Article pubs.acs.org/Langmuir
Impact of Electrostatics on the Adsorption of Microgels at the Interface of Pickering Emulsions Pascal Massé,† Elisabeth Sellier,‡ Véronique Schmitt,*,§ and Valérie Ravaine*,† †
Université de Bordeaux, ISM, CNRS, ENSCBP, 16 Avenue Pey Berland, 33607 Pessac Cedex, France PLACAMAT, Université de Bordeaux, 87 avenue Dr Albert Schweitzer, 33608 Pessac, France § Université de Bordeaux, Centre de Recherche Paul Pascal CNRS, 115 Avenue Dr Albert Schweitzer, 33600 Pessac, France ‡
S Supporting Information *
ABSTRACT: The importance of electrostatics on microgel adsorption at a liquid interface is studied, as well as its consequence on emulsion stabilization. In this work, poly(Nisopropylacrylamide) (pNIPAM) microgels bearing different numbers of charges and various distribution profiles are studied, both in solution and at the oil−water interface of emulsion drops. Charged microgels are compared to neutral ones, and electrostatic interactions are screened by adding salt to the aqueous solution. In solution, electrostatics has a significant impact on microgel swelling, as induced by the osmotic pressure exerted by mobile counterions in the gel network. At the interface of drops, microgels pack in a hexagonal array, whose lattice parameter is independent of the number of charges and range of electrostatic interactions. Microgel morphology and packing are ruled only by the adsorption of the pNIPAM chain at the interface. Conversely, decreasing the charge density of microgels by the protonation of the carboxylic groups leads to unstable emulsions, possibly as a result of the impact of hydrogen bonding on microgel deformability.
I. INTRODUCTION Among the various original properties of responsive colloidal gel particles called microgels, considerable interest has recently been paid to their ability to act as Pickering emulsion stabilizers.1−20 Such particles made of solvent-swollen crosslinked polymers are soft, deformable, and porous, and they can swell or collapse under the action of an external stimulus. These specificities make them emulsifiers of special interest because they offer large versatility to emulsions and materials elaborated thereof.21−23 However, this versatility is also a source of complexity in understanding the underlying mechanisms of stabilization. Recent studies on poly(N-isopropylacrylamide)based microgels have attempted to understand the role of deformability on the behavior at the oil−water interface and have highlighted their peculiarities compared to hard particles acting as Pickering emulsions stabilizers.10,18 Classically, Pickering emulsions are kinetically stabilized by colloidal particles partially wetted by oil and water, with the value of the contact angle being a key parameter in determining the emulsion type.24,25 The particle adsorption at the interface is quasi-irreversible because of the high energy of desorption. The limited coalescence process is a consequence of this strong adsorption and is a special feature of such emulsions produced with small numbers of particles.26 Particles usually densely pack at the interface, providing a solid shell for the drops and conferring good protection against coalescence and Oswald ripening.26,27 Because of lateral interfacial interactions between © XXXX American Chemical Society
adsorbed particles, the interface exhibits plastic behavior at the origin of the outstanding stability of Pickering emulsions.27 Tuning those lateral interactions is a means to modify the emulsion stability.28 Responsive emulsions whose stability can be switched on demand can be produced by this strategy. Microgel-stabilized emulsions exhibit some similarities with hard particle Pickering emulsions but also some important differences. As for hard particles, the anchoring of microgels at the interface is quasi-irreversible, and the emulsions are very stable and may be produced by limited coalescence. In opposition to hard particles that require an energy input to adsorb, particle adsorption has been shown to be spontaneous29−32 and a unique contact angle is not valid and cannot be defined suitably. Indeed, microgels adopt different morphologies at the oil−water interface, being strongly deformed compared to their initial swollen spherical shape in the bulk. Two limiting morphologies have been observed at the interface of emulsion drops: microgels can be adsorbed in a flattened state, and thus occupy an area higher than their equatorial section in the bulk, or in a compressed state, in which they cover an area much smaller than that estimated from the bulk size. It has been shown that the morphology can be controlled by structural parameters (size17 and cross-linking Received: July 31, 2014 Revised: November 13, 2014
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density15) or processing parameters (emulsification temperature and shear rate16) as well as by the concentration of microgels.32 In both types of adsorbed states, despite the difference in polymer density, the microgels form a regular hexagonal array and fully cover the interface, making a 2D network of polymers connected through the peripheral shells of the microgels. The interfacial elasticity arising from this network is crucial to ensuring emulsion stability. For microgel-stabilized emulsions, it has been shown that when the crosslinking density increases, the stability of the emulsion is lost14 likely as a result of the decrease in interfacial elasticity that could result from the hindrance of interconnections. For a given microgel structure, the impact of morphology, flattened or compressed, on the emulsion stability remains unclear, but other emulsion properties depend on this parameter. For example, adhesion between drops can modify flow properties by creating a plug flow.15 Adhesion between drops occurs systematically when adsorbed microgels are in a flattened state and consequently when the polymer surface density at the oil− water interface is low. On the contrary, emulsion drops are fully dispersed when adsorbed microgels are compressed with a higher packing density, which means that a high polymer surface density is reached at the drop surface.15,16 As a result, the flow properties are strongly modified by the morphology at the interface. From a practical point of view, it is thus of special interest to the chemical physicist to identify the parameters controlling the morphology of microgels and their packing at the oil−water interface. The impact of structural parameters, such as the cross-linking density and the size of microgels, has already been clarified. They modify the polymer distribution at the oil−water interface: more irregular interfacial profiles are obtained with the largest microgels and with the highest crosslinking densities, which favor bridging between adjacent drops.18 Conversely, the role of charges remains obscure. Electrostatics has a significant impact on the emulsion stability for hard particles because it governs interactions between adsorbed particles (at the same interface as well as on neighboring drops).28 In the case of gel particles, its contribution to the morphology and packing of adsorbed microgels remains unclear, although various experimental studies have been carried out. 5−8 These studies mainly focused on the comparison of poly(N-isopropylacrylamide-co-methacrylic acid) (pNIPAM-co-MAA) microgels in different pH situations, where microgels bear different electrical charges. Emulsions are more stable at high pH (charged state) than at low pH (neutral state). The enhanced stability at high pH could not be attributed to surface tension because it was lower at low pH.5 Investigations of the packing density of microgels showed significant differences as a function of pH, and the results were consistent with surface tension measurements but were counterintuitive with stability: the packing density was higher at low pH, where the emulsions were unstable, than at high pH, where emulsions exhibited high stability.6 The authors explained the obtained results in terms of interfacial mechanical properties and microgel interactions, and this was confirmed by shear and dilational interfacial rheology7 performed on model interfaces. At high pH where charges are present, the microgels are partially interconnected, providing a soft gel-like interface whereas at low pH the uncharged microgels are more densely packed, leading to a brittle interface that cannot withstand mechanical forces, leading to unstable emulsion. However, it was shown recently that these data have to be interpreted with
care because interfacial elasticity measurements depend on the microgel conformation, which itself depends on the kinetics of adsorption of microgels.32 Because of a difference in interface preparation, microgel conformation at a flat interface might differ from that at emulsion drop one. In an attempt to tune lateral interactions and to probe the role of charges, Schmidt et al. studied the behavior of microgels bearing charges located selectively in the core or in the shell.8 They could show that electrostatic repulsion between microgels at the interface was not necessary to stabilize emulsions but that the presence of charges within the particles was a requirement for emulsion stability. They concluded that electrostatics plays a role in the swelling of the particles rather than in their mutual interactions. Recent investigations on charged microgels at model flat interfaces have also raised confusion. On one hand, imaging by FreSCa cryo-SEM indicated that the morphology of microgels at the oil−water interface was the same, whatever the pH.11 In this case, the microgels had spontaneously adsorbed in the dilute state at the interface. On the other hand, compression isotherms performed at different pH values showed a strong difference in the compressibility: charged microgels were more compressible than neutral ones, indicating a minor role of Coulomb repulsion and the possible importance of microgel swelling.33 The present work aims to investigate the role of charges on microgel packing at the oil−water interface of emulsions. A systematic study of microgels bearing different charges and distributions is proposed. Charges are brought to microgels by incorporating pH-sensitive monomers, namely, acrylic acid (AAc) or vinylacetic acid (VAA). Thanks to their different reactivity, the final distribution of carboxylic groups inside the microgels is expected to be different. Charges are randomly distributed throughout the microgels in the AAc system, whereas charges are expected to be mainly located at the microgel periphery for VAA. The impact of electrostatic interactions is probed by adding salt to the aqueous phase or by switching the pH from a value above the pKa of carboxylic groups to a lower value. It is unambiguously shown that charges have a strong impact on microgel swelling but do not influence the organization of microgels at the oil−water interface.
II. MATERIALS AND METHODS 1. Chemicals. All reagents were purchased from Sigma-Aldrich. NIsopropylacrylamide (NIPAM) was recrystallized from hexane (ICS) and dried under vacuum overnight prior to use. N,N′-Methylenebis(acrylamide) (BIS), acrylic acid (AAc), vinylacetic acid (VAA), potassium persulfate (KPS), n-dodecane (purity >99%), n-heptane, and isopropanol were used as received. Milli-Q water was used for all synthesis reactions, purification, and solution preparation. 2. Particle Synthesis and Purification. Charged pNIPAM microgels were synthesized by incorporating different amounts of charged comonomer (5 and 10 mol %) at a given cross-linking density (2.5 mol %). In order to get microgels with different distributions of charges, two types of comonomers bearing carboxylic acid were introduced: acrylic acid (AAc) and vinylacetic acid (VAA) having different copolymerization kinetics with NIPAM. The following notation has been adopted: pNIPAM-co-X Y%, where X = AAc or VAA and Y is the comonomer molar ratio. According to previous work by Pelton et al.,34,35 pNIPAM-co-VAA has a predominantly surfacelocalized radial −COOH distribution and a highly delocalized chain distribution. pNIPAM-co-AAc exhibits a relatively low block content and a relatively homogeneous radial distribution of the charges in the gel matrix. The microgels were obtained by an aqueous free-radical precipitation polymerization classically employed for the synthesis of B
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thermoresponsive microgels and especially pNIPAM microgels.36−38 Polymerization was performed in a 500 mL three-necked roundbottom flask equipped with a magnetic stir bar, a reflux condenser, a thermometer, and an argon inlet. NIPAM and BIS were dissolved in 98 mL of water. The solutions were purified through a 0.2 μm membrane filter to remove residual particulate matter. The solutions were then heated to 70 °C with thorough argon bubbling for at least 1 h prior to initiation. An appropriate number of carboxylic acid comonomer AAc or VAA was introduced. The initial total monomer concentration was held constant at 62 mM. 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. To eliminate possible chemical residues, the microgels were purified by centrifugation−redispersion cycles at least five times (21000g for 1 h, where g is 9.81 m s−2). For each cycle, the supernatant was removed and its surface tension was measured by the pendant drop method. The purification was repeated until the surface tension of the supernatant reached a value close to that of pure water, i.e., above 70 mN/m, showing that the microgel dispersions were free of surface-active impurities. 3. Emulsion Preparation. Typical emulsion batches were composed of 13 mL of an aqueous phase containing microgels at a concentration Cp and 7 mL of oil (dodecane but also heptane for cryo-SEM observations). This mixture was then stirred with an UltraTurrax T25 mixer equipped with an S25 kV-25F shaft at constant speed (9500 rpm) for 30 s. The temperature was kept constant and equal to 25 °C. 4. Characterization of Microgels and Emulsions. The methods used to characterize the microgels and emulsions are described in the Supporting Information.
Figure 1. Schematic view of the charge distribution in microgels: pNIPAM-co-AAc microgel (a) and pNIPAM-co-VAA (b).
Figure 2. pH dependence of the electrophoretic mobility in 4 mM phosphate buffer at appropriate pH for four different microgels: pNIPAM (black circles), pNIPAM-co-VAA5% (green full triangles), pNIPAM-co-AAc5% (red full squares), and pNIPAM-co-AAc10% (red empty squares).
III. RESULTS AND DISCUSSION 1. Microgel Characterization: Discussion of the Localization of Charges. To provide a full description of the impact of charges on emulsion stabilization, four batches of pNIPAM microgels were prepared: one batch of simple pNIPAM microgels and three batches of pNIPAM microgels bearing carboxylic groups. Two batches were prepared with the same comonomer, acrylic acid, at different contents, namely, 5 and 10 mol %. The other batch was prepared with another monomer, vinylacetic acid at 5 mol %. Vinylacetic acid and acrylic acid both provide for the incorporation of carboxylic functions but with different distributions inside the microgel network,34,39−41 which is a consequence of their different kinetic reactivity with NIPAM. Because VAA reacts much more slowly than NIPAM whereas AAc reacts slightly faster, COOH groups are located in the periphery of the pNIPAM-co-VAA microgels and present a homogeneous radial distribution in pNIPAM-co-AAC microgels (Figure 1). Those structures were characterized in depth by Hoare and Pelton.34,35,39,40 The microgels prepared in this work were characterized first by electrophoresis. The electrophoretic mobility was measured as a function of pH for all four batches (Figure 2). All microgels were synthesized in such a way that they could be compared because they had the same size at 50 °C and the same crosslinking density (2.5 mol %) (Table 1). All microgels showed a pH dependence: they were uncharged at low pH (pH
