Article Cite This: Langmuir XXXX, XXX, XXX−XXX
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Role of Electrostatic Interactions in Oil-in-Water Emulsions Stabilized by Heteroaggregation: An Experimental and Simulation Study Manuella Cerbelaud,* Anne Aimable, and Arnaud Videcoq
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Université Limoges, CNRS, IRCER, UMR 7315, F-87000 Limoges, France ABSTRACT: Oil-in-water emulsion stabilization by heteroaggregation of hydrophilic particles without a surfactant is of importance in a wide range of applications; however, the stabilization mechanism is little described. To shed light on the early stage of the stabilization mechanism, a model system composed of an oil wax phase dispersed in water with oppositely charged colloidal particles is studied experimentally and numerically. Experiments show that the colloids do not penetrate deeply in the oil phase, suggesting that adsorption of the colloidal particles on the wax droplets is mainly due to electrostatic interactions. Experiments and Brownian dynamics simulations show also that when oppositely charged colloidal particles are present in the emulsion, a multilayer coating of heteroaggregated colloidal particles is formed around the wax droplets. This protective coating is expected to prevent from the oil droplet coalescence and therefore to stabilize the emulsion.
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INTRODUCTION Emulsions stabilized by colloidal particles, also named Pickering emulsions, are present in a variety of applications ranging from food and pharmacology to agriculture. They can also be used for material manufacturing, leading to specific microstructures and therefore new properties. For example highly porous ceramics and colloidosomes have been obtained by using emulsions stabilized by particles.1 Another example is the synthesis of Janus particles2 by trapping some inorganic particles at a wax/water interface. Stabilization of Pickering emulsions is generally achieved by the strong adsorption of particles at a liquid−liquid interface, which happens when particles are partially wetted by the two liquids. Stability depends on the contact angle value (θ) between the colloidal particles and the dispersed liquid phase, which is related to the penetration depth of the particles into the liquid droplets.3 The θ value can be varied by modifying the hydrophobicity of particles. Generally, to stabilize an oil-inwater emulsion, particles have to be hydrophobic enough to adsorb at the liquid−liquid interface, but not too much to not aggregate nor inverse the emulsion leading to the water-in-oil emulsion.3,4 The highest stability for an oil-in-water Pickering emulsion is obtained for a θ value comprised between 70° and 86°.5 Several approaches are proposed to modify the surface chemistry of colloidal particles to produce more stable Pickering emulsions, such as using surfactants4,6,7 or by a chemical grafting.8 Some authors have also shown that Pickering emulsions could be obtained by using very hydrophilic particles, without any addition of hydrophobic species to modify their surface chemistry. In that case, stabilization is promoted by using a suspension composed of oppositely charged particles leading to heteroaggregation. © XXXX American Chemical Society
Abend et al. have described stable Pickering emulsions stabilized by the heterocoagulation of clay minerals and layered double hydroxides.9 Binks et al. have obtained stable dodecane-in-water emulsions by using suspensions of silica and alumina particles, which were heteroaggregated.10,11 Similar systems were also used by Barg et al. to prepare cellular ceramic materials,12 and different microstructures were obtained depending on the nature of the particles, either heteroaggregated particles, or particles modified with a surfactant. Experimental and numerical studies have already been performed to understand the mechanism of emulsion stabilization by heteroaggregation. Nallamilli et al. have studied the stabilization of Pickering emulsions with oppositely charged latex particles. They have observed that stabilization is obtained when a close-packed monolayer of particles around droplets is formed.13 Moreover, they have shown that the droplet size depends on the composition of the heteroaggregated suspension used to stabilize the emulsion.13,14 Pushpam et al. have also performed Monte Carlo simulations to determine the patterns formed in Pickering emulsions stabilized by oppositely charged colloids.15 However, all these studies use partially hydrophobic colloids, and the stabilization mechanism of Pickering emulsions by heteroaggregation of hydrophilic colloids remains very little presented and described. In that context, this paper proposes a study of the mechanisms underlying the stabilization of Pickering emulsions by using heteroaggregated systems of hydrophilic colloidal particles, especially at the early stage, to better Received: August 28, 2018 Revised: November 28, 2018 Published: December 3, 2018 A
DOI: 10.1021/acs.langmuir.8b02922 Langmuir XXXX, XXX, XXX−XXX
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Langmuir understand and control the microstructure of the final materials. An experimental model system is investigated composed of oppositely charged silica particles mixed with paraffin wax. This system presents the advantage to keep the emulsion as it is formed because wax droplets solidify when the temperature decreases. It is also interesting because contact angles between colloidal particles and the wax phase can be measured, as already done in refs 2 and 16, which is generally more difficult to do when the two phases remain liquid. This paper is a comparative study, with an experimental part presenting the results of Pickering emulsions prepared with different mixtures of colloidal particles and their characterizations. Then, in a second part, Brownian dynamics simulations are used to better understand the role of the interactions between the different components and the local organization of the particles around the wax droplets in the Pickering emulsions.
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Figure 1. Zeta potential as a function of pH. Si−F: silica marked by FITC dye and Si−R: APS surface modified silica marked by RIBTC dye.
EXPERIMENTAL STUDY
be adjusted with HCl to pH 6 to get a zeta potential value ζ1 = 42 mV for the Si−R particles, and ζ2 = −53 mV for the Si−F population. Heteroaggregation between Si−R and Si−F is thus expected. Emulsion Preparation. Pickering emulsions using paraffin wax as the dispersed phase are prepared. To probe the effect of the suspensions compositions, binary suspensions of Si−F and Si−R particles are prepared with various proportions of the two populations. In the following, the different suspensions will be identified by the Si−R amount (R = mSi−R/(mSi−R + mSi−F)). A suspension containing only Si−R particles will be noted R = 100% whereas a suspension containing only Si−F particles will be noted R = 0%. In total, 140 mg of silica powder with defined ratios of Si−R and Si−F are dispersed in 14 mL of osmosed water using an ultrasonic treatment. As already mentioned, the pH of suspensions is adjusted to 6 to promote heteroaggregation. After their preparation, it is observed that suspensions composed of only one kind of particles are quite stable whereas when mixed suspensions are used, a fast settling is observed in the vessel in agreement with the presence of heteroaggregation (see Figure 2a). To prepare the emulsions, 1 g of paraffin wax was added to the silica suspension and the whole was heated at 90 °C, above the melting point of the wax which is around 60 °C. When wax is melted, emulsions are obtained by using ULTRA-TURRAX stirring at 19 000
Materials. Tetraethyl orthosilicate 99% (TEOS), 3-aminopropyltriethoxysilane 99% (APS), fluorescein isothiocyanate isomer I 90% (FITC), and rhodamine B isothiocyanate (RBITC) were purchased from Sigma-Aldrich (Germany). Ammonium hydroxyde 28% was supplied by VWR. Absolute ethanol was obtained from BDH Prolabo. Paraffin wax used for emulsion preparation was purchased from Fisher Scientific. To compare emulsions stabilized by heteroaggregation with other stabilized emulsions, the surfactant didodecyldimethylammonium bromide (DDAB) from Sigma-Aldrich and the alumina powder (ref. AKP30) supplied by Sumitomo Chemical Company (d50 = 400 nm, purity 99.99%, specific surface area 7 m2 g−1) were also used. Silica Particle Synthesis. In this study, we used home-made fluorescent silica particles. The silica synthesis protocol is described in detail in ref 17. It is a modified Stöber method based on two steps. First, monodispersed silica particles are synthesized by the Stöber method, which consists in the hydrolysis of TEOS in an ethanol/ water/ammoniac mixture. According to ref 17, at this step, particles are spherical and their diameter is around 500 nm. Then, the fluorescent dyes (RIBTC or FITC), previously bonded to APS as a coupling agent, are introduced to label the silica particles. The fluorescent dyes being small compared with the size of the synthesized silica particles, the functionalization does not alter significantly the size of silica particles. The second step of the synthesis consists in growing a silica shell by a dropwise addition of TEOS until the final particle diameter attains around 600 nm. The final silica particles are thus core−shell structures, which reduces the presence of the organic fluorescent dyes on their surface. Following the protocol described in ref 17, in this study, two different types of particles were synthesized: one labeled with the RIBTC dye (Si−R) leading to red fluorescent particles, and one with the FITC dye (Si−F) leading to green fluorescent particles. Analysis of size distribution performed on 100 silica particles by image analyses gives a mean diameter of 571 ± 65 and 598 ± 56 nm for the Si−F and Si−R particles, respectively. Naturally, both populations of silica particles are negatively charged. To obtain particles with a positive surface charge, the RIBTC-grafted population of silica is modified with APS. Si−R powder (1.48 g) and 3.7 g of APS are introduced in 200 mL of osmosed water in a beaker and left under a magnetic stirring for 24 h. After that time, the particles are rinsed three times with osmosed water to remove the excess of APS and left for drying at 60 °C over the night. Zeta potential of silica particles was measured using AcoustoSizer IIs from Colloidal Dynamics. pH was adjusted by adding HCl 0.5 M or NaOH 0.25 M. Results are shown in Figure 1. Si−F is negatively charged over a wide range of pH as expected for silica. Its isoelectric point is around pH 3.5. Si−R has an isoelectric point around 8, which is expected from the APS grafting, leading to a positive surface charge between pH 4 and 8. In the following, the pH of the suspensions will
Figure 2. (a) Appearence of the suspensions obtained after 5 min as a function of the Si−R amount (R) and (b) pictures of the corresponding emulsions after 24 h (s = 0.14). B
DOI: 10.1021/acs.langmuir.8b02922 Langmuir XXXX, XXX, XXX−XXX
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Langmuir rpm during 1 min and then cooled down to room temperature, so that paraffin wax becomes solid. In the following, the ratio between the mass of silica (ms) and the mass of paraffin wax (mw) will be expressed by s = ms/mw. If nothing is mentioned, s = 0.14 (1 g of paraffin for 140 mg of silica particles). Characterization Techniques. Just after preparation, emulsion solid droplets are observed by optical microscopy. The droplets size was determined by image analysis performed with ImageJ. Because particles are fluorescent, emulsions are observed by an epifluorescence microscope Axio Imager 2 from Zeiss. These observations are performed just after the emulsion preparation. The resolution of the microscope is not high enough to distinguish clearly the positioning and the organization of particles in the emulsion; however, it allows to check the distribution of both populations (Si− R; positively charged particles in red, and Si−F; negatively charged particles in green) around the solidified wax droplets. For a better resolution, suspensions are also observed by a confocal microscope, LSM880 from Zeiss. Observations are performed between 2 and 5 h after their preparation, and images are treated with ImageJ. The emulsions are also observed with a scanning electron microscope (SEM) MEB-Quanta FEG 450 in the low vacuum mode. A droplet of emulsion is deposited on a sample stub and is directly observed without any metalization. Experimental Results. To better understand the role of the particles in the emulsion stabilization, an emulsion was prepared without silica particles. It is observed that, after cooling, a film of paraffin is formed at the surface of the emulsion (see Figure 3). The
Figure 4. Optical microscopy images of emulsions obtained with s = 0.14 and different ratios of Si−F and Si−R particles as a function of the Si−R amount (R): (a) R = 25%, (b) R = 50%, (c) R = 75%, and (d) R = 100%.
Figure 5. (a) Picture of the emulsion obtained with alumina AKP30 after 15 min and (b) corresponding optical microscopy image. positive alumina particles are also obtained. Solid wax droplets are observed by optical microscopy, proving that APS is not necessary to stabilize wax-in-water emulsion. Using positively charged particles like pure alumina leads to stable Pickering emulsions. Analysis of size distribution performed on 200 droplets by optical microscopy is shown in Figure 6a. The droplets size is found to decrease with the increase of R. More interesting, a very large distribution of droplets size is observed, with a relative standard deviation reaching 55%. To understand the role of the solid content on the droplets size, different emulsions with R = 50% have been prepared by varying the mass of paraffin wax (i.e., s). For the ratios s < 0.047, a thin film of paraffin is observed at the surface of emulsions, which let us think that there are not enough silica particles to stabilize the emulsions. For the ratios s ≥ 0.047, the analysis of the droplets size is shown in Figure 6b. The size of droplets decreases when s increases. As previously, a large distribution of sizes is also observed. These results will be discussed in the following. Emulsions prepared with fluorescent silica particles have been observed by epifluorescence and confocal microscopy. Pictures obtained with different ratios R and s = 0.14 are shown in Figure 7. Because of the low resolution and the Brownian motion in water, it is not possible to distinguish clearly each silica particle. However, these techniques allow checking the distribution of particles around solid wax droplets. When a mixture of positive and negative silica particles are present, solid wax droplets are always covered by both types of particles, as shown by the presence of fluorescein (green) and rhodamine (red) lights on the surface of droplets. The size of the fluorescent layer is comprised between 1300 and 1800 nm, which corresponds to a stack of two or three silica particles. When R = 100%, droplets are covered by Si−R particles, which proves that positive particles can adsorb onto the wax droplets and stabilize the Pickering emulsion. In that case, the fluorescent layer is thinner (around 650 nm) than that with heteroaggregated particles and corresponds to a monolayer of Si−R particles. To compare these emulsions based on an heteroaggregation mechanism of colloidal particles with more classical emulsions, where particles are modified with a surfactant to increase their hydro-
Figure 3. Pictures of an emulsion prepared with 1 g of paraffin and 14 mL of a HCl solution at pH 6 without silica particles. A film of solid paraffin is obtained at the surface. presence of this film proves that this emulsion prepared only with water and paraffin is not stable. Paraffin droplets quickly coalesce during cooling forming this film. Pictures of emulsions obtained with s = 0.14 and different ratios of Si−F and Si−R (R) are shown in Figure 2b. Corresponding optical microscopy images are presented in Figure 4. When only negatively charged silica particles are used (R = 0%), the emulsion is not stabilized and a film of solid paraffin wax is observed at the surface as in the case of the emulsion prepared without silica particles. In all other cases (R ≥ 25%), an emulsion is obtained, solid wax droplets are stabilized and can be clearly observed by optical microscopy. Contrary to what was observed by Binks et al.,10 positive particles only are able to promote the wax-in-water emulsion (see R = 100%). In this case, positive particles are modified with an APS chemical grafting on their surface, which could modify a little bit their hydrophobicity. To verify if the presence of APS explains more the emulsion stabilization than the positive surface charge, another system is used for comparison. Suspensions are prepared with AKP30 alumina particles from the Sumitomo Chemical Company. The pH of the aqueous suspension was adjusted to 6, where this powder is positively charged (see ref 18). Results of emulsification are shown in Figure 5. Emulsions prepared with only C
DOI: 10.1021/acs.langmuir.8b02922 Langmuir XXXX, XXX, XXX−XXX
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Figure 6. (a) Analysis of the solid wax droplet sizes as a function of R for s = 0.14. The mean droplet size obtained for emulsions prepared with DDAB modified silica particles is reported in green. (b) Analysis of the solid wax droplet sizes obtained with R = 50% as a function of s = ms/mw. The size is determined by image analysis of optical microscopy images. Results are averaged on 200 particles. The relative standard deviation is noted directly in the graph.
Figure 8. (a) Picture of the emulsion obtained with silica particles modified with DDAB after 15 min and (b) corresponding optical microscopy image. without a surfactant, it is also observed that the droplet size distribution is narrower. This point will be discussed in more details in the following. Emulsions were also observed after aging (45 days) by SEM in low vacuum mode (see Figure 9). This mode imposes less stresses to the
Figure 7. Epifluorescence microscopy images (first column) and confocal microscopy images (second and third column) of emulsions obtained with Si−F and Si−R particles as a function of the Si−R amount (s = 0.14): (a−c) R = 25%; (d−f) R = 50%; (g−i) R = 75%, and (j−l) R = 100%. Positive Si−R particles marked with rhodamine appear in red, and negative Si−F particles marked with fluorescein appear in green. Pictures in the first column show several wax droplets, pictures in the second column show a single wax droplet, and pictures in the third column show a zoom of the wax droplet surface. In all cases, silica particles are adsorbed on the wax droplets.
Figure 9. SEM pictures of different emulsions aged 45 days: (a,d) obtained with Si−F and Si−R particles at R = 50%; (b,e) obtained with Si−R particles R = 100% and (c,f) obtained with DDAB modified Si−F particles. samples, which is necessary to observe a system where an organic phase is present (here paraffin wax). SEM pictures show that with aging, when no surfactant is used, silica particles detach from the surface of solid wax droplets. Some of them become indeed completely nude, whereas just after preparation, epifluorescence microscopy showed that droplets were completely covered by colloidal particles. One interesting point is that, after being detached from the surface, no clear marks are observed into the solid wax surface. Indeed, in case of stabilization with DDAB, particles penetrate the liquid oil phase during the emulsification process and remain trapped inside the solid wax after cooling. When they are removed from the solid droplets, by ethanol rinsing, for example, some spherical marks are observed on the surface of the solid phase. In
phobicity, some emulsions were prepared with Si−F particles modified with DDAB. This system was chosen because it was already used for wax-in-water Pickering emulsions with silica particles.2 For that, silica suspensions were prepared with 140 mg of Si−F silica particles and 0.14 mg of DDAB dispersed in 14 mL of osmosed water. This suspension was left under magnetic stirring during one night. Then, the emulsion was prepared as described in the previous section with 1 g of paraffin. One emulsion obtained with DDAB is shown in Figure 8a. As expected, the emulsion is stable and wax droplets are observed by optical microscopy. They look not totally spherical and are wrinkled. Analysis of size distribution performed on 200 droplets is reported in Figure 6. By comparison with emulsions stabilized D
DOI: 10.1021/acs.langmuir.8b02922 Langmuir XXXX, XXX, XXX−XXX
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Figure 10. (a) Interaction potentials between different silica particles and (b) interaction potentials between silica and paraffin wax.
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previous studies, the diameter of these marks was used to determine the contact angle value.2,16 Therefore, it can be concluded that the particles without a surfactant do not penetrate into the liquid wax during the emulsification process. For the emulsion stabilized by DDAB-modified silica particles, no detachment is observed after 45 days. Particles are strongly adsorbed. Previous studies have indeed shown that for this quantity of DDAB, silica particles penetrate the wax,16 which could explain that there is no release of these particles with time, and thus no aging phenomenon of the emulsion. From all these experimental results, it can be concluded that the stabilization mechanism of Pickering emulsions by heteroaggregation of hydrophilic particles is different from classical stabilization obtained using colloidal particles with a defined hydrophilic/hydrophobic balance. When hydrophobic particles are used, it is indeed well known that stabilization of emulsions is obtained when particles are deeply penetrated into the droplets which leads to a strong adsorption energy defined from3 ΔG = πa2γ(1 − cos θ )2
BROWNIAN DYNAMICS SIMULATIONS
Simulation Method. The system is simulated thanks to Brownian dynamics,21 which has already been used to study Pickering emulsions.22 A cubic box with periodic boundary conditions is used. The time step is fixed at 10−7 s, and results are analyzed at t = 30 s. In this study, simulations are performed with 1 wax droplet of 10 μm and 2600 silica particles of 600 nm. At the beginning, only 600 silica particles are randomly placed in the simulation box avoiding overlapping. Then, every 5 × 10−3 s, one silica particle is randomly inserted into the system to mimic the progressive adsorption of particles during the mixing. Thus, in these simulations, only the local environment of the droplet is considered to understand the structuration of the particles. Heteroaggregation in bulk, which should be perturbed by the emulsification, is not taken into account. Interactions between the components are considered to be only electrostatic and are described by a Derjaguin−Landau−Verwey−Overbeek (DLVO) potential,23,24 which is developed for colloids in bulk. For emulsions stabilized by partially hydrophobic particles which penetrate deeply the oil phase, it is often necessary to consider other interaction potentials adapted for particles at a surface as done in ref 15. However, as already mentioned, in the present study, the silica particles do not penetrate the oil droplets and therefore the DLVO potential seems sufficient. The DLVO potential used in this study is the same as in our previous studies18
(1)
where a is the radius of particles, γ is the interfacial tension, and θ is the contact angle. This energy can indeed be considerably large with contact angles near 90° (more than several 103kBT). In these conditions, particles cannot not desorb which promotes the emulsions stabilization. However, when θ is lower than 30°, the adsorption energy is considered as negligible, and oil-in-water emulsions are generally not stable.3 In this study, when emulsions are stabilized by heteroaggregation, it is observed that particles do not penetrate the solid wax; therefore, adsorption energy is very low and cannot explain the emulsion stabilization. This low adsorption energy is also responsible for the detachment of particles during the emulsion aging. In conclusion, in these systems, stabilization mechanism may be due to the electrostatic interactions instead of a strong adsorption into the oil phase. Indeed, positively charged particles can electrostatically adsorb on the wax droplets, which are negatively charged.20 The difference of adsorption energies can also explain the droplet shapes observed in Figures 4, 8 and 9, where emulsions are stabilized by heteroaggregation or by using the DDAB surfactant. During the cooling of emulsions, the density of wax droplets will increase inducing a shrinkage. If silica particles are strongly adsorbed on their surface (i.e., when DDAB is used), they cannot desorb, which leads to creases on the surface when solidification occurs. On the contrary, if silica particles are only electrostatically adsorbed on the droplets surface (as supposed for the emulsions stabilized by heteroaggregation in this study), when solidification occurs, particles will rearrange themselves allowing to save a spherical shape of droplets. In the following, Brownian dynamics simulations will help discussing the mechanism of stabilization for these different systems, to better understand particles arrangement and final microstructures.
VijDLVO = VijHamaker + VijHHF
with VijHamaker
(2)
ÄÅ Aij ÅÅÅÅ 2aiaj 2aiaj = − ÅÅÅ 2 + 2 2 6 ÅÅÅ rij − (ai + aj) rij − (ai − aj)2 Ç É ij rij 2 − (ai + aj)2 yzÑÑÑÑ zzÑÑ + lnjjjj 2 zÑ j rij − (ai − aj)2 zzÑÑÑ (3) k {ÑÖ
and E
DOI: 10.1021/acs.langmuir.8b02922 Langmuir XXXX, XXX, XXX−XXX
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Figure 11. Simulations snapshots at t = 30 s: (a) R = 0%, (b) R = 25%, (c) R = 50%, (d) R = 75%, and (e) R = 100%.
Figure 12. Number of silica particles situated at a surface−surface distance rws from the wax droplet surface at t = 30 s: (a) R = 0%, (b) R = 25%, (c) R = 50%, (d) R = 75%, and (e) R = 100%.
VijHHF
ÄÅ ÅÅ 2ψψ ij 1 + exp( −κhij) yz Å i j jj zz ln = πεaeff (ψi 2 + ψj 2)ÅÅÅÅ 2 j z ÅÅ ψ + ψ 2 jj 1 − exp( −κhij) zz j k { ÇÅÅ i ÉÑ ÑÑ Ñ + ln(1 − exp( −2κhij))ÑÑÑÑ ÑÑ ÑÑÖ (4)
surface to surface interparticle distance between particles i and j and aeff = aiaj/(ai + aj). The zeta potential used for the liquid wax is fixed at ψw = −60 mV20 and the Hamaker constant for wax−wax interactions to Aww = 6 × 10−20 J.25 For silica particles, previously experimentally determined zeta potentials are used; therefore, ψSiF = −53 mV is used for negative Si−F particles and ψSiR = 42 mV is used for positive Si−R particles. The Hamaker constant is taken at Ass = 4.6 × 10−21 J for silica particle interactions and to A ws = A ww A ss = 1.66 × 10−20 J for paraffin and silica
where Aij is the Hamaker constant, ϵ is the dielectric constant, κ is the inverse Debye length, ψi is the surface potential of the particles (assimilated here to the zeta potential), and hij is the F
DOI: 10.1021/acs.langmuir.8b02922 Langmuir XXXX, XXX, XXX−XXX
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Langmuir particle interactions. A temperature of T = 363 K and a viscosity of ν = 3 × 10−4 Pa·s are considered (emulsions are prepared at 90 °C). Interaction potentials used for this study are shown in Figure 10. The graph Figure 10a indicates that interactions between identical silica particles are repulsive, whereas they are attractive between different silica particles. The graph Figure 10b shows that Si−F and paraffin interactions are repulsive, whereas interactions between Si−R particles and paraffin are attractive. VDLVO is infinite at the ij contact between particles. As in our previous studies, DLVO is thus cut at −14kBT at short interparticle distance to avoid divergence during simulations and a linear repulsive potential is added to avoid the interpenetration.22 Numerical Results. Simulation snapshots taken at t = 30 s are shown in Figure 11. In this numerical study, because the droplet size and the colloid number are fixed, simulations cannot be used directly to understand the evolution of size according to the composition; however, they show the local organization of particles around droplets, which minimizes the energy and therefore the formation of the covering that can be expected in experiments. To observe silica particle arrangements on the wax droplet surface, the number of silica particles whose surface is situated at a distance rws from the wax surface is reported as a function of rws in Figure 12. In simulations, it is clearly observed that when no positive particles are present, no adsorption on the wax droplet surface is obtained, which is in agreement with the previous experimental characterizations and the results of literature, which show that emulsions are not stable.10 When only positive silica particles are present, it can be observed that a single layer of adsorbed silica is obtained around the wax droplet. It is in agreement with the experimental results obtained for R = 100% as shown in Figure 7. This layer is not compact, which is explained by the strong repulsion between the Si−R silica particles. Let us now discuss the results found in the literature. Binks et al. have indeed shown that dodecane-in-water emulsions are not stabilized with only positive particles.10 The difference in the wax-in-water emulsion is that just after emulsification, the wax droplets are solidified, the liquid paraffin becoming solid at ambient temperature. This avoids any later coalescence. In the case of dodecane-in-water system, positive particles may electrostatically adsorb onto the oil surface droplets, as it is explained in ref 22. Because of the low covering of the droplets and the strong rearrangement of particles around droplets, coalescence should not be hindered when two liquid oil droplets encounter, leading to a nonstable emulsion. With R = 25, 50 and 75%, simulation results show that both positive and negative silica particles are present on the wax droplet surface, which is in agreement with the epifluorescence and confocal microscopy observations (see Figure 7). By analyzing more precisely their mutual arrangement in Figure 12, it is observed that particles form a multilayer coating on the wax surface. In all cases, a first layer of positive particles is found directly in contact with the wax (see rws = 0) and a second layer of negative particles is then formed. When positive particles are in large quantity, a third positive layer can also be found (see R = 50 and 75%). This organization is explained by the nature of interactions between the various components. The first layer is formed thanks to the strong attraction between the negative wax droplet and positive Si−R particles, as in the case of R = 100%. Then, negative particles Si−F are attracted by Si−R adsorbed particles by hetero-
aggregation. This new layer can in turn attract positive particles to form a third layer. This mechanism allows the formation of heteroaggregated layers of particles on wax droplets leading to a good covering. The thickness of the silica layer formed in simulations can be determined from Figure 12. By adding a distance of d = 600 nm (diameter of the silica particles) to the highest distance rws where a pick in the curve is observed, a thickness of around 1.85, 1.7, 1.7, and 0.67 μm is obtained, respectively, for R = 25, 50, 75, and 100%. These thicknesses are in agreement with the experimental observations (see Figure 7), which suggests that the number of silica layers formed on the droplets in simulations is similar to those obtain experimentally. The multilayer coating formation around the wax droplets let us think that emulsion stabilization in the presence of heteroaggregation is due to the formation of a protective layer around the droplets or/and a gelification of the colloidal suspensions as already proposed in other studies. 9,26 Gelification is indeed a widely observed phenomenon in heteroaggregated suspensions.27
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DISCUSSION Numerical simulations and experimental characterizations show that the stabilization of emulsions by heteroaggregation seems more dictated by electrostatic interactions than by the hydrophobicity of particles. Let us now discuss and explain different behaviors due to this kind of stabilization. On the one hand, it has already been explained that because electrostatic interactions are weak, particles are not strongly adsorbed on the wax surface, which leads to more spherical droplets than when particles are strongly adsorbed. On the other hand, it is well known that when emulsions are stabilized by partially hydrophobic colloids which are insufficient to fully cover the oil−water interfaces, the range of droplets size is generally narrow (three to one) and in this case, coalescence is generally referred as “limited coalescence”.19 This phenomenon is explained by a strong attachment of colloids at the interface in such a way that colloids are likely irreversibly adsorbed. Results obtained in Figure 6 with emulsions stabilized by DDAB show indeed a low dispersion in the droplet sizes with a size ratio between the smallest and the largest droplets around 3, confirming that in this case coalescence can be considered as limited. However, in Figures 4 and 6, a large dispersion in the droplet sizes is clearly observed. A factor above 8 up to 19 between the sizes of the smallest and the largest droplets is observed depending on the composition. In this case, limited coalescence is thus unlikely. As already mentioned, in these emulsions, particles adsorb weakly at the interface and desorption is likely, which favors the coalescence of droplets. We will now discuss the size of droplets obtained in Figure 6. In refs 13 and 14, Nallamilli et al. have shown the dependence of droplet size in emulsions stabilized by oppositely charged latex particles. They observe that the size of droplets decreases when the ratio between the opposite particles increases up to R = 50%, and then, the droplet size increases with R until R = 100%. This behavior has been explained by modifying the theory of limited coalescence, taking into account the concentration of the heteroaggregates.14 The kinetics of heteroaggregation between colloids is indeed fast as shown in ref 28, and aggregates are supposed to be the entities which stabilize the droplets. G
DOI: 10.1021/acs.langmuir.8b02922 Langmuir XXXX, XXX, XXX−XXX
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Langmuir Similar to the observations of Nallamilli et al.,14 Figure 6 shows that when R ≤ 50% the droplets size decreases with R. However, when R > 50%, that is, when Si−R particles are in excess, the droplets size continues to decrease as a function of R in contrast to the observations of Nallamilli et al. One of the reasons of this difference can come from the fact that in this study positive Si−R particles are able by themselves to stabilize emulsions which is not the case of positive particles used by Nallamilli et al. In our study, the size of emulsion droplets depends on the mass ratio between the positive particles (mrh) and the paraffin, noted in the following as rh = mrh/mw. Data obtained in Figure 6 have been plotted as a function of rh in Figure 13. It is clearly observed that the size of droplets is
well known that particles can penetrate into the oil forming a monolayer of particles if they do not aggregate. Such emulsions are then stabilized thanks to a good covering of the droplets and a high energy of desorption of particles.3 Because only a monolayer of particles around the droplets can stabilize the emulsion, this system can lead to thin interconnectivity in final materials obtained after removing the liquid phases. On the contrary, if heteroaggregation is used to stabilize the emulsion, as previously shown experimentally and numerically, a multilayer coating of particles will form on the surface of the droplets and the connectivity will be larger in the final material. This multilayer formation is allowed only because particles are mainly hydrophilic and therefore do not adsorb strongly on the oil droplet. In case of partially hydrophobic particles, the adsorption of particles has to be taken into account because the adsorption energy becomes high and dominates. If all the particles are partially hydrophobic, the limited coalescence theory can be applied and monolayers of particles can be obtained. The heteroaggregation should then be responsible for different patterns of the layer. Nallamilli et al. have indeed observed patterned monolayers of particles around droplets in Pickering emulsions stabilized by oppositely charged latex particles.13 If heteroaggregation is obtained with both hydrophobic and hydrophilic particles, a more complicated mechanism of stabilization has to be considered. The hydrophobic particles will indeed penetrate the oil phase, whereas the hydrophilic particles will remain in suspension promoting sometimes the destabilization of the emulsion as shown by Whitby et al.29
Figure 13. Analysis of the solid wax droplet sizes as a function of ratio between the mass of the Si−R silica particles and the mass of paraffin rh. In red, data with R = 50% and different ratios s, and in green, data with s = 0.14 and different ratios R.
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CONCLUSIONS The stabilization of emulsions by heteroaggregation using hydrophilic particles has been studied experimentally and numerically thanks to a model system consisting of a wax-inwater emulsion stabilized by silica particles. Because of the solidification of the wax, this system allows to keep the emulsion as it is formed, preventing from long-time coalescence of oil droplets. On the one hand, experimental characterizations have shown that wax droplets present a wide size distribution and that particles do not penetrate the wax, revealing that hydrophobicity is very low in the system and should not be responsible for the droplet stabilization. On the other hand, numerical simulations show that the electrostatic interactions between various silica particles and wax droplets play a key role in the droplet stabilization. Because of attractive interactions between the oil droplet and positive particles, these latest can form a layer around the droplet, allowing then the attraction of the negative particles present in the emulsion. The build-up of a multilayered structure forms a protective layer which can also gelify preventing further from the droplet coalescence. This study contributes to our understanding of the stability of oil-in-water emulsions by heteroaggregation. It reveals that the hydrophobic/hydrophilic nature of particles has to be considered to understand the mechanism of emulsion stabilization by heteroaggregation. It can help experimentalists to understand microstructures of their emulsions and of the products manufactured from them. For example, in case of porous ceramics processed by a Pickering emulsion route, this study can help to understand the thickness of the interconnectivity and the pore size distribution according to the nature of components.
similar for the same ratios rh, and the droplet size decreases when rh increases. According to this remark, the quantity of positive particles which can adsorb directly on the paraffin oil should fix the size of droplets and the other particles form the multilayer structure. In this case, the majority of Si−R particles introduced in emulsions will stabilize the droplets, whatever the quantity of Si−F particles introduced. Si−F particles will then form a second layer. In addition, if some Si−R particles remain in bulk, because of for example an insufficient emulsification, these particles will be attracted in turn by the adsorbed Si−F particles to form a third layer. With this scenario, when positive and negative particles are present, large multilayers around droplets are unlikely. It is expected that multilayers are formed of 2 to 3−4 layers, as observed in the confocal microscopy images (Figure 7). Moreover the second layer will be more compact when Si−F is in excess. When Si− R particles are indeed in minority, they will be adsorbed on droplets and then fully covered by the Si−F particles. On the opposite, when Si−R particles are in excess, they will adsorb in priority on the droplets and the Si−F particles which are in minority will not fully cover the adsorbed layer. The electrostatic model used in this study also allows explaining the structure of cellular ceramics presented in ref 12. In this paper, microstructures of cellular ceramics obtained from emulsified suspensions stabilized either by surfactant molecules or by heteroaggregated colloidal suspensions are compared. One of the main differences in the porous ceramic structure rises in the interconnectivity between pores. When a surfactant is used, the interconnectivity is thin whereas when heteroaggregated suspensions are used interconnectivity is larger. This difference can be explained by the stabilization mechanism, which is different. When a surfactant is used, it is H
DOI: 10.1021/acs.langmuir.8b02922 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Manuella Cerbelaud: 0000-0001-6934-1872 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
The authors thank CALI and its team for computing facility (CALI has been financed by the region Limousin, the institutes XLIM, IPAM, GEIST, and the University of Limoges). The ̈ Hyvernaud, and authors also thank Claire Carrion, Eloise Amandine Magnaudeix for their help on the different microscopes. Figure 11 has been obtained by VMD, a molecular graphics program originally designed for the interactive visualization and analyses of biological materials, developed by the Theoretical Biophysics Group in the Beckman Institute for Advanced Science and Technology at the University of Illinois at Urbana-Champaign.30
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DOI: 10.1021/acs.langmuir.8b02922 Langmuir XXXX, XXX, XXX−XXX
