Water-in-Oil Emulsions Stabilized by Water-Dispersible Poly(N

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Water-in-Oil Emulsions Stabilized by Water-Dispersible Poly(N-isopropylacrylamide) Microgels: Understanding Anti-Finkle Behavior Mathieu Destribats,*,† Veronique Lapeyre,‡ Elisabeth Sellier,|| Fernando Leal-Calderon,§ Veronique Schmitt,† and Valerie Ravaine*,‡ †

Centre de Recherche Paul Pascal, Universite de Bordeaux, 115 Av. A. Schweitzer, 33600 Pessac, France Institut des Sciences Moleculaires, Universite de Bordeaux, ENSCBP, 16 Av. Pey Berland, 33607 Pessac Cedex, France § Laboratoire de Chimie et Biologie des Membranes et des Nano-objets, Universite de Bordeaux, CNRS, Allee Geoffroy St Hilaire, Bât B14, 33600, Pessac, France CREMEM, Universite Bordeaux 1, Bât. B8, Avenue des Facultes, 33405 Talence, France

)

‡

bS Supporting Information ABSTRACT: Emulsions were prepared using poly(N-isopropylacrylamide) microgels as thermoresponsive stabilizers. The latter are well-known for their sensitivity to temperature: they are swollen by water below the socalled volume phase transition temperature (VPTT = 33 °C) and shrink when heated above it. Most of the studies reported in the literature reveal that the corresponding emulsions are of the oil-in-water type (O/W) and undergo fast destabilization upon warming above the VPTT. In the present study, whereas O/W emulsions were obtained with a wide panel of oils of variable polarity and were all thermoresponsive, water-in-oil (W/O) emulsions were found only in the presence of fatty alcohols and did not exhibit any thermal sensitivity. To understand the peculiar behavior of emulsions based on fatty alcohols, we investigated the organization of microgels at the oilwater interface and we studied the interactions of pNIPAM microgels with octanol. By combining several microscopy methods and by exploiting the limited coalescence process, we provided evidence that W/O emulsions are stabilized by multilayers of nondeformed microgels located inside the aqueous drops. Such behavior is in contradiction with the empirical Finkle rule stating that the continuous phase of the preferred emulsion is the one in which the stabilizer is preferentially dispersed. The study of microgels in nonemulsified binary water/octanol systems revealed that octanol diffused through the aqueous phase and was incorporated in the microgels. Thus, W/O emulsions were stabilized by microgels whose properties were substantially different from the native ones. In particular, after octanol uptake, they were no longer thermoresponsive, which explained the loss of responsiveness of the corresponding W/O emulsions. Finally, we showed that the incorporation of octanol modified the interfacial properties of the microgels: the higher the octanol uptake before emulsification, the lower the amount of particles in direct contact with the interface. The multilayer arrangement was thus necessary to ensure efficient stabilization against coalescence, as it increased interface cohesiveness. We discussed the origin of this counterexample of the Finkle’s rule.

’ INTRODUCTION Materials that can adapt their behavior in response to an external stimulus have received considerable attention over the past few years. Likewise, emulsions that can evolve “on demand” are particularly sought after. These mestable materials, made of two immiscible fluids, such as oil and water, can be destabilized or structurally modified by simply changing an intensive variable in the system. Emulsions are generally stabilized by surface-active species like surfactant molecules, amphiphilic polymers, or proteins,1 but it is now well-established that solid particles of colloidal size, which can be wetted by both fluids, are also suitable stabilizers. The so-called Pickering emulsions1,2 can be obtained with a wide variety of solid organic particles, mineral powders, and naturally occurring particles.3,4 Quite recently, microgels have been identified as a new class of responsive particulate stabilizers. These particles made r 2011 American Chemical Society

of weakly cross-linked polymers swollen in a solvent may undergo volume phase transitions upon changes in their environment. A well-known example is the case of poly(N-isopropylacrylamide) (pNIPAM) microgels, which are soft colloidal particles. They are swollen in water below the so-called volume phase transition temperature (VPTT = 33 °C) and shrink when heated above it due to a change in the polymersolvent affinity. Other microgels bearing acidic or basic functions in the network experience volume phase transitions depending on the pH.512 Interestingly, various examples of pH-sensitive512 or thermosensitive1315 microgels have been reported to provide stimulus responsiveness in emulsions. Received: September 4, 2011 Revised: October 16, 2011 Published: October 21, 2011 14096

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Langmuir Despite the complex relationships between microgel chemistry and emulsion properties, we recently proposed a unifying view of the emulsion stabilization by soft microgels and showed that their deformability was a key parameter in controlling emulsion stability.15 We provided experimental evidence that the microgels adopt a flattened conformation at the oil/water interface. The most stable emulsions were obtained with the most deformable microgels, which tended to form 2D connected networks characterized by significant overlapping of their peripheral chains. When the deformability was lost, either by increasing the cross-linking density or decreasing the swelling ratio through an external stimulus, the stabilization efficiency was considerably reduced. More generally, this study also clearly revealed some fundamental differences between hard and soft colloidal particles as emulsions stabilizers. The concept of threephase contact angle valid for solid particles is not relevant anymore when dealing with soft colloids because of their deformation at the interface. Most of the emulsions stabilized by microgels are of the oil-inwater (O/W) type, in agreement with the empirical Bancroft or Finkle rules. These latter predict that the continuous phase of the preferred emulsion is the one in which the stabilizer is preferentially soluble (Bancroft) or preferentially dispersed (Finkle).16 Water-swollen cross-linked polymers are thus expected to stabilize O/W emulsions preferentially. However, several examples of water-in-oil (W/O) emulsions were reported. For instance, weakly cross-linked poly(4-vinylpyridine)silica nanocomposite microgel particles could stabilize water-in-undecanol emulsions, when the particles were in their neutral state,17 and cross-linked poly(N-isopropylacrylamide)-co-methacrylic acid (pNIPAM-co-MAA) particles were reported to stabilize water-in-octanol emulsions in their neutral state and possibly in their charged state, when the charges were hidden in the core of the particles.18 In this paper, we intend to clarify the parameters governing the occurrence of W/O emulsions. We fabricated emulsions stabilized by cross-linked poly(N-isopropylacrylamide) microgels using a wide panel of oils. Among them, W/O emulsions were produced with fatty alcohols only. To study the stabilization mechanisms, we investigated the microgel organization at the oil/water interface, via both direct observations of isolated oil/ water droplet interfaces and by exploiting the features of the limited coalescence process occurring in Pickering emulsions.4,19,20 Our results revealed significant differences in the deformability and in the interfacial arrangement of the microgels depending on the oil type. Finally, in an attempt to elucidate the unexpected type of emulsions based on fatty alcohols, we investigated the behavior of pNIPAM microgels in nonemulsified binary fatty alcoholwater systems. The contact with fatty alcohol drastically modified the microgels properties, and we discuss the impact of such changes on the emulsion type.

’ EXPERIMENTAL SECTION Materials. All the reagents were purchased from Sigma-Aldrich, unless otherwise specified. N-Isopropylacrylamide (NIPAM) was recrystallized from hexane (provided from ICS) and dried under vacuum prior to use. N,N0 -Methylenebisacrylamide (BIS) and potassium persulfate (KPS) were used as received. Milli-Q water was used for all synthesis reactions, purification, and solution preparation. Hexadecane, dodecane, heptane, 2-octanone, 5-nonanone, toluene, bromocyclohexane, dichlorobenzene, poly(dimethylsiloxane) (20 mPa s), 1-hexanol,

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1-heptanol, 1-octanol, 1-nonanol, 1-decanol, and 1-undecanol (SigmaAldrich, purity >99%) were used as received. Particles Synthesis and Purification. Poly(NIPAM) microgels with various cross-linking densities (from 1.5 to 10 mol % BIS) were synthesized by an aqueous free-radical precipitation polymerization.21 Polymerization was performed in a 500 mL three-neck round-bottom flask, equipped with a magnetic stir bar, a reflux condenser, thermometer, and argon inlet. The initial total monomer concentration was held constant at 62 mM. NIPAM and BIS were dissolved in 98 mL of water. In some other cases, a fluorescent monomer (containing acryloylfluorescein, 0.1 wt % with respect to the thermoresponsive monomer) was added during the synthesis to discern the microgels using confocal fluorescence microscopy. In all cases, the solution was purified through a 0.2 μm membrane filter to remove residual particulate matter. The solutions were then heated up to 70 °C with argon thoroughly bubbling during at least 1 h prior to initiation. Free radical polymerization was then initiated with KPS (2.5 mM) dissolved in 2 mL of water. The initially transparent solutions became progressively turbid as a consequence of the polymerization process. The solutions were allowed to react for a period of 6 h in the presence of argon under stirring. The microgels were purified by centrifugationredispersion cycles at least five times (21 000g for 1 h, where g is the acceleration due to gravity). For each cycle, the supernatant was removed and its surface tension was measured by the pendant drop method. The purification was repeated until the surface tension of the supernatant reached that of pure water, i.e., above 70 mN m1. Particle Dispersion Characterization. Particle size and polydispersity were measured by photon correlation spectroscopy (PCS) at a detection angle of 90°, using a Zetasizer Nano S90 Malvern Instrument equipped with a HeNe laser. The hydrodynamic diameters were calculated from diffusion coefficients using the StokesEinstein equation. All analyses were performed with the software supplied by the manufacturer. The polydispersity index (PDI) was derived from the cumulant analysis method. Electrophoretic mobility measurements were carried out using the Zetasizer Nano ZS (from Malvern Instruments, UK) at the appropriate temperature, after allowing the mixture to equilibrate for 10 min. Each value results from at least five cycles of 15 measurements each. To determine the particle concentration, we used eq 1, which takes into account the hydration of the polymer in its collapsed state. Following the work published by Lele et al.,22 we consider that a particle is composed of 71 wt % of polymer and 29 wt % of bound water at 50 °C. From the hydrodynamic particle diameter, d50°C, measured by PCS at 50 °C, and the polymer content, cpolymer (in g cm3), determined by the drying method, the particle concentration cparticles (expressed in cm3) of the dispersions was estimated as cparticles ¼

6cpolymer 1 0:29 þ 0:71Fwater πd50°C 3 Fpolymer

! ð1Þ

where Fpolymer = 1.269 g cm3 and Fwater = 0.988 g cm3 are respectively the polymer and water densities at 50 °C. Emulsion Production and Droplet Size Measurements. In a typical batch, well-defined amounts of the oil phase and of the aqueous microgels dispersion were introduced in the same vessel. The mixture was then stirred by an Ultra-Turrax T25 mixer, at constant speed (9500 rpm) for 30 s. The emulsion type was determined by dilution tests and by visual inspection of the creaming or sedimentation behavior due to the density mismatch between oil and water. Most of the oils used in this study had lower density than water. For W/O (respectively O/W) emulsions, the drops tended to sediment (cream) and thus, in the final state, an almost clear upper (bottom) phase coexisted with a turbid sediment (cream). Nevertheless, the inverse situation was observed in 14097

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Table 1. List of Synthesized Microgels and Main Characteristics hydrodynamic diameter (nm) and corresponding PDI cross-linking (mol %)

T = 25 °C

T > VPTT

1.5%

770 (0.046)

293 (0.014)

2.5% 3.5%

660 (0.073) 650 (0.045)

260 (0.043) 276 (0.056)

5.0%

650 (0.056)

274 (0.156)

10%

725 (0.056)

324 (0.027)

2.5%

833 (0.029)

304 (0.033)

the presence of bromocyclohexane and dichlorobenzene, since the densities of these two halogenated oils were higher than that of water. The size distribution of the emulsion drops was determined by direct imaging using an inverted optical microscope (ZeissAxiovert X100) and a video camera. Images were recorded and the dimensions of about 50 droplets were measured. Both the surface average diameter D and the polydispersity, P, defined by eq 2, were estimated.

∑i Ni Di 3 D¼ ∑i Ni Di 2

1 P ¼ Dm

∑i Ni Di 3 jDm Di j ∑i Ni Di 3

ð2Þ

where Ni is the total number of droplets with diameter Di. Dm is the median diameter, i.e., the diameter for which the cumulative undersized volume fraction is equal to 50%. In the remainder, unless otherwise specified, the experiments were carried out at room temperature (20 °C). Cryo-SEM Observations. Cryo-SEM observations were carried out with a JEOL 6700FEG electron microscope equipped with liquid nitrogen cooled sample preparation and transfer units. A small amount of emulsion was first deposited on the aluminum specimen holder. The sample was frozen in the slushing station with boiling liquid nitrogen. The specimen was transferred under vacuum from the slushing station to the preparation chamber. The latter was held at T = 150 °C and P = 105 Pa and was equipped with a blade used to fracture the sample. Once the sample fractured, it was coated by a layer of AuPd and was then inserted into the observation chamber equipped with a SEM stage cold module held at 150 °C.

’ RESULTS Influence of the Oil Nature on the Emulsion Type. Several batches of pNIPAM microgels were prepared with various crosslinking densities expressed as the molar % of BIS incorporated during the synthesis (1.5, 2.5, 3.5, 5, and 10 mol % BIS). To rule out the size contribution, all the synthesized pNIPAM microgels had similar diameters, approximately 700 nm at room temperature (see Table 1), and as expected, the microgels shrank when heated above their volume phase transition temperature (VPTT = 33 °C). First, the influence of the oil nature was investigated with microgels containing 2.5 mol % BIS. The microgels were initially dispersed in the aqueous phase. Their ability to stabilize emulsions was investigated using different oil types and by varying systematically the volume fraction of water between 20 and 80%, at constant microgel concentration (0.02 wt % relative to the overall emulsion mass). Most of the oils, except fatty alcohols and dichlorobenzene, resulted in O/W emulsions, as long as the

Figure 1. Examples of O/W and W/O emulsions stabilized with 2.5 mol % BIS microgels (microgel concentration = 0.02 wt % of the total mass): (a) macroscopic view of the sample at rest, (b) macroscopic view of samples under flow, (c) optical microscopy images of the drops, scale bars are 200 μm; (1) hexadecane-in-water emulsion (oil volume fraction = 40%, adapted from ref 15) and (2) water-in-octanol emulsion (water volume fraction =30%).

volume fraction of the aqueous phase remained below approximately 60%. Above this value, the two immiscible phases were fully separated once the agitation was stopped (See Supporting Information, Figure S1). An example of O/W emulsion is shown on Figure 1a. In this case, the droplets were very large and strongly aggregated and, as a consequence, they were subjected to fast creaming. After a few minutes of settling, the cream became very firm, as revealed by Figure 1b (sample 1): when the vial was tipped over, the cream did not flow; instead it formed a rigid chunk that could hardly flow under its own weight. The O/W emulsions fabricated at oil fractions below 60% were kinetically stable at rest and could resist mechanical disturbances (either compression and/or shear): the oil droplet volume fraction could be raised up to 90% by centrifugation. Conversely, W/O emulsions were obtained when the oil phase was a fatty alcohol, irrespective of the length chain from 1-hexanol to 1-decanol, and when the volume fraction of aqueous phase, ϕ, was lower than approximately 60%. No stable emulsion could be prepared above this volume fraction. For ϕ < 60%, the aqueous drops rapidly formed a sediment at the bottom of the vessel. These emulsions were not aggregated, as suggested by their ability to flow (Figure 1b, sample 2) and by the observations under the microscope (Figure 1c) (See also Supporting Information, Figure S2). The W/O emulsions were stable at rest: they did not undergo phase separation after a settling period as long as 2 years, although a slow increase of the drop size was measured by optical microcopy (from 100 to 400 μm). However, they could not withstand mechanical compaction. For example, they were destabilized in less than 30 min when submitted to a very low centrifuge acceleration of 400g. Emulsions formulated with dichlorobenzene, or with 1-undecanol were unstable. For undecanol, a kind of bicontinuous structure was obtained shortly after preparation. After 1 day, phase separation occurred and the microgels migrated in the oil phase (See Supporting Information, Figures S2 and S3). Table 2 indicates the emulsion type as a function of the oil’s chemical nature. In our previous studies,15 we investigated the structure of the microgels adsorbed on free interfaces as well as on thin films of 14098


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Table 2. Emulsion Type and Relative Dielectric Constant as a Function of the Oil’s Nature oil type

ε

emulsion type

alkanes (heptane to hexadecane)

O/W

1.922

ketones (2-octanone and 5-nonanone) aromatic hydrocarbon (toluene)

O/W O/W

10.3 2.4

halogenated oil (bromocyclohexane)

O/W

7.9

halogenated oil (dichlorobenzene)

phase separation

9.9

silicone oil (PDMS 20 mP s)

O/W

2.32.8

fatty alcohols (1-hexanol to 1-decanol)

W/O

8.113

fatty alcohol (1-undecanol)

phase separation

8.0

alkane-in-water emulsions. Following the same strategy, we now explore the interfacial structure of W/O emulsions based on fatty alcohols in order to get insight into the origin of this emulsion type and to identify some key differences with respect to W/O emulsions. Arrangement of the Microgels in the Case of Water-in-Oil Emulsions. Due to the synthesis process, the cross-linking density of the microgels is higher in the center of the particles than at the periphery, resulting in a coreshell structure.2326 In the case of O/W emulsions, we highlighted the impact of microgel deformability on emulsion stabilization.15 This was achieved by measuring the packing density of the microgels at the interface through a combination of microscopy imaging and surface coverage determination using the limited coalescence process. When emulsification was performed at room temperature, i.e., below the VPTT, the less cross-linked microgels spread at the interface in a flattened “fried-egg-like” conformation, with the protruding “egg yolk” (core) being less deformed than the “egg white” (shell). These soft particles made of weakly crosslinked polymer can extend their chain to maximize the contacts of hydrophobic polymer segments with the interface. This ability to spread is counterbalanced by the elastic response of the polymeric network due to partial cross-linking. It was argued that the overlapping of the peripheral shells created a 2D elastic network that protected the drops against coalescence.15 We adopted a similar approach based on direct imaging to investigate the interfacial arrangement in W/O emulsions. A typical batch was prepared with 70 vol % octanol and 30 vol % aqueous phase containing 0.5 wt % microgels (2.5 mol % BIS). Because of the weak refractive index mismatch between the swollen microgels and water, simple optical microscopy was inappropriate to visualize them. The adsorption of microgels at the oilwater interface was thus revealed by means of confocal microscopy using fluorescent labeling (Figure 2). Figure 2a shows an image obtained at low magnification. The interface appears highly fluorescent, whereas the continuous phase is very dark, indicating that most of the microgels are located in the vicinity of the interface. In Figure 2a a very limited number of free particles can be distinguished in the continuous phase. We would like to emphasize that this was an artifact due to sample loading. Indeed, some droplets coalesced on the glass slides, and particles were then released into the continuous phase. This phenomenon was really marginal, as evidenced by the very low level of fluorescence of the continuous phase in the low magnification image of Figure 2a. At higher magnification (Figure 2b), one can see that the adsorbed particles form at least one hexagonally packed layer, where apparently the microgels are not in close contact. This observation may be due to an

Figure 2. Confocal microscopy of a water-in-octanol emulsion stabilized by 2.5 mol % BIS microgels: (a) large view (scale bar is 200 μm) and (b) 3D reconstruction of a single water drop stabilized by fluorescent microgels (scale bar is 3 μm).

inhomogeneous spatial distribution of the fluorescent dye within the particles, the concentration being larger in the center than in the periphery.15 As a consequence, the colored spots are actually not revealing the whole microgel volume. The average center-tocenter distance estimated from the image is about 820 nm, which is close to the value of the hydrodynamic diameter of the fluorescent pNIPAM microgel at 25 °C. The interfacial packing of the microgels is thus very different from that of O/W emulsions. In this latter case, the center-tocenter distance was much larger than the hydrodynamic diameter, due to the flattening of the microgels (using the same microgels to stabilize hexadecane-in-water emulsion, the lattice parameter was 1200 nm). We carried out a quantitative approach to measure the surface coverage, based on the limited coalescence process. This phenomenon occurs in any colloidal system stabilized by surfaceactive species irreversibly adsorbed, as is the case in Pickering emulsions. If the system is emulsified with a low amount of particles, the newly created droplets are insufficiently protected. When the agitation is stopped, the droplets coalesce, thus reducing the total amount of interface. Since the particles are irreversibly adsorbed, the coalescence process stops as soon as the interface is sufficiently covered, and the resulting emulsions exhibit remarkably narrow size distributions (P < 30%).27 The final surface area of the droplets depends on the initial amount of particles and on their arrangement at the interface. Generally, particles adsorb either as a dense monolayer or as multilayers,20,27 but cases where the coverage is surprisingly low have been reported as well (much less than one layer).28,29 For emulsions undergoing limited coalescence, the interfacial particle coverage can be directly deduced from the average drop size. Indeed, the total interfacial area, Sint, of the emulsion is directly linked to the average drop size, D (Sint = 6Vd/D where Vd is the oil volume). Assuming that all the particles are adsorbed, the interface area that the particles may cover is estimated from their total equatorial section Seq = nπ(d25°C/2)2, where n is the total number of particles and d25°C is their hydrodynamic diameter at 25 °C. The surface coverage, C, defined as C = Seq/Sint, can thus be estimated after measuring the drop size D: Seq 1 nπd25°C 2 ¼ ¼ D 24CVd 6CVd

ð3Þ

Equation 3 predicts that the average inverse droplet diameter resulting from limited coalescence is proportional to Seq/Vd. 14099


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Figure 3. Optical microscopy images of water-in-octanol emulsions stabilized with increasing microgel (2.5 mol % BIS) amounts: (a) 1.51, (b) 2.26, (c) 4.48 and (d) 6.12 1011 microgels per cm3 of water. Scale bar is 200 μm.

The parameter C characterizing the particle packing can be directly obtained from the slope of the experimental 1/D vs Seq/Vd plot. The linear dependence is generally observed at low particle concentration.27 In that limit, all the particles are adsorbed at the water/oil interface, as shown in Figure 2. Conversely, when particles are in excess, the linearity is lost and the obtained emulsions become very polydisperse (P > 30%). In this limit, the size distribution is less sensitive to the total particle amount and depends mainly on the stirring intensity. Water-in-octanol emulsions were prepared with various amounts of particles. Below 0.6 wt % of microgels in the aqueous phase, emulsions were monodisperse, with a drop size increasing as the particle concentration decreased (Figure 3), in qualitative agreement with eq 3. The droplets had a narrow size distribution, as revealed not only by the microcope images but also by the low value of the polydispersity index (P < 25% for all the systems). Moreover, in Figure 3 the droplets seem to be surrounded by a thick shell, which would indicate the presence of aggregates or of multilayers at the interface. In Figure 4, we plot the evolution of 1/D vs Seq/Vd in the low particle concentration regime. Both the linear dependence and the relative monodispersity of the emulsions are the main fingerprints of limited coalescence. Results for the different cross-linker densities were gathered and collapsed within a single linear plot, indicating that the surface coverage C was independent of the microgel cross-linking density. A similar result was obtained with O/W emulsions.15 However, the C value of 250% deduced from the linear plot of Figure 4 is about 6 times higher than that obtained for O/W emulsions (≈40%).15 For the sake of comparison, the expected value for a hexagonally close-packed 2D monolayer of monodisperse hard spheres is 90%. In O/W emulsions, the low surface coverage was due to the deformation of the adsorbed microgels. As a consequence, the particles covered a surface area much larger than their equatorial section estimated from the hydrodynamic diameter in solution. In the case of W/O emulsions, two hypotheses can be proposed to explain the comparatively large coverage: (i) microgels are aggregated or form a layered structure with three strata on average and (ii) microgels are deformed along the normal to the interface or are collapsed and consequently they cover a much smaller area than their equatorial surface estimated from the hydrodynamic diameter in solution. This latter hypothesis

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Figure 4. Evolution of the inverse drop diameter (1/D) of water-inoctanol emulsions as a function of total equatorial surface area of particles normalized by the dispersed volume, for four cross-linking densities: 9 = 1.5%, 2 = 2.5%, b = 5%, and [ = 10 mol % BIS.

can be ruled out by the confocal microscopy experiment (Figure 2), indicating that the center-to-center distance is on the same order as the nondeformed equatorial radius. In order to improve the resolution, imaging of the microgel organization at the oilwater interface was obtained by cryoSEM experiments. Emulsions of the W/O type were frozen and fractured prior to observation. The fracture was planar (crosssection), making it possible to visualize the inner part of the droplets. A few typical images are shown on Figure 5. Spherical and nondeformed microgels are visible inside the water droplet. They are mostly concentrated in the interfacial region, forming a multilayer assembly. The first layer is very dense and uniform and seems to be in contact with the oil phase. The number of layers varies locally within the same drop between one and six with an average value of three layers. From the slope of 1/D vs Seq/Vd, we also deduce that the average number of layers is equal to three: indeed, the value of 250% is almost 3-fold the surface coverage of a densely packed single monolayer. Likewise for confocal microscopy, we estimated the center-to-center distance between microgels of the first layer from cryo-SEM imaging. A distance of 685 ( 20 nm was obtained, which is again comparable to the hydrodynamic diameter at 25 °C (670 nm in this case), suggesting the absence of deformation of the microgels in W/O emulsions. We also obtained cryo-SEM images showing the outer part of the interface, i.e., the face that is exposed to oil (in that case, the fracture was not planar). In Figure 6, we can observe that the first layer microgels are slightly protruding toward the oil phase (this image was obtained with heptanol instead of octanol). The surface coverage was obtained for various water-in-fatty alcohol emulsions. In all cases, the emulsions followed the limited coalescence process, allowing a precise determination of the parameter C. The results are reported in Table 3. The surface coverage was much higher than 90%, again confirming the presence of multilayers, whatever the alcohol chain length from heptanol to decanol. This result is original compared to the Pickering emulsions reported so far. First, emulsions reported here do not obey the Finkle’s law. Even after emulsification, the microgels remain 14100


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Figure 5. Cryo-SEM images of two water-in-octanol drops stabilized by microgels (2.5 mol % BIS) and observed a few hours after emulsification: large views of drops (scale bar are 10 μm) and closer views of the interfaces (scale bar are 5 μm). The microgels form a stratified structure at the interface.

Table 3. Interfacial Surface Coverage of Water-in-Oil Emulsions as a Function of the Oil Chain Length and as a Function of the Delay before Emulsification surface coverage (%)

continuous phase

Figure 6. Cryo-SEM of a heptanolwater interface viewed from the oil side. Scale bar is 2 μm.

dispersed in the aqueous phase but the emulsions are of the W/O type. Second, the final average droplet size is determined by the limited coalescence process, but the microgels adopt a multilayered (stratified) structure at the interface. The condensed state of the microgels is at first glance surprising considering that they were initially dispersed in the aqueous phase (before emulsification). It can be stated that the direct or indirect contact of the microgels with the alcohol phase has modified their properties and induced some attractive interaction between them. Behavior of the Microgels in the Presence of Water and Octanol. In order to probe the influence of the oil phase, we studied the colloidal behavior of microgel dispersion at rest after contact with octanol. For that purpose, pure octanol (1 mL) was gently deposited at the top of an aqueous suspension (4 mL) containing 0.1 wt % of microgels in a quartz cell, at 20 °C. As

emulsification immediately after

emulsification after contact of

contact of oil/water

oil/water for 24 h

1-hexanol

164

unstable

1-heptanol

110

163

1-octanol

230

220

1-nonanol

111

152

1-decanol

153

181

shown in Figure 7, the aqueous phase was initially almost transparent owing to the weak light-scattering ability of the water-swollen microgels. After a few hours, the turbidity increased drastically nearby the oil/water interface. The extent of this turbid region progressed over time, until the whole aqueous phase became opaque. This evolution took place within 1 week. Then, a second phenomenon occurred. The bottom of the aqueous phase tended to become transparent again, with a clarification front progressing upward. After 18 days, the sample was composed of two transparent phases (oil and water) with a turbid thin layer in between them. This experiment is revealing a progressive uptake of octanol by microgels after its diffusion across the water phase owing to its slight solubility in it (0.053% w/w). The kinetics of the process described in Figure 7 was rather slow because of the small interfacial surface area between the two bulk phases that limits the octanol exchange rate. It is probable that saturation of the microgels by octanol occurs much more rapidly when the aqueous droplets are dispersed in oil because of the significantly larger surface-to-volume ratio (23 orders of magnitude). The incorporation of octanol molecules in the microgels has two main consequences: (i) it increases the 14101


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Figure 7. Evolution of the aspect of the microgel dispersion in the aqueous phase after contact with oil: (a) octanol progressively diffuses across the aqueous phase and is incorporated into the microgels. The refractive index of the microgels increases and the dispersion becomes turbid. (b) A phase separation progressively occurs due to the creaming of microgels containing octanol. (c) Comparison of the aspect of microgel dispersions in contact with different oils: after 1 week the aqueous phase in contact with octanol is entirely turbid, whereas the turbidity of the dispersion in contact with octanone did not evolve.

refractive index (the refractive index of octanol, 1.429, is much larger than that of water) and (ii) it decreases the average microgel density at a point that it becomes smaller than that of water (the density of octanol is 0.827 g cm3). To check the evolution of the microgel density, a centrifugation experiment was carried out. Two samples were submitted to centrifugation (28 000g for 10 min): one contained a suspension of native microgels (0.1 wt %), while the other one contained the same amount of microgels but was exposed to octanol for 7 days prior to centrifugation. As expected, the first sample sedimented, whereas the other creamed (Figure 8). The evolution of the refractive index mismatch between microgels and water was confirmed qualitatively by optical microscopy. The mismatch was enhanced after contact with octanol, and the microgels could be easily discerned under the microscope (See Supporting Information, Figure S4). For the sake of comparison, the aqueous dispersion was held in contact with other oils (of various polarities and solubilities in water) like octane (low polarity, low solubility) or octanone (higher polarity, higher solubility) for more than 1 week, but the dispersion remained clear and did not exhibit any visible evolution. On the whole, we can state that there is a correlation between the ability of the microgels to incorporate oil and their propensity to produce O/W (no incorporation) or W/O (incorporation) emulsions. It must be underlined that even after a very long settling period, our pNIPAM microgels remained always dispersed in the water phase, with absolutely no transfer into the octanol phase, which remained perfectly transparent (see for instance Figure 7b). It has been demonstrated that pNIPAM-co-MAA microgels become dispersible into octanol at low pH, i.e., in their uncharged state.5 This difference in behavior can be explained by the presence of covalently bonded

Figure 8. Centrifugation test for microgels dispersed in water, before (left-hand side sample) and after (right) contact with octanol: (a) before centrifugation and (b) after centrifugation.

electrically charged groups (sulfate and carboxyl) at the working pH (5.5 in our case), originating from the ionic free radical polymerization initiator (KPS)30 and its further hydrolysis known as Kohltoff’s reaction.31 We performed electrophoretic mobility measurements at two different cross-linking densities (0.5 and 2.5 mol % BIS), 14102


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Figure 10. Stability of hexadecane-in-water (left-hand side vial) and water-in-octanol (right-hand side vial) emulsions initially at 20 °C (a) and after 3 days at 50 °C (b). Figure 9. Electrophoretic mobility (absolute value) of pNIPAM microgels with two different cross-linking degrees (0.5 and 2.5 mol % BIS), before and after octanol uptake, below and above the VPTT (T = 25 and 40 °C) at pH 5.5.

before and after contact with octanol, as well as below (25 °C) and above (40 °C) the VPTT of the native microgels. The data reported in Figure 9 confirm the presence of charges. The electrophoretic mobility increased considerably after octanol uptake and/ or above the VPTT. It is well-known that the electrophoretic mobility of microgels depends strongly on their swelling state.32 Oshima proposed a mobility formula valid for “soft particles”,33 according to which the mobility is determined not by the total amount of fixed charges but by their volume density. Thus, below the VPTT, the decreased charge density leads to a large reduction of the mobility, even if the total amount of charges remains constant. Our experimental results concerning pNIPAM microgels in water (no octanol) are in qualitative agreement with this model. Upon octanol uptake, the electrophoretic mobility, μE, becomes insensitive to temperature, which suggests that the microgels no longer undergo the usual shrinkage around 33 °C. Moreover, below the VPTT, the mobility is significantly larger than that of native microgels in water. Sierra-Martin et al. studied pNIPAM microgel cross-linked with 0.5 wt % BIS and proposed that particles consist of a nondraining core covered by an ionpermeable shell.32 The compression of this shell would affect the charge density and thus the resulting μE behavior upon salt addition or temperature raise. In our case, the presence of octanol probably induces conformational rearrangements that tend to displace the charged groups toward the outer regions of the shell, where the contact with water molecules is more likely. The accumulation of charges within a reduced shell thickness would provoke the increase of μE. Influence of the Temperature on the Emulsion Stability. We compared the effect of temperature on both the W/O system containing octanol and the O/W system prepared with hexadecane (the two emulsions were prepared with the 2.5 mol % BIS microgels). The volume fraction of dispersed phase was 30%. The emulsions remained stable at rest at 20 °C for at least 1 year. Upon heating at 50 °C for 3 days, only the W/O emulsion based on hexadecane exhibited phase separation (Figure 10). Microgels were desorbed from the interface and were released back into the aqueous phase, as proved by its significant turbidity. In contrast, the W/O emulsion containing octanol remained stable, without

Figure 11. Evolution of the hydrodynamic diameter as a function of temperature. The microgels were dispersed in water and the aqueous dispersion was (diamonds) or was not (squares) in contact with octanol (for 24 h under gentle stirring) prior to the measurement.

any evolution of the drop size. It can be deduced that W/O emulsions are not thermoresponsive in the explored temperature range (1550 °C). In parallel, we measured the influence of temperature on the hydrodynamic diameter of the microgels after octanol uptake (24 h under gentle magnetic stirring). In contrast to native pNIPAM microgels, the size of octanol-filled microgels did not exhibit any sharp transition (Figure 11), as already suggested by the evolution of the electrophoretic mobility with temperature (Figure 9). These two results explain the loss of thermoresponsiveness of the corresponding emulsion, since they both demonstrate that the microgels do not undergo the volume phase transition upon warming. The interfacial structure of the emulsions at high temperature (40 °C) was observed using cryo-SEM imaging. Figure 12 corresponds to a water-in-octanol emulsion directly fabricated at 40 °C and maintained at that temperature prior to observation. Again, the multilayered structure is visible together with some free particles in the aqueous droplet bulk. In this case, the surface coverage is rather homogeneous and is made of a regular fourlayer stacking. The resolution of the image is good enough to discern a slight but regular increase of the lateral center-to-center distance from the first layer (in direct contact with oil) to the fourth one. The size increment between these two layers is on the order of 20%. From the image in Figure 12, we deduce that the average diameter of the interfacial microgels is about 700 nm, a value which is on the same order as the hydrodynamic diameter 14103


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properties. The oil:water volume ratio was equal to one in all these experiments, and the amount of microgels (2.5 mol % BIS) remained constant (0.5 wt % with respect to the aqueous phase). We report here the type of emulsion as a function of octanol molar fraction in the oil phase, X (see Supporting Information, Figure S5): For 0% < X < 5%, O/W emulsions were obtained; the oil drops rapidly creamed, as classically observed in this emulsion type (see section 1). For 5% < X < 25%, the preferential emulsion type could not be determined because of the intrinsic instability of the systems. After the stirring was applied (emulsification), all the mixtures underwent phase separation within a few tens of minutes. For 25% < X < 100%, W/O emulsions were obtained; the droplets rapidly sedimented for this whole range of fractions. It is worth noting that the W/O region is rather extended. Indeed, when the molar ratio of octanol exceeded a low threshold value close to 25%, W/O emulsions were systematically obtained. This threshold is certainly representing the minimum amount of octanol required to swell the microgels and modify their physical properties. Figure 12. Cryo-SEM image of a water-in-octanol drop stabilized by 2.5 mol % BIS microgels, emulsified, and stored at 40 °C prior to observation. Scale bar is 5 μm.

Figure 13. Scheme illustrating the interfacial stratification of the particles. For the sake of simplicity, the oil/water droplet interface is represented as a plane. (a) Mechanism 1: progressive adsorption of the particles on the interface after octanol uptake. (b) Mechanism 2: orogenic displacement of the adsorbed microgels as the oilwater interfacial area, represented by the solid line, is progressively reduced.

of the microgels in suspension. Hence, it can be stated that neither the interfacial microgels nor the microgels in suspension are thermoresponsive. Influence of the Oil Phase Composition: Binary Mixtures. The nature of the oil is a key parameter in controlling the emulsion type. Only fatty alcohols as oils could give rise to W/O emulsions, whereas O/W emulsions were stabilized with other oils (Table 2). Among the large compound repertoire that was probed, fatty acids belong to the family of protic molecules. Polar protic molecules have hydrogens attached to electronegative atoms, namely, F, O, or N. Polar aprotic molecules like ketones still contain dipoles, but there are no hydrogens attached to F, O, or N. We also investigated the evolution of the emulsion type as a function of the oil composition, using mixtures of octanol and octanone, two oils with similar polarities but different protic

’ DISCUSSION To the best of our knowledge, only two examples of antiFinkle behavior were reported so far.34,35 In each case, hydrophobic polystyrene latexes stabilized with sulfate charges were shown to stabilize W/O emulsions, while remaining dispersed in the aqueous phase. However, two important differences with respect to our systems can be underlined. First, according to Golemanov et al., the presence of salt in the aqueous solution was a prerequisite for W/O emulsions to be obtained.35 Second, latex particles were adsorbed as a single monolayer. The W/O emulsions that were obtained in the present work are thus a new counterexample of the Finkle’s rule. The following section aims at discussing the necessary conditions to obtain anti-Finkle W/O emulsions stabilized by pNIPAM microgels and at understanding the origin of that unusual behavior. Influence of the Oil Nature. The occurrence of W/O emulsions is strongly related to the fact that the oil can penetrate into the microgels. The polarity of the oil should thus be large enough for the molecules to withstand the aqueous environment. We thus tried to find a correlation between the emulsion type and some physical parameter reflecting the oil polarity. Generally, the relative dielectric constant provides a rough evaluation of a solvent’s polarity. As can be deduced from Table 2, a large oil polarity is certainly a necessary but not sufficient condition for obtaining W/O emulsions. For example, octanone and octanol have very close relative dielectric constants, 10.4 and 10.3, respectively, but they give rise to emulsions of opposite curvature. However, there is a correlation between the protic and aprotic nature of the oils and the emulsion type. Indeed, pNIPAM microgels can stabilize W/O emulsions with fatty alcohols only, which are protic oils. Such correlation can be understood considering that protic oils are prone to hydrogen bonding, which obviously facilitates their interaction with pNIPAM segments and thus their incorporation into the microgels. Further experiments will be necessary to consolidate the empirical correlation by using other protic solvents like amines and amides of carboxylic fatty acids. Influence of Electrostatics. Electrostatic interactions between the microgels do not play a major role in the stabilization mechanism. We carried out an experiment with octanol as the oil 14104


Langmuir phase and a 0.1 M NaCl aqueous suspension within the prospect of screening the electrostatic interaction arising from the presence of electrical charges in the microgels. The hydrodynamic diameter of the microgels remained close to 700 nm at that electrolyte concentration. Despite the highly electrolyte concentration and the presumable strong electrostatic screening effect, we obtained the same emulsion type (W/O) and the same surface coverage as the emulsion without added electrolyte. Impact of Octanol Uptake on the Amphiphilic Character, on the Volume Transition, and on the Adsorption Capacity of the Microgels. The multilayered structure involves that only a limited fraction of the microgels is in direct contact with the oilwater interface: for octanol-in-water emulsions, only one out of three microgels is involved in the most peripheral layer. In our previous studies about O/W emulsions based on different aprotic oils,15,36 it was demonstrated that all the microgels were in direct contact with the interface forming a single hexagonally packed layer. Thus, it must be admitted that the anchoring capacity of the particles is weaker with fatty alcohols than with aprotic oils. This may be due to the fact that fatty alcohols interact with the hydrophobic (isopropyl) side group carried by each monomer and establish hydrogen bonds with the amide functions, thus modifying the amphiphilic character of pNIPAM.3741 The disappearance of any sharp volume transition in Figure 11 reveals that octanol uptake has profound consequences for the internal molecular interactions. Another possible explanation for the change in adsorption properties is that the incorporation of oil within the microgels reduces their deformability and consequently the number of “hydrophobic” anchoring segments decreases. The osmotic pressure exerted by the octanol molecules within the particles may also induce some deformability loss. To verify that the adsorption becomes weaker upon octanol uptake, the microgel aqueous dispersions were preincubated for 1 day in contact with the oil phase prior to emulsification, under gentle magnetic stirring. The values of the coverage deduced from 1/D vs Seq/Vd plots are on average larger than those obtained without preincubation (see Table 3). In the case of even longer incubation times with octanol (6 days), the surface coverage could raise up to 550%. Only one microgel out of six is in direct contact with oil in this case. This result proves that the adsorption of the microgels at the oilwater interface is progressively reduced as they incorporate increasing amounts of octanol before emulsification. Using shorter alcohol chains, the effect was even more pronounced, since it was impossible to stabilize any emulsion after 1-day incubation with 1-hexanol, possibly due to faster uptake of this molecule owing to its higher water solubility. Origin of the Multilayered Structure Condensed at the Interface. We now propose two plausible mechanisms to explain the interfacial stratification of the particles. Mechanism 1 (Figure 13a). The particles in the multilayers can be regarded as ordered crystallites coexisting with free particles (see Figure 5). Ordered packings may arise when the total pair interaction between particles becomes appreciably attractive at a given separation.42 If the depth of the attractive well is on the order of kBT (where kB is the Boltzmann constant and T is the absolute temperature), the thermal energy allows reaching an equilibrium state with permanent exchange between aggregated and free particles. In contrast, when the depth of the attractive well is large compared to kBT (i.e., more than 10 kBT), the particles are strongly bound to the aggregates and cannot be redispersed by thermal motion. After a short period of time, all the particles are

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entrapped within large and tenuous fractal clusters that fill the whole space.43 In our experiments, both the crystalline structure nearby the interface and the presence of free particles in Figure 5 would indicate that the interaction energy between the microgels is on the order of kBT. Below the VPTT, the native particles are swollen and thus water is the main component. Under these conditions, the van der Waals attractive forces are relatively small. The electrophoretic mobility measurements reveal (Figure 9) that the particles are slightly charged and they are thus submitted to electrostatic repulsive forces. Furthermore, the polymer tails extend to act as steric stabilizers, further enhancing colloid stability. The net force balance being repulsive, the microgels do not aggregate. The situation is different after octanol uptake, as revealed by the images of Figures 5 and 12. The octanol content of the gels probably gives a higher van der Waals attraction but other types of attractive interactions cannot be excluded. We were able to obtain cryo-SEM images at large magnification (see Supporting Information, Figure S6) showing that particles are linked through very thin filaments formed by overlapping of the peripheral shells. The particles do not seem in close contact, but this observation has to be considered with caution, since in this very precise experiment we cannot exclude that a light sublimation occurred (therefore producing some retraction of the particles) before the observation was made. To explain the origin of the interfacial stratification, it can be envisaged that only a fraction of the particles is adsorbed. The incorporation of octanol in the nonadsorbed microgels would induce an attractive interaction between them on the order of kBT. The particles would then tend to progressively self-assemble into the multilayered structure. Within this scheme, the particle layer in direct contact with oil acts as a nucleation substrate for two-dimensional crystal growth. Mechanism 2 (Figure 13b). Alternatively, the interfacial stratification could result from a progressive displacement of the microgels. In the following scenario, all the particles are supposed to be initially adsorbed (i.e., in direct contact with the interface). To fabricate the emulsion, the mixture of oil and aqueous phase is stirred and once the agitation is stopped, the droplets undergo limited coalescence. Due to the interfacial area reduction, the microgels are compressed. They also potentially experience strong attractive capillary interactions due to the local deformation of the oil/water,44 forcing the peripheral shells to overlap. The thin filaments connecting the drops could be remnants of this highly compressed state (see Supporting Information, Figure S6). The droplets recombine and the interfacial coverage increases until a point that the surface pressure of the monolayer becomes large enough to squeeze individual microgels or microgel patches away from the interface. The attractive links between the particles counteract their tendency to escape from the interface. In such conditions, it has been demonstrated theoretically that the compressed interfacial film forms an adsorbed layer that can be several particles thick.45 The initial monolayer becomes “orogenically” displaced (orogeny is the process by which mountain chains are formed), leading to a significant increase in film thickness, while the film as a whole remains associated with the interface. Such a mechanism has already been proposed to explain the progressive displacement of proteins by low molecular weight surfactants.46 In our case, displacement of the microgels would be produced by the interfacial compression resulting from coalescence (Figure 13 b). Concomitantly, after each coalescence event, the shape relaxation process driven by surface tension (leading two emulsions drops to 14105


Langmuir merge into a single one) induces a viscous stress that facilitates reordering of the particles. Origin of the W/O Emulsion Type. We finally discuss the origin of the preferential W/O emulsion type. In a previous paper,15 we hypothesized that the emulsion stability is mainly determined by the overlapping of the microgels, and especially by the surface density of interparticle bridges (interconnected digitations) resulting from the close contact of the shells. For the most deformable microgels, the shells are easily interpenetrable and may form a dense interconnected network. Owing to this strong connectivity, the interfacial layer is characterized by a high 2D elasticity, which efficiently protects the drops against coalescence. Conversely, for the least deformable microgels (high cross-linking densities or collapsed state at T > VPTT), the internal elastic resistance of the particles is considerably enhanced, which is detrimental to their spreading, their anchoring at the interface, and their lateral overlapping. Chain interpenetration becomes less favorable, resulting in a reduced interfacial elasticity that may become insufficient to avoid film rupturing (coalescence). In presence of fatty alcohols, both the characteristic distance between the adsorbed microgels and the images of Figures 5 and 12 reveal that there is almost no deformation. Hence, according to our previous statements, the emulsions are expected to be quite unstable, whatever their type. It is likely that the interfacial elasticity is increased by the formation of densely packed particle layers in direct contact with the interface, which may provide sufficient resistance to coalescence in quiescent storage conditions. The mobility of the microgels directly adsorbed on the interface and of the adjacent layers is thus a key parameter as particles can respond to the compression of film separating two drops by moving sideways. In the O/W configuration, during the film formation process, water is squeezed away from the film and the adsorbed microgels layers tend to be dragged toward the peripheral parts until the two interfacial monolayers are in direct contact. The multilayered structure cannot be restored anymore because of the confinement (the film thickness becomes smaller than the particle diameter). The cohesiveness of the monolayers then becomes insufficient to avoid coalescence after the withdrawal of the particle layers. In the W/O configuration, film thinning may produce internal circulation of the aqueous fluid, but the multilayered structure is not necessarily disrupted. Consequently, the interfacial cohesiveness is partially preserved, providing enhanced resistance to coalescence. To sum up, the possibility to maintain or rapidly restore the multilayered structure after film thinning is possible in the W/O configuration and almost impossible in the W/O one, which could explain the preferential formation of W/O emulsions. An alternative explanation can be proposed, based on a purely geometrical argument. Let us consider a collection of particles densely packed at the vicinity of a planar oil/water interface. In the densest configuration, which is also the most stable one (considering attractive interactions), the coordination number of each particle is equal to 12. Moreover, the center-to-center distance between a given particle and its 12 nearest neighbors is exactly the same. This is not the case anymore if the interface is curved. In Figure 14, we represent the two possible configurations considering that the particles are always immersed in the water phase. In the W/O configuration, the curvature tends to compress the particles located in the same layer and to increase the packing density. At the opposite, in the O/W configuration, the curvature is increasing the distance between the particles

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Figure 14. Scheme illustrating the impact of curvature on the distance between microgels in a layer.

located in the same layer. The center-to-center distance, xn, between adjacent particles located in the same layer with index n (n g 0 for O/W emulsions and n e 0 for W/O emulsions) is given by: pffiffiffi d xn ≈d 1 þ n 3 ð4Þ D where d and D are the particle and emulsion droplet diameters, respectively; n = 0 corresponds to the layer in direct contact with the interface and in this case, x0 = d (assuming that the particles adopt a hexagonally close-packed arrangement). Equation 4 is valid in the limit D . d and assumes a constant particle diameter. For d = 700 nm, D = 70 μm (situation corresponding to Figure 4), we find that the distance variation, |xn d|, due to curvature is equal to 12 and 24 nm for n = 1 and 2, respectively. Such distances are significant considering that the extension of the peripheral shell of the microgels is generally smaller than 30 nm.25,26 Hence, the W/O configuration tends to increase the cohesiveness of the multilayer owing to the confinement imposed by the negative curvature. Obviously, this argument is only valid for the layers closest to the interface (low n value). For more distant layers, the confinement is no longer compatible with the sterical constraints and the particles are forced to adopt a more disordered arrangement with many defects in it. On the other hand, the O/W curvature tends to reduce the packing density and thus to decrease the cohesiveness of the multilayer, which is detrimental to the stabilization of this emulsion type.

’ CONCLUSION In this paper, we provided experimental evidence that W/O emulsions can be stabilized by water-dispersible microgels. This unusual (anti-Finkle) situation was encountered under specific conditions that can be summarized as follows: (1) the oil phase is a fatty alcohol like octanol, (2) the microgels are partially swollen by the oil phase, (3) the microgels self-assemble on the droplet surfaces and form a densely packed multilayered structure, reflecting the existence of attractive interactions, and (4) there is almost no interfacial deformation of the microgels in direct contact with the oil phase. This latter condition suggests that the pNIPAM microgels behave like attractive, mostly hydrophilic solid colloidal particles. For conventional solid particles, the three-phase contact angle3 is the key parameter for controlling both the emulsion type (according to Finkle’s rule) and the interfacial anchoring energy. However, this is not the case for soft particles like microgels, even if they remain spherical when adsorbed at the oil/water interface, and consequently, an effective contact angle can be defined. The 14106


Langmuir ability of microgels to incorporate oil necessarily makes the adsorption thermodynamics much more complex. Those remarks highlight the original behavior of this new type of emulsion stabilizer, as already stated in our previous studies.15,36 Forthcoming studies will be devoted to characterize the bulk and interfacial interparticle interactions after octanol uptake and to elucidate the stratification mechanism. It is also within the reach of future work to characterize the 2D rheological properties of the interfacial multilayered structures and to validate the importance of the protic nature of the oil phase to induce preferential formation of W/O emulsions, by testing other solvents like amines and carboxylic fatty acids.

’ ASSOCIATED CONTENT

bS

Supporting Information. Figure S1, heptane-in-water emulsions stabilized with pNIPAM microgels as a function of the aqueous phase volume fraction; Figure S2, optical microscopy images of W/O emulsions stabilized by 2.5 mol % BIS microgels as a function of the oil’s nature at constant microgel concentration; Figure S3, migration of the microgels from the aqueous phase to undecanol; Figure S4, optical microscopy images of microgels before and after octanol uptake; Figure S5, evolution of the emulsion type as a function of oil composition for octanol/octanone mixtures; Figure S6, cryo-SEM image of a water-in-octanol emulsion obtained at high magnification. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors gratefully acknowledge the regional council of Aquitaine and the French Ministery of Higher Education and Research for financial support and Prof. Serge Ravaine for fruitful discussions. ’ REFERENCES (1) Ramsden, W. Proc. R. Soc. 1903, 72, 156. (2) Pickering, S. U. J. Chem. Soc., Trans. 1907, 91, 2001. (3) Binks, B. P. Curr. Opin. Colloid Interface Sci. 2002, 7, 21. (4) Leal-Calderon, F.; Schmitt, V. Curr. Opin. Colloid Interface Sci. 2008, 13, 217. (5) Ngai, T.; Auweter, H.; Behrens, S. H. Macromolecules 2006, 39, 8171. (6) Ngai, T.; Behrens, S. H.; Auweter, H. Chem. Commun. 2005, 331. (7) Binks, B. P.; Murakami, R.; Armes, S. P.; Fujii, S. Langmuir 2006, 22, 2050. (8) Fujii, S.; Armes, S. P.; Binks, B. P.; Murakami, R. Langmuir 2006, 22, 6818. (9) Fujii, S.; Read, E.; Binks, B.; Armes, S. In Polymer Preprints, Japan, 1 ed.; The Society of Polymer Science, Japan: Tokyo, 2006; Vol. 55, p 1127. (10) Brugger, B.; Richtering, W. Langmuir 2008, 24, 7769. (11) Brugger, B.; Rosen, B. A.; Richtering, W. Langmuir 2008, 24, 12202. (12) Brugger, B.; R€utten, S.; Phan, K. H.; M€oller, M.; Richtering, W. Angew. Chem., Int. Ed. 2009, 48, 3978. (13) Koh, A. Y. C.; Saunders, B. R. Langmuir 2005, 21, 6734. (14) Tsuji, S.; Kawaguchi, H. Langmuir 2008, 24, 3300.

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Water-in-Oil Emulsions Stabilized by Water-Dispersible Poly(N

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