Responsive Emulsions Stabilized by Stimuli ... - ACS Publications

Invited Feature Article pubs.acs.org/Langmuir

Responsive Emulsions Stabilized by Stimuli-Sensitive Microgels: Emulsions with Special Non-Pickering Properties Walter Richtering* Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, D-52056 Aachen, Germany S Supporting Information *

ABSTRACT: Recent studies revealing the unique properties of microgel-stabilized responsive emulsions are discussed, and microgels are compared to classical rigid-particle Pickering stabilizers. Microgels are strongly swollen, lyophilic particles that become deformed at the oil− water interface and protrude only a little into the oil phase. Temperatureand pH-sensitive microgels allow us to prepare temperature- and pHsensitive emulsions and thus enable us to prepare and break emulsions on demand. Although such emulsions are sensitive to pH, the stabilization of droplets is not due to electrostatic repulsion, instead the viscoelastic properties of the interface seem to dominate droplet stability. Being soft and porous, microgels behave distinctly differently from rigid particles at the interface: they are deformed and strongly flattened especially in the case of oil-in-water emulsions. The microgels are located mainly on the water side of the interface for both oil-in-water and water-in-oil emulsions. In contrast to rigid, solid particles, the behavior of microgels at oil−water interfaces does not depend only on the interfacial tension but also on the balance among the interfacial tension, swelling, elasticity, and deformability of the microgel, which needs to be considered. It is obvious that microgels as soft, porous particles are significantly different from classical rigid colloidal stabilizers in Pickering emulsions and we suggest avoiding the term Pickering emulsion when swollen microgels are employed. Microgel-stabilized emulsions require the development of new theoretical models to understand their properties. They open the door to new sophisticated applications.

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INTRODUCTION Emulsions stabilized by solid particles have been known for a very long time; they are often termed Pickering or Ramsden emulsions.1,2 Here particles adsorb to the interface of two immiscible liquids as oil and water and lead to kinetically stabilized emulsions. Many different types of solid particles (e.g., naturally occurring or synthetic organic or inorganic particles) can be used as stabilizers.3−5 Particle-stabilized emulsions offer several advantages over surfactant-stabilized systems such as improved stability against coalescence, reduced foaming, and less skin irritation. Very recently, microgel-based particles were employed as emulsion stabilizers. Here the term microgel refers to crosslinked polymer particles that are swollen by a solvent. Typically, they have diameters of less than a micrometer and thus are sometimes also called nanogels. Although microgels have already been described by Staudinger,6 functional microgels have attracted great interest recently as model systems in colloid science because of their polymer−colloid duality.7−9 In this feature article, we discuss recent studies where soft stimuli-sensitive microgel particles were employed as stabilizers for emulsions, and we will focus on the differences between soft and rigid particles. Stimuli-sensitive microgels provide a unique approach to emulsions, the stability of which can easily be controlled by parameters such as temperature and pH. Because © XXXX American Chemical Society

microgel properties can be tailored during synthesis, they can be adapted to the specific requirements of a target application. There are similarities between emulsions stabilized by rigid and soft particles, respectively. However, there are also significant differences, as will be outlined below. The crucial difference between microgels and rigid colloidal particles is their structure. Microgels are cross-linked polymer particles that are swollen by the solvent. Therefore, they are lyophilic (solvent-loving) colloids that form stable solutions even at high concentration. Colloidal particles, however, are lyophobic particles, and their surfaces need to be functionalized such that electrostatic and/or steric stabilization is sufficient to overcome the van der Waals attraction. The structural integrity of a gel “particle” is provided by the chemical cross-links, but often the microgels are highly swollen, leading to a polymer volume fraction as small as 10% or less. Furthermore, the swelling of microgels might depend on the temperature. Often temperature-responsive hydrogels become less swollen at high temperatures and display a strong change in size above the volume phase transition temperature (VPTT). The most prominent example of a temperature-sensitive Received: June 8, 2012 Revised: September 27, 2012

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dx.doi.org/10.1021/la302331s | Langmuir XXXX, XXX, XXX−XXX

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Invited Feature Article

Figure 1. (Left) Illustration of the microgel structure and (right) the internal polymer segment density profile.

the particle, as illustrated in Figure 2; however, the microgel will carry nearly no effective charge.32,33 The pH influence on

microgel is poly(N-isopropylacrylamide) (PNIPAM), which was first described by Pelton et al.10 Microgel properties can be modified by incorporating functional comonomers and/or by changing the microgel architecture. They can be prepared with great precision and can also be modified as hybrid particles.11,12 PNIPAM microgels are often prepared via precipitation polymerization using N,N′-methylenebisacrylamide as a crosslinker, which reacts faster than the monomer.13 This leads to an inhomogeneous distribution of cross-links inside the microgel,14,15 and the conditions of the polymerization process influence the structure and size of the particles.16,17 Small-angle neutron scattering experiments revealed that the segment density of PNIPAM microgels prepared by batch precipitation polymerization is not homogeneous but decays smoothly, leading to strongly swollen particles with fuzzy surfaces, as schematically shown in Figure 1.15 The strong swelling by the solvents allows for deformation as well as for interpenetration and thus affects the interaction between particles.8 In addition, the porosity alters the draining and hydrodynamic interactions.18,19 Furthermore, ionizable functional groups can be incorporated into microgels by proper copolymerization with functional groups leading to microgels with charged moieties located at the surface and/or inside the microgel.20,21 Preparing core−shell particles with different compositions of repeat units in the core and shell, respectively, can further modify the functionality of microgels. Because both the core and shell are cross-linked networks, the swelling properties are strongly coupled and mutually influence each other.22−26 The incorporation of acidic or basic comonomers leads to pH-sensitive microgels. Their solution properties such as surface charge and colloidal stability strongly depend on the location of the functional groups inside the microgels.27−31 When they are located at the surface of the microgel, the pHdependent ionization will lead to a swelling of the particle especially at the surface. Ionization also increases the effective charge of the microgel because some counterions will leave the microgel as described by the Donnan equilibrium. However, the counterions will be mostly located inside the microgel when the functional groups are located in the core of the microgel. The increased osmotic pressure will also lead to a swelling of

Figure 2. Illustration of the influence of the spatial distribution of acidic groups on the pH-dependent swelling of microgels. In the upper case, the acidic groups are located near the surface of the microgel; in the lower case, they are located near the center of the microgel. Depronotation leads to swelling that is mainly due to the osmotic pressure of the counterions, and the distribution of counterions is given by the Donnan equilibrium. The microgels will carry a significant effective negative charge only when the acidic groups are located in the shell.

microgel properties caused by ion formation inside the particle is an important difference compared to that of the rigid colloidal particle. The different distribution of charges also influences particle interactions and phase behavior, rendering charged microgels different from conventional charged colloids.34,35

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EMULSIONS STABILIZED BY RIGID PARTICLES Several excellent reviews on particle-stabilized emulsions are available in the literature,3,4,36,37 and some general rules have been summarized: (i) the particles have to be partially wettable B


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by both water and oil phases; (ii) the continuous phase of the preferred emulsion is normally the one in which the particles are preferentially dispersed. (which is similar to the case of surfactant molecules); and (iii) the interactions between particles are important, and flocculated dispersions are reported to be more efficient in stabilizing emulsions.36 The peculiar properties of particle-stabilized emulsions arise from the strong anchoring of the particles to the interface. The energy ε required to desorb the particle from the interface can be estimated by ε = γπa 2(1 − |cos θow|)2

There are, however, many cases in which emulsions need to be broken, and recently much effort was expended in preparing emulsions on demand. As mentioned above, surface properties are relevant and thus particles have been developed with wettability that can be changed reversibly. A very interesting approach involves block-copolymer-stabilized polystyrene latexes that lead to pH-dependent emulsions (i.e., the stability of the emulsion depends on the pH during preparation).47 However, the emulsion stability does not respond to pH changes after emulsion preparation and thus these systems were pH-dependent but not pH-responsive.48

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(1)

with γ being the oil−water interfacial tension, a being the particle radius, and θow being the contact angle. The energy of attachment for a single particle to the interface can be extremely high compared to the thermal energy kBT, and often the particles can be considered to be irreversibly attached.38,39 It is obvious that the particles need to be partially wetted by both liquids in order to adsorb at the interface. The position of the particle at the interface mainly depends on the contact angle θow that the particle makes with the interface as schematically shown in Figure 3. Usually the fluid that

EMULSIONS STABILIZED BY NANOCOMPOSITE MICROGELS Fujii et al. used nanocomposite microgel particles as stabilizers for emulsions.49,50 The particles were prepared by polymerizing 4-vinylpyridine in the presence of ultrafine aqueous silica sols leading to lightly cross-linked P4VP/SiO2 nanocomposite microgels with a mass content of silica of around 35%. The silica nanoparticles had a size of 20 nm. The size of the nanocomposite microgels depends on the protonation of the 4vinylpyridine residues and is thus pH-dependent. For example, values for the hydrodynamic diameter of 230 nm at pH 8.8 instead of 550 nm at pH 2.5 demonstrate the pH-dependent swelling of these nanocomposite microgels. These particles were used as emulsifiers: stable o/w emulsions were obtained with methyl myristate and n-dodecane whereas w/o emulsions were found with 1-undecanol. Interestingly, the nanocomposite microgels stabilized emulsions only at high pH whereas the particles did not stabilize emulsions at low pH, leading to pH-sensitive emulsions. Obviously, the emulsions became unstable under conditions where the microgels are highly swollen.49,50 Properties of these nanocomposite microgels have been studied in detail, and they reveal a pH-dependent ζ potential, being positive at low pH and negative at high pH with an isoelectric point at ca. pH 6. The particles are strongly swollen at low pH, but the particle size stays constant above pH 4 and thus the charge reversal at pH > 6 does not lead to reswelling of the nanocomposite microgels.50 The particles flocculated near the isoelectric point, so the particles are colloidally stable either at low pH when strongly swollen and cationic or at high pH when collapsed and strongly anionically charged. The influence of pH and salt concentration on the stability of o/w emulsions employing the nanocomposite microgels as a sole emulsifier was also investigated and demonstrated that the particles are not adsorbed to the oil−water interface in their swollen form. From these results, one can conclude that the stable emulsions obtained with this system are similar to Pickering emulsions obtained with other rigid particles and these particles do not function as stabilizers when they are in the swollen microgel state.

Figure 3. (Top) Position of a small spherical particle at a planar oil− water interface for a contact angle θow (measured through the aqueous phase) of less than 90° (left), equal to 90° (center), and greater than 90° (right). (Bottom) Corresponding probable positioning of particles at a curved interface. For θow < 90°, solid-stabilized o/w emulsions may form (left). For θow > 90°, solid-stabilized w/o emulsions may form (right). Reprinted from ref 4 with permission from Elsevier.

preferentially wets the particle becomes the continuous phase of the emulsion. The surface properties of the particle can be manipulated chemically, and thus the interfacial properties of colloidal particles and consequently emulsion properties can be tailored. Various parameters such as size, shape, roughness, particle wettability, particle interactions, and surface coverage influence the effectiveness of the particle as a stabilizer and the interfacial energy; in addition, viscoelastic properties of the interface are relevant.40−43 Surface -active materials become compressed at the interface when the droplet volume is reduced (e.g., by desiccation or Ostwald ripening). Because particles are irreversibly attached to the interface, the volume reduction leads to a transition from a fluid interface, which is characterized only by surface tension, to a solid interface that possesses elastic moduli.44 The viscoelastic properties of the particle-covered interface depend strongly on the interaction between the particles,45 and finally shape and buckling transitions occur.46

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EMULSIONS STABILIZED BY STIMULI-SENSITIVE MICROGELS Stimuli-sensitive microgels with hydrodynamic diameters between 300 nm and 1 μm have been employed as stabilizers for emulsions only very recently.51−60 The stimuli sensitivity of the microgels can lead to responsive emulsions, and mainly two systems have been investigated in the literature: PNIPAM and PNIPAM-co-MAA (methacrylic acid) microgels. Obviously, the incorporation of acidic or basic repeating units into PNIPAM C


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Microgel flocculation allows for simple separation and recycling of the colloidal emulsifier, an important feature for many applications. The addition of salt did not influence the emulsion stability, which indicates that emulsion stability is not due to electrostatic repulsion between droplets. Salt, however, influences the colloidal stability of the microgels at temperatures above the VPTT and can thus be used to promote microgel flocculation. These results demonstrate that microgels are extremely interesting stabilizers for emulsions. These microgels are characteristically different from the nanocomposite microgels by Fujii et al. that were discussed above: at low temperature, the PNIPAM-co-MAA microgels are not collapsed in the uncharged state, and most importantly, the microgels lower the interfacial tension independently of pH.55 In other words, they adsorb to the interface independently of MAA deprotonation. Microgels obtained by copolymerizing NIPAM with 2aminoethylmethacrylate (AEM) become cationic at low pH. They are also swollen in the uncharged state by analogy to the PNIPAM-co-MAA microgels and also adsorb to the interface independently of the degree of protonation.61 Of course, the opposite influence of pH on the swelling of PNIPAM-co-AEM microgels, as compared to PNIPAM-co-MAA microgels, results in the formation of very stable emulsions at low pH. The behavior of the cationic PNIPAM-co-AEM microgels again reveals the difference as compared to the P4VP/SiO2 nanocomposite microgels discussed above. Both types of microgels are swollen at low pH; however, the PNIPAM-coAEM microgels stabilize emulsions at low pH whereas the P4VP/SiO2 nanocomposite microgels do not. Microgel particles can be visualized by confocal microscopy, and Figure 5 shows an example for an o/w droplet. The microgels are organized at the droplet surface, and excess microgels are present in the continuous water phase. It is, however, rather difficult to obtain detailed information on the microgel structure because of the inhomogeneous labeling of the microgel with the fluorescent dye and especially because of the different refractive indices of water and oil, respectively. Fluorescence labeling of microgels enables the dynamics of microgels at oil−water interfaces to be investigated by fluorescence bleaching experiments. Figure 6 shows the results obtained from heptane-in-water emulsions. An emulsion was prepared using fluorescently labeled microgels, and the sample was placed in a fluorescence microscope such that the droplet equator was in the focal plane. A small region at the interface (inside the white circle in Figure 6) was bleached, and fluorescent images were recorded afterward for 10 min. The aqueous phase contained excess microgels that revealed Brownian motion. (A video is provided as Supporting Information in the study by Liu et al.61) However, no recovery of fluorescence intensity was detected within the bleached area. This indicates that the mobility of the microgels at the interface is very small and that there is no exchange with excess microgels present in the water phase.61 The results presented above show that microgels are excellent stabilizer for emulsions. We now turn to the question of whether microgel-stabilized emulsions are just a different example of Pickering emulsions or if microgels are different from rigid particles. As mentioned above, PNIPAM-co-MAA microgels adsorb to the oil−water interface independently of pH, and oil droplets can be stabilized in the uncharged state, although the long-time stability is not as good as when the microgels are in the charged

microgels introduces pH sensitivity into the microgels and emulsions. On first sight, one might assume that the charged moieties could give rise to the electrostatic stabilization of emulsion droplets. However, as will be outlined below, the experimental results reported in the literature demonstrate that the stability of emulsion droplets stabilized by charged microgels cannot be explained by electrostatic stabilization. PNIPAM-co-MAA microgels are swollen by water at temperatures below the VPPT, and the deprotonation of the acidic groups leads to further swelling due to the osmotic pressure of the counterions inside the microgels.32 Thus, the composition of the microgel is relevant and depends on the conditions during microgel polymerization.54 Such microgels stabilize emulsions (mostly o/w emulsions) with oils of very different polarities; the droplet size and rheological properties depend on the oil/water ratio as well as the emulsification procedure, as is generally typical of emulsions. The microgels are dissolved in the aqueous phase, typically at concentrations of 0.01 to 1 wt %. Most importantly, the stability can be controlled via pH and temperature. Figure 4 illustrates this point with emulsions prepared with octanol and heptane as the oil.55

Figure 4. Emulsions prepared with temperature- and pH-sensitive microgels. (Top) Octanol-in-water emulsion. The emulsion decomposes after adding acid or by heating to well above the VPTT. (Bottom) A heptane-in-water emulsion prepared at pH 9.3 fully separated into oil, water, and a flocculated microgel after decreasing the pH to 2.8 and successive heating to 60 °C. The pink color originates from the fluorescent label that is incorporated into the microgel (adapted from Brugger et al.).55

Creaming is observed for some emulsions depending on the density of the oil, but the emulsion droplets are extremely stable (up to years) at low temperature especially at high pH when the microgels are strongly swollen. However, the emulsions can be broken by reducing the pH and elevating the temperature, and the sensitivity of the emulsion stability with respec to these parameters depends on the polarity of the oil. Figure 4 demonstrates that complete separation into oil and water phases can be achieved, and the microgels flocculate.55 D


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Figure 5. Confocal microscopy images from an octanol-in-water emulsion droplet stabilized by PNIPAM-co-MAA microgels with MAA located in the shell at pH 3. Images are taken at different vertical positions, as adapted from Schmidt.62

Figure 6. Confocal fluorescence micrograph of a microgel-stabilized heptane-in-water droplet after photobleaching. The white circle indicates the region of the droplet where the fluorescent dye, which is chemically attached to the microgels, was bleached. Adapted from Liu et al.61

state. Even pure PNIPAM microgels below their VPTT (i.e., in the swollen state) are surface-active; they adsorb to air−water and oil−water interfaces and lower the interfacial tension.63,64 Recently, the Bordeaux group published a very interesting series of papers reporting on emulsions stabilized by PNIPAM microgels.59,65,66 These emulsions were prepared via the technique of limited coalescence that uses fewer microgels as compared to the quantity used in other studies. It has the advantage that the number of excess microgels in the aqueous phase is strongly reduced. However, this often leads to very large droplets, the aggregation of drops, and flocculated emulsions. Although the microgels and the emulsion preparation techniques employed by Destribats et al. are different from those used in the studies by our group, very important similarities are observed. The most relevant feature observed in both systems is related to the structure of microgels at the oil−water interface. Figure 7 shows images obtained by means of cryogenic freeze−fracture scanning electron microscopy (cryo-SEM).56,59,67 All images show microgels at the surface of heptane droplets in water: PNIPAM-co-MAA microgels at pH 9 (top) and PNIPAM microgels (center) and PNIPAM core−shell microgels with MAA groups located in the core at pH 9 (bottom).

Figure 7. Cryo-SEM images from microgels at surfaces of heptane-inwater emulsion droplets: (a) PNIPAM-co-MAA microgels at pH 9;56 (b) PNIPAM microgels.59 (Reproduced by permission from The Royal Society of Chemistry). (c) PNIPAM core−shell microgels with MAA groups located in the core.67

These images can be compared to micrographs obtained by means of direct imaging cryo-transmission electron microscopy from microgels in aqueous solution as displayed in Figure 8. The different studies demonstrate three important features of microgels at the surface of emulsion droplets: (i) the microgels E


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be compared to the interfacial tension values reported by Brugger et al. for PNIPAM-co-MAA microgels: the interfacial tension was higher at higher pH.55,56 Thus, the lower interfacial tension was in both cases found under conditions where microgels are less swollen. However, emulsions were less stable under these conditions, which clearly indicates that the emulsion stability (as a function of temperature and/or pH) does not correlate with the interfacial tension. Instead, it has been proposed that the viscoelastic properties of the microgel-covered interface are relevant to emulsion stability. Georgieva et al. measured the elastic responses of various surfactant layers adsorbed at oil−water interfaces.41 They observed that the emulsion stability was better the higher the low-frequency elastic modulus of the layer was. Brugger et al. investigated the interfacial rheology properties of microgelcovered oil−water interfaces by means of the pendent drop technique as well as by shear deformation with double-wall ring geometry. They also observed the presence of a highly elastic interface, and the modulus was higher under conditions where stable emulsions were obtained.55,71 However, further studies are necessary that investigate the influence of the cross-link density on the viscoelastic properties of the interfacial microgel layer. The temperature-dependent viscoelastic properties are illustrated by videos given in the Supporting Information. Video S1 shows the behavior of microgel-stabilized emulsion droplets inside a microfluidic mixer. Videos S2 and S3 show results from pendent drop experiments. Figure 9 shows a

Figure 8. Cryo-TEM micrograph obtained from a PNIPAM microgel in bulk aqueous solution at 25 °C. The scale bar represents 100 nm.

are strongly deformed, (ii) the microgels interpenetrate, and (iii) the microgels are linked by filaments. It is obvious that the microgels are not rigid particles and that adsorption to the interface induces a significant deformation. Destribats et al. compared microgels with different content of cross-linker and concluded that increasing the degree of crosslinking reduced the stabilization efficiency.59 This agrees with the observation that PNIPAM-co-MAA microgels stabilize emulsions better when they are charged and thus more swollen as compared to when they are in the uncharged state. The core−corona type of microgel structure observed in cryo-SEM, together with the strong interpenetration and formation of filaments at the interface, can be related to the structure of such microgels in aqueous solution. Small-angle neutron scattering studies revealed that the cross-link density is higher in the core and decays with increasing distance from the center.15 Obviously, this softness and the fuzzy density profile with dangling chains at the surface enables the strong deformation at the oil−water interface. The similar behavior of the pure PNIPAM and the PNIPAMco-MAA microgels indicates again that the origin of enhanced emulsion stability in the latter case is not due to electrostatic repulsion between droplets but rather to other properties of the interface covered by soft and deformed microgels. Rheological properties of interfaces are known to influence emulsion stability.42,68 The contraction of the droplet surface eventually brings the particles into close proximity to each other, leading to a fluid-to-solid transition and a buckling transition once the particles jam together.46 One can easily imagine that the soft and porous nature of microgels as well as the deformability can lead to rather complex viscoelastic properties of the interface, which are relevant to the emulsion stability. The interfacial tension was measured by means of the pendent drop technique by different groups.55,56,64 The adsorption of microgels to the oil−water interface leads to a stronger reduction of the interfacial tension, as was reported for rigid nanoparticles and microparticles.69,70 Monteux et al. investigated the interfacial tension of PNIPAM microgels at the oil (decane)−water interface. Interestingly, the interfacial tension is lowered by the microgels at all temperatures with a minimum near the VPTT. The interfacial tension at temperatures well below the VPPT was higher than at temperatures well above the VPTT.64 This can

Figure 9. Irregular shape of a pendent water-in-heptane drop with PNIPAM microgels adsorbed to the interface. The drop was prepared at 35 °C (i.e., above the VPTT of the microgels).

picture of a pendent drop, which was prepared at 35 °C. Emulsion droplets are not stable at this temperature, and the pendent drop reveals an irregular shape. Video S2 shows how the rigidity of the interfacial layer strongly affects the shape of the droplet when the droplet volume is changed. Video S3 demonstrates the tremendous effect of temperature on the microgel-covered interface. The pendent drop reaches a regular shape upon cooling, indicating that the microgels are able to rearrange at the interface as compared to the packing at 35 °C. On first sight, this is rather surprising because one would expect that the microgels jam even more at the interface because they swell in bulk solution upon cooling. However, the softness and deformability of the microgels are also temperature-dependent. F


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considered as a dispersion of a concentrated (and thus viscous) microgel suspension in oil rather than a water-in-oil emulsion.55 Recently, two publications reported on water-in-oil emulsions stabilized by PNIPAM-based microgels, both with octanol as the oil phase.65,67 The two studies are concerned with slightly different microgel systems (MAA-containing PNIPAM microgels and pure PNIPAM microgels, respectively), and different routines for emulsion preparation were used. Nevertheless, the most important general findings are rather similar. Figure 10 shows some cryo-SEM images from these studies.

It is obvious that emulsion stability depends on the viscoelastic properties of the microgel layer at the interface. The interfacial viscoelasticity is connected to the structure and interactions of these soft and deformable microgel particles. However, quantitative data that allow us to correlate the frequency-dependent storage and loss moduli of the interface with the modulus of microgels and with the interaction between microgels are not yet available. Also, the nonlinear rheological properties and shape and buckling transitions are hardly explored. One can expect that soft microgels behave very differently from rigid particles.

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CORE−SHELL MICROGELS The difference between microgels and rigid particles is also important when particles with core−shell morphology are discussed. The preparation of core−shell particles is well known in colloid science, and shell formation allows the surface properties of the particles to be modified. Consequently, it is the surface that determines the stabilization behavior of rigid core−shell particles in Pickering emulsions. With microgels, however, the interfacial behaviors also depend on the internal properties of the solvated, swollen particles. Recently, Schmidt et al. prepared pH- and temperaturesensitive PNIPAM-co-MAA core-shell microgels with different morphologies: one with a neutral P(NIPAM) core and MAAcontaining shell (called an MS microgel) and the other with an MAA-containing core and neutral P(NIPAM) shell (called an MC microgel).67 The influence of microgel architecture on the pH-dependent properties of such microgels has already been mentioned in Figure 2. Heptane-in-water emulsions were prepared with these two types of microgels under two different pH conditions with the microgels uncharged (pH 3) or charged (pH 9). Emulsions prepared at pH 3 were of limited stability, similar to what was observed for pure PNIPAM microgels,54 whereas the emulsions are very stable at pH 9. The emulsion stabilized with the MC microgel remained stable at high temperature at pH 9, whereas the emulsion broke at pH 3. Obviously, the deprotonation of the MAA units strongly enhances the emulsion stability even for the MC microgel. This is rather surprising because the pH does not significantly influence the effective charge of the MC microgels as observed in electrophoretic mobility. As already mentioned above, most of the counterions are located inside the core of the microgel and thus the MC microgels have almost zero effective charge.67,72 However, the increased osmotic pressure due to the counterions leads to enhanced swelling, which affects the emulsion stabilization efficiency. These experiments demonstrate again the fundamental difference between soft microgels and rigid particles.

Figure 10. Cryo-SEM images from water-in-octanol emulsion droplets. (Top) The panel shows four images of w/o emulsion droplets at pH 3 stabilized by PNIPAM-co-MAA microgels with different morphologies: MS microgels with MAA located in the shell (top) and MC microgels with MAA located in the core (bottom); images with two different magnifications are shown. Adapted from ref 67. The bottom panel shows images from a water-in-octanol drop stabilized by PNIPAM microgels: large view of the drop and closer views of the interfaces. Adapted from ref 65.

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WATER-IN-OIL (W/O) EMULSIONS Most of the microgel-stabilized emulsions reported in the literature are of the oil-in-water type (o/w), which agrees with empirical rules that the outer, continuous phase of an emulsion usually is the phase in which the stabilizer is more soluble (Bancroft rule) or preferentially dispersed (Finkle rule). The microgels discussed above are typically strongly swollen in water and are expected to stabilize o/w emulsions. Water-inheptane emulsions were found at low water-to-heptane ratios and rather high microgel concentrations. Strong droplet aggregation was observed, and such systems can be best

Most importantly, both studies reveal that no microgels were found in the continuous octanol phase. Instead, all microgels were found at the octanol−water interface and inside the water droplet. This clearly shows that water is the preferred phase for the microgels and thus the water-in-octanol emulsions can be regarded as anti-Bancroft or anti-Finkle. Both studies mention the different behavior of n-octanol as compared to that of nonpolar oils, especially with respect to the interaction with the microgels. Destribat et al. investigated in G


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great detail the behavior of PNIPAM microgels in the presence of water and octanol, revealing a progressive uptake of octanol by microgels.65 Obviously, the uptake of octanol alters the properties of the microgels, enabling them to stabilize w/o emulsions. Nevertheless, the microgel always remained in the aqueous phase, in agreement with other reports.54 The PNIPAM-microgel-stabilized w/o emulsions reported by Destribats et al. were stable for a long time on standing but were sensitive to mechanical compaction. Interestingly, they remained stable upon heating to 50 °C, thus these w/o emulsions were not thermoresponsive. As a working hypothesis, the authors speculate about the relevance of hydrogen bonding between microgel segments and octanol, which can influence the amphiphilicity of the polymer as well as the deformability of the microgels.65 The investigations by Brugger et al. and Schmidt et al. employing MAA-containing PNIPAM microgels with different internal structures also revealed different interactions of the microgels with octanol as compared to those of nonpolar oils.54,55,67 These microgels also take up octanol and are even swollen by octanol in the uncharged state, indicating that the presence of protonated MAA groups enhances the interaction with octanol most probably via hydrogen bonding. The w/o emulsions stabilized by the MAA-containing microgels are, however, sensitive to temperature. The w/o emulsions phase separated when heated to 60 °C. The microgels migrated to the octanol phase when the aqueous phase was at pH 3. However, the MC microgels did not migrate to the octanol phase when the aqueous phase was at pH 9.67 This indicates that the uncharged shell enabled the formation of the w/o emulsion at low temperature, but the charged core prevents partitioning to the octanol phase at high temperature when the emulsion breaks.

Figure 11. Emulsion stabilized with microgels that contain magnetic nanoparticles: the left image shows that the droplets can be separated with a magnet; the right image shows the phase-separated system caused by heating when the emulsion was exposed to an alternating magnetic field. Adapted from Brugger et al.73

the anionic microgels were labeled with Sudan yellow, and droplets stabilized by the cationic microgels were labeled with Sudan blue. Then the two emulsions were mixed, and the optical micrograph in Figure 12 demonstrates that these

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NEW OPPORTUNITIES OF MICROGEL-STABILIZED EMULSIONS FOR APPLICATIONS Finally, we refer to two studies illustrating the great opportunities for microgel-stabilized emulsions. Microgels can be used as nanoreactors for the synthesis of nanoparticles inside the microgels, and such hybrid microgels also stabilize emulsions. Brugger et al. employed microgels based on poly(N-isopropylacrylamide) with incorporated magnetic iron oxide nanoparticles (MNP).73 Emulsions stabilized by magnetic nanoparticles are of great interest74,75 because they allow for manipulation via magnetic fields. These microgels are very efficient stabilizers for emulsions when in the swollen state, in contrast to the P4VP/SiO2 nanocomposite particles mentioned above. The properties on the PNIPAM/ MNP microgels with respect to emulsion stabilization are very similar to microgels without the nanoparticles discussed above. However, the presence of the nanoparticles leads to the possibility of remotely controlling droplet separation and emulsion stability via magnetic fields as shown in Figure 11.73 Liu et al. used the unique properties of microgels as emulsion stabilizers to prepare stable emulsions with oppositely charged droplets.61 Anionic and cationic microgels were synthesized by copolymerizing NIPAM with either methacrylic acid or 2aminoethylmethacrylate, respectively. At intermediate pH 7, both microgels are charged but oppositely, and thus stable o/w emulsions can be prepared with both types of microgels. Heptane-in-water emulsions stabilized by the cationic and anionic microgels were separately prepared at pH 7, and the droplets were labeled with different dyes. Droplets stabilized by

Figure 12. Optical micrograph of an n-heptane-in-water emulsion prepared by mixing an emulsion stabilized by cationic microgels (oil phase colored with Sudan blue) with an emulsion stabilized by anionic microgels (oil phase colored with Sudan yellow). Adapted from Liu et al.61

emulsions remain stable upon mixing. The experiments showed that no coalescence of droplets occurred. Furthermore, there were no indications for significant aggregation of droplets although they are oppositely charged. Liu et al. performed additional microfluidic experiments and observed that the oppositely charged droplets did not coalesce even when squeezed together in the Y junction of a microfluidic device.61 Instead, the droplets bounced off each other. However, droplet coalescence was observed at low and high pH (i.e., when one microgel species was not charged). The results again show that electrostatic interactions do not determine droplet coalescence. Instead, the pH -dependent emulsion stability correlates well with the above-mentioned elastic behavior of microgel-covered interfaces. The results H


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equilibrium shape of the microgel particles is given by the balance among the solvation of the hydrogel in the two liquid phases, its interfacial activity, and the internal elasticity of the particle. Geisel et al. argued that this can lead to a particle conformation as sketched in Figure 13. The polymer corona is lying flat at the oil−water interface, and the microgel is stretched and protrudes only a little into the oil phase.

further demonstrate the very peculiar properties of stimulisensitive microgels with respect to preparing emulsions that can be broken on demand.

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MICROGELS AT FLAT INTERFACES The special properties of microgels as stabilizers for emulsions are far from completely understood. Thus, experiments at flat oil−water interfaces will be extremely important. Very recently, Geisel et al. studied the assembly of MAA-containing PNIPAM microgels at a flat heptane−water interface by means of freezefracture shadow-casting cryo-SEM (FreSCa).76 This technique involves the preparation and freezing of millimeter-sized planarparticle-laden oil/water interfaces that are subsequently exposed upon fracturing and imaged with cryo-SEM. Threedimensional information on the particle position relative to the interface is obtained by coating the fractured interface with a thin tungsten layer at a 30° angle relative to the interface. Features protruding from the interface, such as colloidal particles, leave a shadow behind them; by directly measuring the particle size at the interface and the shadow length, one can calculate the vertical position of individual nanoparticles at the interface with subnanometer accuracy.77 Two different microgels were investigated: (i) a PNIPAM-coMAA microgel and (ii) a core−shell microgel with a PNIPAMco-MAA core and a PNIPAM shell. Figure 13 shows a FreSCa

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CONCLUSIONS Recent studies published by various groups on different systems demonstrate unambiguously that microgel-stabilized emulsions have rather unique properties and that microgels behave characteristically different from classical rigid-particle Pickering stabilizers. Microgels are strongly swollen, lyophilic particles that become deformed at the oil−water interface and protrude only a little into the oil phase. The deformability is an important feature controlling the emulsion stability. Temperature- and pH-sensitive microgels allow us to prepare temperature- and pH-sensitive emulsions and thus enable emulsions to be prepared and broken on demand. Although such emulsions are sensitive to pH, the stabilization of droplets is not due to electrostatic repulsion. Instead, the viscoelastic properties of the interface seem to dominate the droplet stability. Microgels of different temperature-sensitive polymers have been employed as emulsifiers in addition to the PNIPAM-based systems described above: poly(N-vinylcaprolactam) (PVCL) microgels were used by Berger et al., 57 and poly(Nisopropylmethacrylamide) (PNIPMAM) and poly(N,N-diethylacrylamide) (PDEAAM) microgels were used by our group.80,81 This shows that the results discussed in this feature article are not unique to PNIPAM but might be generic. However, the repeat units of all of these polymers contain hydrophilic as well as hydrophobic moieties and this might be relevant to the adsorption to oil−water interfaces. It still has to be explored whether other polymer microgels can be employed for emulsion stabilization. Oil-in-water emulsions can be prepared with many oils, and water-in-oil emulsions have been reported mainly for fatty alcohols. The microgels always prefer the aqueous phase over the oil phase, thus the w/o emulsions are of the anti-Bancroft or anti-Finkle type. Figure 14 provides a schematic of microgels at the surface of emulsion droplets as an attempt to indicate the differences as

Figure 13. FreSCa cryo-SEM image of a core−shell microgel with a PNIPAM-co-MAA core and a PNIPAM shell at a water/heptane interface at pH 3. The scheme at the bottom shows the deformation of the microgels at the interface and the protrusion height h. di denotes the diameter of the microgel at the interface (as obtained from FreSCa cryo-SEM), and dw denotes the hydrodynamic diameter in bulk water (as obtained from dynamic light scattering).76

image of the core−shell microgel at pH 3. A significant deformation of the particles, comprising pronounced flattening and stretching of the polymer corona at the interface, was found. Interestingly, Geisel et al. did not observe an influence of pH on the size that the microgels take up at the interface or on the protrusion height into the oil phase, although the microgel size in water and the emulsion stability depend on the pH.76 Soft particles or particles with soft shells can deform at an interface, and preferentially adsorbing surface chains stretch out at the interface.78,79 Microgels can deform fully, and the

Figure 14. Schematic of microgels at the surface of emulsion droplets.

compared to the behavior of rigid particles in classical Pickering emulsions as shown in Figure 3. Several points should be noted: the microgels, which are spherical in solution, become deformed at the interface. In particular, they are strongly flattened in the case of o/w emulsions. The degree of deformation seems to be smaller for w/o emulsions, but I


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further cryo-SEM studies are required for a better understanding. The microgels are located mainly within the aqueous phase for both o/w and w/o emulsions, which is clearly different from the classical behavior of rigid particles in Pickering emulsions as shown in Figure 3. However, the exact location of the oil−water interface is not clear. In contrast to rigid particles, the oil−water interface could be located slightly inside the microgel, especially in the case of octanol-in-water emulsions. In contrast to rigid, solid particles, the behavior of microgels at oil−water interfaces does not depend only on the interfacial tension. Instead, the balance among the interfacial tension, swelling, elasticity, and deformability of the microgel needs to be considered. Thus, a full theoretical description of microgels at fluid interfaces is rather challenging, but we hope that the experimental findings will stimulate theoreticians to study such systems. The results of several different studies discussed above demonstrate that microgel-stabilized emulsions and capsules57,82 are unique systems. They have a tremendous potential for many different applications that need to be explored in the future. It is obvious that microgels as soft, porous particles are significantly different from classical Pickering emulsions, and the term “Mickering“ emulsion was used in a previous publication as an attempt to symbolize the differences and analogies to Pickering emulsions. Emulsions stabilized by soft microgels require the development of new theoretical models to understand their stability and open the door to new applications.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Three videos: (S1) The flow of microgel-stabilized heptane-inwater emulsion droplets inside a microfluidic mixing channel illustrating the highly elastic interface. (S2) A pendent drop experiment at 35 °C, which is above the VPTT of the microgels. The microgel layer at the water−oil interface is very rigid, leading to a strongly distorted shape of the droplet. (S3) A pendent drop experiment; the water−oil interface is covered with microgels. The droplet is cooled from 35 °C (video S2) to 25 °C, illustrating the influence of temperature on microgel solubility and interfacial properties. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +49 241 80 94760. Fax: +49 241 80 2327. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support by the Deutsche Forschungsgemeinschaft, the Collaborative Research Center “Functional Microgels and Microgels Systems“ (SFB 985), and the excellence initiative of the German federal and state governments is gratefully acknowledged. We thank the DWI, Aachen, for the support with the cryo-SEM studies, the group of D. A. Weitz, Harvard, for the collaboration concerning the behavior of emulsions in microfluidic devices, and D. Danino, Technion Haifa, for the cryo-TEM image. J


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