work was partially funded by Rhodia Services (France) for S.A. The authors thank D. Monin and B.P. Binks for fruitful discussions and J.-Y. Chane-Ching for providing the S1 particles.
References 5. Conclusion It has been observed that solid particles may generate new materials that are not available with standard surfactant molecules. Besides the field of applications which is considerably widespread, most of the properties of these materials remain unexplored and it is clear that some basic concepts governing the behavior of emulsions (aging, bulk elasticity, etc.) need to be revisited in the presence of solid particles. The rheological data reported in the present paper provide indirect evidence of the existence of a rigid interfacial layer adsorbed at the oil–water interface. The rigidity of the interface is most probably due to strong attractive interactions between the solid particles. For coalescence to occur, elongation of the interface is necessary and this can be achieved on compression through centrifugation or on application of a high shear. Therefore, the resistance of a solid-stabilized
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
S.U. Pickering, J. Chem. Soc. 91 (1907) 2001. B. Binks, Curr. Opin. Colloid Interface Sci. 7 (2002) 21. R.M. Wiley, J. Colloid Sci. 9 (1954) 427. D.E. Ballast, R.M. Wiley, S.I. Bates, U.S. Patent 2,968,066, 1961. M.J. Garvey, I.C. Griffiths, I. Thompson, European Patent 0309054, 1989. T.H. Whitesides, D.S. Ross, J. Colloid Interface Sci. 169 (1995) 48. S. Arditty, C.P. Whitby, B.P. Binks, V. Schmitt, F. Leal-Calderon, Eur. Phys. J. E 11 (2003) 273. K. Pays, J. Kahn, B. Pouligny, J. Bibette, F. Leal-Calderon, J. Control. Release 79 (2002) 193. T.M. Mason, J. Bibette, D.A. Weitz, Phys. Rev. Lett. 75 (1995) 2051. H.M. Princen, J. Colloid Interface Sci. 91 (1983) 160. T. Dimitrova, F. Leal-Calderon, Langmuir 17 (2001) 3235. L. Bressy, P. Hébraud, V. Schmitt, J. Bibette, Langmuir 19 (2003) 598. P.A. Kralchevsky, K. Nagayama, Adv. Colloid Interface Sci. 85 (2000) 145. S.R. Raghavan, J.B. Jun Hou, G.L. Baker, S.A. Khan, Langmuir 16 (2000) 1066.
Materials based on solid-stabilized emulsions Stéphane Arditty,a Véronique Schmitt,a Joanna Giermanska-Kahn,a and Fernando Leal-Calderon b,∗ a Centre de Recherche Paul Pascal, CNRS, Avenue A. Schweitzer, 33600 Pessac, France b Laboratoire des Milieux Dispersés Alimentaires, ISTAB, Université Bordeaux 1, Avenue des Facultés, 33405 Talence, France
Received 1 December 2003; accepted 4 March 2004 Available online 6 May 2004
Abstract Solid-stabilized emulsions are obtained by shearing a mixture of oil, water, and solid colloidal particles. In this article, we present a large variety of materials, resulting from a limited coalescence process. Direct (o/w), inverse (w/o), and multiple (w/o/w) emulsions that are surfactant-free and monodisperse were produced in a very wide droplet size range, from micrometers to centimeters. These materials exhibit original properties compared with surfactant-stabilized emulsions: outstanding stability with respect to coalescence and unusual rheological behavior. For such systems, the elastic storage modulus G is not controlled by interfacial tension but by the interfacial elasticity resulting from the strong adhesion between the solid particles adsorbed at the oil–water interface. Due to the wide accessible droplet size range, concentrated emulsions can be extremely fluid while emulsions with low droplet volume fraction can behave as solids. 2004 Published by Elsevier Inc. Keywords: Pickering emulsions; Solid particles; Biliquid foams; Monodispersity; Multiple emulsions; Outstanding stability; Rheological behavior
1. Introduction Emulsions are used in many different fields such as road surfacing, food industry, paints, coatings, cosmetics, and pharmaceutics. It is now well established that solid particles of colloidal size may be employed to kinetically stabilize emulsions. There is growing interest in the so-called Pickering or solid-stabilized emulsions [1–3], as they may advantageously replace conventional emulsions containing organic surfactants [4,5]. In Pickering emulsions, the stabilizing film in between the droplets comprises very rigid layers that provide a mechanical barrier against coalescence. The solid particles are irreversibly anchored at the oil–water interface and develop strong lateral interactions [2]. A rule similar to that in the case of surfactant molecules holds: the continuous phase of the preferred emulsion is normally the one in which the particles are preferentially dispersed. Indeed, the continuous phase of an emulsion stabilized by particles having a high degree of hydrophilicity will be the water phase, whereas hydrophobic particles will preferentially stabilize water drops dispersed in an oil phase.
The aim of this article is to exploit the solid particles as stabilizing agents to easily obtain a large variety of new materials such as direct (o/w), inverse (w/o), and multiple (w/o/w) emulsions that are surfactant-free, monodisperse, and all in a very wide droplet size range: from micrometers to centimeters. These materials exhibit original properties in terms of rheology and kinetic stability. The remainder of this article is as follows. Section 2 is devoted to the description of the preparation method, which is based on the so-called limited coalescence process initially described by Whitesides et al. [6] and recently extended by Arditty et al. [7]. We then describe, in Section 3, the different types of materials that can be obtained following this process. In Section 4, we focus on the main physical properties of the materials obtained, especially the outstanding stability toward coalescence. To probe the properties of the adsorbed interfacial layer, we measure the bulk elastic moduli (G ) of concentrated emulsions. We deduce the elastic coefficient characterizing the interfacial layer and we discuss its impact on kinetic stability. 2. Preparation method
* Corresponding author. Fax: +33-05-56370336.
E-mail address: [email protected] (F. Leal-Calderon). 0021-9797/$ – see front matter 2004 Published by Elsevier Inc. doi:10.1016/j.jcis.2004.03.001
The emulsions were fabricated following the basic principles described in Refs. [6,7]. Only solid colloidal parti-
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S. Arditty et al. / Journal of Colloid and Interface Science 275 (2004) 659–664
cles were used as stabilizing agents. In the present study, two different types of particles, termed S1 and S2, were used. Both types are monodisperse spherical silica particles that have been chemically modified to promote their adsorption at the oil–water interface. The contact angles of these particles could not be measured because of their small diameter. S1 particles, used to stabilize direct oilin-water (o/w) emulsions, were obtained from an aqueous dispersion of hydrophilic precipitated silica particles with a mean diameter of 25 nm (Klébosol 30R25 from Clariant). These particles were partially hydrophobized by grafting an n-octyltriethoxysilane with an average surface density of 5 octyl chains nm−2 . The particles were transferred from the synthesis medium (ethanol) into pure water by dialysis. The dispersions obtained were considerably flocculated, with a floc size of about 0.2 µm. S2 particles were used to stabilize inverse water-in-oil (w/o) emulsions. They consist of a hydrophobic fumed silica powder purchased from Degussa (R972), the hydrophobic coating agent being dichlorodimethylsilane. The average primary particle diameter is 16 nm, corresponding to a specific surface area of 110 m2 g−1 . They were dispersed in the oil phase under agitation and formed large aggregates of 1 µm. As the oil phase, we used polydimethylsiloxane (PDMS) with variable viscosity—350 cP (Rhodorsil 47V350, Rhodia silicone), 100 cP, or 10 cP (both from Fluka)—or alkanes such as octane and dodecane. In all the experiments, the water was passed through a Millipore purifier (Milli-Q Reagent Water System). The method employed to obtain solid-stabilized monodisperse emulsions is based on a limited coalescence process [6,7]. It consists of producing a large excess of oil– water interface compared with the amount that can be covered by the solid particles. For this process to occur, the systems were always formulated in the presence of a very small amount of solid particles (approximately 20–200 mg for 100 g of dispersed phase). When the agitation is stopped, the partially unprotected droplets coalesce, thus reducing the total amount of oil–water interface. Since the particles are irreversibly adsorbed, the coalescence process stops as soon as the oil–water interface is sufficiently covered [7]. The resulting emulsions are stable over months and remarkably monodisperse. Although simple, this preparation process is reproducible and allows the stabilization of droplets with an average diameter ranging from 1 µm to 1 cm.
3. Different types of materials 3.1. Simple emulsions Direct (o/w) emulsions were prepared by manually shaking equal volumes of oil and aqueous dispersion of S1 particles for a few seconds. The oil mass fraction could be increased up to 90% by progressive addition of oil, and the concentrated materials remained stable. The image in Fig. 1
Fig. 1. Image of an o/w emulsion containing 5 g of water, 45 g of 100-cP PDMS, and 24.4 mg of S1 particles. On the scale bar, the separation between two consecutive tips is 1 mm. (Some droplets contain visible small air bubbles.)
shows a typical o/w emulsion obtained following the partial coalescence process. The average droplet size increases and saturates 2–3 min after the agitation is stopped [7], ultimately leading to the system exhibited in Fig. 1. For those emulsions in which the drops were visible by eye, images were recorded using a Nikon digital camera (Coolpix 950). By measuring the dimensions of about 50 droplets, we evaluated the surface-weighted average diameter D as well as the droplet size distribution. We verified that 50 droplets were enough to obtain the size distribution with sufficient accuracy: larger populations lead exactly to the same size distribution. For micron-sized emulsions the droplet distribution was determined by using a commercial static light scattering laser granulometer (Malvern, Mastersizer S). The polydispersity was characterized by means of the parameter U defined as Vi i Di |D − Di | , U= Vi D iD i where Vi is the relative volume and Di is the mean diameter of droplets of size class i. One can observe in Fig. 2 that the droplet size distribution is narrow, as revealed by the value of the polydispersity parameter, U = 11%. Such a low polydispersity is quite unusual in materials deriving from either fragmentation or coalescence processes. In a similar way, inverse (w/o) emulsions could be obtained by manually shaking equal volumes of oil dispersion of S2 particles and pure water. The water fraction can be increased up to 75 wt% and stable inverse monodisperse emulsions are obtained, with an average drop diameter ranging from several tenths micrometers to 1 cm. The process of limited coalescence can also be exploited to produce droplets of colloidal size. For example, direct (o/w) emulsions were produced using a jet homogenizer (Microfluidics M110S) with an inlet air pressure of 7 bars, corresponding to a pressure of 1600 bars in the homogenizing chamber. The very important energy input enables
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Fig. 2. Droplet size distribution of an o/w emulsion containing 5 g of water, 45 g of 100-cP PDMS, and 24.4 mg of S1 particles. The value of the uniformity index U and the average drop diameter D are given.
Fig. 4. Multiple (w/o/w) emulsion. Dodecane droplets stabilized by S1 particles contain sedimented water drops stabilized by S2 particles. Bar = 4 mm.
Fig. 3. Direct (o/w) emulsion of 350-cP PDMS stabilized by S1 particles, with an average drop diameter of 5 µm, viewed by optical microscopy.
the stabilization of monodisperse emulsions with a much reduced mean diameter: from 1 to 10 µm (Fig. 3). 3.2. Double emulsions
double emulsions can encapsulate a hydrophilic species on the scale of at least 1 year. This is a quite surprising behavior considering that the process of release in double emulsions stabilized with surfactants generally occurs within hours or days [8]. The origin of this unexpected property is discussed in Section 4.2. In general, any practical application of double emulsions would require release of the entrapped species under controlled conditions. In the case of solid-stabilized double emulsions, the release can be produced mechanically on application of a shear.
4. Characterization and physical properties 4.1. Control of droplet size
Multiple (w/o/w) macroscopic drops were obtained following a two-step procedure. We first prepared an inverse (w/o) emulsion in the presence of S2 particles. Then, this primary emulsion was dispersed in a water phase containing S1 particles. In Fig. 4, we can clearly distinguish the oil globules, each of them containing sedimented internal droplets. Due to their double (or multiple) compartment structure, interest in double emulsions has been increasing, as they can be considered as reservoirs of encapsulated substances to be released under variable conditions. Copper sulfate (CuSO4 ) was used as a tracer to probe the potentiality of macroscopic double emulsions to maintain an encapsulated species inside the internal water droplets. The initial concentration of CuSO4 (0.1 mol L−1 ) provides an intense bluish color to the internal droplets. Even after 1 year, the bluish color persisted with the same apparent intensity, while the external water phase remained uncolored. It can be concluded that such
For all the materials presented, one can note the remarkable monodispersity despite the use of uncontrolled flow for the preparation. As was already underlined, this monodispersity is surprising considering that the droplet growth is driven by coalescence. Whitesides et al. [6] have proposed a theoretical analysis for either diffusional or turbulencedriven droplet collisions. Assuming that the coalescence probability between two drops is simply proportional to their individual uncovered surface fraction, Monte-Carlo simulations predict droplet size distributions that are much narrower than those resulting from unlimited coalescence. For all the previously mentioned materials, the final mean droplet diameter can be perfectly controlled by adjusting the amount of particles. Because the solid particles are totally and irreversibly adsorbed (no free particles in the continuous phase), the inverse average droplet diameter must vary
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Fig. 5. Evolution of the inverse diameter, 1/D, with mp for direct (o/w) emulsions containing 75 wt% 350-cP PDMS and stabilized by S1 particles. The emulsions were shaken manually. From the experimental slope of the curve, we deduce sf = 8.7 m2 g−1 .
linearly with the amount of particles [7], 1/D = (sf mp )/6Vd , where mp is the mass of particles, Vd is the volume of dispersed phase, sf is the specific surface area, i.e., the droplet surface covered per unit gram of silica. This latter quantity depends on the intensity of the agitation [7]. Fig. 5 provides an example confirming the validity of the previous relation. For this particular system obtained by manual shaking, sf = 8.7 m2 g−1 . A densely packed layer of adsorbed particles (2-D compacity of 0.9) would provide an sf value of 30–35 m2 g−1 . Therefore, the particles do not form a hexagonal 2-D array, but rather adsorb as large aggregates with low covering capacity [7]. 4.2. Outstanding stability of the thin liquid films The highly concentrated emulsions reported in Section 3.1 are analogous to foams, although they comprise two liquids. We should like to stress that biliquid foams of comparable diameter undergo rapid destruction through coalescence when stabilized by standard molecular surfactants, their lifetime being generally shorter than 1 h. Besides the outstanding stability, biliquid foams stabilized by solid particles exhibit a relatively low viscosity (<0.1 Pa s) despite the high droplet volume fraction. This is due to the extremely large diameter of the dispersed objects which provides a very low surface area-to-volume ratio. The inherent fluidity of these biliquid foams is an appreciable property for all applications where emulsions have to be spread or poured. Once the coalescence process is achieved, the emulsions are very stable due to the rigid barrier created by the solid particles anchored at the droplet surface. The micron-sized emulsions can even be dried without being destroyed until almost all the water is evaporated, as can be observed in Fig. 6. The original shape of the sample is preserved during the drying process, with absolutely no oil leakage, revealing the existence of a strong protective layer around the oil droplets. After removal of the oil phase, it is even possible to obtain macroporous materials composed of air compartments separated by silica aggregates. The unusual long-term encapsulation of the macroscopic double emulsions described in Section 3.2 can also be corre-
Fig. 6. Direct (o/w) emulsion of 350-cP PDMS stabilized by S1 particles, with an average drop diameter of 2 µm, dried in an oven at 40 ◦ C during 3 days.
lated to the outstanding stability of the solid-stabilized films. It is now well established that two mechanisms are responsible for the release of chemical substances in w/o/w double emulsions [8]. (i) One is due to the coalescence of the thin liquid film separating the internal droplets and the globule surfaces. (ii) The other mechanism, termed compositional ripening, occurs by diffusion and/or permeation of the chemical substance across the oil phase. The first mechanism, coalescence, did not take place in our double emulsions since we did not visualize any variation either in the number or in the size of the droplets inside the globules (Fig. 4). Concerning compositional ripening, several models have been proposed, all of them are in agreement with Fick’s law [8]. The permeation could be due to reversible and thermally activated holes that are continuously formed in the thin liquid films [8]. The characteristic hole size and lifetime may allow the passage of small hydrophilic substances. Among the many holes that are permanently formed, the smaller ones are evanescent and contribute to the permeation process, while the larger are irreversible and grow, producing coalescence events. It is unlikely that such holes are formed in our systems since we did not observe any sign of coalescence on the scale of 1 year. Another possible mechanism for the permeation could result from the solubilization of the salt ions in the oil phase followed by molecular diffusion across the globule. Since the solubility of the encapsulated species (CuSO4 ) in dodecane is negligible, the permeation following this latter mechanism occurs at extremely low rate. Another reason for the longterm encapsulation capacity lies in the fact that our double emulsions comprise droplets and globules of macroscopic size with a very low surface area-to-volume ratio. Under such conditions, the exchange capacity at the water/oil interfaces is quite reduced [8] and only application of a shear can induce the release of the entrapped species. 4.3. Elasticity and rheological properties With the aim of understanding the origin of the unusual resistance to coalescence in solid-stabilized emulsions, we
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performed rheological measurements. In concentrated emulsions, the drops are partially deformed and the rheological G and G are determined mainly by the properties of the interfacial layers. 4.3.1. Experimental procedures The rheological experiments were carried out with a controlled strain RFSII rheometer using a parallel plate geometry with a gap of 1 mm. Despite the fact that the applied stress is not constant over the whole sheared volume, this type of geometry was preferred due to the very high rigidity of the materials and to avoid problems related to confinement. The frequency of the applied strain was 0.1 Hz. We checked that, for all the measurements, the stress always varied linearly with the strain (linear regime). The interfacial tension γ between PDMS oil and pure water was determined at equilibrium using a Wilhelmy balance (T = 20 ◦ C). 4.3.2. Results Despite being composed of fluids, emulsions consisting of micron-sized concentrated droplets can possess a striking shear rigidity that is characteristic of a solid. Mason et al. [9] have studied the rheological properties of monodisperse emulsions, stabilized by sodium dodecyl sulfate. The normalized data for the elastic storage modulus, G , all fall onto a single curve as shown by the continuous line in Fig. 7. The scaling with γ /R (R is the droplet radius) confirms that the elasticity results from the droplet interfaces. The scaling indicates that the elasticity of these compressed monodisperse emulsions is universal. Shear elasticity exists because the repulsive droplets have been compressed and thus concentrated up to a sufficiently large volume fraction, φ, which permits the storage of interfacial energy [9,10]. The droplet system minimizes its total free energy by reducing the repulsion at the expense of creating some additional surface area by deforming the droplet interfaces.
Fig. 7. Storage modulus G (normalized by γ /R) of emulsions stabilized by surfactants (continuous line) and solid particles (symbols). Solid-stabilized o/w emulsions were obtained using a jet homogenizer in presence of S1 particles.
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In the following, we describe the rheological properties of micron-sized PDMS-in-water emulsions obtained using a jet homogenizer in the presence of S1 particles. From the slope of the curve 1/D versus mp , we deduce sf = 40 m2 g−1 . Such a value is consistent with a monolayer of solid particles anchored at the oil/water interface with a compacity close to 0.75 (the maximum 2-D compacity of monodisperse spheres is 0.9). In Fig. 7, we report only the values obtained for the elastic storage modulus G . G was at least 10 times larger than the dissipation modulus G , the difference increasing with the droplet volume fraction. This reflects the essentially elastic nature of the materials. The G moduli normalized by γ /R (γ = 0.03 N m−1 for the silicone oil/water interface) do not fall onto the master curve of Mason et al. [9]. Therefore, the energy storage is not controlled by the Laplace pressure of the droplets. Let us mention here that the probed emulsions were substantially flocculated, which explains the significant elasticity measured at volume fractions well below the random close packing (φ ∗ = 64%). At large volume fractions (φ > φ ∗ ), the normalized elasticity is more than one decade above Mason’s curve. This behavior is reminiscent of that already reported for proteinstabilized emulsions [11,12]. It can be argued that strong lateral interactions between the adsorbed solid particles renders the surface extremely rigid so that the deformation is not controlled by γ /R but rather by ε/R, ε being an effective elastic coefficient characterizing the droplet surface. Our rheological data fall on the master curve of Mason et al. [9] if we take ε = 0.3 N m−1 . The surface can be regarded as a compact 2-D network of solid particles with strong lateral attractive interactions: van der Waals forces, capillary forces [13], and the attractive interactions arising from the interpenetration of the octyl chains grafted on the solid particles [14]. Assuming that the spacing between the particles is equal to the length of an extended octyl chain (0.6 nm), the van der Waals attraction can be evaluated as 4–10kT . Considering the characteristic size of the solid particles (25 nm), we deduce that the capillary interactions are not larger than 10kT [13]. We therefore believe that the strongest interaction arises from the interpenetration of the octyl chains. Indeed, the octyl chains are in bad solvent in the water continuous phase and tend to overlap. The resulting attractive interaction can be as large as 104 kT [14] for particles of 25 nm densely covered by octyl chains. It is likely that this interaction is also responsible for the aggregation of the solid particles in the water phase (prior to the emulsification) and of the oil drops once the emulsions are fabricated. The very high value of the effective elastic coefficient ε can be correlated to the outstanding stability of the droplets against coalescence. Let us first remember that there are no free particles in the continuous phase and that their adsorption is irreversible [7]. The occurrence of coalescence events should imply the formation of “patches” at the oil/water interface that are not covered by the solid particles. It is clear that thermal fluctuations are unable to disrupt the lateral
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links between the adsorbed particles, and, consequently, the spontaneous formation of uncovered patches is completely inhibited. The only way to form unprotected zones is by deforming the droplets to a sufficient degree so that their total surface area is extended. This may be achieved on application of either a stress or a compression of the order of ε/R, i.e., 3 × 105 Pa for micron-sized drops. To test this idea, an emulsion with R = 1 µm was introduced into a tube and submitted to a centrifugational field (Optima TLX Ultracentrifuge). After 1 h, the drops were strongly compressed at the bottom of the tube (the average density of the droplets covered by the silica particles was larger than 1). From the oil volume fraction and the height of the sediment, we deduced the maximum osmotic pressure exerted at the bottom of the sample [11]. We observed that coalescence (release of macroscopic oil) occurs only when the osmotic pressure reaches a threshold value close to 2.4 × 105 Pa, in excellent agreement with the previous prediction.
emulsion to coalescence is strongly dependent on particles attraction. This conclusion is in agreement with the general phenomenology reported in the field of solid-stabilized emulsions: the most stable emulsions are very often obtained with solid particles that are weakly or strongly aggregated in the bulk continuous phase. Finally, we stress that monodisperse solid-stabilized emulsions are certainly appropriate precursors for the fabrication of elaborated materials like templates, solid foams, and macroporous aerogels.
Acknowledgments This work was partially funded by Rhodia Services (France) for S.A. The authors thank D. Monin and B.P. Binks for fruitful discussions and J.-Y. Chane-Ching for providing the S1 particles.
References 5. Conclusion It has been observed that solid particles may generate new materials that are not available with standard surfactant molecules. Besides the field of applications which is considerably widespread, most of the properties of these materials remain unexplored and it is clear that some basic concepts governing the behavior of emulsions (aging, bulk elasticity, etc.) need to be revisited in the presence of solid particles. The rheological data reported in the present paper provide indirect evidence of the existence of a rigid interfacial layer adsorbed at the oil–water interface. The rigidity of the interface is most probably due to strong attractive interactions between the solid particles. For coalescence to occur, elongation of the interface is necessary and this can be achieved on compression through centrifugation or on application of a high shear. Therefore, the resistance of a solid-stabilized
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