Pickering Emulsions: What Are the Main ... - American Chemical Society

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Pickering Emulsions: What Are the Main Parameters Determining the Emulsion Type and Interfacial Properties? Mathieu Destribats,*,†,∥ Stéphane Gineste,†,⊥ Eric Laurichesse,† Hugo Tanner,†,# Fernando Leal-Calderon,‡ Valérie Héroguez,§ and Véronique Schmitt*,† †

Centre de Recherche Paul Pascal, Université de Bordeaux, CNRS, UPR 8641, 115 Avenue Dr Albert Schweitzer, 33600 Pessac, France ‡ Laboratoire de Chimie et Biologie des Membranes et des Nano-objets, Université de Bordeaux, CNRS, UMR 5248, Allée Geoffroy St Hilaire, Bât B14, F-33600 Pessac, France § Laboratoire de Chimie des Polymères Organiques, UMR CNRS 5629-IPB-ENSCBP-Université Bordeaux 1, 16 avenue Pey-Berland, F-33607 Pessac Cedex, France S Supporting Information *

ABSTRACT: We synthesized surface-active lipophilic core−hydrophilic shell latex particles, and we probed their efficiency as emulsion stabilizers. The relative weight percentage of the shell, RS/P, was varied to trigger the balance between lipophilicity and hydrophilicity of the particles. Particle wettability could concomitantly be tuned by the pH of the aqueous phase determining the surface charge. Emulsions covering a wide range of RS/P and pH values were fabricated, and their type, oil-inwater (O/W) or water-in-oil (W/O), and kinetic stability were systematically assessed. By adapting the particle gel trapping technique to pH-variable systems and by exploiting the limited coalescence process, we were able to determine the proportion of oil/water interfacial area, C, covered by the particles as well as their contact angle, θ. All of these data were gathered into a single generic diagram showing good correlation between the emulsion type and the particle contact angle (O/W for θ < 90° and W/O for θ > 90°) in agreement with the empirical Finkle rule. Interestingly, no stable emulsion could be obtained when the wettability was nearly balanced and a “bipolar”-like behavior was observed, with the particles adopting two different contact angles whose average value was close to 90°. For particles such that θ < 90°, O/W emulsions were obtained, and, depending on the pH of the continuous phase, the same type of particles and the same emulsification process led to emulsions characterized either by large drops densely covered by the particles or by small droplets that were weakly covered. The two metastable states were also accessible to emulsions stabilized by particles of variable origins and morphologies, thus proving the generality of our findings. bijels,17or porous materials.18,19 The resulting materials may become trapped in deep metastable states such as colloidal glasses, and gels, exhibiting robustness to subsequent changes in thermodynamic conditions. However, by grafting suitable chemical functions on the particles, the droplet surfaces may become sensitive or responsive to external stimuli, and emulsion (foam) destabilization can be produced “on demand”.2 To adsorb at the interface, particles are required to be wetted by both liquids; if the particles are completely wetted by water or oil, they remain dispersed in either phase, and no stable emulsion can be obtained. Following the empirical Finkle’s rule,20 the emulsion type (O/W or W/O) is mainly determined by the relative particle wettability in both liquids, expressed in terms of the contact angle θ that particles make with the oil−

1. INTRODUCTION Solid particles of colloidal size can be strongly adsorbed at the interfaces of immiscible fluids such as oil and water.1,2 The consequence is that they are irreversibly anchored, unlike surfactant molecules, which adsorb and desorb within very short time scales under the effect of thermal fluctuations. Because of this peculiar property, colloidal particles may become efficient species for the kinetic stabilization of metastable materials. Since the pioneering work of Ramsden3 and Pickering,4 various studies have reported the fabrication of kinetically stable emulsions1,2and foams5−7using particles alone. The application field of such materials is considerably widespread.1,2,7,8 For instance, hazardous organic surfactants can be completely avoided as required in cosmetic and pharmaceutical fields8 and replaced by more environmentalfriendly species. The association of oil (or gas), water, and particles allows a large set of materials to be obtained, as, for example, double emulsions,9,10 liquid marbles,11 powdered 13−16 ̈ emulsions and dry water,7,12 capsules and colloidosomes, © 2014 American Chemical Society

Received: May 6, 2014 Revised: July 22, 2014 Published: July 23, 2014 9313

dx.doi.org/10.1021/la501299u | Langmuir 2014, 30, 9313−9326

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Figure 1. (a) Optical microscopic image of spherical latex particles, uniform in size and characterized by a mean diameter equal to 3.5 μm. (b) PAA shell wt % of the different particle batches determined by TGA.

charged latex particles of micrometer size are spread at an oil−water interface, they usually form a 2D hexagonal lattice.24,25 This shows that long-range repulsion prevents the particles from aggregating through van der Waals and capillary attractive forces26(due to deformation of the fluid interface around the particles). Because of the differential polarity of oil and water, the surface charge distribution of the particles becomes anisotropic, thus giving rise to highly repulsive permanent dipoles.25 Horozov et al.27 reported a transition from disordered to well-ordered horizontal silica particle monolayers with an increase of particle hydrophobicity and attributed it to Coulombic repulsion acting through the oil phase as a result of charges at the particle−oil interface.28,29 All of the previous examples illustrate the importance of charge effects when dealing with particles at interfaces. Even if wettability and charge effects have been explored in single monolayer experiments,27−29 there is still insufficient knowledge about how these factors concomitantly determine the emulsion type and stability. The work related in this Article aims at identifying the different metastable states accessible to ternary mixtures of oil, water, and solid particles and to gain deeper understanding about the main factors controlling the properties of solidstabilized emulsions. For that purpose, we fabricated a set of organic particles made of hydrophobic cores and hydrophilic shells. For this type of particle, the shell-to-particle mass ratio, RS/P, could be varied, and their preferred wettability could be quantified. Because of the presence of carboxylic functions (COOH, pKa ≈ 4.5) in the shells, the particles are also stimulus responsive: they are neutral at low pH, and they acquire a negative charge for pH > pKa. We examined the particle contact angle, the emulsion type (O/W or W/O), the fraction of interfacial area covered by the particles, and the stability of emulsions. We show that the behavior of the system can be rationalized in a two-axes diagram, where RS/P and the pH of the aqueous phase are used as control parameters to tune both the wettability and the surface charge density of the particles. Finally, we discuss the generality of our results by considering other particle types (mineral and hybrid organic−inorganic).

water interface through the water phase. The most wetting liquid becomes the continuous phase. Considering mixtures of equal volumes of the two immiscible fluids, if the particles are relatively hydrophilic (hydrophobic, respectively), θ < 90° (θ > 90°, respectively) and O/W (W/O, respectively) emulsions are preferentially obtained. The wettability at the balanced state defining the transition from O/W to W/O emulsion types is attained at θ = 90°. This angle also corresponds to the maximum anchoring energy of the particles at the interface: Edesorption = γπa 2(1 − |cos θ|)2

(1)

where γ is the interfacial tension of the nude interface and a is the particle radius. Note that this expression is symmetric with respect to θ = 90° due to the particle symmetry with respect to its equator. The stability of emulsions based on particles very close to the balanced state is still raising challenging questions. For instance, it is well-known that surfactant molecules at the balanced state are generally inefficient emulsion stabilizers.21 It is thus interesting to know whether the same phenomenology holds for amphiphilic particles. The inversion from one emulsion type to the other can be realized either by a progressive change in particle wettability at constant oil/water ratio (transitional inversion) or by varying the oil/water ratio at constant particle wettability (catastrophic inversion). Chemically modified silica particles were used to trigger both types of emulsion inversion.22,23 The silanol (SiOH) groups present on the surface of the hydrophilic particles were converted to hydrophobic groups by reaction with silane reagents. Interestingly, transitional inversion was also observed on air−water surfaces containing adsorbed particles of variable wettability.7 This inversion consisted of a continuous transition from air bubbles dispersed in water, as in conventional foams, to water drops dispersed in air. It this latter case, it was shown that highly hydrophobic particles could encapsulate isolated macroscopic water drops in air, thus producing a free-flowing powder of coated water drops. So for, transitional inversion phenomena were provoked by tuning a unique variable, which is the relative percentage of hydrophilic groups on the surfaces. In addition to the intrinsic hydrophilicity/hydrophobicity of the functional groups, the presence of surface charges may have a strong influence on the interfacial behavior of adsorbed particles. For instance, when 9314


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Figure 2. SEM images of (a) spherical silica particles of 790 nm diameter and (b) neighborite cubes with an edge length of 1.6 μm. characterized. To vary the final shell-to-particle mass ratio, RS/P, several particle batches have been produced with different shell synthesis durations ranging from 0 (corresponding to unmodified cores) to 150 h. Deprotection of the tert-butyl groups through acidolysis in dichloromethane in the presence of trifluoroacetic acid finally led to polymer particles with a hairy and hydrophilic poly(acrylic acid) (PAA) shell. After each synthesis step, the particles were washed successively three times with different solvents such as dichloromethane, acetone, ethanol, and water. The resulting particles were then dried overnight under vacuum. A more detailed description of the two synthesis steps is reported as Supporting Information (1). The final shell-to-particle mass ratios were determined by thermogravimetric analyses (TGA) (Setaram TAG-16 analyzer). Measurements were done after purging the sample unit (vacuum purge and fill with argon) and a stabilization isotherm at 30 °C for 1 h. Argon flow was set to 15 cm3 min−1, and temperature ramps of +5 °C min−1 up to 600 °C were applied. A thermogram example is given in Supporting Information (2): the weight losses, respectively, due to the particle shell and core occurred over sufficiently distinct temperature ranges to allow simple analyses. The identification of the different contributions was first checked by analyses of core particles and commercial PAA molecules (Mw ≈ 2000 g mol−1), separately. The first weight loss between 150 and 280 °C corresponds to the shell degradation, and the second weight loss between 280 and 800 °C is due to the degradation of the cross-linked core. Above 600 °C, the sample weight did not evolve anymore, and only the degradation residues that did not dissociate into gas during the temperature increase remained. The same characterization protocol was systematically performed with all of the synthesized particle batches; their characteristics are reported in Supporting Information (3), and the evolution of RS/P ratio as a function of the batch number is plotted in Figure 1b. The weight loss attributed to the shell degradation was normalized by the total particle weight loss (shell + core) without taking into account the residual matter (99%), bipyridine (99%), copper bromide (>99%), and trifluoroacetic acid (99%) purchased from Sigma-Aldrich. Acetonitrile (ACN, HPLC grade) and dichloromethane were from Xilab (purissimum grade pur). These chemicals were used as received without further purification. Free-radical initiator 2,2-azobis(2-methylpropionitrile) (AIBN) was purchased from SigmaAldrich and used after purification by crystallization in methanol. tertButyl acrylate (tBA, Sigma-Aldrich, >98%) was trap-to-trap distilled under reduced pressure prior to use, and tetrahydrofuran (THF, analyzed grade, J.T. Baker) was distilled over sodium under argon. For silica synthesis, tetraethoxysilane (TEOS, Fluka, >99%), aminopropyltriethoxysilane (APTES, Aldrich, >98%), absolute ethanol (Prolabo), and ammonia solution (29% in water, JT Baker) were used. Neighborite particles were synthesized using sodium fluoride (>99%) and magnesium chloride (>98%) from Sigma-Aldrich. For emulsions, hexadecane from Aldrich (ρ = 0.77 g cm−3) of high purity (>99%) was adopted as the oil phase. Water (resistivity 18.2 MΩ cm) was deionized by a Milli-Q system. Sylgard 184 curable silicone elastomer (polydimethylsiloxane, PDMS), a gift from Dow Corning, was used as polymerizable oil for particle contact angle measurements. 2.2. Stimuli-Responsive Particles’ Synthesis and Characterization. Latex Particles. The model particles used in this study are micrometer-sized polymer particles with a core−shell structure. They were prepared through a two-step polymerization process. First, core particles were prepared through precipitation polymerization of a mixture of monomers (divinylbenzene (DVB), ethylene glycol dimethacrylate (EGDMA), and para-chloromethylstyrene (pCMS) in 54/23/23 molar ratio) in hot acetonitrile, following a procedure described elsewhere.30 The resulting core particles were hydrophobic, highly cross-linked, and stable over the whole considered range of pH. They were remarkably uniform in size and characterized by a mean diameter around 3.5 μm, determined by optical microscopy (see Figure 1). Incorporation of para-chloromethylstyrene in the mixture of monomers endowed the polymer core particles with reactive chloro functions at their surface, allowing in a second step the atom transfer radical polymerization (ATRP) of tert-butyl acrylate (tBA) catalyzed by a complex of copper bromide/bipyridine in tetrahydrofuran. Among the different available controlled radical polymerization techniques, atom transfer radical polymerization (ATRP) has been most extensively used to produce polymer brushes. This technique was chosen here to produce polymer chain with low polydispersity and uniform shell thickness even if this parameter could not be 9315


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mp 1 = D 4Cρp d pVd

adsorption at the oil−water interface (pKB of the amine ∼4). The added quantity of grafting agent was adjusted to obtain 15 functions per nm2 of the nanoparticle surface. The obtained particles were characterized by a statistical analysis from SEM observation (HITACHI TM-1000 apparatus), and their mean diameter was estimated to be 790 ± 10 nm (Figure 2a). The particles were purified by centrifugation−redispersion cycles, with each successive supernatant being replaced by pure water. The extent of purification was assessed by measuring the surface tension of the supernatant until the value was close to that of pure water. The particle concentration was determined by measuring the loss of water upon drying. Neighborite Cubes. Nearly monodisperse NaMgF3 cubes with an average edge length of 1.6 μm (Figure 2b) were produced following an experimental procedure already reported by Hsu et al.32 The final particles were washed several times by centrifugation−redispersion cycles in water. 2.3. Zeta Potential Measurements. The ζ-potential was estimated from the measurement of the particles electrophoretic mobility at 25 °C with a Malvern Zetasizer 3000 HSA instrument employing a dip cell and using the Smoluchowski equation for converting the measured mobilities. To avoid multiple-scattering, the dispersions were diluted down to 0.01 wt % with an aqueous phase at the appropriate pH, also keeping the ionic strength constant prior to the measurements. 2.4. Emulsions Characterization. Observation and characterization of emulsions were done using an inverted optical microscope (Zeiss Axiovert X100) and a video camera. The size distributions were estimated by image analysis; the dimensions of about 50 droplets were measured to determine the surface average diameter D and the polydispersity P, defined by the following equations:

D=

P=

where mp is the mass of particles, ρp is the particle density, dp is the particle diameter (dp = 2a), Vd is the volume of the dispersed phase, and C is the surface coverage, that is, the fraction of the droplet interfacial area covered by the particles. This parameter characterizes the packing density of the particles at the interface. For example, for hexagonally close-packed monodisperse particles, C is equal to 0.9. If C is lower than 0.9, it means that particles are loosely packed, whereas a value larger than 0.9 reflects the presence of particle multilayers or aggregates. 2.6. Particle Trapping and Contact Angle Measurement. Measurement of the contact angle of a colloidal particle at interface is a challenging issue, and various methods with restrictive conditions of application have been proposed in the literature. Among them, we can mention indirect methods, such as ellipsometry34,35 and reflectometry36 techniques or the recording of the pressure/area isotherm in a Langmuir trough,37,38 which measure global properties of model particle-laden interfaces and deduce the average contact angle from educated hypotheses and models. Direct methods such as atomic force microscopy (AFM)39,40and interferometry41,42 also provided alternative ways enabling the contact angle measurement of individual particle to be measured. Recently, a seducing method has been also proposed by Isa et al.43 based on the freeze-fracture of an interface, the subsequent shadow-casting by a unidirectional metal deposition at a known oblique angle, and the determination of the shadow length by cryo-SEM visualization. This technique offers accurate measurements over a wide range of particles sizes (from several micrometers to 10 nm). In our study, we adapted an ingenious method named the “gel trapping technique” (GTT) designed by Paunov44 that allows imaging particles entrapped at the surface of an oil phase replica. One of the many attractive features of this technique is its simplicity. The technique involves first the spreading of particles at an oil−water interface at 50 °C and subsequently the gelling of the aqueous phase with a nonadsorbing polysaccharide (gellan) as the system is cooled to room temperature. The adsorbed particles are then “frozen” by the gel at their equilibrium contact angle at the interface. The top liquid oil phase is then carefully removed and replaced by curable silicone oil, which after cross-linking can be easily peeled off together with the trapped particles and imaged by SEM. Because the gelling properties of the polysaccharide are affected by pH, we simplified the technique by suppressing the aqueous phase solidification step and replacing the gellan solution by pure water at the desired pH. As a consequence, we directly used PDMS Sylgard 184 elastomer (Dow Corning) as model oil. The major improvement of the adapted method lays in the possibility to study the evolution of particle contact angles at the PDMS−water interface over a wider range of pH (from 1 to 13) as a function of RS/P. The main steps of the method are described below and summarized in Supporting Information (4). PDMS base was first mixed with the corresponding curing agent in a 10:1 mass ratio. A model interface was then produced into a beaker by pouring softly the oil mixture over 40 mL of water (for which the pH had been previously adjusted by HCl or NaOH solutions) to form an approximately 1 cm thick oil layer. Dispersions of latex particles (1 wt %) were prepared in the spreading solvent (ethanol and chloroform 80/20 wt % mixture) by sonication. Around 50 μL of the dispersions was carefully spread at the interface using a micropipette, and the system was left to cure at room temperature for 48 h. Finally, the solidified PDMS layer formed a macroscopic elastomeric disc that was collected (Supporting Information (4)), washed with distilled water, and dried before observation. Because of the use of a spreading solvent allowing deposition of the particles at the interface and the slow curing kinetics, the particles are trapped at the interface at their equilibrium position. By changing the kinetics of curing, we could not observe any significant difference in the measured contact angle. The drawback of the adapted gel trapping technique, as compared to the original one, resides in the low choice of possible oils. However, even if the interface is not exactly the same as for emulsions, the wettability of particle with

3 ∑i ND i i 2 ∑i ND i i

(2)

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

(3)

(4)

where Ni is the total number of droplets with diameter Di. The median diameter Dm is the diameter for which the cumulative undersized volume fraction is equal to 50%. The emulsion type was determined by dilution tests in both water and oil. The solvent in which the emulsion can be dispersed corresponds to the continuous phase. The results were confirmed by gravity effects induced by the large density mismatch between hexadecane and the aqueous phase: droplets in O/W emulsions tended to form a cream, whereas they sedimented in W/O emulsions. In the present study, because of the low amount of particles, the droplet average diameters were rather large (several tens or hundreds of micrometers), and so the surface-to-volume ratio of the drops was insufficient for the adsorbed particles to invert the density mismatch between oil and water. 2.5. Surface Coverage C. We checked that particle anchoring at the oil−water interface was always complete and irreversible. Indeed, we did not observe the presence of free particle in the continuous phase after the emulsification step. As a consequence, monodisperse solid-stabilized emulsions were produced exploiting the limited coalescence process.9,33 It consists of producing a large excess of oil−water interface as compared to the amount that can be covered by the solid particles. For this process to occur, a very small amount of solid particles is required as is the case in our experiments. When the agitation is stopped, the partially unprotected droplets coalesce, thus reducing the total amount of oil−water interface. Because the particles are irreversibly adsorbed, the coalescence process stops as soon as the oil−water interface is sufficiently covered. The resulting emulsions exhibit a drop diameter that is controlled by the mass of particles and their packing at the interface. Assuming that all of the particles are adsorbed at the oil−water interface, from simple geometrical considerations, the final drop diameter D is given by9 9316


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various amounts of shell and at various aqueous phase pH may be compared. Cuts of elastomeric discs could be observed either by SEM for large particles or by AFM (Supporting Information (5)) for smaller particles. In the case of large latex particles, the cuts were coated by a layer of Au−Pd and placed onto a specimen holder with a 60° inclination angle before visualization using an EVO 50 scanning electron microscope (Zeiss). On the basis of the SEM images (Figure 3), the particle contact angles were extracted from at least 50

part of the particles can be determined by atomic force microscopy with sufficient accuracy (see Supporting Information (5) and (6b)). In this latter case, the resolution is determined by the detection threshold of AFM and by the flatness of the PDMS film.

3. RESULTS AND DISCUSSION As shown in the previous section, the synthesis pathway allowed us to produce several batches of calibrated latex particles made of “hydrophobic cores” and PAA shells. We had two levers to continuously modify the particles characteristics, either during the synthesis by varying the particle shell wt %, or equivalently the intrinsic hydrophilic/hydrophobic balance, or in situ by changing the aqueous pH (varying the charge density borne by the carboxylic functions of the shell). In the following, we examine the influence of these two parameters on (i) the preferential wettability of the particle (expressed by the particle contact angle), (ii) the emulsion type (O/W or W/O), (iii) the surface coverage, and (iv) the stability of the emulsions. 3.1. Particle Contact Angle Diagram. By exploiting the above-mentioned trapping technique, the particle contact angle was systematically measured as a function of the shell wt % and of the pH of the aqueous phase. The main trends are detailed below. 3.1.1. Adsorption of Core Particles. To check the reliability of the method and to rule out any possible artifact that could affect the measurement, we measured the evolution of the contact angle θ of core particles (shell wt % = 0) as a function of pH. Because these particles have no PAA shell, they were expected to be rather hydrophobic and insensitive to pH. This behavior was indeed confirmed by the observations using the adapted trapping technique (see Figure 4a). As expected, the particles devoid of PAA were preferentially immersed into the PDMS phase, generating a high contact angle with the interface (θ ≈ 140°) that remained constant over the considered range of pH (Figure 4a). These results validate the fact that the method can be applied in a wide pH range without introducing any artifact. 3.1.2. Influence of the Shell Proportion for Uncharged Particles). To study the influence of RS/P independently and to rule out any contribution of shell ionization, contact angles were systematically measured at pH = 1 (viz., well below the pKa of the carboxylic groups). Some SEM observations are given as examples in Figure 4b, and the variation of the contact angle is plotted in Figure 5. The RS/P ratio is reported in the same graph and can be read on the right-hand side axis. The trend is very clear: θ decreases continuously from 140° to 25° when the shell proportion increases from 0 to 16 wt %. Grafting a hydrophilic PAA shell allowed a continuous tuning of the uncharged particle wettability from hydrophobic to hydrophilic with a sharp transition occurring for RS/P between 6.8 and 7.7 wt %. Surprisingly, the particles showed a peculiar “bipolar” behavior in the transition domain (see hatched zone in Figure 5). Indeed, within the same interface, some particles adopted a low contact angle (θ = 46° ± 6°), whereas the others showed high contact angle (θ = 122° ± 6°). Instead of evolving continuously, the contact angle swang between two welldefined values whose average is close to 90°. In the following, we address the evolution of θ as a function of pH for two particle batches whose RS/P ratios are close to 8 wt % and on both sides of the wettability transition occurring at pH = 1. 3.1.3. Influence of pH and Surface Charges for Initially Rather Hydrophilic Particles (θ < 90° at pH = 1). Figure 6

Figure 3. Examples of SEM images of latex particles embedded in PDMS layer: (a) large view of a particle-laden surface and (b) close view of one of the particles anchored at the interface. measurements for each sample (at given RS/P ratio and pH). Estimations of θ were based on simple geometric considerations according to the following equations that depend on whether the particles were embedded within the PDMS above (θ > 90°, eq 5) or below (θ < 90°, eq 6) their equatorial line (Figure 3b and Supporting Information (6a)): θ=

⎛d ⎞ Π + arccos⎜ ⎟ ⎝ 2a ⎠ 2

⎛d ⎞ θ = arcsin⎜ ⎟ ⎝ 2a ⎠

(5) (6)

where d is the particle anchoring diameter (contact line) and a is the particle radius (Figure 3b). Because a could not be measured directly on SEM images when θ > 90°, only d was determined in that case, and the averaged value of a was measured independently on particle dispersions by optical microscopy. This technique allowed us, for the first time, to form homogeneous PDMS layers over a wide pH range, from 1 to 13. Because the surfaces were highly laden in particles, observations were easy and statistically meaningful to get a reliable value of the contact angle. It is worth noticing that the technique is still valid for smaller particles provided that the height of the embedded 9317


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Figure 4. (a) SEM pictures of latex particles from batch 1 (no shell, RS/P = 0) embedded in PDMS as a function of pH. For pH = 1, θ = 140° ± 5°; for pH = 4, θ = 140° ± 6°; for pH = 7, θ = 139° ± 4°; for pH = 10, θ = 139° ± 5°; and for pH = 13, θ = 135° ± 5° (figure not shown). (b) SEM pictures showing the evolution of the contact angle as a function of RS/P at pH = 1.

Figure 5. Contact angle and shell wt % at pH = 1 of latex particles deriving from the different batches. The contact angle decreases continuously with respect to the particle shell wt %, but this variation exhibits a singularity near the wettability transition domain (dashed area), where the particles of the same batch adopt simultaneously two contact angles (90°).

Figure 6. SEM pictures of latex particles from batch 6 (RS/P = 8.5 wt %) embedded in PDMS as a function of pH. For pH = 1, θ = 54° ± 11°; for pH = 4, θ = 55° ± 8°; for pH = 7, θ = 42° ± 6°; and for pH = 13, θ = 43° ± 10°. At pH = 10, no particle was visible on the interface (θ = 0°) (image not shown).

shows typical SEM pictures of latex particles with the same shell wt % (batch 6: RS/P = 8.5 wt %) embedded into PDMS as a function of pH. The corresponding contact angle values are indicated in the caption. When the pH was increased from 1 to 10, the particles became progressively more hydrophilic (θ decreased from 54° to 0°) probably because the particle shell got progressively charged, inducing conformational rearrangements of the PAA polyelectrolytes grafted at the particle surface and leading to better solvation. Ionic dissociation increased the overall particle affinity for water and thus promoted the decrease of the contact angle. For pH < pKa of the PAA carboxylic groups, the particles were neutral or slightly charged

(see ζ-potential measurements in Supporting Information (7)), and the contact angle remains constant (55°). For pH ≥ pKa, the shell was composed of polyelectrolytes with an increasing number of ionized carboxylic groups, and the contact angle decreases continuously. At pH = 10, the particles were even too hydrophilic to adsorb at the interface and remained in the aqueous phase: the final PDMS film was particle-free (θ = 0°). Finally, if the pH was increased further, the particles recovered their surface activity as shown in Figure 6 for pH = 13. It is our understanding that adsorption of the latex particles at such a 9318


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Figure 7. SEM pictures of latex particles from batch 4 (RS/P = 6.8 wt %) embedded in PDMS as a function of pH. At pH = 1, the particles are mainly hydrophobic and θ = 100°. At the intermediate pH = 7, the latex particles exhibit a “bipolar” behavior with two distinct contact angles, one higher and the other lower than 90°. At pH = 10, the particles are hydrophilic and θ = 33°.

Figure 8. State diagrams of (a) particles at interface and (b) type of emulsions stabilized by these particles as a function of the particle shell wt % (RS/P) and pH. The domains are delimited by sharp boundary lines to facilitate the interpretation. The dotted lines represent the extrapolated tendencies because no accurate transition could be observed in the absence of particles with RS/P between 0 and 4.5 wt %. The dashed lines delimit 3 subdomains of the O/W type.

charged surfaces). However, at an intermediate pH value (pH = 7), the particles displayed the same “bipolar” behavior as that described previously at pH = 1 for batch 5: some particles adopted a low contact angle (θ = 46° ± 15°), whereas others exhibited a high contact angle (θ = 123° ± 7°). Figure 7b shows a typical SEM picture of particles with these two different contact angles coexisting on the same interface. This observation confirms the fact that near the balanced state in terms of wettability (θ = 90°), particles adopt a bistable behavior: two wetting states are accessible, corresponding to two minima of the adsorption energy probably separated by an energy barrier. To our knowledge, no other similar observation has been reported so far in the literature. It is worth noticing that the desorption energy is almost the same for the two coexisting angles. We have no unambiguous explanation for this phenomenon, but experimental artifact can be ruled out as we reproduced many times the measurements to achieve a large

high pH is helped by the high ionic strength leading to surface charge screening. The shell configuration is then similar to the one obtained for less charged particles. This latter hypothesis was confirmed by the following simple experiment: at pH = 10, where latexes did not adsorb at the interface, NaCl was added to the aqueous phase at a concentration of 0.1 M before particle spreading. As a consequence of the ionic strength increment, the particles were found to adsorb again at the water−PDMS interface with an average contact angle θ = 66° ± 6°. 3.1.4. Influence of pH and Surface Charges for Initially Rather Hydrophobic Particles (θ > 90° at pH = 1). Another illustration of how surface charges influence the contact angle is shown in Figure 7 for rather hydrophobic particles (batch 4, RS/P = 6.8 wt %). Their behavior at the interface was very pHsensitive: at pH = 1 (neutral surfaces), they were preferentially wetted by PDMS (θ = 100° ± 7°), whereas they were preferentially wetted by water at pH = 10 (θ = 33° ± 7°; 9319


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statistics. Moreover, this phenomenon could be observed for at least five occurrences with particles differing by their amount of shell and at different pH. Also, the position in the phase diagram of the two coexisting contact angle makes sense as it separates the more hydrophilic and more lipophilic particles. Interestingly, we also noticed that the particles adopted a “gregarious” behavior as they tended to gather and to form 2D clusters with similar contact angle as can be seen in Figure 7b. It is likely that preferential interactions between particles of similar contact angle are at the origin of this segregation that minimizes the interface deformation. 3.1.5. Contact Angle Diagram of Particles at Model Interface. Systematic analyses of contact angles as a function of pH and particle shell wt % allowed us to draw the state diagram that is plotted in Figure 8a. The domains are delimited by sharp transition lines to facilitate the readability of the diagrams. The location of the experimental points within this diagram and the associated θ values are reported in Supporting Information (8). Four main areas are observed: at low RS/P and/or low pH, particles are mainly wetted by the oil phase and θ > 90°; at high RS/P and low pH or intermediate RS/P and high pH, particles are preferentially wetted by the water phase and θ < 90°; between, there is a zone where charged particles show a bipolar behavior with two different contact angles coexisting within the same interface; finally, at high RS/P and high pH, the particles are too hydrophilic to adsorb at the PDMS−water interface and θ = 0°. Overall, the increases of both shell grafting and surface ionic dissociation (pH > pKa) are promoting a decrease of the contact angle. 3.2. Emulsion State Diagram. After having described the particle behavior on model isolated interfaces, we now consider the ability of those latexes to stabilize real liquid−liquid interfaces and the characteristics of the resulting emulsions. The particles were initially dispersed either in the oil or in the water phase depending on their preferential wettability and sonicated in a bath to help dispersion. Typical emulsions were prepared in batches of 20 g with identical volumes of hexadecane and water. The pH of the aqueous phase was previously adjusted by HCl or NaOH solution additions, and the particle concentration was fixed at 20 mg/g of dispersed phase for all samples. The mixture was vigorously shaken by hand during 30 s. Although simple, this method produced emulsions with reproducible average diameter and size distribution, as reported elsewhere.45 The results in terms of emulsion type are gathered in Figure 8b as a function of the RS/P ratio and pH. The liquid phase where the polymer particles have been dispersed prior to emulsification did not matter because the same emulsion type was finally obtained whatever the phase where the particles have been dispersed. 3.2.1. Description of the Domains. Four domains have been identified and are described below. a. W/O Emulsion Domain. W/O emulsions stabilized by core particles (RS/P = 0 wt %) showed no pH sensitivity whatever the pH, in good agreement with contact angle measurements. After emulsification, the water drops quickly sedimented due to the density mismatch between oil and water and formed a concentrated sediment, stable for months (see Figure 9a). The inverse drop diameter evolved linearly with the initial particle amount, and, from eq 4, we deduced a surface coverage of approximately 90% (data not shown) corresponding to a monolayer of close-packed monodisperse particles, consistent with the optical microscopy observations (Figure 10a). Emulsions of similar type were produced with other low

Figure 9. Macroscopic pictures of emulsions in the different domains of the phase diagram reported in Figure 8b. (a) W/O emulsion (20 vol % water, 10 mg/g, RS/P = 0 wt %, pH = 7); (b) climbing film (RS/P = 6.8 wt %, pH = 7); (c) emulsion in O/W1 domain (40 vol % hexadecane, 10 mg/g, RS/P = 10.8 wt %, pH = 1) and (f) close-up view of the cream layer; (d) emulsion in the O/W3 domain (40 vol %, 10 mg/g, RS/P = 10.8 wt %, pH 6) and (g) close-up view of the cream layer; and (e) phase separation in the PS2 domain (40 vol %, 10 mg/g, RS/P = 10.8 wt %, pH = 10). The oil and particle fractions were not strictly the same as in Figure 8b. Specific values of these formulation parameters were adopted to better illustrate the differences between the different parts of the diagram. Yet, the images reported in this figure are fully representative of the subdomains.

RS/P particles (0 < RS/P < 7.7 wt %). Their kinetic stability tended to become poorer at the approach of the upper boundary line in Figure 8. b. Phase Separation 1 (PS 1) Domain. At higher pH, a thin intermediate unstable zone was identified, where the systems underwent phase separation a few seconds after emulsification. Even if the particles were not able to stabilize emulsions, they all remained anchored at the macroscopic interface, forming a film between water and oil phases that climbed up along the inner wall of the container as long as the phase separation progressed (see Figure 9b). This wetting phenomenon of container wall by a climbing film of particles has already been reported in the literature.46,47 It happens when the particles are strongly absorbed at the oil−water interface and when the layer they form is very rigid or incompressible. The film then tends to spread/expand at the hydrophilic glass surface. The aqueous film is delimited on one side by the glass surface and on the other by the particle adsorbed at the water/air or water/oil interface. It cannot be excluded that a thin layer of oil is dragged along during the film formation to ensure a complete wetting of the particles. c. O/W Emulsions Domain Composed of Three Subdomains. i. O/W 1 and O/W 2 Subdomains. Even if these two domains occupy distinct zones in the RS/P/pH diagram, they both correspond to O/W emulsions with similar characteristics. After emulsification, the oil drops rapidly formed a dense cream 9320


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Figure 10. Optical microscopy pictures of representative emulsions in the different stability domains. (a) W/O emulsion (RS/P = 0 wt %, pH = 7), (b and c) O/W emulsions and close-up views on single drops belonging to domains (b) O/W 1 (RS/P = 8 wt % at pH = 1) and (c) O/W 3 (RS/P = 8 wt % at pH = 6).

measurements. Increasing the RS/P ratio obviously increases the hydrophilicity of the particles. Alternatively, increasing the pH makes the particle more hydrophilic as it promotes a progressive ionization of the shell. In the following, we first describe the behavior of batches 6 to 10 (RS/P from 8 to 16 wt %), and we propose some hypotheses to explain the observed phenomenology in the RS/P−pH diagram. i. O/W 1−O/W 3 Transition. For a given particle batch (constant RS/P ratio), the transition between the highly and poorly covered regimes was found at pH = 4.5 (see dashed line in Figure 8b), a value close to the pKa of the carboxylic groups. This result suggests that the interfacial surface coverage of the O/W emulsions is controlled by electrostatic or dipolar effects. To corroborate this hypothesis, the ζ-potential of the particles was measured as a function of the pH and is reported in Supporting Information (7). At pH < pKa, the particles are neutral or slightly charged, and the electrostatic or dipolar repulsions are negligible. Particle interfacial monolayers adopt a tightly packed configuration after successive coalescence events that reduce the interfacial area, until the coverage C reaches a value close to 90%. At pH values close to 6 (>pKa), the poly(acrylic acid) chains in contact with the aqueous phase are almost fully dissociated, whereas the charge density is certainly much weaker in the oil phase because of its low polarity. The dissymmetric charge distribution on the particle surface is at the origin of a long-range dipolar moment. We will discuss further the mechanism explaining how effective dipoles can be at the origin of the emulsion stabilization even when the interfacial surface coverage of the drops is very low (as low as 10%45). ii. O/W 1−PS 2 Transition. The transition occurred at high RS/P ratios (≥13.9 wt %), that is, for particles that were already highly hydrophilic even for pH < pKa. As soon as the chains acquired charges, the particles became too hydrophilic and remained suspended in the aqueous phase. iii. O/W 3−PS 2 Transition. For smaller RS/P ratios ( pKa and RS/P > 8.5 wt %. Unlike in the PS 1 domain, the particles were not able to absorb at the interface and remained in the water phase (Figure 9e), leading to rapid destabilization. After a few minutes, the two phases were separated by a clear interface (deprived of particles), and the aqueous subnatant, containing the particles, was turbid. 3.2.2. Origin and Evolution of the Domain Boundaries. The previous observations can be understood considering that the hydrophilic−lipophilic balance of the particles varies with both pH and RS/P as previously revealed by the contact angle 9321


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90°, respectively), W/O (O/W, respectively) emulsions were obtained in perfect agreement with the well-known Finkle rule. Between, the particles displayed a bipolar behavior and adopted two different contact angles. This behavior led to emulsion instability and to the formation of particle-laden climbing films. Because the required energy to detach a particle from the interface reaches its maximum value for θ = 90° (see eq 1), it is often thought that this is the optimal situation to impart emulsion stability. To the best of our knowledge, this hypothesis has never been explored experimentally because of the difficulty in measuring reliably the contact angle and in finely tuning surface wettability to reach the balanced state. In our case, the use of two different parameters (pH and RS/P) made it possible to approach the balanced state with sufficient accuracy and to unambiguously demonstrate that the emulsions become unstable. Within the frame of Finkle’s rule, this instability is not really surprising considering that the two angles adopted by the particles within the same interface are imposing incompatible constraints as they favor emulsions of opposite type. It is within the reach of future work to determine whether the bipolar behavior inducing emulsion instability at the approach of θ = 90° is generic or is limited to specific particle types. Finally, the domain where the particles were not adsorbed on the PDMS surface (θ = 0°) because they were too hydrophilic was well correlated with the instability zone PS 2. 3.4. Surface Coverage in O/W Emulsions. As shown previously, the particle contact angle is not the only microscopic parameter to be considered to account for the whole phenomenology observed in emulsions. The behavior of the O/W systems stabilized by predominantly hydrophilic particles (θ < 90°) can be summed as follows: When the particle shells were protonated (O/W 1) or when their charges were screened by high electrolyte concentration (O/W 2), the resulting drops were densely covered by the particles and well dispersed (individual). In this case, the final drop size resulted from a coalescence process limited by the hard sphere interaction of particles organized in close-packed monolayers at the interface. This dense interfacial coverage gave resistance against deformation to the interface: for example, this type of emulsion could be concentrated up to 88 vol % by progressive incorporation of hexadecane (see Supporting Information (9)). When the particle shells were deprotonated (O/W 3), the resulting drops were adhesive, and their surface coverage was surprisingly low. The particles were nonhomogeneously distributed, leaving large interfacial areas uncovered. The emulsions were stable at rest for months, but, without surprise, they could not be concentrated above a critical volume fraction close to the random close packing of the drops (64%). Above this limit, the drop interfaces are compressed and stretched. The films separating the drops did not withstand this compression and broke, leading to a partial destabilization of the emulsions. To elucidate the nature of the stabilization mechanism of poorly covered emulsions at rest, we added salt in the continuous phase of an emulsion stabilized by particles with RS/P = 8 wt % at pH 6 (O/W 3 domain). Coalescence between drops started spontaneously until achieving a stable state similar to that observed in acidic conditions (large and individual drops, dense interfacial monolayer of particles). It can therefore be concluded that for pH > pKa, the stability of the poorly protected emulsions has an electrostatic origin.

more hydrophilic. The contact angle progressively decreases with pH, as reported in the previous section, until a point such that the particles do not adsorb anymore at the interface. The loss of surface activity is indirectly reflected by emulsion destabilization and by the turbidity of the aqueous phase (Figure 9e). iv. PS 2−O/W 2 Transition. This transition has only been observed for one batch of particles (RS/P = 8.5 wt %). Between roughly pH = 8 and 12.5, the particles were too hydrophilic to adsorb, and the systems rapidly destabilized. However, at high pH, the particles recovered their surface activity as has been previously shown in section 3.1 dedicated to contact angle measurements. Upon addition of concentrated NaOH, the high ionic strength (0.1 M at pH = 13) induced charge screening, promoting particle adsorption and significant attenuation of the electrostatic or dipolar interactions. The situation was then equivalent to the neutral particle regime (pH < pKa), where the interface was covered by a compact monolayer. The impact of the ionic strength on the particle surface activity was confirmed via a very simple test: addition of NaCl up to 0.1 M to an unstable sample at pH = 10 allowed the particles to adsorb again (see section 3.1) and concomitantly stabilized emulsions with properties similar to those in the O/W 2 domain. v. Transition from O/W 3 to O/W 2. This transition originates from the peculiar stabilization mechanism of the poorly covered emulsions based on electrostatic and effective dipole repulsive interactions. The ionic strength concentration for pH > 12 became probably large enough for charge screening to provoke a transition from the poorly to the highly covered interfacial regime. vi. W/O-PS 1-O/W 2 Transition. For lower RS/P ratios ( 90° (θ < 9322


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Figure 11. Optical microscopy pictures of hexadecane-in-water emulsions based on solid mineral particles. Emulsions stabilized either by silica particles (a1 and a2) or by neighborite cubes (b1 and b2) also exhibit two kinetically stable states characterized by two different interfacial surface coverages as a function of salinity or pH of the aqueous phase. In the poorly covered regime, a closer view of the contact zones between drops reveals the preferred location of the particles. They form dense monolayer patches or organize as crowns surrounding unprotected films. (c1 and c2) Pictures at two different focal planes of one drop (c1) separated from three neighbors (c2) by monolayer patches (RS/P = 10.6 wt %, pH = 4.9); (c3 and c4) examples of crown structures indicated by white arrows (RS/P = 8.0 wt %, pH = 6 for (c3) and RS/P = 10.6 wt %, pH = 8 for (c4)). Crowns are also visible in the contact zones of emulsion stabilized by Brownian silica particles (diameter = 790 nm) at pH 6: (d1) side view of 2 drops in contact; the particles remain clearly located in the hemisphere near the contact zone, leaving the other hemisphere completely uncovered; (d2) front view of 2 adhesive films between 3 drops; the particles concentrate at the border of the adhesive films, and the central circular zones are deprived of particles.

pH and Salt Cycles. To probe the reversibility of the two stability regimes, we performed cyclical variations of either the pH or the salinity of the aqueous phase (Supporting Information (10)). For pH cycling, the lower phase of an emulsion at pH = 3 (O/W 1 domain) was replaced, after creaming, by an aqueous solution at pH = 6. Without energy supply, after varying the pH, no significant evolution of the emulsion could be observed: the particles remained at the interface, fully protecting the large size drops. Nevertheless, if energy was brought to the system by hand-shaking, a new interface was created, and an emulsion with small drop size

characteristic of the O/W 3 domain was produced. The newly formed emulsion had features similar to those directly prepared at pH = 6 (same drop size, flocculated state, and low surface coverage). The same procedure was then applied to vary the pH from 6 to 3 with no energy supply. Only a gentle manual inclination of the vessel was applied to carefully homogenize the sample avoiding any drop fragmentation: spontaneously the emulsion evolved through coalescence within a few seconds toward the large drop size emulsion. The final stable state was identical to the starting one at the same pH (O/W 1 domain). 9323


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effective dipole moment oriented perpendicular to the fluid interface. Dipolar interactions between neighboring particles adsorbed at the same interface are repulsive because the dipoles are parallel. To explain relocation effects in emulsions, we recently proposed that dipoles originating from particles adsorbed at two opposite interfaces would induce an attraction between drops.45,48 Indeed, the interaction energy between antiparallel dipoles anchored on two opposite interfaces becomes increasingly attractive if the angle α between the dipoles direction and the particles center-to-center line is larger than 54.7° (see Supporting Information (12)). So the system tends to adopt a configuration where α takes the largest accessible value. This situation occurs when the particles anchored at one interface are in contact with the opposing interface. We are aware that a description based on pair interactions is certainly oversimplifying the complex nature of this issue. Nevertheless, the proposed mechanism may account not only for the electrostatic origin of the stability but also for particle settling in the contact zones and for the flocculated state of the emulsions due to attractive interaction between drops. In the remainder, we propose some arguments to account for the packing morphology of the particles based on the dipole attraction mechanism. Two packing types were evidenced in Figure 11: (i) crowns surrounding a circular depleted film and (ii) disks made of a compact monolayer. Such configurations have already been reported in experiments where model interfaces were brought into contact.29,52−55 For instance, Stancik and Fuller54 observed the behavior of particles adsorbed on a flat interface as a drop was approached. Horozov and coworkers29 examined vertical films stabilized by charged particles, which were either opened by incorporating fluid or closed by suction of the fluid. In both studies, the two morphologies (disk and crown) were identified. They resulted in the formation of a dimple and reorganization of the particles at the periphery of the dimple or at its center. From the study reported by Horozov et al.,29 it can be concluded that disks are preferentially observed for thick films or when the incorporation of fluid is slow, whereas crowns are obtained for thin film and for fast variations of the film thickness. In the low coverage regime, where the interfaces remain fluid, coalescence, droplet shape relaxation, and drainage are expected to occur rapidly, leading mainly to crowns. During the draining process, hydrodynamics should have a great impact on particle packing in the contact zone. To identify the conditions favoring the formation of crowns, we varied the oil viscosity, the oil type, the particle, and drop sizes, and we estimated by optical microscopy the crown diameters. The results are listed in Supporting Information (13). None of the studied parameters had a significant influence, suggesting that crown formation is a complex process involving coupled physical phenomena at a very short time scale. To determine the central depleted film morphology, the thickness of the protruding oil end-cap should be compared to the steric repulsion range arising from the particles composing the crowns. From simple geometrical considerations, we show that the undeformed curved caps of adjacent drops should be in contact and irremediably lead to coalescence (see Supporting Information (13) and (14)). As emulsions are stable, the bare oil−water interfaces must necessarily be submitted to a strong repulsive interaction. The stability of such films can only be explained considering that the two interfaces are flat and parallel and are supported by the particles located in the

The pH was varied at least five times with a good reproducibility of the resulting emulsions. An analogous experiment was performed to cyclically replace the aqueous phase of an initial emulsion belonging to O/W 3 domain (pure water RS/P = 8 wt %, pH = 6) by brine (0.1 M NaCl at pH = 6) (Supporting Information (10)). The replacement of the lower phase was carried out successively several times with the same solution to ensure a precise control of the salt concentration. As for pH cycling, the transition from small to larger drops occurred without an external energy supply, whereas the transition from large to smaller drops required manual shaking. For both stimuli (pH and salt concentration), the resulting sizes were reproducible at each cycle, showing that the emulsions can adopt two kinetically stable states, characterized by two different degrees of surface coverage, triggered by electrostatic interactions. Generalization of the Observations. Such bistability as a function of pH and brine was also observed with different particle types such as pH-sensitive silica spheres or NaMgF3 cubes. Both particle types can stabilize either highly covered individual drops (Figure 11a1 and b1) or poorly covered flocculated drops (Figure 11a2 and b2) depending on the pH or salinity of the aqueous phase.45,48 Preferential Location of the Particles (O/W 3 Domain). Because of their large size, we could visualize by optical microscopy the particles at the drop surface and in the contact regions between emulsion droplets. A common feature of the investigated systems was the preferred location of the charged particles in the vicinity of the drop contact zones (Figure 11 c and d). We observed that 3.5 μm-sized latex particles concentrated in the adhesive films between the flocculated drops leaving the rest of the drop interface completely unprotected (Figure 11c). Two types of organization were identified depending on the pH: the particles formed either dense monolayer patches for pH values close to the O/W 1− O/W 3 transition (Figure 11c1 and c3) or crowns surrounding an inner uncovered film when the pH was higher, close to the O/W 3−PS 2 transition (Figure 11c2 and c4). Crowns were also obtained in the presence of Brownian pHsensitive silica particles (Figure 11d). Despite their intense motion on the interface, the particles tended to remain located in the vicinity of the contact zone between two drops, leaving the rest of the interface completely uncovered as can be seen in Figure 11d1. Also, the localized diffusion was evidenced by the examination of the adhesive films, revealing that particles diffused within a limited region around a circular uncovered central film (Figure 11d2). Movies of these observations can be found in Supporting Information (11). Such preferential location of particles was also reported by Vignati et al.,49 who provided evidence for “reallocation effect” of Brownian particles associated with drops closeness. Furthermore, kinetically stable Pickering emulsions with low surface coverage have already been reported by several authors.38,45,49−51 The distribution of particles at an interface is determined by a large set of colloidal forces (van der Waals, capillary, electrostatic, dipolar, etc.). Different repulsive interactions have been inferred from the compressibility and the 2D spatial distribution on model particle-laden interfaces: (i) a long-range repulsion between particles may arise from the Coulombic force through the oil caused by surface charges,24 and (ii) according to Pieranski,25 the asymmetric distribution of the free ions around the interfacial particles in water should lead to an 9324


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surrounding crown. This configuration is probably induced by the electrostatic disjoining pressure acting in the thin liquid films. Electrostatic repulsion of bare alkane−water interfaces has already been reported in the literature56−58 and was attributed to the presence of hydroxyl groups. Roger et al.59 demonstrated that the charges in alkanes originate from oleic acid (carboxylic groups) type impurities at very low content (an amount as low as 1 mM is enough to cause charged interfaces). In the case of PDMS, charges may result from ionizable terminal groups of the silicone chains as well.60 The incorporation of charged particles in thin films delimited by charged surfaces of the same sign is electrostatically unfavorable. Under such conditions, crown formation under the effect of the above-described attractive dipolar interaction is the most probable scenario. As the surface charge densities of the particles and of the oil−water interfaces decrease, for instance, by lowering the pH, the electrostatic constraint is released, and the entrapment of particles in the thin films and thus the formation of patches become possible.

= 8.5, 10.6, and 13.9 wt %. (8) Detailed diagram and experimental values of contact angle as a function of RS/P and pH. (9) Incorporation of oil up to 88 vol % in the highly covered regime. (10) pH and salt cycles. (11) Movies showing the preferential location of the particles near the contact area between adhesive drops. (12) Proposed stabilizing mechanism for poorly covered drops based on dipolar interactions. (13) Crown diameters obtained for various systems. (14) Geometrical demonstration that the curved water/oil interfaces beyond the crows may come into contact. To ensure thin film stability, the surfaces must adopt a flattened configuration. (15) Examples of crowns observed by optical microscopy in various systems. This material is available free of charge via the Internet at http://pubs.acs.org.

CONCLUSION In this Article, we proposed a mapping of the metastable states accessible to oil/water/particle mixtures over a wide range of particle wettability and surface charge. This was achieved by varying the hydrophilic shell-to-hydrophobic core mass ratio of the particles and the pH of the aqueous phase. Two different metastable states were identified: (i) O/W or W/O emulsions with tightly packed particles at the interfaces; and (ii) O/W emulsions with very low interfacial coverage. The correlation between the emulsion type and the contact angle of the particles (θ < 90° for O/W emulsions and θ > 90° for W/O emulsions) at the interface was demonstrated, in agreement with Finkle’s rule. We demonstrated that the stability of the poorly covered regime is governed by electrostatics and is due to a preferential localization of the particles at the droplet junctions. We provided experimental evidence that the transitional phase inversion from O/W to W/O emulsion is not continuous. Instead, the O/W and W/O domains are spaced out by a region, where emulsions are unstable and undergo fast phase separation. In this domain, particles adopt two specific contact angles on the same interface. Up to our knowledge, this is the very first evidence of such “bipolar” behavior. In terms of practical applications, the existence of this unstable region is potentially exploitable to provoke the destabilization of the emulsions on demand. Finally, we demonstrated that for θ < 90°, depending on the salinity or the pH of the aqueous phase, two different metastable states characterized by very distinct interfacial coverages were accessible. The generality of this latter observation was verified considering particles of different nature (organic or inorganic), different sizes (from 100 nm to 3 μm), and different shapes (spheres or cubes).

Present Addresses

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*E-mail: [email protected]. *E-mail: [email protected].

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AUTHOR INFORMATION

Corresponding Authors

∥

L’Oréal, 11-13 rue Dora Maar, 93400, Saint-Ouen, France. Laboratoire IMRCP, UMR 5623, Université Paul Sabatier, 118 route de Narbonne, 31062 Toulouse cedex 9, France. # L’OCCITANE en Provence ZI Saint Maurice, 04100 Manosque, France. ⊥

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Alain Deré for his help in performing the TGA analyses, Serge Ravaine and Jean-François Dechézelle for their contribution in synthesizing silica particles and neighborite cubes, Hassan Sasaaoui for AFM experiments, and Marion Instaby for the systematic observations of the crowns using optical microscopy.

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REFERENCES

(1) Binks, B. P. Particles as surfactants - Similarities and differences. Curr. Opin. Colloid Interface Sci. 2002, 7, 21. (2) Leal-Calderon, F.; Schmitt, V. Solid-stabilized emulsions. Curr. Opin. Colloid Interface Sci. 2008, 13, 217. (3) Ramsden, W. Proc. R. Soc. 1903, 72, 156. (4) Pickering, S. U. CXCVI-Emulsions. J. Chem. Soc., Trans. 1907, 91, 2001. (5) Alargova, R. G.; Warhadpande, D. S.; Paunov, V. N.; Velev, O. D. Foam superstabilization by polymer microrods. Langmuir 2004, 20, 10371. (6) Binks, B. P.; Horozov, T. S. Aqueous foams stabilized solely by silica nanoparticles. Angew. Chem., Int. Ed. 2005, 44, 3722. (7) Binks, B. P.; Murakami, R. Phase inversion of particle-stabilized materials from foams to dry water. Nat. Mater. 2006, 5, 865. (8) Frelichowska, J.; Bolzinger, M. A.; Chevalier, Y. Pickering emulsions with bare silica. Colloids Surf., A 2009, 343, 70. (9) Arditty, S.; Schmitt, V.; Giermanska-Kahn, J.; Leal-Calderon, F. Materials based on solid-stabilized emulsions. J. Colloid Interface Sci. 2004, 275, 659. (10) Aveyard, R.; Binks, B. P.; Clint, J. H. Emulsions stabilised solely by colloidal particles. Adv. Colloid Interface Sci. 2003, 100-102, 503. (11) Aussillous, P.; Quéré, D. Liquid marbles. Nature 2001, 411, 924. (12) Murakami, R.; Moriyama, H.; Yamamoto, M.; Binks, B. P.; Rocher, A. Particle stabilization of oil-in-water-in-air materials: Powdered emulsions. Adv. Mater. 2012, 24, 767. (13) Destribats, M.; Lapeyre, V.; Wolfs, M.; Sellier, E.; LealCalderon, F.; Ravaine, V.; Schmitt, V. Soft microgels as Pickering

ASSOCIATED CONTENT

* Supporting Information S

(1) Synthesis of the core−shell latex particles. (2) Example of a TGA thermogram. (3) Table listing the shell wt % for all batches. (4) Scheme describing the film preparation technique with embedded particles. (5) Example of the use of AFM to observe a film comprising smaller particles. (6) Schemes describing the determination of the contact angle from geometrical considerations using either (a) SEM or (b) AFM. (7) ζ-Potential as a function of pH for latex particles with RS/P 9325


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