Effect of Adsorbed Amphiphilic Copolymers on the Interfacial Activity of Superparamagnetic Nanoclusters and the Emulsification of Oil in Water

CO2-Triggered Hierarchical-Pore UiO-66-Based Pickering Emulsions for Efficient and Recyclable Suzuki-Miyaura Cross-Coupling in Biphasic Systems

Hierarchical-pore metal-organic frameworks (H-MOFs) are considered to be emerging stabilizers for Pickering emulsion formation because of their hierarchically arranged pores, tailorable structures, and ultrahigh surface areas. However, stimulus-triggered Pickering emulsions built by H-MOFs have been seldom presented to date despite their great significance in diverse applications. Herein, by grafting Pd(OAc)2 on the hierarchical-pore zirconium MOF UiO-66, namely, H-UiO-66, with the aid of 1-alkyl-3-methylimidazolium 2-cyanopyrrolide salts ([CnMIM][2-CN-Pyr], n = 4, 6, and 8), a series of Pd(OAc)2-[CnMIM][2-CN-Pyr]@H-UiO-66 have been developed and utilized as emulsifiers for constructing CO2-switching Pickering emulsions. It was found that Pd(OAc)2-[CnMIM][2-CN-Pyr]@H-UiO-66 was able to stabilize the n-hexane-water mixture to form a Pickering emulsion even at an amount of 0.5 wt %. Upon alternate addition of CO2 and N2 at normal pressure, Pickering emulsions could be smartly converted between demulsification and re-emulsification. Through combining varieties of spectroscopic techniques, the mechanism of the switchable phase transformation lay in the acid-base reaction of ionic liquids with CO2 on H-UiO-66 and the creation of more hydrophilic salts, which reduced the wettability of the emulsifier and destabilized the emulsion. As an example of application, the stimulus-triggered Pickering emulsion was employed as a palladium-catalyzed Suzuki-Miyaura cross-coupling microreactor to achieve the combination of chemical reactions, isolation of products, and recovery of catalysts.

CO2-Switchable Hierarchically Porous Zirconium-Based MOF-Stabilized Pickering Emulsions for Recyclable Efficient Interfacial Catalysis

Stimuli-responsive Pickering emulsions are recently being progressively utilized as advanced catalyzed systems for green and sustainable chemical conversion. Hierarchically porous metal-organic frameworks (H-MOFs) are regarded as promising candidates for the fabrication of Pickering emulsions because of the features of tunable porosity, high specific surface area and structure diversity. However, CO₂-switchable Pickering emulsions formed by hierarchically porous zirconium-based MOFs have never been seen. In this work, a novel kind of the amine-functionalized hierarchically porous UiO-66-(OH)₂ (H-UiO-66-(OH)₂) has been developed using a post-synthetic modification of H-UiO-66-(OH)₂ by (3-aminopropyl)trimethoxysilane (APTMS), 3-(2-aminoethylamino)propyltrimethoxysilane (AEAPTMS) and 3-[2-(2-aminoethylamino)ethylamino]propyl-trimethoxysilane (AEAEAPTMS), and employed as emulsifiers for the construction of Pickering emulsions. It was found that the functionalized H-UiO-66-(OH)₂ could stabilize a mixture of toluene and water to give an emulsion even at 0.25 wt % content. Interestingly, the formed Pickering emulsions could be reversibly transformed between demulsification and re-emulsification with alternate addition or removal of CO₂. Spectral investigation indicated that the mechanism of the switching is attributed to the reaction of CO₂ with amino silane on the MOF and the generation of hydrophilic salts, leading to a reduction in MOF wettability. Based on this strategy, a highly efficient and controlled Knoevenagel condensation reaction has been gained by using the emulsion as a mini-reactor and the emulsifier as a catalyst, and the coupling of catalysis reaction, product isolation and MOF recyclability has become accessible for a sustainable chemical process.

Brine-Soluble Zwitterionic Copolymers with Tunable Adsorption on Rocks

Injection of aqueous fluids into reservoirs as an enhanced oil recovery (EOR) tool has been of great interest in petroleum engineering. EOR using viscous polymer solutions improves the volumetric sweep efficiency. However, significant polymer adsorption on reservoir rock surfaces is one of the greatest challenges in polymer-flooding EOR. We have synthesized and characterized five zwitterionic copolymers and studied their static adsorption on limestone surfaces in seawater at high temperatures and salinities. Our results indicate that polymer adsorption directly correlates to a small percentage of functional co-monomers on the polymer backbone. One particular copolymer shows negligible static adsorption on limestone surfaces.

Degradable sugar-based magnetic hybrid nanoparticles for recovery of crude oil from aqueous environments

We designed and fabricated a sugar-based magnetic nanocomposite material that is capable of tackling environmental pollution posed by marine oil spills, while minimizing potential secondary problems that may occur from microplastic contamination.

Interfacial Phenomena of Natural Dispersants for Crude Oil Spills

A natural surfactant was studied to simulate the dispersion process of crude oil in water. The interfacial phenomena of this natural dispersant was compared with a commercially available chemical dispersant, COREXIT EC9500A. This functional surfactant was extracted from the mucilage of the Opuntia ficus-indica cactus species. The evaluation to determine the efficacy to disperse crude oil of the cactus-based mucilage extract (nongelling extract, NE) was based on characterizing surface and interfacial tension, dispersion efficiency, mixing effects, salinity effects, stability, and droplets size distributions. We found that surface tension values follow a linear relationship with respect to the natural logarithm of the concentrations of NE. The application of NE in the water phase led to decreasing oil/water interfacial tensions. Surface tension tests were also used to quantify the effect of oil-in-water (O/W) emulsion ratios once either natural or commercialized dispersants were added. A key finding of our work is that the surface tension between typical 6% and 3% v/v O/W emulsions was significantly reduced with the addition of discrete amounts of NE. This result indicated that the dynamic balance between O/W and water-in-oil (W/O) emulsions was thermodynamically more stable toward O/W emulsion states with NE. We also found that O/W emulsions with higher dispersion effectiveness were formed for both 10 and 35 practical salinity units, as the dispersant to oil ratios increased, with a significant correlation to the mixing energy. We observed that the O/W emulsions with natural dispersants had a significantly smaller weighted average diameter compared to those with COREXIT EC9500A. Such a phenomenon can be explained by understanding intermolecular interactions due to the structure and type of dispersant. In conclusion, cactus-based mucilage extracts could be used as environmentally benign dispersants and, therefore, reduce negative social perceptions of the application of dispersants to clean up spilled oil.

Biosurfactant-modified palygorskite clay as solid-stabilizers for effective oil spill dispersion

An effective and conventional remediation technique in marine oil spills is to apply chemical dispersants to emulsify oil slicks into small oil droplets. Still, the potential hazards of chemical dispersants onto the marine ecosystem have motivated the research for environmentally friendly alternative while keeping exceptional dispersion ability. Here, we showed that the mixture of palygorskite (PAL) and rhamnolipid (Rha) formed a biocompatible alternative to synthetic surfactants used for oil spill dispersion. The oil droplets dispersed by R-PAL presented a small average size and long-term stability, which illustrated the synergistic interactions between Rha and PAL acting as an efficient dispersant in artificial sea water (ASW). Due to the strong flocculation caused by high salinity, PAL alone was not effective emulsifiers in ASW. A small amount of Rha could played a major role in modifying the surface characteristics of PAL and decreasing oil-water interfacial tension. Therefore, PAL particles irreversibly adsorbed onto the oil-ASW interface and formed a rigid interfacial film around oil droplets in the presence of Rha, which offered an efficient barrier to droplet coalescence. The synergistic interactions between PAL and Rha could enable the dispersion of tetradecane in ASW. Such a functionality was further tested in dispersing crude oil in ASW. The study presents a new strategy of using a mixture of PAL and Rha for oil dispersion, thus providing an ecofriendly alternative to conventional dispersants.

Synthesis, Characterization, and Pickering Emulsifier Performance of Anisotropic Cross-Linked Block Copolymer Worms: Effect of Aspect Ratio on Emulsion Stability in the Presence of Surfactant

Reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization is used to prepare epoxy-functional PGMA-P(HPMA- stat-GlyMA) diblock copolymer worms, where GMA, HPMA, and GlyMA denote glycerol monomethacrylate, 2-hydroxypropyl methacrylate, and glycidyl methacrylate, respectively. The epoxy groups on the GlyMA residues were ring-opened using 3-aminopropyltriethoxysilane (APTES) in order to cross-link the worm cores via a series of hydrolysis-condensation reactions. Importantly, the worm aspect ratio can be adjusted depending on the precise conditions selected for covalent stabilization. Relatively long cross-linked worms are obtained by reaction with APTES at 20 °C, whereas much shorter worms with essentially the same copolymer composition are formed by cooling the linear worms from 20 to 4 °C prior to APTES addition. Small-angle X-ray scattering (SAXS) studies confirmed that the mean aspect ratio for the long worms is approximately eight times greater than that for the short worms. Aqueous electrophoresis studies indicated that both types of cross-linked worms acquired weak cationic surface charge at low pH as a result of protonation of APTES-derived secondary amine groups within the nanoparticle cores. These cross-linked worms were evaluated as emulsifiers for the stabilization of n-dodecane-in-water emulsions via high-shear homogenization at 20 °C and pH 8. Increasing the copolymer concentration led to a reduction in mean droplet diameter, indicating that APTES cross-linking was sufficient to allow the nanoparticles to adsorb intact at the oil/water interface and hence produce genuine Pickering emulsions, rather than undergo in situ dissociation to form surface-active diblock copolymer chains. In surfactant challenge studies, the relatively long worms required a thirty-fold higher concentration of a nonionic surfactant (Tween 80) to be displaced from the n-dodecane-water interface compared to the short worms. This suggests that the former nanoparticles are much more strongly adsorbed than the latter, indicating that significantly greater Pickering emulsion stability can be achieved by using highly anisotropic worms. In contrast, colloidosomes prepared by reacting the hydroxyl-functional adsorbed worms with an oil-soluble polymeric diisocyanate remained intact when exposed to high concentrations of Tween 80.

Polymer-Coated Nanoparticles for Reversible Emulsification and Recovery of Heavy Oil

Heavy crude oil has poor solubility and a high density, making recovery and transport much more difficult and expensive than for light crude oil. Emulsifiers can be used to produce low viscosity oil-in-water emulsions for recovery and transport, but subsequent demulsification can be challenging. Here, we develop and implement interfacially active, pH-responsive polymer-coated nanoparticles (PNPs) to reversibly stabilize, recover, and break oil/water emulsions through variation of solution pH. Silica particles with poly(2-(dimethylamino)ethyl methacrylate) (DMAEMA) chains covalently grafted to the surface are prepared although a reversible addition fragmentation chain transfer grafting-through technique. The resulting DMAEMA PNPs can stabilize emulsions of high viscosity Canadian heavy oil at PNP concentrations as low as 0.1 wt % and at neutral pH. The performance of the DMAEMA PNPs exceeds that of DMAEMA homopolymer additives, which we attribute to the larger size and irreversible adsorption of DMAEMA PNPs to the oil/water interface. After recovery, the emulsion can be destabilized by the addition of acid to reduce pH, resulting in separation and settling of the heavy oil from the aqueous phase. Recovery of more than 10 wt % of the crude heavy oil-in-place is achieved by flooding with aqueous solution of 0.1 wt % DMAEMA PNPs without any additional surfactant or chemical. This work demonstrates the applicability of PNPs as surface active materials for enhanced oil recovery processes and for heavy oil transport.

Light and Magnetic Dual-Responsive Pickering Emulsion Micro-Reactors

Emulsion droplets can serve as ideal compartments for reactions. In fact, in many cases, the chemical reactions are supposed to be triggered at a desired position and time without change of the system environment. Here, we present a type of light and magnetic dual-responsive Pickering emulsion microreactor by coadsorption of light-sensitive titania (TiO₂) and super paramagnetic iron oxide (Fe₃O₄) nanoparticles at the oil-water interface of emulsion droplets. The droplets encapsulating different reactants in advance can be driven close to each other by an external magnetic field, and then the chemical reaction is triggered by UV illumination due to the contact of the isolated reactants as a result of droplet coalescence. An insight into the incorporation of hydrophobic TiO₂ and hydrophilic Fe₃O₄ nanoparticles simultaneously at the emulsion interface is achieved. On the basis of that, an account is given of the coalescence mechanism of the Pickering emulsion microreactors. Our work not only provides a novel Pickering emulsion microreactor platform for triggering chemical reactions in a nonintrusive and well-controlled way but also opens a promising avenue to construct multifunctional Pickering emulsions by assembly of versatile building block nanoparticles at the interface of emulsion droplets.

CO2 and Redox Dual Responsive Pickering Emulsion

Herein, we described for the first time a CO₂ and redox dual responsive paraffin oil-in-water Pickering emulsion stabilized by the modified silica nanoparticles with Se-containing tertiary amine, SeTA, in which the tertiary amine serves as a CO₂-sensitive group, and the Se atom serves as a redox-sensitive center. The Pickering emulsion can be reversibly switched between stable and unstable states by bubbling CO₂ and N₂ in the reduced state, or with the addition of H₂O₂ and Na₂SO₃ in the absence of CO₂, because of the adsorption and desorption of SeTA on the silica surface. The former is mainly attributed to a CO₂-controllable electrostatic attraction, resulting from the transition of molecules between cationic and nonionic states; whereas, the latter is ascribed to a redox-tunable hydrogen bonding, originating from the transition of molecules between selenide and selenoxide. However, in the presence of CO₂, redox can only induce a change in the droplet size, not demulsification. These interesting and unique multiresponsive behaviors endow the Pickering emulsion with a capacity for intelligent control of emulsification and demulsification, as well as the droplet size, which may be an asset for a myriad of technological applications in biomedicine, microfluidics, drug delivery, and cosmetics.

Dual responsive Pickering emulsions stabilized by constructed core crosslinked polymer nanoparticles via reversible covalent bonds

In this study, pH- and glucose-responsive Pickering emulsions stabilized by core crosslinked polymer nanoparticles, which were constructed via reversible covalent bonds, were presented for the first time. Firstly, well-defined PDMA-b-PAPBA (poly(N,N-dimethylacrylamide)-b-poly(3-acrylamidophenylboronic acid)) diblock copolymers were synthesized via sequential reversible addition-fragmentation chain transfer (RAFT) polymerization reactions. By means of complexation of PBA units of PDMA-b-PAPBA with PVA in basic water, core crosslinked polymer nanoparticles (CCPNs) with a core-shell structure were formed. The PAPBA/PVA crosslinked network and PDMA acted as the core and shell, respectively. Because of the reversible B-O chemical bonds in the core, the as-produced CCPNs showed structural transitions in response to the external stimuli involving pH and glucose. Investigation of the interfacial activities revealed that CCPNs exhibited high emulsifying performances, and oil in water (o/w) Pickering emulsions could be formed at a low particle content. The formed Pickering emulsions showed high stability at room temperature without any disturbances, whereas de-emulsification was observed upon improving the pH or adding glucose at a given pH. This is the first report on a responsive Pickering emulsion whose stability can be manipulated by glucose, and this type of fabricated manipulative Pickering emulsions are expected to provide useful guidance in the fields of oil recovery, interfacial reactions, etc.

Tuning Amphiphilicity of Particles for Controllable Pickering Emulsion

Pickering emulsions with the use of particles as emulsifiers have been extensively used in scientific research and industrial production due to their edge in biocompatibility and stability compared with traditional emulsions. The control over Pickering emulsion stability and type plays a significant role in these applications. Among the present methods to build controllable Pickering emulsions, tuning the amphiphilicity of particles is comparatively effective and has attracted enormous attention. In this review, we highlight some recent advances in tuning the amphiphilicity of particles for controlling the stability and type of Pickering emulsions. The amphiphilicity of three types of particles including rigid particles, soft particles, and Janus particles are tailored by means of different mechanisms and discussed here in detail. The stabilization-destabilization interconversion and phase inversion of Pickering emulsions have been successfully achieved by changing the surface properties of these particles. This article provides a comprehensive review of controllable Pickering emulsions, which is expected to stimulate inspiration for designing and preparing novel Pickering emulsions, and ultimately directing the preparation of functional materials.

Pickering Emulsions Responsive to CO2/N2 and Light Dual Stimuli at Ambient Temperature

A dual stimuli-responsive n-octane-in-water Pickering emulsion with CO2/N2 and light triggers is prepared using negatively charged silica nanoparticles in combination with a trace amount of dual switchable surfactant, 4-butyl-4-(4-N,N-dimethylbutoxyamine) azobenzene bicarbonate (AZO-B4), as stabilizers. On one hand, the emulsion can be transformed between stable and unstable at ambient temperature rapidly via the N2/CO2 trigger, and on the other hand, a change in droplet size of the emulsion can occur upon light irradiation/rehomogenization cycles without changing the particle/surfactant concentration. The dual responsiveness thus allows for a precise control of emulsion properties. Compared with emulsions stabilized by specially synthesized stimuli-responsive particles or by stimuli-responsive surfactants, the method reported here is much easier and requires a relatively low concentration of surfactant (≈1/10 cmc), which is important for potential applications.

Steric stabilization of nanoparticles with grafted low molecular weight ligands in highly concentrated brines including divalent ions

Whereas numerous studies of stabilization of nanoparticles (NPs) in electrolytes have examined biological fluids, the interest has grown recently in media with much higher ionic strengths including seawater and brines relevant to environmental science and subsurface oil and gas reservoirs. Given that electrostatic repulsion is limited at extremely high ionic strengths due to charge screening, we have identified ligands that are well solvated in concentrated brine containing divalent cations and thus provide steric stabilization of silica nanoparticles. Specifically, the hydrodynamic diameter of silica nanoparticles with grafted low molecular weight ligands, a diol ether, [3-(2,3-dihydroxypropoxy)propyl]-trimethoxysilane, and a zwitterionic sulfobetaine, 3-([dimethyl(3-trimethoxysilyl)propyl]ammonio)propane-1-sulfonate, is shown with dynamic light scattering to remain essentially constant, indicating lack of aggregation, at room temperature and up to 80 °C for over 30 days. An extended DLVO model signifies that steric stabilization is strongly dominant against van der Waals attraction for ∼10 nm particles given that these ligands are well solvated even in highly concentrated brine. In contrast, polyethylene glycol oligomers do not provide steric stabilization at elevated temperatures, even at conditions where the ligands are soluble, indicating complicating factors including bridging of the ether oxygens by divalent cations.

Stimuli-responsive Pickering emulsions: recent advances and potential applications

Pickering emulsions possess many advantages over traditional surfactant stabilized emulsions. For example, Pickering emulsions impart better stability against coalescence and, in many cases, are biologically compatible and environmentally friendly. These characteristics open the door for their use in a variety of industries spanning petroleum, food, biomedicine, pharmaceuticals, and cosmetics. Depending on the application, rapid, but controlled stabilization and destabilization of an emulsion may be necessary. As a result, Pickering emulsions with stimuli-responsive properties have, in recent years, received a considerable amounts of attention. This paper provides a concise and comprehensive review of Pickering emulsion systems that possess the ability to respond to an array of external triggers, including pH, temperature, CO2 concentration, light intensity, ionic strength, and magnetic field. Potential applications for which stimuli-responsive Pickering emulsion systems would be of particular value, such as emulsion polymerization, enhanced oil recovery, catalyst recovery, and cosmetics, are discussed.

In situ measurement and simulation of nano-magnetite mobility in porous media subject to transient salinity

Nanotechnologies have been proposed for a variety of environmental applications, including subsurface characterization, enhanced oil recovery, and in situ contaminant remediation. For such applications, quantitative predictive models will be of great utility for system design and implementation. Electrolyte chemistry, which can vary substantially within subsurface pore waters, has been shown to strongly influence nanoparticle aggregation and deposition in porous media. Thus, it is essential that mathematical models be capable of tracking changes in electrolyte chemistry and predicting its influence on nanoparticle mobility. In this work, a modified version of a multi-dimensional multispecies transport simulator (SEAWAT) was employed to model nanoparticle transport under transient electrolyte conditions. The modeling effort was supported by experimental measurements of paramagnetic magnetite (Fe3O4) nanoparticle, coated with polyacrylamide-methylpropane sulfonic acid - lauryl acrylate (nMag-PAMPS), mobility in columns packed with 40-50 mesh Ottawa sand. Column effluent analyses and magnetic resonance imaging (MRI) were used to quantify nanoparticle breakthrough and in situ aqueous phase concentrations, respectively. Experimental observations revealed that introduction of de-ionized water into the brine saturated column (80 g L(-1) NaCl + 20 g L(-1) CaCl2) promoted release and remobilization of deposited nanoparticles along a diagonal front, coincident with the variable density flow field. This behavior was accurately captured by the simulation results, which indicated that a two-site deposition-release model provided the best fit to experimental observations, suggesting that heterogeneous nanoparticle-surface interactions governed nanoparticle attachment. These findings illustrate the importance of accounting for both physical and chemical processes associated with changes in electrolyte chemistry when predicting nanoparticle transport behavior in subsurface formations.

High interfacial activity of polymers "grafted through" functionalized iron oxide nanoparticle clusters

The mechanism by which polymers, when grafted to inorganic nanoparticles, lower the interfacial tension at the oil-water interface is not well understood, despite the great interest in particle stabilized emulsions and foams. A simple and highly versatile free radical "grafting through" technique was used to bond high organic fractions (by weight) of poly(oligo(ethylene oxide) monomethyl ether methacrylate) onto iron oxide clusters, without the need for catalysts. In the resulting ∼1 μm hybrid particles, the inorganic cores and grafting architecture contribute to the high local concentration of grafted polymer chains to the dodecane/water interface to produce low interfacial tensions of only 0.003 w/v % (polymer and particle core). This "critical particle concentration" (CPC) for these hybrid inorganic/polymer amphiphilic particles to lower the interfacial tension by 36 mN/m was over 30-fold lower than the critical micelle concentration of the free polymer (without inorganic cores) to produce nearly the same interfacial tension. The low CPC is favored by the high adsorption energy (∼10(6) kBT) for the large ∼1 μm hybrid particles, the high local polymer concentration on the particles surfaces, and the ability of the deformable hybrid nanocluster cores as well as the polymer chains to conform to the interface. The nanocluster cores also increased the entanglement of the polymer chains in bulk DI water or synthetic seawater, producing a viscosity up to 35,000 cP at 0.01 s(-1), in contrast with only 600 cP for the free polymer. As a consequence of these interfacial and rheological properties, the hybrid particles stabilized oil-in-water emulsions at concentrations as low as 0.01 w/v %, with average drop sizes down to 30 μm. In contrast, the bulk viscosity was low for the free polymer, and it did not stabilize the emulsions. The ability to influence the interfacial activity and rheology of polymers upon grafting them to inorganic particles, including clusters, may be expected to be broadly applicable to stabilization of emulsions and foams.

Modified montmorillonite clay microparticles for stable oil-in-seawater emulsions

Environmentally benign clay particles are of great interest for the stabilization of Pickering emulsions. Dodecane-in-synthetic seawater (SSW) emulsions formed with montmorillonite (MMT) clay microparticles modified with bis(2-hydroxyethyl)oleylamine were stable against coalescence, even at clay concentrations down to 0.1% w/v. Remarkably, as little as 0.001% w/v surfactant lowered the hydrophilicity of the clay to a sufficient level for stabilization of oil-in-SSW emulsions. The favorable effect of SSW on droplet size reduction and emulsion stability enhancement is hypothesized to be due to reduced electrostatic repulsion between adsorbed clay particles and a consequent increase in the continuous phase (an aqueous clay suspension) viscosity. Water/oil (W/O) emulsions were inverted to O/W either by decreasing the mass ratio of surfactant-to-clay (transitional inversion) or by increasing the water volume fraction (catastrophic inversion). For both types of emulsions, coalescence was minimal and the sedimentation or creaming was highly correlated with the droplet size. For catastrophic inversions, the droplet size of the emulsions was smaller in the case of the preferred curvature. Suspensions of concentrated clay in oil dispersions in the presence of surfactant were stable against settling. The mass transfer pathways during emulsification of oil containing the clay particles were analyzed on the droplet size/stability phase diagrams to provide insight for the design of dispersant systems for remediating surface and subsurface oceanic oil spills.

Synergistic formation and stabilization of oil-in-water emulsions by a weakly interacting mixture of zwitterionic surfactant and silica nanoparticles

Oil-in-water emulsions were formed and stabilized at low amphiphile concentrations by combining hydrophilic nanoparticles (NPs) (i.e., bare colloidal silica) with a weakly interacting zwitterionic surfactant, caprylamidopropyl betaine, to generate a high hydrophilic-lipophilic balance. The weak interaction of the NPs with surfactant was quantified with contact angle measurements. Emulsions were characterized by static light scattering to determine the droplet size distributions, optical photography to quantify phase separation due to creaming, and both optical and electron microscopy to determine emulsion microstructure. The NPs and surfactant acted synergistically to produce finer emulsions with a greater stability to coalescence relative to the behavior with either NPs or surfactant alone. As a consequence of the weak adsorption of the highly hydrophilic surfactant on the anionic NPs along with the high critical micelle concentration, an unusually large surfactant concentration was available to adsorb at the oil-water interface and lower the interfacial tension. The synergy for emulsion formation and stabilization for the two amphiphiles was even greater in the case of a high-salinity synthetic seawater aqueous phase. Here, higher NP adsorption at the oil-water interface was caused by electrostatic screening of interactions between (1) NPs and the anionic oil-water interface and (2) between the NPs. This greater adsorption as well as partial flocculation of the NPs provided a more efficient barrier to droplet coalescence.

Stabilization of iron oxide nanoparticles in high sodium and calcium brine at high temperatures with adsorbed sulfonated copolymers

A series of sulfonated random and block copolymers were adsorbed on the surface of ~100 nm iron oxide (IO) nanoparticles (NPs) to provide colloidal stability in extremely concentrated brine composed of 8% wt NaCl + 2% wt CaCl2 (API brine; 1.4 M NaCl + 0.2 M CaCl2) at 90 °C. A combinatorial materials chemistry approach, which employed Ca(2+)-mediated adsorption of anionic acrylic acid-containing sulfonated polymers to preformed citrate-stabilized IO nanoclusters, enabled the investigation of a large number of polymer coatings. Initially a series of poly(2-methyl-2-acrylamidopropanesulfonate-co-acrylic acid) (poly(AMPS-co-AA)) (1:8 to 1:1 mol:mol), poly(styrenesulfonate-block-acrylic acid) (2.4:1 mol:mol), and poly(styrenesulfonate-alt-maleic acid) (3:1 mol:mol) copolymers were screened for solubility in API brine at 90 °C. The ratio of AMPS to AA groups was varied to balance the requirement of colloid dispersibility at high salinity (provided by AMPS) against the need for anchoring of the polymers to the iron oxide surface (via the AA). Steric stabilization of IO NPs coated with poly(AMPS-co-AA) (1:1 mol:mol) provided colloidal stability in API brine at room temperature and 90 °C for up to 1 month. The particles were characterized before and after coating at ambient and elevated temperatures by a variety of techniques including colloidal stability experiments, dynamic light scattering, zeta potential, and thermogravimetric analysis.