Influence of Microgel Architecture and Oil Polarity on Stabilization of

ARTICLE pubs.acs.org/Langmuir

Influence of Microgel Architecture and Oil Polarity on Stabilization of Emulsions by Stimuli-Sensitive Core
Shell Poly(N-isopropylacrylamideco-methacrylic acid) Microgels: Mickering versus Pickering Behavior? Sabrina Schmidt,† Tingting Liu,† Stephan R€utten,‡ Kim-Ho Phan,‡ Martin M€oller,‡ and Walter Richtering*,† † ‡

Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, D-52056 Aachen, Germany DWI e.V. and Institute of Technical and Macromolecular Chemistry, RWTH Aachen University, Pauwelstr. 8, D-52074 Aachen, Germany

bS Supporting Information ABSTRACT: Charged poly(N-isopropylacrylamide-co-methacrylic acid) [P(NiPAM-co-MAA)] microgels can stabilize thermoand pH-sensitive emulsions. By placing charged units at different locations in the microgels and comparing the emulsion properties, we demonstrate that their behaviors as emulsion stabilizers are very different from molecular surfactants and rigid Pickering stabilizers. The results show that the stabilization of the emulsions is independent of electrostatic repulsion although the presence and location of charges are relevant. Apparently, the charges facilitate emulsion stabilization via the extent of swelling and deformability of the microgels. The stabilization of these emulsions is linked to the swelling and structure of the microgels at the oil
water interface, which depends not only on the presence of charged moieties and on solvent polarity but also on the microgel (core
shell) morphology. Therefore, the internal soft and porous structure of microgels is important, and these features make microgel-stabilized emulsions characteristically different from classical, rigid-particle-stabilized Pickering emulsions, the stability of which depends on the surface properties of the particles.

’ INTRODUCTION Stimuli-sensitive emulsions have attracted much research attention in recent years. They hold promising application potential in drug delivery, controlled catalytic reactions, and many other processes in which triggering emulsion destabilization by external conditions is necessary or advantageous.1
10 A convenient way to produce this kind of emulsions is to use stimuli-sensitive microgels as emulsifiers. Poly(N-isopropylacrylamide-co-methacrylic acid) [P(NiPAM-co-MAA)] microgels are both temperature and pH-sensitive. They can adsorb to the oil
water interface and have been employed as stabilizers for emulsions using heptane and octanol as model oils.11
14 The microgels can be charged or uncharged depending on whether the methacrylic acid (MAA) groups are deprotonated or protonated at the given pH. In turn, the stability of the emulsions prepared with these microgels can be switched from one state to another.15
19 Previous studies have shown notable differences between the interfacial behaviors of P(NiPAM) microgels and those of common Pickering stabilizers that are rigid particles.20
23 When the charge density of pH-sensitive rigid particles is increased, the particles leave the interface, followed by destabilization of the emulsions.24 On the contrary, studies so far have shown that P(NiPAM) microgels require charges to produce stable emulsions:18 in this case, the charges do not lead to desorption of the particle-stabilizers but to increased interparticle distance r 2011 American Chemical Society

on the interface. This is accompanied by a formation of clusters, a structure proven to provide a considerable increase of the elasticity of the interface, compared to uncharged microgels that pack densely at the interface leading to brittle interfaces.16,17 It remains unclear if the presence of charges is truly necessary for the emulsion stability and through what mechanism they provide the stability. Recent experiments with oppositely charged microgels indicated that the Coulomb repulsion between microgels is not required for emulsion stability. Thus, it might be that other effects of charges, for example, the deformability of microgels at the oil
water interface, are responsible for emulsion stability. The presence of charges and thus of counterions inside microgels leads to enhanced swelling and thus to a softer particle which will affect the viscoelastic properties of the interface. To clarify the influence of charges, we prepared two P(NiPAM-co-MAA) microgels with different architectures: one with a neutral P(NiPAM) core and MAA-containing shell (MS microgel) and the other with a MAA-containing core and neutral P(NiPAM) shell (MC microgel). Emulsions with water and heptane or water and 1-octanol were prepared with these two types of microgels under two different pH conditions with the microgels uncharged (pH 3) or charged (pH 9). The structure of Received: May 15, 2011 Revised: July 5, 2011 Published: July 08, 2011 9801

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microgels at the oil
water interface was visualized by means of cryogenic field emission scanning electron microscopy (CryoFESEM) to study how the architecture of the microgels and their charge properties influence the emulsion stability. Understanding this mechanism is fundamental to designing stimuli-sensitive emulsions with desired properties.

’ EXPERIMENTAL SECTION Materials. Methacryloxyethyl thiocarbamoylrhodamine B (MRB, Polysciences, Inc.), 1-octanol (99%, Gr€ussing), n-heptane (>99%, Merck), potassium persulfate (99%, Merck), N-isopropylacrylamide (NiPAM) [stabilized with 500 ppm 4-methoxyphenol (MEHQ 99%), Acros Organics], N,N0 -methylene-bis-acrylamide (BIS, molecular biology grade, Applichem), and methacrylic acid (MAA, 99%, stabilized with 100
250 ppm hydroquinone, ABCR Lancaster Avocado) were used as received; doubly distilled Milli-Q-water (bidistilled water) was used in all procedures. Microgel Synthesis. The two microgels used in this paper were synthesized by surfactant free emulsion polymerization (SFEP) in a semibatch synthesis to obtain microgels with a core
shell structure. The synthesis was performed in a 1000 mL three-necked glass reactor equipped with a reflux condenser, a mechanical stirrer, and a gas inlet. For each of the microgels, two monomer solutions were prepared, for the microgel core and the shell respectively. For the MS microgel, MAA was added in the shell solution; for the MC microgels, MAA was added in the core solution. The detailed quantities of the reactants are shown in Table S1 of the Supporting Information. After the core solution was heated up to 70 °C and purged with nitrogen for 1 h, the initiator was added. Fifteen minutes after the first turbidity of the reaction mixture appeared, the shell solution was added with a speed of 6.67 mL/min using a syringe pump (Kd Scientific). After the addition was complete, a second initiator solution was injected to the reaction mixture and the polymerization was left to proceed for 5 h. The final reaction mixture was filtered through glass wool to remove particulate matters and further purified by three centrifugation cycles at 30 000 rpm for 20 min at 20 °C. Each centrifugation step was followed by decantation and redispersion in bidistilled water. Characterization of the MS and the MC Microgels. The weight percentage of MAA was determined by pH titration. A known mass of microgels was dispersed in bidistilled water overnight. Excess HCl was added to neutralize all the charges of the microgels. Subsequently, the sample was titrated with 0.1 M NaOH under nitrogen atmosphere. The titration was performed with a Metrohm autotitrator. The hydrodynamic radius (Rh) of the microgels was determined by dynamic light scattering (DLS) in an ALV-5000 instrument with a light of wavelength of 632.8 nm. Highly dilute samples were prepared using syringe filters in a flow box to avoid dust contamination. Electrophoretic mobility measurements were performed using a NANO ZS zetasizer (Malvern Instruments, U.K.). The voltage applied for the measurement in the disposal flow cell was 150 V. The characterization data of the microgel are provided in Figures S1 and S2 in the Supporting Information. Preparation and Observation of Emulsions Made by UltraTurrax. Oil and aqueous microgel dispersions at certain pH were added together at the desired ratio in a sample glass. The total volume of the mixture was 10 mL. We used an IKA Ultra-Turrax T-25 instrument with a 10 mm head to mix the two phases at 8000 rpm for 2 min. The emulsion type was examined with a Leica microscope with the assistance of an oil soluble dye Sudan Blue. Preparation of Cryo-FESEM Samples. The Cryo-FESEM images of the microgel-stabilized emulsions were taken in a Hitachi S-4800 electron microscope. This device is equipped with a vacuum sample chamber with temperature control. The chamber is connected to

Figure 1. Schematic drawing of the core
shell microgels with different architectures. Left: P(NiPAM)-core/P(NiPAM-co-MAA)-shell (MS). Right: P(NiPAM-co-MAA)-core/P(NiPAM)-shell (MC). The red color indicates the fluorescence label in the microgels introduced by the dye MRB. The negative charges illustrate the MAA groups at pH 9; the counterions are not shown. the microscope, so that the sample can be observed in the same environment as in the chamber. A drop of the emulsion (∼10 μL) was added into a cylinder copper cuvette. The volume of the drop is slightly larger than that of the cuvette, and therefore, an excess of liquid is held at the top of the cuvette by surface tension. The sample was frozen in liquid nitrogen and transferred into the sample chamber (T =
140 °C, p = 10
6 Pa), where a part of the frozen drop on top of the cuvette was cut away with a razor blade. The temperature in the chamber was raised to
90 °C to create a clean surface. After 5
10 min, the sample was cooled again to
140 °C and maintained at this temperature during the observation. Fluorescence Lifetime. The microgels were labeled with a fluorescent dye methacryloxyethyl thiocarbamoylrhodamine B (MRB). The solvent dependent fluorescence lifetime of the dye incorporated in the microgels was measured using a MicroTime200 fluorescence spectroscopy and microscopy system (PicoQuant). A pulsed laser with a wavelength of 532 nm was used. A detailed description was given by M€uller et al.25 All measurements were performed at room temperature. For each sample, five measurements for duration of 1 min each were made. As solvents, water (chromatography grade), 1-octanol (99%), and their saturated mixtures were used.

’ RESULTS AND DISCUSSION Two types of core
shell P(NiPAM-co-MAA) microgels were synthesized by semibatch precipitation polymerization:26 MAA was added either at the beginning or at the end of the polymerization. Thus, one microgel has a neutral P(NiPAM) core with MAA-containing shell (MS) and the other has an MAA-containing core with a neutral P(NiPAM) shell (MC); see Figure 1. The core
shell structure of the microgel was confirmed by dynamic light scattering (DLS) and electrophoretic mobility measurements (details are summarized in the Supporting Information). Both microgels are pH-sensitive: at pH 3, both of them are uncharged; at pH 9, both of them are charged. The charges are located on the microgel surface in the case of the MS microgel, but are in the core of MC microgels. The neutral P(NiPAM) shell of the latter (MC) leads to a vanishing zeta potential. Nevertheless, the osmotic pressure of the counterions present inside the core of this microgel at pH 9 still lead to stronger swelling, as revealed by the increase in its hydrodynamic radius (Rh = 610 nm at pH 3; Rh = 785 nm at pH 9). Emulsions with these two types of microgels can be prepared with a high speed mechanical stirrer (Ultra-Turrax). Two types of oils were employed: n-heptane as an example of nonpolar oil; 9802

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and 1-octanol as one of polar oil. The emulsion type is determined by optical microscopy (listed in Table 1); images obtained by confocal fluorescence microscopy are shown in the Supporting Information. When heptane is used as oil, stable emulsions are only obtained with lower oil
water ratio and pH 9. Both microgels form oil-in-water (O/W) emulsions. At this condition with octanol as oil, the emulsion type differs for the two microgels: the MS microgel forms an oil-in-water (O/W) emulsion, and the MC microgel forms a water-in-oil (W/O) emulsion. The structures formed by the microgels at the interface are investigated in detail by Cryo-FESEM. In the following, we will discuss the properties of the emulsions with respect to three aspects: (i) the oil type; (ii) whether the microgels are charged, that is, whether the pH is 9 or 3; and (iii) the location of charges inside the microgel. N-Heptane Emulsions. Neither of the MAA-containing P(NiPAM) microgels (MS or MC) stabilize heptane emulsions when they are uncharged (pH 3). The same behavior has been reported for pure P(NiPAM) microgels.18 It is only when the MAA units are charged (pH 9) that oil-in-water (O/W) emulsions are obtained.16 The emulsions formed by both core
shell Table 1. Emulsion Types of the Heptane
Water and Octanol
Water Emulsions at Different pH and Oil
Water Volume Ratio emulsion type oil type

pH

oil
water volume ratio

MS microgel

heptane

pH 9

2:8

O/W

O/W

octanol

pH 9

2:8

O/W

W/Oa

4:6

O/W

W/O/W

6:4

O/W

W/O

8:2

W/O

W/O

2:8

W/O

W/O

6:4

W/O

W/O

8:2

W/O

W/O

pH 3

a

MC microgel

The sample consists partially of water-in-oil-in-water (W/O/W) multiple emulsion droplets.

microgels show the same pH sensitivity: at pH 3, when they are uncharged, no stable emulsion is obtained; whereas at pH 9, stable O/W emulsions are formed. Cryo-FESEM images of MS and MC microgels on the droplet interfaces in these stable emulsions are shown in Figure 2. It can be recognized that both microgels form clusters at the interface and there is defined space between the clusters, indicating balanced attractive and repulsive forces between the interfacial microgels (Figure 2a and d). In both cases, there are regions where microgels are ordered (see, e.g., Figure 2b) and regions where the microgels are clustered (see, e.g., Figure 2e). However, there is no qualitative difference between the structures formed by MS microgels and those formed by MC microgels. Both structures are the same as observed before for P(NiPAM-coMAA) microgels prepared with batch synthesis and thus with a more homogeneous distribution of the MAA groups.19 It is not surprising that MS microgels behave this way; however, it is unexpected that MC microgels do. According to electrophoretic mobility measurements, there are virtually no surface charges in the MC microgels due to the neutral shell (Figure S2, Supporting Information). Nevertheless, the presence of charges in the microgel core still enables the MC microgels to stabilize heptane-water emulsions. The MC microgels even form similar interfacial structures as the surface-charged microgels (MS). This result strongly suggests that the stabilization of these emulsions is not due to electrostatic repulsion between the interfacial microgels on different droplets. However, the observation that microgels that do not carry charges at all (both MS and MC microgels at pH 3) do not stabilize emulsions indicates that the presence of charges is necessary for emulsion stability. The most plausible explanation is that the presence of the charges and counterions in both microgels influences the deformability, thus the viscoelasticity of the interface,17,18 no matter whether the charges and counterions are in the shell (MS) or in the core (MC). This unexpected behavior of MC microgels clearly demonstrates that even microgels without surface charges can be employed as stabilizers if they are appropriately modified in the core. One could expect that the Coulomb repulsion between MS microgels at high pH leads to a different packing at the interface as compared to that between MC microgels. Indeed the pictures

Figure 2. MS and MC microgels on the droplet surface of n-heptane-in-water (O/W) emulsions at pH 9. (a
c) MS microgels on the surface of an O/W emulsion droplet, overview and close-up images; the white line indicates the distance between two interfacial microgels, d = 1.1 μm. (d
f) MC microgels on the surface of an O/W emulsion droplet, overview and close-up images. The white circles show the hydrodynamic diameters of the microgels in water at pH 10. 9803

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Table 2. Hydrodynamic Radius and Fluorescence Lifetime of the Microgels in 1-Octanol, Water, and Their Saturated Mixtures hydrodynamic radius (nm)a fluorescence lifetime (ns) solvent

MS microgel MC microgel

MS microgel

668 ( 12

714 ( 23

1.98 ( 0.05

octanol saturated 530 ( 32

603 ( 35

2.11 ( 0.01

water (pH 10) water (pH 10) water saturated

563 ( 21

765 ( 26

2.79 ( 0.02

octanol octanol

790 ( 110

782 ( 72

2.96 ( 0.02

a

The viscosity and refractive index of the solvents are listed in Table S2 in the Supporting Information.

in Figure 2 indicate that MC microgels form larger clusters, packing less homogeneously as compared to MS microgels. However, this difference is difficult to quantify without a large number of images from different droplets. A striking difference between emulsions stabilized by microgels and classical Pickering emulsions with rigid particles can be seen in the images with the highest magnification (Figure 2c and f). These images show that the interfacial microgels are strongly deformed and there are interconnecting filaments between them. Apparently, these deformed microgels cover the interface more efficiently than rigid particles. 1-Octanol Emulsions. Next we discuss the structure of the P(NiPAM-co-MAA) microgels at the 1-octanol
water interface. The polarity of the oil is known to influence the emulsion properties drastically. Octanol
water emulsions stabilized by P(NiPAM-co-MAA) microgels have been found to be more sensitive to pH change than heptane emulsions.18 In addition, both microgels are soluble in octanol; thus, the affinity of the microgels to the octanol
water interfaces might be different from that to the heptane
water interfaces. Table 2 shows the change in the hydrodynamic radius of the two microgels as well as the change in the fluorescence lifetime of MS microgels in different solvent environments. The results on the size of the microgels show that both water and octanol swell the microgels. This means that the microgels at the interface can be penetrated by both water and octanol, in contrast to the case for heptane, where only water swells the microgels. The microgels are labeled with a fluorescence dye MRB. It is known that the lifetime of the fluorescence depends on the chemical environment which the fluorophore is in. The fluorescence lifetime of MS microgels in different solvents shows a consistent increase with increasing amount of octanol, indicating the increasing penetration of the microgels by octanol. As expected, the properties of octanol
water emulsions are notably different from those of heptane
water emulsions. At pH 9, MS microgels form O/W emulsions (Figure 3a and b), similar to the heptane-in-water emulsions discussed above. The distance between two microgels, as illustrated by the white line in Figure 3b (1.5 μm), is larger as compared to that in the heptane
water emulsions (Figure 2c, 1.1 μm), indicating enhanced repulsion at the octanol
water interface. At the same pH, the MC microgels show different behaviors: a water-in-oil (W/O) emulsion is obtained. Apparently the different location of charges inside the microgels influences the affinity of the microgel to water and oil, and this might lead to a switch of

Figure 3. MS and MC microgels on the droplet surface of 1-octanol
water emulsions at pH 9. (a, b) MS microgels on the surface of an O/W emulsion droplet, overview and close-up image; the dashed lines show the hydrodynamic diameter of the microgels in 1-octanol saturated water at pH 10; the white line indicates the distance between two interfacial microgels, d = 1.5 μm. (c, d) MC microgels on the surface of a W/O emulsion droplet, overview and close-up image. The solid white circle shows the hydrodynamic diameter of the microgels in water at pH 10.

emulsion type in the octanol
water system. Due to the charges in the shell, the MS microgel will have a stronger affinity to the aqueous phase as compared to the MC microgel and the MS microgel can form O/W emulsion whereas the MC microgel forms W/O emulsions. The formation of W/O emulsion with the MC microgels can be considered as an anti-Bancroft behavior.27,28 Although the MC microgels are soluble in octanol, one observes no microgels in the oil phase. The microgels are confined inside the droplets, generating a densely packed interface (Figure 3c and d). The formation of an octanol
water emulsion at pH 3 is even more unexpected. At this pH, both microgels are uncharged. Note that no heptane
water emulsion can be formed at this pH with either microgel. Furthermore, pure P(NiPAM) microgels with no dissociable group cannot form stable octanol
water emulsions either.18 In contrast to these two cases, stable water-inoctanol (W/O) emulsions are obtained with both MAA-containing microgels, MS and MC (Figure 4). This finding suggests that the presence of the MAA groups enables the stabilization of octanol emulsions, even if they are not deprotonated. To explain this, one needs to consider that the pure P(NiPAM) microgels from the previous study do not dissolve in octanol, whereas the P(NiPAM-co-MAA) microgels in this study do. This indicates that the presence of the MAA increases the swelling of the microgels by octanol and thus enhances the stability of the interface. Figure 4 displays Cryo-FESEM images from water-in-octanol (W/O) emulsions formed at pH 3 by the MS and the MC microgels. For these two emulsions, it was possible to obtain images visualizing the cross section of the droplets. It can be seen that both microgels are confined and concentrated in the water droplets. The interfacial MS microgels seem to be located mainly inside the aqueous inner phase. The side of the microgels that is attached to the interface is deformed in a rather elongated instead of flattened shape (Figure 4b and c), demonstrating again that the microgels as soft and porous particles behave differently from rigid particles in classical Pickering emulsions. 9804

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Figure 4. MS and MC microgels on the droplet surface of water
1-octanol (W/O) emulsions at pH 3. (a
c) MS microgels on the surface of a droplet, overview, cross section, and close-up image. (d
f) MC microgels on the surface of droplets, overview, cross section, and close-up image. The white circles show the hydrodynamic diameters of the microgels in water.

’ CONCLUSIONS The characteristics of emulsions prepared with core
shell P(NiPAM-co-MAA) microgels reveal the peculiar behaviors of soft polymer particles at oil
water interfaces. In the case of the heptane
water system, no stable emulsion is obtained at pH 3, when the microgels are not charged. At pH 9, when they are charged, stable emulsions are obtained, even if there are no surface charges (as in the case of a core-charged microgel, MC). These observations clearly show that the presence of charges in the microgel is important for obtaining stable emulsions, whereas the location of charges (i.e., volume vs surfaces charges) is not relevant. This also demonstrates that emulsion stability is not due to electrostatic repulsion. As already mentioned above, this behavior demonstrates a distinct difference between microgel stabilized emulsions and classical Pickering emulsions, where the surface properties of the rigid particles are decisive. In the case of microgels, the internal morphology is relevant as well. The octanol
water system is more complicated: stable emulsions can be obtained at both pH 9 and pH 3. Octanol can swell the microgels in contrast to heptane. This might be the reason why stable emulsions can also be formed at pH 3 with octanol
water system. When the microgels are charged (pH 9), microgel architecture and location of the charges are relevant for the type of emulsion that is formed with octanol. In the presence of surface charges (MS microgels), oil-in-water emulsions are favored, whereas water-in-oil emulsions are obtained in the absence of surface charges (MC microgels) It is important to note that the microgels are deformed at the oil
water interface. This indicates that the soft and porous nature of the microgels plays an important role in emulsion stabilization. When the microgels are not strongly swollen and thus less deformable, no stable emulsions are formed; when the swelling and thus the deformability is enhanced, stable emulsions are obtained. As already mentioned in the Introduction, emulsion stabilization with solid particles is well-known from Pickering emulsions. In that case, wetting of the particle surface by oil and water, respectively, and thus the contact angle made by the oil
water interface on the particle surface is the key parameter.24 The adsorption of microgels to oil
water interfaces strongly resembles this Pickering behavior; however, it does not seem to be straightforward to apply the concept of surface tensions between particle and the two liquids, which is used in the Young equation,

to microgels. First, P(NiPAM) based microgels are known to have a density profile that is characterized by a smooth decay leading to a fuzzy surface.29 Second, the shape of the microgel can change at the interface. Depending on the internal structure of the microgel, they can deform differently at the interface. In addition, microgel deformations and conformational rearrangements of the dangling chains can lead to induced local amphiphilicity depending on the comonomer distribution in along the chains. Third, the presence of volume charges and the osmotic pressure of counterions located inside the microgel will influence solvent penetration and thus microgel swelling and softness as well as the interaction between microgels. In other words, the soft and porous nature of microgels leads to significant differences as compared to solid particles and also leads to emulsions the properties of which are different from Pickering emulsions. Therefore, we suggest using the term “Mickering emulsion” in order to stress similarities and differences to Pickering emulsions stabilized by rigid particles. We hope that the results described in this contribute will stimulate further experimental and theoretical studies on the behaviors of soft and porous microgels at oil
water interfaces. In conclusion, the results clearly show that (i) the stabilization of the microgel-stabilized emulsions does not depend on electrostatic repulsion although the presence and location of charges are relevant. (ii) The stabilization of these emulsions is linked to the swelling and structure of the microgels at the oil
water interface, which depends not only on the solvent’s polarity but also on microgel (core
shell) morphology. These features make the microgel-stabilized emulsions (“Mickering emulsions”) characteristically different from classical, rigid-particle-stabilized Pickering emulsions.

’ ASSOCIATED CONTENT

bS

Supporting Information. Characterization data of the microgels as well as confocal fluorescence microscopy images of emulsions droplets. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Telephone: +49 (0) 241 809 4760. Fax: +49 (0) 241 802 327. E-mail: [email protected]. 9805

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