Pickering Emulsions Stabilized by Soft Microgels: Influence of the

Article pubs.acs.org/Langmuir

Pickering Emulsions Stabilized by Soft Microgels: Influence of the Emulsification Process on Particle Interfacial Organization and Emulsion Properties Mathieu Destribats,‡,§ Mélanie Wolfs,†,⊥ Florent Pinaud,† Véronique Lapeyre,† Elisabeth Sellier,∥ Véronique Schmitt,*,‡ and Valérie Ravaine*,† †

Institut des Sciences Moléculaires, ENSCBP, Université de Bordeaux, 16 Av. Pey Berland, 33607 Pessac Cedex, France Centre de Recherche Paul Pascal, Université de Bordeaux,115 Av. A. Schweitzer, 33600 Pessac, France ∥ CREMEM, Université Bordeaux 1, Bât. B8, Avenue des Facultés, 33405 Talence, France ‡

S Supporting Information *

ABSTRACT: This work reports a new evidence of the versatility of soft responsive microgels as stabilizers for Pickering emulsions. The organization of microgels at the oil−water interface is a function of the preparation pathway. The present results show that emulsification energy can be used as a trigger to modify microgel deformation at the oil−water interface and their packing density: high shear rates bring strong flattening of the microgels, whereas low shear rates lead to dense monolayers, where the microgels are laterally compressed. As a consequence, the resulting emulsions have opposite behavior in terms of flocculation, which arises from bridging between neighboring drops and is strongly dependent on their surface coverage. This strategy can be applied to any microgel which can sufficiently adsorb at low shear rates, i.e. small microgels or lightly cross-linked ones. The control of the organization of microgels at the interface does not only modify emulsion end-use properties but also constitutes a new tool for the development of Janus-type microgels, obtained by chemical modification of the adsorbed microgels.

1. INTRODUCTION Emulsion stabilized by solid particles have been known for a very long time and termed Pickering or Ramsden emulsions.1,2 A few years ago, a new class of particles started to be investigated: responsive microgels, i.e. colloidal particles made of swollen cross-linked polymers, were shown to stabilize emulsions that can break on demand, upon application of a stimulus. This phenomenon was discovered using pHresponsive microgels.3−6 Since then, various microgel compositions and structures were studied. Among them, significant attention has been devoted to poly(N isopropylacrylamide) (pNIPAM) based microgels7−14 which exhibit a well-known volume phase transition temperature (VPTT) at approximately 32 °C.15 Such stabilizers not only offered the possibility to get responsive emulsions but also revealed unique features arising from their deformability. Although similarities can be drawn between solid particles and microgels ability to stabilize emulsions, these soft structures also display fundamental differences owing to their intermediate position between colloidal particles and molecular polymers. Both types of particles are irreversibly anchored at the oil−water interface, with an energy being several kT, the thermal energy at room temperature. Whereas solid-stabilized emulsion types (oil-inwater or water-in-oil) are mainly ruled by particles contact angle at the oil−water interface,16,17 this parameter is not relevant anymore for microgels. Indeed, microgels did not © 2013 American Chemical Society

appear as spherical smooth particles at the oil−water interface but were strongly deformed, as a result of a compromise between maximization of chain adsorption and minimization of gel deformation owing to its elasticity induced by cross-linking. The deformability was found to be a crucial criterion since the loss of deformability, either by increasing the cross-linking density during the synthesis or by triggering the microgel swelling via temperature or pH, leads to emulsion destabilization.12 The link between microgel deformation and emulsion stability was attributed to the emulsion drop surface elasticity resulting from microgel overlapping of their peripheral parts. Soft and deformed microgels provided enhanced surface elasticity compared to more rigid ones, which induced brittle surfaces. This finding supported surface rheology measurements,10,18 showing that the elastic and loss modulus of microgel covered interfaces were higher in conditions where emulsions exhibited the highest stability. Surface elasticity was shown to be a crucial parameter compared to interfacial tension between the two liquids.7,19 Microgel softness and responsiveness is also a means to control emulsion end-use properties. In a previous paper we showed how triggering the microgel swollen state during the Received: August 1, 2013 Revised: September 19, 2013 Published: September 19, 2013 12367

dx.doi.org/10.1021/la402921b | Langmuir 2013, 29, 12367−12374

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tension of the supernatant reached that of pure water, i.e. above 70 mN·m−1. Particles Dispersion Characterization. Particles size and polydispersity were determined by photon correlation spectroscopy (PCS) at a detection angle of 90°, using a ZetasizerNano S90 Malvern instrument equipped with a HeNe laser (λ = 633 nm). The hydrodynamic diameters were calculated from diffusion coefficients using the Stokes−Einstein equation. All analyses were performed with the software supplied by the manufacturer. The polydispersity index (PDI) was derived from the cumulant analysis method. The particles were also characterized using transmission electronic microscopy (TEM) in order to visualize their morphology and to check their monodispersity (Figure S1). A drop of diluted suspension was deposited on a copper grid covered by a carbon membrane. The electronic contrast of microgels was enhanced by labeling with uranyl acetate. The polymer content cpolymer (in g·cm−3) in aqueous dispersions was determined by the drying method. Following the work published by Lele et al.,22 we considered 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, the particle concentration cparticles (expressed in cm−3) of the dispersions was estimated as

emulsification step rather than during storage was another way of exploiting their versatility.13 Emulsion prepared in the swollen state of the microgels, and yet covered with highly flattened microgels, exhibited strong adhesive bonds between drops due to bridging of opposite interfaces by microgels. As a result these emulsions were flocculated and could not flow. When emulsification was carried out in the shrunk state and followed by a rapid swelling of the interfacial microgel due to a temperature quench, the microgels were trapped at the oil− water interface in a dense and rigid configuration due to the forced interpenetration of their lateral polymer chains. In this case, bridging was suppressed, and emulsions were composed of individual drops which were able to flow freely. Interfacial packing density and microgel interconnectivity were thus strongly controlling the dynamics of film draining and thinning. These parameters became path functions owing to microgel sensitivity. Such path-dependence was also illustrated by another example, where water-in-oil emulsions could be stabilized by pNIPAM microgels.8,11 These water-swollen microgels had become octanol-swollen microgels after contact with this oil phase, with fundamentally different adsorption properties.11 In this paper, we propose to investigate the influence of emulsification energy on the path-dependence of surface coverage. We show that this parameter is an easy way to trigger the packing of microgels at the interface and thus the end-use properties of emulsions. Controlling the 2D-organization also brings significant improvement in the synthesis of well-defined Janus-type microgels, as obtained from regionselective chemical modification of adsorbed microgels.

c particles =

⎞ 6c polymer ⎛ 1 0.29 ⎟ ⎜ + 3⎜ 0.71ρwater ⎟⎠ π(d50 ° C) ⎝ ρpolymer −322

(1) −3

and ρwater = 0.988 g·cm are where ρpolymer = 1.269 g·cm respectively the polymer and water densities. Emulsion Production. Typical emulsion batches were composed of 14 g of aqueous phase containing microgels and 6 g of oil (mainly hexadecane for macroscopic and optical microscopy observations, and dodecane for cryo-SEM observations). This mixture was then stirred by an Ultra-Turrax T25 mixer, at constant speed (9500 rpm) for 30 s. To modify the emulsification energy, two rotor heads were employed: one with a large diameter (S25 KV-25F), named UT-LA for large axis and one with a small diameter (S25N-10G) named UT-SA for small axis. Considering the rotor and gap dimensions, the shear rate corresponding to the rotation speed (9500 rpm) was about 11 × 103 s−1 and 18 × 103 s−1 respectively for small and large axis. Emulsification was carried out at 25 °C. We checked that stirring in such conditions did not induce a temperature raise above the microgels VPTT. Moreover, by prolonging the agitation time up to 1 min we did not observe any modification. Emulsions were usually prepared exploiting the limited coalescence process. The system was emulsified with a low amount of particles (between 1.5 × 1010 and 8.0 × 1010 microgels per cm3 of oil) so that the newly created droplets were insufficiently protected by the particles, as described below. Optical Microscopy Observations. The size distribution of the emulsions was estimated by direct imaging using an inverted optical microscope (Zeiss Axiovert X100) and a video camera. Images were recorded, and the dimensions of about 50 droplets were measured, so that both the surface average diameter D and the polydispersity P, defined by eq 2, could be estimated

2. 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,N′-Methylenebisacrylamide (BIS), acrylic acid (AA), sodium dodecylsulfate (SDS), and potassium persulfate (KPS) were used as received. Milli-Q water was used for all synthesis reactions, purification, and solution preparation. Hexadecane and dodecane (Sigma-Aldrich, purity >99%) were used without further purification, and water was double-distilled. Particles Synthesis and Purification. The microgels were synthesized by an aqueous free-radical precipitation polymerization classically employed for the synthesis of thermo-responsive microgels and especially pNIPAM microgels.20,21 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, containing a surfactant, sodium dodecylsulfate SDS (1 mM). To obtain pH-sensitive microgels, the same procedure was employed without surfactant, but 10 mol.% NIPAM was substituted by acrylic acid (AA) in the monomer solution. Various BIS concentrations were used to prepare microgels with different cross-linking densities (0.5 and 2.5 mol.%). In all cases, the solution was filtered 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 (500 rpm). The microgels were purified by centrifugation-redispersion cycles at least five times (21000 g for 1 h where g is the gravity constant). 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

D=

3 ∑i ND i i 2 ∑i ND i i

P=

3 1 ∑i ND i i |Dm − Di | 3 Dm ∑i ND i i

(2)

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%. Characterization of Microgel Adsorption through Limited Coalescence. Limited coalescence arises in Pickering emulsions where particles are irreversibly anchored at the oil/water interface. If the system is emulsified with a low amount of particles, the newly created droplets are insufficiently protected by the particles. When the agitation is stopped, the 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 12368


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interface is sufficiently covered, and the resulting emulsions exhibit remarkably narrow size distributions (P < 30%).23 The final average drop diameter depends on the initial amount of particles and on their arrangement at the interface. Particle arrangement at the interface can be deduced from the surface coverage, C, which is defined as the ratio between the total area that may be covered by the particles Seq and the emulsion interfacial area Sint. For emulsions undergoing limited coalescence, the final interfacial particle coverage can be directly deduced from the surface average drop size, D. Indeed, the total interfacial area Sint of the emulsion is directly linked to D (Sint = 6 Vd/ 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 equatorial section Seq = nparticlesπ(dH at 25 °C/2)2, where n is the total number of particles and dH at 25 °C is their hydrodynamic diameter at 25 °C. The surface coverage, C, defined as C = Seq/Sint can thus be estimated after measuring D: nparticlesπdH2 at 25 ° C Seq 1 = = D 24CVd 6CVd

We studied the influence of emulsification energy on the properties of emulsions stabilized by pNIPAM-co-AA microgels. As expected, in any condition, oil-in-water emulsions were obtained. After emulsification, the emulsions rapidly formed a creamed layer at the top of the recipient (because of the differential density between oil and water and of the large drop size). None of the emulsions coalesced over months when stored above the crystallization temperature of hexadecane and below the VPTT, showing the irreversibility of microgel anchorage. In most cases, oil was totally dispersed in water and coexisted with a clear subnatant aqueous phase. This latter characteristic indicated that no microgel remained in the aqueous phase; all of them were adsorbed at the oil−water interface. An exception was found for the case of low energy emulsification and high cross-linking density: a thick layer of pure oil was observed on the top of the emulsion and an excess of microgels remained in the aqueous phase, indicating an insufficiently efficient emulsification process and a poor adsorption of microgels at the oil−water interface. The less deformable microgels require more energy to adsorb at the oil− water interface than the softer ones. In the following, we chose to focus on the effect of emulsification conditions on less crosslinked microgels. The flocculation state of the emulsions strikingly depended on emulsification energy. Emulsions prepared with the highest emulsification energy were flocculated, and the emulsion did not flow when the container was tipped over whereas the one prepared with the lowest shearing rate was fully dispersed and could flow. This feature can be also easily noticed on emulsions at rest when considering the limit between the emulsion cream and the lower aqueous phase. This limit is smooth, horizontal, and welldefined for nonflocculated emulsions, whereas the flocs of the flocculated ones cause a rough and nonhomogeneous separation (Figure 1). Optical microscopy observations

(3)

Equation 3 shows that the surface coverage characterizing the particle packing at the interface can directly be deduced from a drop size measurement in the low particle concentration regime where the average inverse droplet diameter is proportional to Seq/Vd. A value of C = 90% corresponds to a monolayer of hexagonally packed hard spheres. 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 = 10−5 Pa and was equipped with a blade used to fracture the sample. Once fractured, the sample was coated by a layer of Au−Pd and was then inserted into the observation chamber equipped with a SEM stage cold module hold at −150 °C. Dodecane was preferred to hexadecane for cryo-SEM observations because of its low melting temperature (−9.6 °C) that avoided oil crystallization during the freezing step. Moreover dodecane was amorphous in the solid state, so the droplet interfaces remained spherical and smooth.

3. RESULTS AND DISCUSSION Poly(N-isopropylacrylamide-co-acrylic acid), referred to as pNIPAM-co-AA, were used as stabilizers. They were dispersed in pure water, i.e. at pH 5.5, above the pKa of acrylic acid (pKa = 4.25).24 In these conditions, the microgels were highly swollen in water at 25 °C, due to Coulombic repulsion between charged groups and the excess of osmotic pressure exerted by mobile counterions inside the polymer matrix. Since we proved previously that cross-linking density was an important criterion to control both microgel deformation at the oil−water interface and emulsion bridging, we also prepared microgels with two different cross-linking ratios (0.5 and 2.5 mol.%), with a diameter of respectively 1100 and 800 nm in the swollen state. Like poly(N-isopropylacrylamide), both microgels were also thermoresponsive (Figure S2). However, they reached the collapsed state only at temperatures far above the volume phase transition temperature (VPTT) of pNIPAM. Indeed, microgels resist to collapse upon temperature because of the presence of mobile counterions inside the matrix.25,26 At high temperature and in particular at 70 °C, i.e. the synthesis temperature, they are in the collapsed state and display similar sizes. The difference of cross-linking density between the two gels is clearly visible when the temperature decreases: in the swollen state, the microgels with the lower cross-linking density have the higher swelling degree.27

Figure 1. Macroscopic picture of emulsions stabilized with pNIPAMco-AA microgels (0.5% BIS), produced with an Ultra-Turrax equipped with (a) a large dimension head and (b) a small dimension head, both operating at 9500 rpm for 30 s.

confirmed the flocculation state of emulsions prepared with a high shear rate, whereas emulsions prepared with a low shear rate exhibited only individual drops (Figure 2). Moreover, in this latter case, the drops size was larger compared to emulsions produced with high energy input, keeping the composition constant. 12369


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Figure 2. Optical microscopy images of emulsions produced with an Ultra-Turrax equipped with (a) a large dimension head and (b) a small dimension head, both operating at 9500 rpm for 30 s. The emulsions are composed of hexadecane and water containing 4.6 × 1010 microgels (0.5% mol. BIS)/cm3. Scale bar = 200 μm (identical for the two images).

90% (Table 1). In the case of a monolayer of particles, this would mean that the microgels are compressed at the interface and occupy a surface lower than that of their initial equatorial plane. In order to verify this hypothesis, the organization of the microgels at the oil−water interface was observed by cryo-SEM, replacing hexadecane by dodecane in order to minimize oil crystallization during the freezing step. This substitution did not change the value of the coverage deduced from limited coalescence. In both emulsification conditions, the microgels formed a monolayer with a hexagonal array at the interface (Figure 4). However, the lattice parameter was very different. This shows that the packing density depends on emulsification conditions. The microgel center-to-center distance dcenter to center corresponding to the lattice parameter was estimated from an average of about 50 measurements. We could also calculate the hypothetical surface coverage assuming that the lattice was filled with nondeformed microgels of diameter dH at 25 °C. The values estimated by the limited coalescence process (70 and 155%) were confirmed by the cryo-SEM technique (82 and 163%) proving the good reliability of the two techniques (as already shown in an previous work12). Values are in good agreement with those calculated by the limited coalescence process. Moreover, the comparison between the center-tocenter distance and the initial diameter of the microgels proves that the microgels were deformed at the oil−water interface (Table 1). In the case of high shear rate, they were flattened at the oil−water interface, whereas they were compressed in the case of low shear rate. Once again, the softness of microgels brings versatility in their interfacial properties. Their packing at the oil−water interface is a path function and can be modified by the emulsification process. This peculiar phenomenon can be explained by the irreversibility of microgel anchorage at the oil−water interface. Once the microgels are adsorbed at the interface, they remain trapped in their configuration. For the more flattened microgels, more polymeric segments are adsorbed at the oil− water interface. This adsorption is ruled by the intrinsic hydrophobicity of the polymeric segments. Thus, flattening seems to be the required configuration to minimize the surface energy, as observed under quiescent conditions for flat films.28 However, the dynamics of segment adsorption is probably slow, due to the restricted configurations of the cross-linked chains. Emulsions are nonequilibrium systems, and their creation involves various dynamic processes. The interfacial organization results from the competition between microgel adsorption as a whole, their relaxation to promote polymeric segments adsorption, and drop recombination during coalescence process. Thus, high emulsification energy may promote the

For emulsions resulting from a limited coalescence process, the drop size is directly related to the surface coverage C for a fixed amount of microgels (see eq 3). In other words, the size is a valuable information to get insight into microgel packing at the oil−water interface. In a previous work, using this method concomitantly with CryoSEM observations, we showed that the surface coverage was independent of the cross-linking density. In the present work, we determined the influence of the emulsification process on the surface coverage. In any conditions considered here, the resulting emulsions underwent the limited coalescence process. At low microgel concentrations, the inverse diameter evolved linearly with the amount of microgels. At high microgel concentrations, the diameter did not depend anymore on the microgel concentration, and the drop diameter became very polydisperse. In the low concentration regime, we could extract the slope of linear plot and determine the surface coverage as explained in the Experimental Section. The slope was found to be strongly dependent on the emulsification conditions (Figure 3). In the

Figure 3. Evolution of the average inverse drop diameter (1/D) of hexadecane-in-water emulsions as a function of the total equatorial surface area of particles normalized by the oil volume. The limited coalescence is observed for emulsions stabilized by pNIPAM-co-AA microgels (0.5%BIS) and prepared with two different shear rates. Squares and diamond symbols correspond to large and small dimension head respectively.

case of high shear rate, a surface coverage of 70% was found for both cross-linking densities. It means that the microgels were stretched at the oil−water interface. Indeed, a value below 90% means that the microgels occupy a surface at the interface which is larger than their equatorial plane estimated from the hydrodynamic diameter in the aqueous suspension. Unexpectedly, for the microgels emulsified at the lowest shear rate, the surface coverage was equal to 155%, a value much higher than 12370


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Table 1. Summary of Surface Coverage As a Function of pNIPAM-co-AA Microgel Cross-Linking Density and Emulsification Conditions % BIS

hydrodynamic diameter in water (T = 25 °C)

emulsification conditions

dispersion state of emulsion

surface coverage C (%) determined by limited coalescence process

microgel center-to-center distance at interface

2.5% 0.5% 0.5%

0.83 μm 1.00 μm 1.00 μm

LA-9500 rpm LA-9500 rpm SA-9500 rpm

flocculation ++a flocculation +a dispersed

70% 70% 155%

1.05 μm 1.12 μm 0.78 μm

The degree of flocculation is described qualitatively. ++ means that the flocculation is very important, leading to a very elastic gel made of adhesive drops; + means that the flocculation is less important, the chunk of adhesive drops being softer. a

Figure 4. CryoSEM of drop covered with pNIPAM-co-AA microgels (0.5% BIS) (a) emulsion prepared with a large dimension head and (b) emulsion prepared with a small dimension head. Scale bar is 2 μm for both images.

As mentioned above, the emulsions prepared at different shear rates with low cross-linked microgels not only exhibited different packing densities but also different flocculation states. In a previous paper,13 we already showed that the packing density and the extent of microgel interconnectivity were related to the flocculation state of the emulsion. Indeed, the flocculation of microgel stabilized emulsions arises from microgels bridging opposite interfaces of drops. This phenomenon is governed by the drainage dynamics of the thin film that is created between two approaching drops. The coverage density controls the dynamics of fluid drainage: the higher the surface coverage, the more rigid the interface and the higher the resistance to drainage. Thus, a high coverage density favors bilayers instead of bridged interfaces and prevents flocculation. In the previous paper, the packing density was controlled by the emulsification temperature and the initial swollen state of the microgels. In the present paper, we show that the energy input is another way of triggering the packing density that again controls the flocculation state. The influence of emulsification process on microgel packing at the interface is a general process, not specific of poorly crosslinked pNIPAM-co-AA microgels. Indeed, we could generalize this effect to the case of pNIPAM microgels, where it was also possible to play with this parameter. This was achieved with “small” pNIPAM microgels, with an initial swollen diameter of 250 nm, which could adsorb more easily than bigger ones.30 These microgels, with a cross-linking density of 2.5 mol. %, stabilized emulsions with moderate flocculation when produced with high shear rate. By decreasing the emulsification shear rate, it was possible to inhibit flocculation (Figures S3 and S4). The measurement of the surface coverage confirmed that the packing density followed the same trend as previously (Figure S5): under high shear rate, the surface coverage was 71%, characteristic of stretched microgels, whereas it jumped to 182% under low shear rate, confirming the link between the highly compacted layer and the absence of bridging. Figure 5 summarizes our observations.

exploration of various configurations, enhancing the efficiency in chain adsorption and resulting in strong microgel deformation in the interfacial plane. Moreover, a high oil− water interface area was produced during this emulsification process: the surface density of microgels was low, and they could extend laterally freely. On the contrary, using low emulsification energy, the microgels could adsorb at the interface, but their dynamics of deformation was too slow compared to new microgel adsorption and drop recombination rate during the limited coalescence process. They had neither enough time nor free space to extend. A high density of microgels was therefore obtained. During the drop coalescence process that occurs after halting the emulsification, the interfacial area is reduced, and microgels are compressed laterally. This process results in a high density of microgels at the interface, highly interconnected through the overlapping of their dandling polymer chains. The coalescence is eventually limited by the microgel resistance against compression when there is sufficient lateral chain penetration − and thus interfacial elasticity. It should be emphasized that this process was only efficient with poorly cross-linked microgels. Microgels with higher crosslinking density were not able to stabilize emulsions at low emulsification energy. Again, this behavior could be understood in the light of deformability and dynamics. Indeed, increasing the cross-linking density increases the microgel rigidity and thus both decreases the deformation and slows down the dynamics of unfolding, necessary to achieve segment adsorption. This point has already been reported by Zhang and Pelton, who measured air−water dynamic surface tensions of pNIPAM microgels bearing different cross-linking densities.29 The lower the cross-linking density, the quicker the surface tension decrease. Thus, at low shear rate, emulsification does not occur because microgel adsorption is too slow. A minimal amount of energy is required to achieve microgel adsorption; it increases with increasing the microgel cross-linking density. Controlling the packing density at the oil−water interface is of utmost importance to control emulsion end-use properties. 12371


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chemically different hemispheres, generally called Janus, can be produced by their regioselective functionalization when they are adsorbed at an interface. A few years ago, Kawaguchi et al. reported the first synthesis of Janus microgels, obtained from microgel-stabilized emulsions.31 In the cited paper, a peptidic coupling was achieved using a water-soluble carbodiimide coupling agent. An amine group could be grafted on the carboxylic acid functions contained in the water side. Convincing images of Janus microgels were shown, but only a few particles could be visible. More recently, Umeda et al. reported the preparation of Janus microgels with a better yield:32 they used crystallized oil to counterbalance a possible rotation of microgels at the interface. However, considering the deformation of microgels at the interface and the number of anchored polymeric segments, rotation of microgels is unlikely to occur. In this work, we explore the feasibility of microgel asymmetric modification using liquid oil. A possible limitation to this method of synthesis could be the lack of accessibility of the reactants to all the microgels. In particular, if bridging occurs, microgels can be trapped at the interface in two regimes: they can sit at a free interface or they can be locked between adjacent drops, thus losing their accessibility to watersoluble reactants. For the purpose of producing well-controlled and uniform Janus microgels, we paid attention to the preparation of nonflocculated emulsions, using poorly crosslinked microgels and low energy emulsification method. pNIPAM-co-AA microgels were chosen for their ability to react with amine function in the presence of ethyl-3-(3dimethylaminopropyl)-carbodiimide hydrochloride (EDC) coupling agent. 5-Aminofluorescein was used as a model compound to facilitate optical visualization. This compound was allowed to react for 2 h in the presence of EDC with the microgel-stabilized emulsion. After emulsion breakage by oil crystallization followed by oil removal, the aqueous suspension of microgels was analyzed by epifluorescence microscopy. Figure 6 shows the success of regioselective grafting. On most of the microgels, fluorescence was more pronounced on an arc at the periphery of the particles, indicating a selective grafting on one side of the microgels. Contrarily to the preparation of Janus particles from hard spheres,33,34 there is no need to use a

Figure 5. Schematic view of microgel packing under high and low shear rates.

- On the one hand, using a high shear emulsification process favors the microgel spreading at interface. As a consequence the drop coalescence process is limited at an early stage resulting in a lower interfacial microgel density. The lateral deformation of the microgels results in a poor overlapping of their side dandling chains. The resistance against drainage during the film formation between approaching drops is then low, and the microgels are more easily squeezed away favoring bridging events between drops and causing emulsion flocculation. - On the other hand, using a low shear emulsification process does not promote the microgel spreading at interface, but the microgels keep presumably a configuration close to the one in their initial dispersed state. The reduction of the interfacial surface area, due to drop coalescence after emulsification halt, concentrates the microgels and is eventually limited by their resistance against compression. This process results in a high density of compressed microgels at interface promoting as a side effect an important overlapping of their lateral chains. As a consequence the resistance against film drainage is good enough to prevent bridges formation: the drops remain individual and can flow easily. Application to the Preparation of Janus Microgels. It was shown that the properties of microgel-stabilized emulsions can be controlled through the emulsification process, by playing either with temperature13 or with the shear rate. The control of emulsion flocculation is of huge importance in many applications. Here, we focus on the production of Janus microgels from well-controlled emulsions. Particles having

Figure 6. Fluorescence microscopy image of Janus microgels functionalized on one side by fluorescein amine. Scale bar is 5 μm. 12372


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solid dispersed state to immobilize the particles at the interface. Thanks to the microgel interconnections and their multiple segment adsorption, the microgels do not rotate during the chemical modification step. To estimate the yield of the chemical modification, a wide field image was observed. It shows that a great number of microgels displayed such a signature. It was difficult to catch a large view of the microgels with a uniform contrast because bleaching of fluorescein was very fast. However, one can clearly see on a few enlargements that many areas presented nonsymmetric particles with random orientation. Considering that the microgels were free to rotate in the aqueous suspension, the observer can probably see only a fraction of nonsymmetric microgels. Therefore, it is expected that the yield of this regioselective grafting was quite good. We believe that a precise control of microgel organization at the oil−water interface is a chance of success for further dissymmetric functionalization.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (V.R.), schmitt@crpp-bordeaux. cnrs.fr (V.S.). Present Addresses §

L’Oréal, 11-13 rue Dora Maar, 92400 Saint-Ouen, France. Université de Nice Sophia-Antipolis & CNRS, Laboratoire de Physique de la Matière Condensée, Equipe Surfaces & Interfaces, Parc Valrose, 06108 Nice Cedex 2, France.

⊥

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the regional council of Aquitaine, the French Ministery of Higher Education and Research and Institut Polytechnique de Bordeaux for financial support, and Pr. Fernando Leal-Calderon for fruitful discussions.

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CONCLUSION This paper provides a new illustration of the versatility of microgels as stabilizers for Pickering emulsions. Their packing at the oil−water interface is a path function, which can be controlled by several parameters. Here, we show that emulsification energy is an easy means to control their deformation state at the interface, thus controlling their packing density and, as a consequence, the macroscopic properties of the emulsions. Emulsions produced with high energy are stabilized by flattened microgels, and their drops are flocculated by bridging. Emulsions produced with low energy are stabilized by compressed microgels, and no bridging occurs. This effect is explained by the change in the dynamics of segment adsorption in the microgels: an increase of the emulsification energy promotes segment adsorption and thus microgel flattening, whereas compressed microgels have a low density of adsorbed segments resulting from low mechanical energy. The dynamics of segment adsorption is also dependent on the cross-linking density of the microgels and on their size. Microgels with the slower dynamics require more energy to be used as efficient emulsifier and are less versatile. The strategy of emulsion properties modulation through emulsification energy can be applied to microgels with the fastest dynamics, using either microgels with low cross-linking density or small size. Controlling the organization of microgels at the oil−water interface is not only important for emulsion end-use properties but also for the design of sophisticated smart materials, obtained by chemical modification of adsorbed microgels. Their deformation is a guarantee for successful dissymmetrization, avoiding their free rotation. The control of their packing will be useful in the future to prepare well-defined responsive Janus microgels.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1, TEM view of pNIPAM-co-AA microgels. Figure S2, temperature dependence of the hydrodynamic diameter for the pNIPAM-co-AA microgels. Figure S3, macroscopic view of the emulsions stabilized with “small” pNIPAM microgels using two different rotor heads. Figure S4, optical microscopy images of emulsions stabilized with “small” pNIPAM microgels using two different rotor heads. Figure S5, limited coalescence process of the “small” pNIPAM stabilized emulsions prepared at two different shear rates. This material is available free of charge via the Internet at http://pubs.acs.org. 12373


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