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Particle-Stabilized Emulsions Comprised of Solid Droplets J. Giermanska-Kahn,† V. Laine,† S. Arditty,† V. Schmitt,† and F. Leal-Calderon*,‡ Centre de Recherche Paul Pascal, CNRS, Av. Schweitzer, 33600 Pessac, France, and Laboratoire des Milieux Disperse´ s Alimentaires, ISTAB, Av. des Faculte´ s, 33405 Talence, France Received January 14, 2005. In Final Form: February 8, 2005 We kinetically stabilize oil-in-water emulsions comprising paraffin crystals by adsorbing solid particles (silica) of colloidal size at the oil/water interface. We obtain a set of emulsions that are quiescently stable for a long period of time (months), while the same emulsions are destabilized after only a few hours in the presence of surfactant molecules alone. The emulsions are submitted to a shear stress in order to probe their stability under flow conditions. Partial coalescence and gelation occur when the shear is applied for a sufficiently long period of time. The experiments reveal the existence of a critical droplet mass fraction, φ*, that defines a sharp transition between slow and fast gelation. The process of gelation is rather slow for φ < φ*, occurring at the scale of hours, and becomes almost instantaneous above φ*.
Introduction It is well-known that the presence of fat crystals in the disperse phase of oil-in-water (O/W) emulsions can cause a significant decrease in the kinetic stability.1-4 The reason is that crystals may protrude into the water phase, and if such a crystal is present in the film between approaching droplets, it may pierce the film, causing the droplets to coalesce. The coalescence is incomplete or partial because the network of crystals within the droplets allows the original shape of the droplets to be maintained in the aggregates. Partial coalescence may produce a rapid destruction of the materials during storage and severely restricts the application field of emulsions in which the dispersed phase is totally or partially crystallized. Proteins are highly efficient in stabilizing triglyceridein-water emulsions against partial coalescence and are commonly exploited in the food industry for the preparation of dairy products. The emulsions exhibit long range kinetic stability under quiescent storage conditions and can even be submitted to high shear forces without being destroyed. Instead, short surfactants generally do not provide a sufficient degree of stabilization and the emulsions become lumpy a few hours after their preparation. The addition of small amounts of surfactant in protein-stabilized emulsions is a common way to monitor the sensitivity toward partial coalescence.5-6 Small molecule emulsifiers are typically present, although they are not required to help formulation. The displacement of proteins from the oil interface by added surfactants contributes to emulsion instability and induces sensitivity to partial coalescence. In that case, shear forces due to the processing can induce irreversible changes in the texture, transforming the initially fluid emulsion into a rigid material (e.g., whipped creams). * Corresponding author. E-mail:
[email protected]. † CNRS. ‡ ISTAB. (1) van Boekel, M. A. J. S. Thesis, University of Wageningen, The Netherlands, 1980. (2) Melsen, J. P. Thesis, University of Wageningen, The Netherlands, 1988. (3) Boode, K.; Walstra P. Colloids Surf., A 1993, 81, 121. (4) Boode, K.; Walstra P.; de Groot-Mosert, A. E. A. Colloids Surf., A 1993, 81, 139. (5) Govin, R.; Leeder, J. G. J. Food Sci. 1971, 36, 718. (6) Goff, H. D.; Jordan, W. K. J. Dairy Sci. 1989, 72, 18.
The present study is concerned with the development of a different strategy to stabilize oil-in-water emulsions comprising totally solidified droplets (paraffin). It consists of adsorbing silica particles of colloidal size at the oil/ water interface. It is expected that the silica particles form a rigid barrier that mechanically protects the droplets against partial coalescence. If the characteristic thickness of the adsorbed layer is sufficiently large compared to the reach of the protruding crystals, these crystals should become unable to bridge the gap between the droplets. The main purpose of this study is to test the effectiveness of silica particles in stabilizing emulsions made of solid paraffin droplets dispersed in an aqueous phase. The hydrophilic surface of the silica particles was made partially hydrophobic by either adsorbing a small quantity of surfactant or grafting a suitable chemical, to promote the anchoring at the oil/water interface. We obtain a set of model emulsions that are quiescently stable for a long period of time (months), while the same emulsions are destabilized after only a few hours in the presence of surfactant molecules alone. The emulsions are then submitted to shear forces in order to probe their stability under flow conditions. Partial coalescence and gelation occur when the shear is applied for a sufficiently long period of time. We followed the kinetic evolution of the viscosity at constant shear stress: the viscosity is weakly varying in the first stages and suddenly undergoes a dramatic increase. We study the influence of various parameters on the characteristic gelation time: droplet size, silica diameter, and paraffin mass fraction. Materials and Methods The dispersed phase was a paraffin wax (Merck, CAS no. 800274-2, mixture of long alkanes) with a very narrow range of melting temperature from 42 to 44 °C and a density of F ) 0.9 g‚cm-3. The surfactant used was cetyltrimethylammonium bromide (CTAB) purchased from Chempur (purity >99%). Solid-stabilized emulsions were obtained using three different hydrophilic silica particles (Aerosil fumed silica) provided by Degussa (France). Their main characteristics are reported in Table 1, and we shall refer to them as P1, P2, and P3. They were delivered in powder form and were dispersed in pure water of Milli-Q quality under manual shaking. It is important to note that the silica dispersions were substantially aggregated, forming clusters at a scale of 1001000 nm in the water phase. The silica density is 2.2 g‚cm-3 for all the particle types, and the specific surface areas, sp, deduced
10.1021/la0501177 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/29/2005
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Table 1. Main Characteristics of the Particles (from Degussa) Used in This Study (See Text for Details) particle name
origin
primary particle sp ccp cf diameter (nm) (m2/g) (m2/g) (m2/g)
P1 P2 P3
Aerosil A380 Aerosil A130 Aerosil Ox50
7 21 55
380 130 50
82 28 11
35 15.2 4.6
from BET adsorption isotherms (provided by the manufacturer) are reported in Table 1. Paraffin-in-water emulsions were fabricated in batches of 60 mL according to the following protocol. The paraffin oil (20-35 wt %) and the aqueous solution containing surfactant and/or solid particles were heated at 60 °C. The paraffin was then deposited at the top of the water solution, and the mixture was submitted to vigorous agitation by means of an Ultra-Turrax homogenizer (T25 JANKE & KUNKEL) equipped with a S25 KV-25F rotor head, operating at 9000 rpm for 1 min. In the presence of solid particles, the obtained emulsions were substantially flocculated, and very often, a thin macroscopic oil layer appeared at the top of the samples. Homogenizing the samples with Ultra-Turrax for a longer period of time did not change the emulsion characteristics. This is why, right after the first treatment, the emulsions were submitted to a second homogenization step with lower energy input. This was achieved by means of a Rayneri mixer (Turbotest 33/300P) working at 3000 rpm for 10 min. It was not possible to produce emulsions with an average diameter smaller than 50 µm by applying this second mechanical treatment directly. However, when associated to the first one (Ultra-Turrax), we obtained emulsions over a wide average diameter range (from 10 to 60 µm) with enhanced stability, since no oil leakage was observed anymore at 60 °C. Once fabricated, the final emulsions were immediately stored at room temperature (T ) 25 °C). The emulsions were observed with a phase contrast optical microscope (Zeiss Axiovert ×100), and their size distribution was obtained using a Malvern Mastersizer granulometer. The size distribution was characterized in terms of the surfaceaveraged diameter
∑N D
3
∑N D
2
i
Ds )
i
i
i
i
i
and polydispersity
∑N D 1 i
P)
3
i
|D h - Di|
i
D h
∑N D i
3
i
i
h is where Ni is the total number of droplets with diameter Di. D the median diameter, that is, the diameter for which the cumulative undersized volume fraction is equal to 50%. Granulometric measurements were performed at T ) 25 °C in a pure water solution. The solid paraffin globules were strongly diluted under the measurement conditions (φ < 0.01%). To probe reproducibility, several successive measurements were performed with the same sample loaded in the granulometer and we got exactly the same size distribution. This suggests that aggregation or partial coalescence of the paraffin droplets did not occur in the measuring cell within the time scale of the experiment (∼1 min). Shear flow experiments were conducted at T ) 25 °C with a controlled stress rheometer (TA Instruments, AR 1000). We adopted a thermostated Couette cell with a gap of 1 mm and rough surfaces in order to limit wall slipping. The cell was equipped with an antievaporating device.
Results and Discussion 1. Surfactant-Stabilized Emulsions. Figure 1 is a microscope image of an emulsion stabilized by CTAB at
Figure 1. Microscopic image of a paraffin-in-water emulsion stabilized by CTAB alone. T ) 25 °C.
room temperature. Solidification induces a significant change in the texture of the droplets. The spherical shape of the warm fluid droplets which is controlled by surface tension evolves into a rough and rippled surface due to the formation of irregularly shaped/oriented crystals. All the emulsions formulated with surfactants alone are rapidly destabilized because of partial coalescence. A few hours after the preparation, the solid droplets irreversibly stick to each other, forming dense macroscopic agglomerates that sit at the top of the sample. The agglomerates cannot be redispersed; instead, they form compact macroscopic clumps that totally separate from the water phase if the emulsion is submitted to mechanical stirring. Moreover, they melt and transform into a liquid macroscopic oil layer when the samples are reheated at 60 °C. In other words, if the aggregated system is rewarmed, the droplets lose their integrity and fuse into a macroscopic fluid phase, thus confirming the formation of paraffin bridges (partial coalescence). 2. Particle-Stabilized Emulsions. 2.1. Influence of Surfactant and Oil Contents. With the aim of improving the stability, we emulsified the same crystallizable oil in the presence of silica particles. Since P1, P2, and P3 particles are intrinsically hydrophilic, it was necessary to partially hydrophobize the surfaces in order to favor particle adsorption at the oil/water interface. To achieve this, surfactant molecules were employed at very low concentration. The surfactant type was adapted to obtain the strongest anchoring of the molecules on the particle surfaces using electrostatics. A cationic surfactant (CTAB) was thus selected to hydrophobize the anionic P1, P2, and P3 bare silica surfaces (pH ≈ 5-6). We observed that the emulsions were rapidly destabilized if the surfactant concentration initially introduced in the water phase was above its critical micellar concentration (cmc ≈ 9 × 10-4 mol‚L-1 for CTAB). It is well-known that molecules such as CTAB form bilayers at the silica/water surface when the free surfactant concentration in the aqueous phase exceeds 1 cmc.7 The polar heads of the external layers are oriented toward the water phase, and the silica surface remains hydrophilic. Under such conditions, the solid particles do not adsorb on the emulsion droplets and the behavior is the same as that for surfactant-stabilized emulsions. For initial concentrations in the water phase lower than the cmc (∼cmc/5), the emulsions were kineti(7) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. Adv. Colloid Interface Sci. 2003, 103, 219.
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Figure 2. Typical size distributions obtained for surfactantstabilized (dashed line) and solid-stabilized (solid line) paraffinin-water emulsions (P2 particles).
cally stable and we got experimental evidence that the solid particles were almost totally adsorbed at the oil/ water interface. Indeed, no silica clusters were visible under the microscope once the emulsification was achieved. Moreover, the emulsions were stored for more than 24 h: after that delay, the droplets formed a dense cream coexisting with a transparent subphase. No solid sediment was visible, thus suggesting that all the particles were attached to the droplet interfaces. In all the experiments, the surfactant concentration was fixed at the minimum level required to stabilize the paraffin-in-water emulsions. The amount of surfactant was adapted to the total mass of particles so as to maintain the same specific coverage: 13 nm2 per CTAB molecule at the silica/water interface. This value was estimated with the assumption that all the surfactant from the bulk is employed to cover the silica surface. Of course, surfactant is also adsorbed at the oil/ water interface, but in our systems, the silica surface area is approximately 10 times larger than the droplet surface area (deducible from eq 1 in section 2.2, with the data given in Table 1). Moreover, the electrostatic attraction between the oppositely charged surfactant molecules and silica surfaces favors adsorption at the silica/water interface. The highest mass fraction of paraffin that can be directly emulsified is about 35%. Above this critical value, the final emulsions become lumpy and cannot be redispersed. However, it was possible to get stable emulsions up to 55%, by emulsifying the oil at a lower fraction ( 45%) resemble solid pastes. 2.2. Influence of the Particle Content. We now consider the influence of the amount of particles on the droplet size distribution. In Figure 2, we report typical size distributions obtained for surfactant-stabilized (dashed line) and solid-stabilized (solid line) paraffin-in-water emulsions. The measurement for surfactant-stabilized emulsions was carried out a few minutes after the preparation to avoid partial coalescence. Surprisingly, the solid-stabilized emulsions possess a relatively narrow size distribution characterized by a low value of the polydispersity index: P ) 0.35 ( 0.05 (see also Figure 3). Usually, emulsions produced under turbulent flow conditions are quite polydisperse (P > 0.5), as is the case for the surfactantstabilized emulsion shown in Figure 1. We varied the total amount of solid particles, at a constant mass fraction of paraffin. In Figure 4, we report
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Figure 3. Microscopic image of a paraffin-in-water emulsion stabilized by P2 particles. Inset: same image taken at T ) 25 °C under crossed polarizers, confirming the presence of crystals in the droplets.
Figure 4. Evolution of the inverse average diameter, 1/Ds, as a function of the mass of solid particles employed to fabricate the emulsion. Volume of the dispersed phase, 12 mL; total volume, 60 mL.
the evolution of the inverse average droplet diameter as a function of the total mass of solid particles. The linear variation and narrow distribution suggest that the emulsions have been obtained following a limited coalescence process.8-10 Limited coalescence occurs when a large excess of an oil/water interface is produced, compared to the amount that can be potentially covered by the solid particles. 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 by the particles. For the sake of clarity, we should like to emphasize the difference between “limited” and “partial” coalescence. Limited coalescence occurs in the liquid state of the droplets which recombine and freely relax their shape. The process is interrupted after a short period of time and leads to a dispersion of individual droplets. Instead, partial coalescence refers to “unrelaxed” coalescence events that irreversibly connect the solidified droplets through the formation of paraffin bridges. (8) Wiley, R. M. J. Colloid Sci. 1954, 9, 427. (9) Whitesides, T. H.; Ross, D. S. J. Colloid Interface Sci. 1995, 169, 48. (10) Arditty, S.; Whitby, C. P.; Binks, B. P.; Schmitt, V.; Leal-Calderon, F. Eur. Phys. J. E 2003, 11, 273.
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The final average droplet diameter can be controlled by adjusting the amount of particles. Because the solid particles are totally and irreversibly adsorbed, the inverse average droplet diameter, Ds, varies linearly with the amount of particles:10
cfmp 1 ) Ds 6Vd
(1)
where mp is the mass of the particles, Vd is the volume of the dispersed phase, and cf is the surface coverage, that is, the amount of droplet surface area covered per unit mass of silica. This latter quantity depends on the mixing intensity.10 Figure 4 provides examples confirming the validity of the previous relation. In Table 1, we report the cf values deduced from the slope of the experimental curves for all the particle types. For the sake of comparison, we also calculate the theoretical surface coverage, ccp, of a closely packed layer of adsorbed particles (2-D compacity of 0.9): ccp ) [(x3)/8]sp. The experimental cf values are clearly smaller than the ones calculated assuming a densely packed arrangement. Since the particles are initially aggregated in the continuous phase, it is likely that clusters are attached to the surface through a reduced number of anchoring particles and that other particles in the clusters are protruding toward the continuous phase. The coverage efficiency revealed by cf is thus reduced compared to a closely packed monolayer.10,11 The silica-stabilized emulsions form networks of aggregated droplets visible under the microscope (Figure 3). The aggregates are not due to partial coalescence but rather to an attractive interaction between the droplet surfaces. Indeed, the clusters are labile, meaning that they can be disrupted upon application of a shear. This occurs, for example, if one of the glass slides that confine the sample under the microscope is displaced with respect to the second one. Immediately, the clusters dissociate into individual droplets which again aggregate when the shear is interrupted. It is worth reminding that surfactantstabilized emulsions are totally unstable upon application of a shear. By comparing Figures 1 and 3, it appears that the texture of the droplet surfaces is more regular for solidstabilized emulsions than for the surfactant-stabilized ones. We believe that the solid particles modify the crystal size and number. Each adsorbed particle may activate the nucleation process, and consequently, more crystals of smaller size should be formed at the scale of the droplets. The surfaces appear smoother because small crystals protrude over reduced length scales. 2.3. Freeze-Thaw Stability and Resistance to Water Elimination. An emulsion stabilized by P2 particles (Ds ) 30 µm, φ ) 35%) was submitted to three successive freeze-thaw cycles in order to probe its thermal stability. It was held for 3 h at room temperature: this delay was sufficient for crystallization to occur, as revealed by the rippled texture of the surfaces as well as the bright spots appearing in each droplet under light-polarized microscopy (see inset of Figure 3). After that delay, the emulsion was heated from 25 to 60 °C (+1 °C/min), maintained at 60 °C for 15 min, and cooled again (-1 °C/min). No visible surface oil was observed after the three cycles. Moreover, the droplet size distribution was immediately measured after each cycle and did not exhibit any variation. This definitely confirms the absence of partial coalescence between the paraffin droplets. These experiments also reveal the (11) Binks, B. P.; Kirkland, M. Phys. Chem. Chem. Phys. 2002, 4, 3727.
Figure 5. Image showing the difference between a fluid emulsion and an emulsion submitted to the “water suction” experiment (see text for details). For the sake of comparison, both samples are deposited on the same nonporous surface. The sample on the right is the initial fluid emulsion (φ ) 20%). The sample on the left was previously deposited on a porous substrate and became immediately solidlike.
remarkable stability of solid-stabilized emulsions with respect to freeze-thaw cycling. As a comparison, surfactant-stabilized emulsions were almost totally destroyed after the first cycle. A very simple experiment was performed to qualitatively assess the stability of solid-stabilized emulsions upon water elimination. A 2 mL portion of emulsion at φ ) 20% was deposited on a porous substrate: either a porous silica plate or a blotting paper. Water is pumped by the substrate under the effect of capillarity, so the droplet mass fraction in the sample rapidly rises. Amazingly, the initially fluid emulsion is transformed in a few seconds into a jammed rigid material that does not flow anymore (Figure 5). We could vary the final droplet fraction by allowing the emulsion to reside on the porous substrate for variable periods of time; the longer the residence time, the larger the droplet fraction in the final state. The experiment was carried out using an emulsion stabilized by P2 particles, with an average diameter of Ds ) 30 µm. The capillary suction provided samples with mass fractions ranging from 50 to 60%. Each concentrated emulsion was diluted again in pure water at about φ ) 5% in order to determine whether redispersion occurs or not. Within reasonable experimental uncertainty, it can be stated that the samples with a volume fraction lower than (54 ( 2)% are totally redispersable. Above that limit, irreversible clumps were observed in the solution and the granulometric measurements revealed the presence of droplets larger than the initial ones. 2.4. Shear Stability of Solid-Stabilized Emulsions. Despite the aggregated structure, the emulsions were perfectly stable with respect to partial coalescence over several weeks at room temperature under quiescent storage conditions. With the aim of probing the stability under flow, the paraffin-in-water emulsions stabilized by P1, P2, and P3 particles were subjected to shear forces using the stress-controlled rheometer at T ) 25 °C. However, this type of experiment could not be performed directly with the emulsions formulated according to section 2.1. Indeed, we observed rapid clumping at the emulsion/ air interface during the course of the experiments, suggesting that the emulsion droplets have a strong affinity toward air. Clumping was clearly initiated at the emulsion/air interface, since no such clumps were present
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Figure 6. Image showing the interfacial clumping phenomenon in paraffin-in-water emulsions. The upper plate of the rheometer was removed after shearing the sample at σ ) 20 Pa for 20 min. (a) Low surfactant content (13 nm2 per CTAB molecule at the silica water interface assuming total adsorption): clumps are initially formed at the emulsion/air interface. They are progressively expelled from the interface, forming a visible ring around the rotating plate of the rheometer. (b) Addition of 1 cmc of CTAB in the continuous phase. The sample does not exhibit clumping anymore.
in the bulk phase. This phenomenon is reminiscent of mineral flotation frequently used in the mining industry to separate valuable minerals from other materials in their host environment. Minerals are first hydrophobized by the adsorption of surfactants at low concentration and separated by attaching themselves to rising air bubbles.12 In our experiments, the silica particles covering the emulsion drops have been partially hydrophobized by CTAB and can therefore adhere to the air/water interfaces. The rigid clumps are progressively expelled out of the rheometer gap, and the process goes on at the level of the emulsion/air interface, until almost complete destruction of the material. The phenomenon is illustrated in Figure 6a. Exceptionally, the emulsion was sheared in parallelplate geometry (instead of Couette geometry) in order to easily visualize the effect: rigid clumps have been expelled several centimeters around the rotating plate. Clumping also occurs if, for example, the emulsions are submitted to vigorous manual shaking. Coupling between agitation and permanent extension of the emulsion/air interface produces irreversible clumping, which again destroys the emulsions after a sufficient period of time. To avoid this phenomenon, we modified the wettability of the droplet surfaces by incorporating CTAB. The emulsions were fabricated according to the formula defined in section 2.1 (low surfactant content). After cooling, that is, when the paraffin droplets return to the solid state, surfactant was added in the continuous phase up to a concentration of 1 cmc. Under such conditions, the shear experiments could be carried out without the above-described artifact (see Figure 6b). We checked that the surfactant addition did not produce any desorption of the solid particles as far as the droplets remained in the solid state. Indeed, no silica sediment was observed after allowing the emulsions to cream for 24 h. Moreover, the emulsions remained stable for at least 1 month at room temperature, while, as explained in section 1, emulsions stabilized by surfactant alone exhibit rapid and irreversible clumping (after some hours). Therefore, although the particle wettability has been varied, the emulsion stability is dominated by the particle layers which remain immobilized or mechanically trapped at the solid oil/water interface. However, warming the solid-stabilized emulsions up to 60 °C followed by cooling at T ) 25 °C resulted in partial destruction of the materials. This behavior suggests that, in the presence 1 cmc of CTAB, silica desorption takes place as soon as the (12) Rosen, M. J. Surfactants and Interfacial Phenomena; Marcel Dekker: New York, 1989.
Figure 7. Evolution of the viscosity for a paraffin-in-water emulsion stabilized by P2 particles, sheared at σ ) 20 Pa. Ds ) 30 µm and φ ) 41.3%. T ) 25 °C.
droplets are melted. Desorption is expected at 1 cmc, since the particle surfaces become hydrophilic.7 If oil is in the liquid state, the particles recover some mobility and can leave the interface. The kinetic stability is then dominated by the adsorption of surfactant, and as stated above, the emulsion is partially destroyed after only a few hours. This is why, before shear experiments, we avoided exposing the emulsions to temperatures higher than 40 °C (paraffin melting temperature) once CTAB was added at 1 cmc. We measured the evolution of the viscosity, η, as a function of time, at constant stress σ ) 20 Pa and T ) 25 °C. A typical curve is presented in Figure 7. The weak decrease in viscosity observed at short times can be attributed to a partial deflocculation of the paraffin globules. After this initial step, the viscosity exhibits a slow monotonic increase that most probably reflects the progressive formation of shear-induced droplet aggregates. Finally, η dramatically increases over at least one decade, reflecting a sudden gelation/destabilization of the system. This was confirmed by moving the rotor up: the emulsions became remarkably rigid. Moreover, they could not be redispersed when diluted in pure water under agitation, thus suggesting that the colloidal gel formed under shear contains partially coalesced droplets. We define the socalled gelation time, τ, as the time corresponding to the maximum value measured for the viscosity. The sudden jump offers a way to measure τ with sufficient accuracy. However, it should be noted that the viscosity measure-
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Figure 8. Evolution of the gelation time, τ, as a function of the droplet fraction for paraffin-in-water emulsions with Ds ) 30 µm stabilized by the three types of particles. T ) 25 °C. The lines are only a guide to the eyes. The abrupt decay of the line indicates that τ dramatically vanishes above a critical fraction, φ* (defined by the intercept with the axis).
ments at longer times were subject to large irreproducible fluctuations: the system was presumably exhibiting either fractures or “plug flow” once gelled (t > τ). It is worth noting that the gelation phenomenon is directly linked to the crystallizable nature of the droplets. We identically prepared an oil-in-water emulsion (Ds ) 30 µm, φ ) 35%) stabilized by P2 particles with a paraffin which is fluid at room temperature (from Merck, CAS no. 8012-95-1, density F ) 0.85 g‚cm-3). Gelation did not occur in the presence of liquid paraffin droplets, and only a small increase in viscosity was observed after shearing the sample for 10 h. The same behavior was obtained for an emulsion comprised of pure fluid dodecane (Merck, purity >99%) at T ) 25 °C. In Figure 8, we plot the evolution of τ as a function of the paraffin mass fraction for emulsions with an average diameter of Ds ≈ 30 µm and the three types of particles. Although the data are a little bit scattered, they reveal interesting issues. In the presence of P2 and P3 particles, the data follow the same qualitative evolution: τ weakly depends on φ for 30% < φ < 45% and dramatically decreases above a critical value, φ* ≈ (47 ( 2)%. We could not measure τ for samples with a mass fraction lower than 30% because of the low viscosity of the emulsions: at the applied stress (σ ) 20 Pa), the resulting shear rate exceeds the upper measurable limit of the apparatus (2500 s-1). For φ > φ*, gelation occurred during sample loading, reflecting the strong shear sensitivity of the most concentrated samples. This is why, in Figure 8, no data are reported above φ*. In the presence of P1 particles, the gelation time exhibits a more regular decrease with φ and is shorter than that in the presence of P2 and P3 particles. This result suggests that P1 particles are less efficient in stabilizing the solid paraffin globules against flow-induced gelation/coalescence. Note that P1 particles possess the smallest primary particle diameter among the three silica types that were probed in the present study. In Figure 9, we examine the influence of the droplet average diameter for P2-stabilized emulsions. Interestingly, the τ values measured for the smallest globules (Ds ) 13 µm) are much larger, while the critical mass fraction is smaller, φ* ≈ (33 ( 2)%. 3. Discussion. We now comment on the characteristic features of the gelation process. Davies et al.13 have explored the shear stability of protein-stabilized emulsions containing triglyceride crystals. Constant stress experi(13) Davies, E.; Dickinson, E.; Bee, R. Food Hydrocolloids 2000, 14, 145.
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Figure 9. Evolution of the gelation time, τ, as a function of the droplet fraction for paraffin-in-water emulsions with variable droplet diameters stabilized by P2 particles. T ) 25 °C. The lines are only a guide to the eyes.
ments were carried out, and the authors observed a slow variation regime followed by a rapid increase in the viscosity, exactly as in our systems. The problem of gelation in flowing suspensions is one of the fundamental problems in colloid science and has motivated several theoretical developments. If we consider the aggregation of dilute hard spheres, two regimes are usually distinguished. At the first stages, the process is dominated by doublet formation and involves collisions between two particles. At longer times, large clusters appear and each cluster can grow by the accretion of any other primary particle or cluster. Potanin14 argues that the evolution of the system can be correctly described considering that all the particles are combined into uniform aggregates of identical radius. In other words, the most probable mechanism of aggregation is of a hierarchical type and involves cluster-cluster aggregation, thus explaining the fast gelation occurring at the approach of τ (see Figure 7). In our case, the process of gelation can be more complex than the simple picture proposed for dilute suspensions of undeformable hard spheres. The paraffin behaves like a soft waxy solid that may undergo plastic deformations. The adsorbed silica particles form a rigid bidimensional network which can also experience plastic deformations during droplet collisions. The flow-induced structures certainly result from a complex interplay between the attractive colloidal interactions, the intrinsic plasticity of the droplets and of the interfaces, and the applied shear. In the following, we shall only comment on some general tendencies, since a more profound comprehension of the kinetic evolution of the emulsion viscosity would require a direct imaging of the structures formed in the sample during the shearing. The gelled materials resemble elastic solids and cannot be redispersed into primary droplets. Therefore, the initial connected structure has been transformed into a new one with enhanced rigidity. This is due to the fact that the emulsions have undergone partial coalescence under the effect of shear; the enhanced rigidity reflects the formation of irreversible bridges between the droplets. It can be argued that some crystals protrude across the adsorbed particle layers during the solidification process. Another possible explanation is that the shear-induced forces drive the droplets together and that the friction experienced by the rough surfaces progressively induces plastic deformations, allowing the formation of uncovered patches. In the shear flow, the droplets undergo collisions and partial coalescence may occur if two uncovered patches are involved in the contact (Figure 10). Irreversible bridges can also be formed if a covered area collides with an (14) Potanin, A. A J. Colloid Interface Sci. 1990, 145, 140.
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Figure 10. Schematic representation of partial coalescence inducing gelation: (a) general scheme; (b) partial coalescence between two uncovered patches; (c) irreversible bridging between a covered patch and an uncovered one.
uncovered patch. The fraction of surface uncovered by the solid particles is certainly small, so the droplets must experience many collisions before a coalescence event takes place or an irreversible bridge forms. This explains the very long induction times measured for φ < φ*. Following the same scheme, larger silica particles should be more efficient in protecting the droplets against partial coalescence, since they maintain the oil surfaces apart at larger distances during collisions, thus lowering the probability for irreversible bridges (of any kind) to be formed. Such a tendency is confirmed in Figure 8, where the characteristic gelation times decrease as the stabilizing silica particles become smaller. For a given particle type, the average oil droplet diameter influences both the quiescent and shear stability of the emulsions: smaller droplets appear to be more resistant against partial coalescence. We tried to fabricate emulsions with Ds ) 60 µm using P1 particles (7 nm), but the systems turned out to be completely unstable. However, we obtained stable emulsions with Ds ) 12 µm using the same particles. The data in Figure 9 correspond to systems with different average droplet diameters fabricated in the presence of P2 particles. Below φ*, τ exhibits a non-monotonic evolution with the average droplet size [τ(30 µm) < τ(60 µm) < τ(13 µm)], certainly resulting from a complex interplay between hydrodynamic and crystallization effects, all of them being dependent on the characteristic droplet size. It is likely that the diameter of the oil globules influences both the size and the characteristic shape of the crystals. In the limit of small droplet diameters (Ds ) 13 µm), an increasing number of silica particles may activate the nucleation process because of the larger surface-to-volume ratio. Consequently, more crystals with smaller size are likely to be formed and protrude through the interface over smaller characteristic lengths, resulting in a more efficient stabilization against partial coalescence at low droplet fraction. At the applied shear stress (σ ) 20 Pa), a sharp transition occurs at a critical droplet fraction, φ*: the induction time, τ, dramatically falls at the approach of φ*, and for φ > φ*, the gelation is almost instantaneous. Note that this phenomenology was observed with solid paraffin and not with the liquid one, as stated in section 2.4. In the absence
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Figure 11. Paraffin droplets stabilized by silica particles (diameter 100 nm) functionalized by hydrophilic (aminopropyl)triethoxysilane groups to an estimated density of 15 chains per nm2 of silica. T ) 25 °C. The surface was hydrophobized by sodium dedecyl sulfate. The continuous phase is a 2:3 (wt) mixture of ethanol and water. The presence of ethanol improves the solvent quality for (aminopropyl)triethoxysilane groups. Consequently, the solid particles are quite well dispersed in the continuous phase,17 and the paraffin droplets are not aggregated.
of shear, we observed a gradual transition of the emulsions from fluid to solid as the droplet fraction increased. Such a transition is clearly illustrated in Figure 5 where the most concentrated sample does not collapse (flow) anymore under its own weight. Oscillatory rheological measurements were performed with emulsions stabilized by P2 particles and Ds ) 30 µm. The emulsions exhibit viscoelastic behavior with the storage, G′, and loss ,G′′, moduli varying in the range 50-2000 and 10-400 Pa, respectively, as the droplet fraction increases from 30 to 45%. We only provide orders of magnitude, since the values of the rheological moduli sensitively depend on the history of the emulsions, that is, on the concentration procedure and the loading rate (in the rheometer). The comparatively large G′ values reveal the essentially elastic nature of the materials and the presence of stress bearing paths in the samples. Above a threshold droplet fraction, φ*, we observed that the gels turned into rigid materials made of irreversibly connected droplets (they could not be totally redispersed into individual droplets). The value of φ* depends on the procedure applied to concentrate the emulsion, being (54 ( 2)% if water is rapidly sucked by the capillary method described in section 2.3. The fast instability occurring in the presence of shear was obtained at a critical droplet fraction of the same order but slightly smaller than the previous one: (47 ( 2)% for P2 particles and Ds ) 30 µm. We believe that the two instabilities have the same origin and that the differences in the critical droplet fraction only reflect the influence of shear. In the presence of solidified droplets, as φ increases, the packing constraints and the intrinsic roughness of the surfaces progressively hinder local rearrangements of the droplets. It can be argued that, in the presence of shear and at a sufficiently high droplet fraction, frictions between the surfaces in contact rapidly deform the particle layers, causing plastic deformations and exposing the droplets to partial coalescence. The droplet fraction at which this phenomenon becomes almost instantaneous is obviously dependent on the mechanical treatment (applied shear) imposed to the emulsion. Conclusion In this paper, we have investigated the efficiency of colloidal particles in stabilizing solid paraffin droplets.
Particle-Stabilized Emulsions
Monodisperse materials with remarkable kinetic stability under both quiescent and flow conditions could be obtained. We could generalize the use of solid particles by varying the type of particles as well as their surface properties.15 An example is given in Figure 11. The stabilizing silica particles were obtained following Sto¨ber’s synthesis method.16 Their surface was initially covered by hydrophilic (aminopropyl)triethoxysilane groups15,17 and was hydrophobized by adsorbing anionic sodium dodecyl sulfate at a very low concentration. Figure 11 shows an emulsion stabilized by such particles (dp ) 100 nm): surprisingly, the paraffin droplets adopt (on average) an ellipsoidal shape in the solid state. This example clearly illustrates a situation where the presence of solid particles at the interface influences the paraffin solidification, since (15) Schmitt, V.; Kahn, J.; Reculusa, S.; Ravaine, S.; Arditty; LealCalderon, F. French patent no. 04 00761, Jan 27, 2004 [assigned to Centre National de la Recherche Scientifique (CNRS)]. (16) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (17) Reculusa, S.; Masse´, P.; Ravaine, S. J. Colloid Interface Sci. 2004, 279, 471.
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the same emulsion droplets stabilized by surfactant adopt irregular shapes (like in Figure 1). We could also obtain kinetically stable emulsions with silica particles hydophobized by n-octyltriethoxysilane.15-18 In this latter case, the hydrophobizing agent is not adsorbed but chemically grafted to the surface through a covalent bond. We hope that the results presented here will widen the application field of oil-in-water emulsions comprised of solid droplets [food, pharmacy, cosmetics, road surfacing (synthetic bitumen emulsions), etc.] and will provide a useful guidance for the formulation of such materials. Acknowledgment. The authors are very grateful to Pr. B. P. Binks for introducing them to solid-stabilized emulsions. Dr. Reculusa and Prof. Ravaine are acknowledged for providing the functionalized silica particles. Financial support from Le Conseil Regional d’Aquitaine was strongly appreciated. LA0501177 (18) Reculusa, S.; Ravaine, S. Chem. Mater. 2003, 15, 598.
