2D Particles at FluidâFluid Interfaces: Assembly and Templating of

Review Cite This: ACS Appl. Mater. Interfaces 2018, 10, 21765−21781

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2D Particles at Fluid−Fluid Interfaces: Assembly and Templating of Hybrid Structures for Advanced Applications Peiran Wei, Qinmo Luo, Katelynn J. Edgehouse, Christina M. Hemmingsen, Bradley J. Rodier, and Emily B. Pentzer*

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Department of Chemistry, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States ABSTRACT: Fluid−fluid interfaces have widespread applications in personal care products, the food industry, oil recovery, mineral processes, etc. and are also important and versatile platforms for generating advanced materials. In Pickering emulsions, particles stabilize the fluid−fluid interface, and their presence reduces the interfacial energy between the two fluids. To date, most Pickering emulsions stabilized by 2D particles make use of clay platelets or GO nanosheets. These systems have been used to template higher order hybrid, functional materials, most commonly, armored polymer particles, capsules, and Janus nanosheets. This review discusses the experimental and computational study of the assembly of sheet-like 2D particles at fluid−fluid interfaces, with an emphasis on the impact of chemical composition, and the use of these assemblies to prepare composite structures of dissimilar materials. The review culminates in a perspective on the future of Pickering emulsions using 2D particle surfactants, including new chemical modification and types of particles as well as the realization of properties and applications not possible with currently accessible systems, such as lubricants, porous structures, delivery, coatings, etc. KEYWORDS: Pickering emulsion, 2D particle, graphene oxide, Janus, composites

1. INTRODUCTION Two-dimensional (2D) particles, i.e., platelets or sheets, have garnered interest for a variety of applications due to their distinct and complementary properties compared to those of more common 0D and 1D particles (i.e., spheres and rods). For example, synthetic polymers reinforced by clay platelets have enhanced mechanical1 and gas barrier properties,2 whereas graphene nanosheets have been slated to revolutionize materials research.3 One application of 2D particles is stabilization of the fluid−fluid interface in Pickering emulsions (Figure 1). Emulsions are critically important in foods, cosmetics, biosensing, drug delivery, coatings, etc.4,5 and are composed of two immiscible liquids, in which a continuous phase of one contains dispersed domains of the other (i.e., droplets). Conventional emulsions (droplet size > 500 nm) are thermodynamically unstable but can be rendered kinetically stable by a small molecule or particle that decreases the interfacial tension. In comparison to emulsions stabilized by small molecules, those stabilized by solid particles (Pickering emulsions) typically have superior stability, lower toxicity, and can be stimuli-responsive.6 Most Pickering emulsions are composed of oil droplets dispersed in water (i.e., oil-in-water) and are prepared by dispersing particles in water, adding an organic solvent (oil), and then agitating the mixture by vortex, bath sonication, or shearing. Emulsions form when particles reside at the fluid− fluid interface rather than disperse in one phase; 2D particles are expected to lay flat at the fluid−fluid interface because of their aspect ratio (Figure 1) and thus are less likely to desorb © 2018 American Chemical Society

Figure 1. Illustration of Pickering emulsion stabilized by 2D particles and application of these assemblies to template armored particles, capsules, and Janus particles.

compared to small molecules or spherical particles. The choice of oil used to prepare an emulsion is dependent on the particle characteristics as well as application: the oil can be passive (e.g., hexane), contain a reagent (e.g., a diisocyanate dissolved Received: May 2, 2018 Accepted: June 13, 2018 Published: June 13, 2018 21765

DOI: 10.1021/acsami.8b07178 ACS Appl. Mater. Interfaces 2018, 10, 21765−21781

Review

ACS Applied Materials & Interfaces

Figure 2. (A) SEM image of exfoliated clay platelets (montmorillonite);14 (B) structure of representative clay showing atomic composition, and routes to functionalize the material; (C) AFM image of graphene oxide (GO) nanosheet; (D) idealized chemical structure GO and routes for modification of different functional groups: red = esterification of alcohol with acid halide; blue = ring opening of epoxide with primary amine; green = deprotonation of carboxylic acid with tertiary amine.

Table 1. Common Techniques Used To Characterize the Chemical Composition and Interfacial Activity of 2D Particles and Their Emulsions, as Well as Caveats To Consider techniques 2D particle characterization (chemical composition and topology)

interfacial activity of 2D particles

emulsion formation

data acquired

other information

FTIR Raman

chemical functional groups functional groups and structure

zeta potential XPS ToF-SIMS TGA

surface charge of particles binding environment of constituent atoms secondary mass ion fragments thermal stability and ligand density of inorganic particles

AFM

diameter and thickness of particles

TEM

diameter of particles

SEM

diameter of particles

pendant drop tensiometry Langmuir− Blodgett (LB) films water contact angle optical microscopy

surface and interfacial tension at air−liquid or liquid−liquid interface particle−particle interactions from surface-area (π−A) isotherms of particles assembled at the air−water interface; particles can be deposited onto substrate relative hydrophobicity of particles

DLS environmental EM cryo-EM

size distribution profile of emulsion in situ morphology characterization without added processing

morphology and size distribution of emulsion droplets

morphology and cross section of emulsion droplets

in toluene), or itself be a reactive (i.e., styrene, a monomer for polymerization). As such, Pickering emulsions can be used to

solvent should be removed can be combined with confocal microscopy for spatial mapping can be impacted by pH or ionic strength of solution surface technique (8) cannot be disentangled from chemical composition, as basic conditions lead to the chemical transformation of GO (e.g., reduction). Multiple researchers have identified that oil identity impacts Pickering emulsions stabilized by GO. Huang and co-workers demonstrated that aromatic oils such as toluene give more

stable emulsions, attributed to π−π interactions with conjugated regions of GO.40 Gao and co-workers evaluated nonaromatic oils and demonstrated that oil-in-water emulsions are favored by more polar oils (e.g., decanol and ethyl heptanoate vs hexane and cyclohexane, Figure 5B).39 Thickett and Zetterlund studied the correlation between emulsion formation and different types of monomer oil phase and found that the more nonpolar the monomer, the “milkier” the emulsion became, which suggests a more stable emulsion with better adsorption of GO at the interface.42 In another study, the same authors used computational and experimental studies to support that GO-stabilized emulsions prefer to form with nonpolar oils (Figure 5C).41 This discrepancy with the results of Gao may be due to the different oils used and/or variations in GO sample (e.g., extent of oxidation, diameter, extent of exfoliation). For example, Gao used n-hexane and decanol as the nonpolar and polar oils, whereas Zetterlund used toluene and ethyl acetate. Other variables that impact the formation of Pickering emulsions stabilized by GO include particle diameter, extent of oxidation, and nanosheet aggregation, though the impact of these factors has not been thoroughly established. For example, Gao et al. observed that the extent of agitation impacted droplet size, with increased sonication leading to smaller droplets, likely attributed to the higher mixing energy used.39 Perhaps most intriguingly, the extent of oxidation directly 21769


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Figure 6. (A) Illustration of GO/PVA hybrid (i) and SEM and photograph of a porous composite 3D printed from a Pickering emulsion gel stabilized by GO/PVA (ii,iii),43 adapted from ref 43 with permission from Elsevier; (B) GO functionalized with primary alkyl amines (i), photograph and optical microscopy image of oil-in-oil emulsions stabilized by these functionalized nanosheets (ii),45 adapted with permission from ref 45 Copyright 2017 ACS. Closed-cell foams prepared by polymerization of the continuous phase (iii),46 adapted from ref 46 with permission from the Royal Society of Chemistry (RSC); (C) illustration of AuNP/GO composite particles (i) and TEM image of polymer particles prepared from Pickering emulsion polymerizations stabilized by these structures (ii),47 adapted from ref 47 with permission from Elsevier.

co-workers, GO decorated with magnetic iron oxide NPs stabilized dodecane-in-water emulsions, with droplet size corresponding to particle concentration.49 Droplet motion was controlled by an external magnetic field, and the system was used to remove organic dye from water. In contrast to clay, GO is more commonly covalently modified, as oxygen functional groups provide handles for functionalization.50 In an early study, GO dispersibility was controlled by modification of alcohols and carboxylic acids with aryl isocyanates;51 functionalized nanosheets were dispersible in polar aprotic solvents such as N,N-dimethyl formamide (DMF), N-methylpyrrolidone, dimethyl sulfoxide, and hexamethylphosphoramide but were not dispersible in water, tetrahydrofuran, acetone, methylene chloride, toluene, or common protic solvents. In a similar vein, GO was modified with an initiator for atom transfer radical polymerization (ATRP), and then, styrene, methyl methacrylate, and butyl acrylate were polymerized from the nanosheet surface;52 the modified nanosheets were dispersible in DMF, toluene, chloroform, and methylene chloride. As such, functionalization can drastically impact dispersibility of GO and, thus, interfacial activity. Of note, modification of GO typically leads to partial reduction, and the interconnectivity of functional groups can complicate reactivity compared to small molecules analogues.53,54 One of the most common modification schemes of GO is the asymmetric functionalization of the two faces, i.e., preparation of Janus nanosheets. Janus particles are those with two halves chemically differentiated, for example, hydrophobic and hydrophilic,55 and are distinct from homogeneously functionalized particles. Du and co-workers functionalized one face of GO with dodecyl amine (DDA) and showed that these tailored particles stabilize toluene-in-water emulsions for at least 3 weeks (Figure 7A); particles with both faces functionalized with DDA did not produce stable emulsions.56 In a similar vein, Pentzer and co-workers selectively polymerized methyl methacrylate from one face of GO.19 The resulting Janus nanosheets lowered the interfacial tension between chloroform and water compared to nonmodified and symmetrically modified GO, as determined by pendant drop tensiometry (Figure 7B). A distinct surface

impacts the ability to stabilize emulsions with GO: partial reduction renders the nanosheets less hydrophilic and more likely to assemble at fluid−fluid interfaces, but extensive reduction yields distorted droplets or nanosheets that are not dispersible.39 Modified GO Nanosheets. GO can be noncovalently functionalized by H-bonding with oxygen groups on the basal plane as well as electrostatic interactions with carboxylates on the nanosheet edges. In 2017, Ren and co-workers used a composite of GO and poly(vinyl alcohol) (PVA) to stabilize dodecane-in-water emulsion gels, which were 3D printed and used for controlled drug release (Figure 6A).43 Alternatively, Liu and co-workers modified GO with primary amines and attributed functionalization to acid−base reactions between amines and carboxylic acids.44 We note that primary amines can covalently modify GO by ring opening of epoxides as well as undergo acid−base reactions with carboxylic acids. The authors found that GO modified with the longest chain alkyl amine (C18) stabilized water-in-toluene emulsions over a broad range of pH (1−13) and ionic strengths (0.1−1000 mM NaCl). Likewise, Pentzer and co-workers modified GO with primary alkyl amines of different chain lengths and found that not only was solubility altered, but also, the nanosheets stabilized oil-in-oil emulsions.45 The length of the alkyl chain dictated the ratio of covalent to noncovalent modification, as determined by XPS, and GO modified with shorter alkyl chains (C6) stabilized octane-in-DMF emulsions, whereas GO functionalized with longer alkyl chains (C18) stabilized DMF-in-octane emulsions (Figure 6B). GO can also be noncovalently modified with inorganic particles, with the functional groups of GO serving as ligands or sites of nucleated nanoparticle (NP) growth. Tang et al. deposited Au NPs on GO by thermoreduction of an aqueous solution of HAuCl4 in the presence of GO; these AuNP/GO hybrids stabilized benzyl chloride-in-water emulsions and also served as surfactants in emulsion polymerizations (Figure 6C).47 The same group established that AgNP/GO hybrids stabilize similar emulsions.48 The mass ratio of NP-to-GO influenced emulsion stability and morphology with a higher mass ratio being beneficial; however, above a critical ratio, the droplets became irregular. In an interesting study by Lee and 21770


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nonfunctionalized and symmetrically functionalized GO (Figure 7C). Other 2D Particles. Emulsions using 2D particle surfactants other than clay and GO are less studied, though receiving increased interest. For example, Kang and co-workers used the oil−water interface to template the synthesis of 2D dendritic platinum nanoparticles at room temperature and demonstrated good electrocatalytic performance.58 Alternatively, Erne et al. accessed gibbsite platelets, a mineral form of Al(OH)3, by hydrothermal synthesis, and demonstrated stabilization of water-in-water emulsions containing immiscible polymers (Figure 8A).59 Platelets that are ∼170 nm in diameter and ∼7 nm thick formed better emulsions than larger or smaller platelets. In another approach, Yang et al. prepared Janus nanosheets by self-assembly of silane-containing small molecules at an oil−water interface followed by chemical crosslinking.60 Crushing the shell yielded silica-based Janus platelets bearing phenyl groups on one face and ammonium groups on the other, which stabilized toluene-in-water and DMF-inhexane emulsions (Figure 8B). This same method was used to access Janus nanosheets with faces bearing phenyl and imidazolium groups, with the interfacial activity in ionic liquid/chlorobutane mixtures dictated by counterion identity.36 In a top down approach, Cheng et al. exfoliated α-zirconium phosphate (α-ZrP) nanosheets by intercalation with NBu4+HO− and demonstrated that these hydroxyl-coated nanosheets stabilized dodecane-in-water emulsions (Figure 8C).61 Alternatively, Mejia et al. modified the exposed hydroxyl groups of aggregated α-ZrP nanosheets with octadecyl isocyanate and then accessed individual nanosheets by exfolitation.62 This resulted in a mixture of nanosheets with one face and edges alkylated (Janus) and those with only edges functionalized that stabilized toluene-in-water emulsions. In a subsequent report, Wang et al. used similarly modified α-ZrP nanosheets to stabilize paraffin-in-water emulsion (Figure 8D) for latent heat storage.63 In contrast, Miele and co-workers used solution-exfoliated hexagonal-boron nitride platelets to

Figure 7. (A) Optical microscopy image of toluene-in-water emulsions stabilized by Janus GO with dodecyl amine on one face,56 adapted from ref 56 with permission from Elsevier; (B) pendant drop tensiometery measurements of Janus GO, in which only one face is functionalized by PMMA (chloroform-in-water),19 adapted with permission from ref 19, copyright 2017 ACS; (C) surface-area (π−A) isotherms of LB films of acrylate-functionalized GO nanosheets (Acr−GO−Acr), Janus GO with PS on one face (Acr− GO−PS), and GO with PS on both faces (PS−GO−PS),57 reproduced from ref 57 with permission from the RSC.

pressure−area (π−A) isotherm of LB films was also observed for these Janus particles, suggesting that the asymmetrically functionalized particles formed dendritic clusters separated by voids. Pentzer and co-workers also prepared Janus GO by assembly of acrylate-functionalized GO at the toluene−water interface and modification of the oil-exposed face with a thiolterminated polystyrene (PS) by thiol−ene chemistry.57 The Janus particles reduced the interfacial tension of water and air and had distinct π−A isotherms compared to those of

Figure 8. (A) TEM image of gibbsite platelets ∼170 nm in diameter and ∼7 nm thick (i) and photograph of gelatin/dextran water-in-water emulsion stabilized by these platelets (ii),59 adapted with permission from ref 59, copyright 2015 ACS; (B) SEM of Janus silica nanosheets prepared by self-assembly of silane-containing monomers, polymerization, and crushing (i) and POM image of paraffin-in-water emulsion stabilized by these particles (ii),60 adapted from ref 60 with permission from the RSC; (C) SEM image of α-ZrP nanosheets (i, inset shows TEM image) and optical microscopy image of dodecane-in-water emulsions stabilized by these structures (ii),61 adapted from ref 61 with permission from Elsevier; (D) TEM image of α-ZrP nanosheets (i) and paraffin-in-water emulsions (ii).63 21771


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Figure 9. (A) SEM image of Laponite-armored PS particles prepared by Pickering miniemulsion polymerization,69 adapted with permission from ref 69, copyright 2007 ACS; (B) TEM image of poly(styrene-co-2-ethylhexyl acrylate) particles armored with Laponite,70 adapted with permission from ref 70 copyright 2011 ACS; (C) GO-armored polymer particles obtained from the miniemulsion polymerization of styrene,72 adapted from ref 72 with permission from Wiley; (D) SEM image of polymer particles armored with GO prepared using alkene-functionalized GO and thiol-yne polyaddition polymerization,73 adapted from ref 73 with permission from Wiley; E) SEM image of GO-armored polymer particles prepared in oilin-oil emulsions using alkylated GO as surfactant (inset shows cross section of particle),46 adapted from ref 46 with permission from the RSC.

stabilize a water-in-ethyl benzoate emulsion;64 emulsion stability was confirmed visually, and rheological tests showed no change after 14 days. Summary. To date, clay and GO are the most commonly studied 2D particles for stabilization of fluid−fluid interfaces in Pickering emulsions. Studies of nonmodified particles address the impact of salt and particle concentration, pH, oil identity, and oil−water ratio on emulsion formation, whereas studies focused on modified nanosheets typically do not discuss these variables. These systems are complicated by intertwining factors such as ionic strength and pH as well as differences between particle samples, such as diameter, distribution, and chemical composition. Furthermore, many functionalized particles are not fully characterized nor directly contrasted to nonmodified particles.

particles, which is especially relevant to coatings, cosmetics, and foodstuff, etc. Solid polymer particles covered with smaller 2D particles are formed from (mini)emulsion polymerizations, in which the dispersed oil phase contains monomer and initiator;65 for a comprehensive discussion of the differences between emulsion and miniemulsion polymerizations, readers are directed to an excellent report by Lotierzo and Bon.66 Discrete polymer particles form when the fluid−fluid interface is adequately stabilized and no exchange between droplets occurs.67 Polymer structures coated with functional particles, such as GO nanosheets, can be used as feedstock for composite films, for example, by 3D printing.68 Clay-Armored Polymer Particles. Emulsion polymerizations using Laponite were explored by Bon and co-workers by the thermally initiated radical polymerization of styrene, lauryl methacrylate, butyl methacrylate, octyl acrylate, and 2ethyl hexyl acrylate.69 The resulting armored particles were 100−200 nm in diameter with a rough particle surface indicating the presence of Laponite (Figure 9A). In a similar vein, Teixeira et al. performed the emulsion polymerization of monomer mixtures (styrene/butyl acrylate, methyl methacrylate/butyl acrylate, styrene/2-ethylhexyl methacrylate) using Laponite as surfactant (Figure 9B);70 clay platelets nucleated polymerization, with a higher platelet concentration yielding smaller average particles, but a broader distribution. In this system, neither methyl methacrylate nor vinyl acetate could be used because of high water solubility and ability to be hydrolyzed, respectively. In 2017, Bourgeat-Lami and coworkers found that in the Laponite-stabilized emulsion polymerization of styrene, increased clay concentration led to more polymer particles of smaller diameter.23 The impact of monomer concentration (ratio of vinyl chloride to methyl acrylate) and clay concentration was also explored. The

4. REACTIONS OF 2D PARTICLES AT THE FLUID−FLUID INTERFACE Pickering emulsions stabilized by 2D particles can be used as templates to architect composite structures for tailored applications: reactions at the interface yield capsules, polymerization of droplets gives armored particles, and modification of the particle from one phase yields Janus particles. These systems are attractive given the scalability of emulsions and the ability to intimately connect dissimilar materials. In these systems, 2D particles can be modified to tailor not only their interfacial activity but also their reactivity. In this section, preparation of structures from Pickering emulsions stabilized by 2D particles and their applications is discussed, specifically armored particles, capsules, and Janus nanosheets. Armored Particles. By far, the most common architecture templated by Pickering emulsions are armored polymer 21772


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Figure 10. (A) Fluorescence microscopy image of hollow capsules with a shell of rhodamine B-labeled PEI/Laponite cross-linked with poly(ethylene glycol)-dicylcidyl ether,33 adapted with permission from ref 33 copyright 2012 ACS; (B) SEM image of a capsule prepared by interfacial polymerization of toluene diisocyanate and diethyl triamine in an emulsion stabilized by Cloisite,79 adapted with permission from ref 79 copyright 2010 ACS; (C) SEM image of hollow capsule of GO stabilized by cross-linking with hexamethylene diisocyanate (inset shows optical microscopy image),80 adapted from ref 80 with permission from Elsevier; (D) SEM image of capsule prepared by interfacial polymerization of glycerol and hexamethylene diisocyanate in an oil-in-oil emulsion stabilized by alkylated GO,46 adapted from ref 46 with permission from the RSC; (E,F) TEM images of hollow capsules prepared by the polymerizations of divinylbenzene under different conditions (i.e., monomer loading and percent conversion),81 reproduced from ref 81 with permission from the RSC.

dium dodecyl benzenesulfonate (cosurfactant), monomers, and hexadecane (hydrophobe).76 The modified GO nanosheets increased intersheet distance, which allowed monomers to intercalate and led to composite nanoparticles, with GO throughout rather than on the surface only. In 2014, Jeong and co-workers modified the epoxide groups of GO with potassium 2-aminoethanesulfonate and then used these modified nanosheets in the emulsion polymerization of methyl methacrylate to access core−shell microspheres with particle size tunable by the amount of modified GO.77 In 2016, Pentzer and coworkers used alkene-modified GO to prepare modifiable armored particles by photoinduced miniemulsion polymerization of dialkyne and tetra-thiol small molecules (Figure 9D).73 The alkene moieties of the functionalized GO exposed to the oil were incorporated into the polymer, whereas alkene functionalities exposed to water were modified with ligands for Fe2O3 and TiO2. Furthermore, in 2018, Pentzer and coworkers reported emulsion polymerizations in an oil-in-oil system: droplets of an isocyanate-containing methacrylate monomer stabilized by GO underwent thermally initiated free radical polymerization (Figure 9E).46 The isocyanate groups were subsequently modified with a thiol-functionalized dye, illustrating that water sensitive functionalities survive. Capsules. 2D particles assembled at the fluid−fluid interface in Pickering emulsions can be stabilized by crosslinking or interfacial polymerization, yielding composite capsule shells. The inner contents can be filled with an active material, such as a phase change material or electrolyte, or removed to yield a hollow capsule, which collapses or remains spherical. As capsules are attractive for use in confined reactions, supercapacitor electrodes, catalytic supports, drug delivery, etc.,78 controlling the chemical composition of the

authors found that 7.5−15 wt % methyl acrylate gave smaller polymer particles with the surface well covered with clay. In 2015, Chakrabarty et al. functionalized Laponite with a cationic reversible addition−fragmentation chain transfer (RAFT) agent and used these modified platelets in the miniemulsion copolymerization of pentafluoropropyl acrylate, methyl methacrylate, and n-butyl acrylate.71 The fluorinated copolymer particles armored with Laponite had lower surface energy (i.e., were more hydrophobic) than particles prepared with a nonionic RAFT initiation, as determined by water contact angle. GO-Armored Polymer Particles. Nonmodified GO has been used as a surfactant for Pickering emulsion polymerizations to prepare armored particles, with the nanosheet diameter impacting product formation. In 2011, Zhao and coworkers prepared GO-coated PS particles and illustrated that smaller diameter nanosheets (∼300 nm) yielded individual PS particles, while larger nanosheets (1 μm) led to multiple PS particles on a single GO nanosheet.74 Likewise, Sharif and coworkers observed that emulsion polymerization of methyl methacrylate using GO as surfactant yielded a mixture of GOcoated PMMA particles and GO nanosheets with adsorbed PMMA particles,75 likely due to the large and nonuniform size of nanosheets used. Zetterlund and co-workers overcame product heterogeneity by preparing GO nanosheets that are ∼100 nm in diameter and using them for a styrene-in-water miniemulsion polymerization (Figure 9C).72 Modified GO nanosheets have also been used as surfactants for (mini)emulsion polymerizations. In 2011, Etmimi and Sanderson used miniemulsion polymerization to prepare poly(styrene-butyl acrylate)/GO composites from GO modified with 2-acrylamido-2-methyl-1-propanesulfonic acid, so21773


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ACS Applied Materials & Interfaces shell and contents will lead to tailor properties and applications. Hollow Capsules with Clay. Although many examples of Pickering emulsions stabilized by clay have been provided, the preparation of capsules has not been as widely reported. In 2012, Armes et al. prepared hollow capsules from sunflower oil-in-water emulsions stabilized by PEI-modified Laponite by cross-linking with either the oil-soluble poly(propylene glycol) diglycidyl ether or the water-soluble poly(ethylene glycol) diglycidyl ether (Figure 10A).33 In a similar vein, McIlroy et al. prepared hollow capsules by interfacial polymerization of toluene diisocyanate in oil and diethylenetriamine in water in water-in-oil Pickering emulsions stabilized by Cloisite (Figure 10B); these capsules were loaded with an epoxy curing agent such that an epoxy film formed upon grinding with a resin.79 Hollow Capsules with GO. More commonly, GO has been used as a particle surfactant to prepare capsules from Pickering emulsions. Pentzer and co-workers prepared hollow capsules of GO by the chemical cross-linking of nanosheets at the toluene−water interface using the toluene-soluble hexamethylene diisocyanate, which reacted with alcohol groups of GO.80 After removal of the oil phase, the cross-linked capsules collapsed, as verified by microscopy (Figure 10C). Alkylated AuNPs (∼5 nm) remained in the capsules after removal of toluene, indicating shell integrity. In compliment, Pentzer and co-workers prepared hollow capsules using an oil-in-oil emulsion stabilized by alkylated GO.46 The DMF and octane phases contained ethylene glycol and hexamethylene diisocyanate, respectively, and after interfacial polymerization, the capsules remained spherical and were isolated by filtration (Figure 10D). Chain growth polymerizations at the fluid−fluid interface can also be used to prepare capsules of GO. Zetterlund and coworkers used droplets of styrene, divinylbenzene (DVB), 2,2′diazobis(isobutyronitrile) (AIBN), and hexadecane in a continuous water phase to prepare a shell of PS-co-DVB/ GO.81 The morphology and thickness of the shell were controlled by composition of the oil phase (Figure 10E,F), and the authors demonstrated that TiO 2 NPs could be encapsulated. The same authors also studied the impact of monomer identity;82 all-aromatic monomer/cross-linker combinations yielded hollow capsules, whereas the corresponding methacrylic system resulted in solid particles. This difference was attributed to the higher rate of polymerization of methacrylates, which resulted in higher viscosity and prevented polymer localization at the interface. Filled Capsules. In addition to hollow capsules, Pickering emulsions can be used to prepare shells of GO filled with other contentsfor example, using a low melting point discontinuous phase, emulsifying above its melting point, and cooling to room temperature. Du and co-workers used GO modified by the polycondensate of diethanolamine and adipic acid to stabilize droplets of the phase change material (PCM) nhexadecane.83 Heating these core−shell particles led to melting of the PCM core, but no leakage. In a similar vein, Zhang et al. encapsulated the PCM paraffin with shells of GO and melamine−formaldehyde via in situ polymerization using a prepolymer approach.84 The capsule diameter was controlled by GO concentration (from 6.3 to 15.9 μm); differential scanning calorimetry (DSC) revealed that the peak melting point increased compared to that of bulk paraffin (Figure 11A), and a >90% reduction in paraffin leakage was observed. Pentzer and co-workers used a similar approach to encapsulate

Figure 11. (A) SEM image of capsules with a GO/polymer shell filled with parafin (i) and DSC profile comparing these capsules (labeled MEPCM) to bulk paraffin (ii),84 adapted from ref 84 with permission from Elsevier; (B) optical microscopy image of capsules of stearic acid coated with reduced GO (i) and photographs of stearic acid (ii) and the encapsulated stearic acid (iii) heated to 125 °C on a hot plate,85 adapted from ref 85 with permission from the RSC.

the PCM stearic acid with a shell of only GO; the authors showed that reduction and cross-linking of GO yielded a powder with improved thermal conductivity that could undergo multiple heating−cooling cycles without leakage of the PCM (Figure 11B).85 In a similar study, Wang et al. used modified α-ZrP platelets and PS to encapsulate the PCM nonadecane, which showed upon melting.63 Janus 2D Particles. As discussed above, Janus functionalization of 2D particles can impact their interfacial assembly, yet interfacial assembly can also be used to prepare Janus particles. Indeed, a facile and scalable route to Janus particles will provide transformative advances in tailored assemblies, selective catalysis, electroresponsive inks, directional particle propulsion, etc. Both Walther and Muller55 as well as Poggi and Gohy provide comprehensive reviews of Janus particles.86 The most common Janus 2D particles use GO and clay as substrates. Janus Clay Platelets. Liu et. al prepared Janus clay by the noncovalent modification of Laponite; immobilizing the negatively charged platelets on a positively charged PS sphere left one face exposed for functionalization with ammoniumterminated polymers (Figure 12A).87 Subsequent dissolution of the template yielded three distinct Janus structures, characterized by TEM and AFM. Breu and co-workers fabricated Janus Kaolinite platelets by exploiting the inherent chemical differences between the faces: the octahedral face was modified with a catechol-containing small molecule, and the tetrahedral face was modified with [Ru(bpy)3]2+ (Figure 12B).88 The two faces were chemically differentiated by ToF-SIMS after deposition onto substrates of tailored hydrophobicity. Kirillova et al. also prepared Janus Kaolinite by modifying the surface hydroxyl groups first with (3aminopropyl)-triethoxysilane and then with an initiator for ATRP.89 These modified platelets stabilized an anisole-inwater emulsion such that 2-(dimethylamino)-ethyl methacrylate (DMAEMA) and lauryl methacrylate were polymerized from the water-exposed and anisole-exposed faces, respectively. Chemical composition was verified by FTIR and TGA, and the Janus character was highlighted by adsorption of poly(acrylic 21774


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Figure 12. (A) Schematic of the preparation of Janus Laponite by attaching the clay platelets to positively charged PS spheres and then functionalizing the exposed face, followed by dissolution of the PS template,87 adapted with permission from ref 87 copyright 2013 ACS; (B) schematic of the preparation of Janus Kaolinite by functionalization of the octahedral face with a catechol and the tetrahedral face with [Ru(bpy)3]2+,88 adapted from ref 88 with permission from Wiley; (C) selective adsorption of PAA-coated spherical NPs on the PDMAEMA face of the Janus particles,89 adapted with permission from ref 89 copyright ACS.

acid) (PAA)-coated particles on the poly(DMAEMA) face (Figure 12C). Coatings of these Janus platelets resulted in lower ice adhesion compared to that of the polymers themselves, likely because of the combined hydrophobicity and hydrophilicity the Janus particles provided. The same strategy was used to prepare polymer-functionalized Janus Kaolinite by functionalizing the tetrahedral face with PDMAEMA-block-PS and the octahedral face with poly(3(2,3-dihydroxybenzoyloxy)propyl methacrylate)-stat-(methyl methacrylate);90 these Janus platelets compatibilized blends of PS and PMMA, assembling at the polymer−polymer interface. Janus GO Nanosheets. Zhang et al. fabricated Janus TiO2-GO nanosheets by growing anatase TiO2 NPs on the water-exposed face of GO assembled at the oil−water interface (oil was a 4:1 ratio of n-octanol:n-butyl alcohol).91 These composite Janus particles photocatalytically degraded tetracycline hydrochloride in water at >96% efficiency. Zhu et al. also prepared Janus GO nanosheets using an emulsion approach;92 Ag/AgBr/GO nanocomposites were prepared and used as plasmonic photocatalysts for the degradation of methyl orange at 94% efficiency. More commonly, Janus GO nanosheets are prepared by emulsification with a low melting point wax at an elevated temperature, followed by cooling, to isolate one face for selective modification and then dissolution of the wax template. Wu et al. used such an approach to functionalize the exposed face with poly(propylene glycol) bis(2-aminopropyl ether), characterizing the structures by SEM, AFM, and FTIR and using them as surfactants for oil recovery.56 Pentzer and co-workers used the wax template method to selectively functionalize one face of GO with PMMA.19 The authors unequivocally established the Janus functionalization by combining LB techniques with ToF-SIMS. In LB films, the Janus particles laid at the interface such that the nonmodified face of GO was exposed to water and the PMMA-functionalized face was exposed to air (Figure 13A), based on relative hydrophobicity. Deposition of the monolayer by LB technique produced the PMMA face for characterization, whereas deposition by Langmuir−Schaffer technique provided the nonmodified face for characterization. The difference between the two faces was established by water contact angle measurements (Figure 13B), as well as ToF-SIMS, with mass fragments associated with GO and PMMA prepared by ATRP readily differentiated (Figure 13C).

Figure 13. (A) Schematic of LB film of Janus GO nanosheets with PMMA on only one face, showing PMMA is exposed to air; (B) illustration of the Janus particles deposited by two different methods from LB films and water contact angles of the two samples; (C) ToFSIMS mapping of nonmodified GO, GO modified with PMMA on both faces, and each face of Janus GO with mass ions corresponding to GO or PMMA (25 m/z), bromide of the polymer chain end (79 m/z), and PMMA (31 m/z),19 adapted with permission from ref 19 copyright 2017 ACS.

Summary. Pickering emulsions stabilized by 2D particles can be used to template different structures depending on the location of the reactive species. To date, the most common architectures are armored polymer particles, capsules, and Janus particles. The novel architectures have applications in thermal energy management, film formation, oil recovery, and emulsion stabilization. Only a handful of reports indicate that particle diameter impacts product formation, but the variables impacting this system are not fully defined. Incorporation of other 2D particles is vital to the preparation of hybrid architectures for tailored applications. Moreover, researchers 21775


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Figure 14. (A) Illustration of the variables of H-bonding and VdW interactions of GO at an oil−water interface,94 adapted with permission from ref 94 copyright 2014 ACS; (B) schematic of Janus 2D particle adsorption at the oil−water interface during emulsion formation: initially, both particle orientations are observed (i), and then, particles with the less favorable orientation desorb upon crowding (ii) and readsorb with the more favorable orientation (iii),95 adapted with permission from ref 95 copyright 2013 ACS; (C) simulation of amphiphilic Janus triangles at an emulsion interface with increasing particle concentration, left to right, showing aggregates of the platelets then crowding and ultimately wrinkling,96 reproduced from ref 96 with permission from PCCP Owner Societies. parallel to the fluid−fluid interface and modeled the system to take into account oil polarity. Their studies revealed that the angle of GO nanosheets at the oil−water interface increases with increasing polarity of the oil phase. Computational studies can also reveal the distinct benefit of the 2D nature of particles at the fluid−fluid interface. Hurt et al. mathematically modeled the adsorption of GO at the oil−water interface to evaluate ultrathin platelets, ignoring nonparallel orientations.94 This work indicated that on an equal mass basis, assembly of 2D particles at the interface is more favorable than spheres. The impact of particle shape is most apparent as the platelet thickness approaches the atomic scale, which is attributed to increased atom efficiency, as every atom is in contact with both phases. Spheres can only achieve this efficiency when the radius is on the atomic scale, and thus, thin platelets are significantly better at stabilizing emulsions than spheres and are less likely to desorb from the interface. Modeling of the van der Waals (VdW) forces of GO at the oil−water interface showed that adsorption of a GO monolayer results in no net destruction of hydrogen-bonding, as destroyed H-bonds between GO nanosheets are compensated by new H-bonds within the water (Figure 14A). For the favorable assembly of multilayers of GO at the oil−water interface, a similar net zero VdW force change must be realized. In compliment to evaluating the favorability of GO adsorption to the oil−water interface, Majumder et al. mathematically modeled the destabilization mechanism using changes in the average droplet size and emulsion lifetime.24 Experimental data on emulsion decay was compared to four possible destabilization mechanisms: flocculation, phase inversion, Ostwald ripening, and coalescence. Flocculation and phase inversion mechanisms were ruled out, as droplets did not aggregate nor did the emulsion invert. The authors observed a wider distribution of droplet size over time, indicating larger droplets absorb smaller droplets and that Ostwald ripening is responsible for destabilization. Experimental data was fit to a mathematical model that indicated an emulsion lifetime of 18 days. Computational Studies of Janus 2D Materials. Bon and associates used free energy simulations to study Janus 2D disc-shaped particles at the toluene−water interface, in which one face of the particle was modified with PS and the other with PMMA.95 The lowest energy orientation of the Janus discs exposed the PS face to

should take care to illustrate the distinct and enhanced performance afforded by these tailored systems.

5. COMPUTATIONAL STUDIES OF 2D MATERIALS AT FLUID−FLUID INTERFACES Understanding the factors that govern the formation and stability of Pickering emulsions with 2D particles as well as opportunities for modification can be facilitated by computational studies. Such analyses are complementary to experiment. This section focuses on computational analyses of 2D particles in Pickering emulsions and includes nonmodified clay and GO, as well as Janus 2D particles. These studies can inspire and guide new directions and illuminate discrepancies between experiment and computation. Computational Studies of Clay. Computational studies on Pickering emulsions stabilized by clay are few. Bon and Colver computationally explored Laponite-stabilized styrene-in-water Pickering miniemulsion polymerizations.69 The authors modeled the impact of the ratio of monomer (styrene) to hydrophobe (hexadecane) and concentration of Laponite on emulsion formation, developing an equation to predict droplet diameter. A linear relationship between clay concentration and droplet size was established, matching experimental results. However, at high styrene-to-clay ratios, this model was not adequate, since curvature is an important parameter in small droplets, but negligible for large droplets. Computational Studies of GO. In contrast, computational studies on the assembly of GO at oil−water interfaces are more common. Zetterlund et al. investigated different oil−water interfaces using surface contribution terms and Hansen solubility parameters.93 Simulating the energy of nanosheet adsorption at the interface revealed that assembly is energetically favorable. However, experimental studies indicate that oil identity impacts emulsion formation, as discussed above; thus, computational models were revised to include dispersive and polar contributions. These modified conditions support that oil polarity impacts emulsion formation: in more polar oils, GO is likely to remain in the water phase. Zetterlund et al. further evaluated GO-stabilized emulsions, based on solubility parameters of GO in oils that do and do not form emulsions. This work revealed that the more compatible GO is with the oil, the less likely for emulsion formation, in line with simulations of adsorption energy. The authors then questioned the assumption that GO nanosheets lie 21776


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Figure 15. Outlook for the use of 2D particles at fluid−fluid interfaces to template hybrid structures beyond armored particles, capsules, and Janus nanosheets, including porous materials, HIPES, and bijels and their use in advanced applications. toluene and the PMMA face to water. Despite this favorable orientation, simulation showed that in early stages of assembly, the Janus discs have random orientations (i.e., some expose the PMMA face to toluene while others expose the PS face). As more particles adsorb to the interface, monolayers aggregate and collide, causing less favorably oriented particles to desorb and then adopt the lower energy orientation upon readsorption (Figure 14B). Thus, a net decrease in interfacial tension was observed, concurrent with emulsion stabilization. In a similar vein, Luo et al. found agreement for dissipative particle dynamic simulations and spatial structure computations in the interfacial assembly of Janus triangles, in which one face was hydrophobic and the other hydrophilic.96 These amphiphilic triangles adsorbed parallel to the fluid−fluid interface at low concentrations and then wrinkled and formed channels with particles protruding into both phases at higher concentrations (Figure 14C). Surface pressure simulations further supported that particle assembly lowers interfacial tension but that increased concentration yields separated aggregates that wrinkle. Summary. New assemblies and materials can be facilitated by a synergistic combination of computation and experiment, with computation guiding experiment and experiment helping refine computation.97 The assembly and orientation of 2D particles at an oil−water interface is influenced by many factors, and more extensive studies are required to fully understand the systems and address discrepancies. Moreover, computational studies have thus far been limited to the assembly of 2D particles at fluid−fluid interfaces and have yet to address the application of these structures.

Researchers should take care to define the characteristics of the particles used, including chemical composition, size (diameter and thickness), and size distribution; this is especially important with GO. For modified particles, the homogeneity and density of functionalization should be addressed. Most commonly, armored polymer particles (polymerization of droplets), capsules (interfacial reactions), and Janus particles (independent functionalization of each face) are templated from Pickering emulsions stabilized by 2D particles. Preparing novel structures should be paired with defining the enhanced properties they allow, especially given the ability to intimately connect dissimilar materials, access high interfacial area, and the potential for stimuli-responsiveness. Access to 2D particles beyond GO and clay and interfaces beyond oil−water will open new applications and possibilities of Pickering emulsions. α-ZrP platelets and those prepared from small molecule precursors are briefly discussed above and illustrate novel particle compositions to be explored. Transition metal dichalcogenides (TMDs), such as MoS2,99 and layered double hydroxide (LDH)100 nanosheets have gained recent attention but have yet to be explored for stabilization of emulsions. For example, Qiu and co-workers prepared highly thermodynamically stable emulsions using LDH nanosheets and carbon nanotubes; the large and stable surface area led to increased catalytic performance.101 To study novel classes of 2D particles in Pickering emulsions, particles must be readily accessed on a large scale and thoroughly characterized before use. Fluids to use for Pickering emulsions beyond oil−water include those containing perfluorocarbons,102 liquid CO2 (LCO2),103 and ionic liquids (ILs).104 For example, Tabor and co-workers used GO to prepare perfluorouhexane-in-water emulsions stable from pH 0.5 to 10.98 Likewise, Cerruti et al. stabilized droplets of perfluorodecalin-in-water with GO and used these structures to deliver O2, demonstrating an order of magnitude slower release than that of current systems.102 Zhang et al. used GO to stabilize LCO2-in-water emulsions and used these assemblies to prepare

6. CONCLUSIONS AND OUTLOOK The study of 2D particles at fluid−fluid interfaces, their use to architect advanced structures, and the applications of these structures are still relatively nascent, especially compared to small molecule and spherical particle surfactants. Most experimental studies focus on clay or GO at oil−water interfaces, whereas a spattering of computational studies evaluated nonmodified and Janus particles. The fundamental properties of emulsion droplets must be understood, including droplet−droplet interactions, for example, using in situ AFM techniques.98 Experimentally, 2D particles have been modified to alter their interfacial assembly and to make them reactive. 21777


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ACS Applied Materials & Interfaces porous scaffolds for catalysts.103 More recently, Pentzer and co-workers used GO to prepare ionic liquid-in-water emulsions, stabilizing the assemblies by interfacial polymerization. The hybrid capsules were isolated as powder and used as the active electrode material in electrochemical double-layer capacitors, showing superior performance at lower temperatures and faster scan rates than a state-of-the art porous material.104 Clearly, increased diversity in the identity of materials used to prepare Pickering emulsions stabilized by 2D particles will open new applications and performance. A number of architectures can be accessed using Pickering emulsions with 2D particle surfactants (Figure 15), such as closed-cell foams,46 poly(HIPEs),105 and bijels.106 Pentzer and co-workers prepared closed-cell foams by the polyaddition polymerization of the discontinuous phase of an oil-in-oil emulsion.46 In a similar vein, macroporous structures were prepared by polymerization in a high internal phase emulsion (HIPE) stabilized by cetyltrimethylammonium bromide (CTAB)-modified GO: subsequent calcination produced a carbon framework with potential applications in energy management.105 Bijels are another architecture with a high amount of interface attractive for applications in catalysis, energy storage, molecular encapsulation, etc.106 Bijels are composed of two fluids arrested in a bicontinuous configuration with particles jammed at the interface and have traditionally been prepared with particles of tailored hydrophobicity and two liquids with a critical solution temperature.107 Recently, Helms and co-workers prepared water−oil bijels with submicrometer domains using carboxylic acidfunctionalized PS particles and amine-functionalized polydimethylsiloxane.108 To date, bijels with the fluid−fluid interface stabilized by 2D particles have not been reported. In addition to a better understanding of factors that govern the assembly of 2D particles at fluid−fluid interfaces, the development and exploration of new particles, interfaces, architectures, and applications are required. As researchers across many fields prepare and use 2D particles, more consistent and standardized characterization is needed so that trends are identified and structure−property-application relationships established. Opportunities abound to control particle (and droplet) diameter and chemical composition to fabricate not only systems with controlled interfacial activity109 but also those that are stimuli-responsive.110 In the future, Pickering emulsions stabilized by 2D particles will find widespread applications in tailored composites and hybrid architectures that will address current and future needs in energy management, coatings, foods, cosmetics, molecular delivery, etc., areas not possible with currently accessible systems.

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was a NASA Harriett G. Jenkins Predoctoral Fellow (Grant #NNX13AR93H).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Peiran Wei: 0000-0001-7820-1716 Emily B. Pentzer: 0000-0001-6187-6135 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank CWRU College of Arts and Sciences and NSF CAREER Award #1551943 for financial support. B. J. R. 21778


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