Separating oil-water nanoemulsions using flux-enhanced hierarchical membranes

The Separation of Oil/Water Mixtures by Modified Melamine and Polyurethane Foams: A Review

Melamine (MA) and polyurethane (PU) foams, including both commercial sponges for daily use as well as newly synthesized foams are known for their high sorption ability of both polar and unipolar liquids. From this reason, commercial sponges are widely used for cleaning as they absorb a large amount of water, oil as well as their mixtures. These sponges do not preferentially absorb any of those components due to their balanced wettability. On the other hand, chemical and physical modifications of outer surfaces or in the bulk of the foams can significantly change their original wettability. These treatments ensure a suitable wettability of foams needed for an efficient water/oil or oil/water separation. MA and PU foams, dependently on the treatment, can be designed for both types of separations. The particular focus of this review is dealt with the separation of oil contaminants dispersed in water of various composition, however, an opposite case, namely a separation of water content from continuous oily phase is also discussed in some extent. In the former case, water is dominant, continuous phase and oil is dispersed within it at various concentrations, dependently on the source of polluted water. For example, waste waters associated with a crude oil, gas, shale gas extraction and oil refineries consist of oily impurities in the range from tens to thousands ppm [mg/L]. The efficient materials for preferential oil sorption should display significantly high hydrophobicity and oleophilicity and vice versa. This review is dealt with the various modifications of MA and PU foams for separating both oil in water and water in oil mixtures by identifying the chemical composition, porosity, morphology, and crosslinking parameters of the materials. Different functionalization strategies and modifications including the surface grafting with various functional species or by adding various nanomaterials in manipulating the surface properties and wettability are thoroughly reviewed. Despite the laboratory tests proved a multiply reuse of the foams, industrial applications are limited due to fouling problems, longer cleaning protocols and mechanical damages during performance cycles. Various strategies were proposed to resolve those bottlenecks, and they are also reviewed in this study.

Contactless Method for Electrocoalescence of Water in Oil

This paper discusses an experimental approach to study the effects of a contactless method on electrocoalescence of water-in-oil mixture/emulsion. A positive corona discharge is utilized using a sharp conductive needle without direct contact with the mixture/solution to avoid potential corrosion of the electrode. This creates a nonuniform electric field, which is further used for the coalescence of water droplets in the range of micro to macro in oil. Two approaches are employed in this study: qualitative analysis conducted by visually studying coalescence patterns in videos captured with a high-speed camera and a quantitative analysis based on calculations obtained from dynamic light scattering measurements. From the behavior of the water droplets under the electric field, it is observed that dipole-dipole interaction, migratory coalescence/electrophoresis, and dielectrophoresis have major roles in promoting the coalescence events. The effects of oil viscosity and power consumption on the coalescence rate are also investigated, suggesting an optimal oil-water separation process. The results of this study pave a path for developing a safe, contactless, rapid, and low-power-consuming separation process, potentially suitable for an offsite application.

Graphene Oxide Membranes for Trace Hydrocarbon Contaminant Removal from Aqueous Solution

The aim of this paper is to shed light on the application of graphene oxide (GO) membranes for the selective removal of benzene, toluene, and xylene (BTX) from wastewater. These molecules are present in traces in the water produced from oil and gas plants and are treated now with complex filtration systems. GO membranes are obtained by a simple, fast, and scalable method. The focus of this work is to prove the possibility of employing GO membranes for the filtration of organic contaminants present in traces in oil and gas wastewater, which has never been reported. The stability of GO membranes is analyzed in water solutions with different pH and salinity. Details of the membrane preparation are provided, resulting in a crucial step to achieve a good filtration performance. Material characterization techniques such as electron microscopy, x-ray diffraction, and infrared spectroscopy are employed to study the physical and chemical structure of GO membranes, while gas chromatography, UV-visible spectroscopy, and gravimetric techniques allow the quantification of their filtration performance. An impressive rejection of about 90% was achieved for 1 ppm of toluene and other pollutants in water, demonstrating the excellent performance of GO membranes in the oil and gas field.

Colloid Separation by CO2-Induced Diffusiophoresis

We present a microfluidic crossflow separation of colloids enabled by the dissolution of CO₂ gas in aqueous suspensions. The dissolved CO₂ dissociates into H⁺ and HCO₃^- ions, which are efficient candidates for electrolytic diffusiophoresis, because of the fast diffusion of protons. By exposing CO₂ gas to one side of a microfluidic flow channel, a crossflow gradient can be created, enabling the crossflow diffusiophoresis of suspended particles. We develop a simple two-dimensional model to describe the coupled transport dynamics that is due to the competition of advection and diffusiophoresis. Furthermore, we show that oil nanoemulsions can be effectively separated by utilizing highly charged particles as a carrier vehicle, which is otherwise difficult to achieve. These results demonstrate a portable, versatile method for separating particles in broad applications including oil extraction, drug delivery, and bioseparation.

Bioinspired Slippery Lubricant-Infused Surfaces With External Stimuli Responsive Wettability: A Mini Review

Responsive slippery lubricant-infused surfaces (SLIS) have attracted substantial attention because of the high demand of fundamental research and practical applications, such as controllable liquid-repellency, intelligent, and easy-to-implement wettability switching. In this review, advanced development of responsive slippery surfaces is briefly summarized upon various external stimuli, including stress, electrical field, magnetic field, and temperature. In addition, remaining challenge and prospect are also discussed.

Low-Voltage Surface Electrocoalescence Enabled by High-K Dielectrics and Surfactant Bilayers for Oil-Water Separation

Processes for separating oil-water mixtures are critical to operations in energy and water. However, existing separation methods pose efficiency limitations as well as environmental and safety challenges. Here, we present a low-voltage surface electrocoalescence approach that triggers coalescence of surfactant-stabilized emulsions by combining high-K dielectrics with surfactant bilayers. In this system, the high-K dielectric reduces the electrocoalescence voltage, while the surfactant bilayer functions as a self-healing, high capacitance film that prevents pinning of droplets on the dielectric surface. This high capacitance system maximizes the electric field between neighboring droplets, exerting high electrostatic pressure that overcomes the disjoining pressure between droplets, thereby enabling rapid electrocoalescence. We demonstrate electrocoalescence of surfactant-stabilized microscale droplets of saline water in oil using single volts. We expect our results may find application in the energy sector, wastewater treatment, and purification industries.

Efficient Separation of Ethanol-Methanol and Ethanol-Water Mixtures Using ZIF-8 Supported on a Hierarchical Porous Mixed-Oxide Substrate

This work reports a new approach for the synthesis of a zeolitic imidazolate framework (ZIF-8) composite. It employs the direct growth of the crystalline ZIF-8 on a mixed-metal oxide support TiO₂-SiO₂ (TSO), which mimics the porous structure of Populus nigra. Using the natural leaf as a template, the TSO support was prepared using a sol-gel method. The growth of the ZIF-8 layer on the TSO support was carried out by the seeds and second growth method. This method facilitates the homogeneous dispersion of ZIF-8 crystals at the surface of the TSO composite. The ZIF-8@TSO composite adsorbs methanol selectively, mainly due to the hierarchical porous structure of the mixed oxide support. As compared with the as-synthesized ZIF-8, a 50% methanol uptake is achieved in the ZIF-8@TSO composite, with only 25 wt % ZIF-8 loading. IAST simulations show that the ZIF-8@TSO composite has a preferential adsorption toward methanol when using an equimolar methanol-ethanol mixture. An opposite behavior is observed for the as-synthesized ZIF-8. The results show that combining MOFs and mixed-oxide supports with bioinspired structures opens opportunities for synthesizing new materials with unique and enhanced adsorption and separation properties.

Ultrastable Underwater Anti-Oil Fouling Coatings from Spray Assemblies of Polyelectrolyte Grafted Silica Nanochains

Surfaces that have superhydrophilic characteristics are known to exhibit extreme oil repellency under water, which is attractive for applications including anti-fogging, water-oil separations, and self-cleaning. However, superhydrophilic surfaces can also be easily fouled and lose their extreme oil repellency, which limits their usage in practical applications. In this work, we create an anti-oil fouling coating by spray coating poly(acrylic acid) (PAA)-grafted SiO₂ nanochains (approximately 45 nm wide and 300 nm long) onto solid surfaces, forming a nanoporous film exhibiting superhydrophilicity (water contact angle in air ≈ 0°) and underwater superoleophobicity (dichloroethane contact angle ≥ 165°). The polymer-grafted nanochain assemblies exhibit extremely low contact angle hysteresis (<1°) and small adhesion hysteresis (-0.05 mN m^-1), and thus, oil can readily roll off from the surface when the coating is immersed in water. Compared to other superhydrophilic surfaces, we show that both the unique structure of spray-assembled nanochains and the hygroscopic nature of PAA are essential to enable ultrastable anti-oil fouling. Even after the PAA-grafted nanochain coating is purposely fouled by oil, oil can be readily and completely expelled and lifted-off from the coating within 10 s when placed under water. Further, we show that our coating retains underwater superoleophobicity even after being subjected to shearing under water for more than 168 h. Our approach offers a simple yet versatile method to create an ultrastable superhydrophilic and anti-oil fouling coating via a scalable manufacturing method.

Stabilization of Dispersed Oil Droplets in Nanoemulsions by Synergistic Effects of the Gemini Surfactant, PHPA Polymer, and Silica Nanoparticle

Nanoemulsion systems comprising n-heptane (oleic component), stabilized by the {gemini surfactant (14-6-14 GS) + polymer [partially hydrolyzed poly-acrylamide (PHPA)] + silica (SiO₂) nanoparticle} shell and dispersed in aqueous phase, were synthesized by ultrasonication (high-energy method). Influence of ultrasonication time on nanoemulsion kinetics was investigated to predict the saturation droplet diameter. Morphological analysis by transmission electron cryomicroscopy imaging showed that oleic phase appears as uniformly dispersed spherical droplets in 14-6-14 GS-stabilized nanoemulsion, which on PHPA addition changes into a network structure consisting of larger oil droplets. 14-6-14 + PHPA + SiO₂ nanoemulsion systems show more effective packing arrangement with irregular-shaped (nonspherical) droplets. Dynamic light scattering studies identified droplet size distribution profiles in the range 4.2-25.4 nm for the surfactant-stabilized nanoemulsion, 125.9-358.8 nm for the surfactant-polymer nanoemulsion, and 88.4-222.3 nm for the surfactant-polymer-nanoparticle-based nanoemulsion in optimal dosage(s). Statistical analyses were performed using normal, log-normal, and Cauchy-Lorentz distribution functions. A modified form of Hinze theory was employed to model droplet behavior in analyzed nanoemulsion systems. Zeta potential values of nanoemulsions were studied at different time intervals to determine kinetic stability as well as corroborate Hinze model findings. In summary, this article aims at investigating nanoemulsion droplet stability by thorough examination of electrostatic repulsive barrier and steric hindrance effects.

Fabrication of Janus Membranes for Desalination of Oil-Contaminated Saline Water

Desalination of oil-contaminated saline water using membrane distillation requires hydrophobic membranes with underwater superoleophobic surfaces. For designing such membranes, the chemistry and morphology of the interfacial domains in contact with the contaminated water need to be adjusted such that a stable water layer, adhering to the surface, prevents oil droplets from wetting the membrane. In this article, we present an approach that relies on the controlled functionalization of the surface of polyvinylidene fluoride (PVDF) membranes; we adjust the surface topography of the membranes and introduce chemical heterogeneity to them. We show that the morphology of the PVDF surface can be altered by adjusting the composition of the nonsolvent bath used for the phase inversion process. Also, we render the surface of the membranes hydrophilic by using an alkaline chemical bath solution. The membrane morphology and effectiveness of our chemical treatment were confirmed by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), Fourier-transformed infrared spectroscopy (FTIR), and zeta potential measurements. A stable underwater contact angle, higher than 150°, was observed for both canola oil (ρ ≈ 0.913 g cm^-3, γ ≈ 31.5 mN m^-1) and hexane (ρ ≈ 0.655 g cm^-3, γ ≈ 18 mN m^-1). We evaluated the performance of both pristine and functionalized membranes in a laboratory-scale direct contact membrane distillation (DCMD) setup and desalinated a saline solution contaminated with 500 ppm canola oil. Our results show that oil does not wet the functionalized membrane during the desalination process. The average permeate flux and salt rejection values for the functionalized membranes were 45 ± 5 Lm^-2h^-1 ( T_feed = 70 °C, T_distillate = 20 °C) and 99.99%, respectively.

Preparation of PSF/FEP mixed matrix membrane with super hydrophobic surface for efficient water-in-oil emulsion separation

The PSF/FEP membrane with super hydrophobic and super oleophilic surface had an outstanding separation performance for water-in-oil emulsion.

Separation of Oil-in-Water Emulsions Using Hydrophilic Electrospun Membranes with Anisotropic Pores

It has been challenging to separate oil from oil/water emulsions with droplet size less than 1 μm using conventional porous membranes. Membranes with small pores are preferred, but the trade-off is a dramatic reduction of volumetric flux. Here, we prepared membranes from electrospun polycaprolactone (PCL) fibers with high porosity (∼88%). When the membranes were stretched uniaxially at different strain levels, the pores became anisotropic with an aspect ratio (pore length/width) up to 5.3 ± 3.0. To improve their wettability, we added Tween 80, a hydrophilic surfactant, to PCL solutions for electrospinning. The modified PCL membranes showed excellent mechanical properties with a tensile strength at 6.59 ± 1.67 MPa and the elongation at break up to 130 ± 21%, warranting their use as free-standing separators. We narrowed the pore gap while maintaining the high porosity by stretching the membranes. Scanning electron microscopy (SEM) images of the stretched membranes show changes of pore geometry without altering the fiber size and fiber network integrity with strain up to 80%. The anisotropic membrane could exclude oil from oil-in-water emulsion droplets with a diameter as small as 18 nm without reduction of the volumetric flux in comparison with the nonstretched one.

Nanoemulsion-induced enzymatic crosslinking of tyramine-functionalized polymer droplets

In situ gelation of water-in-oil polymer emulsions is a key method to produce hydrogel particles. Although this approach is in principle ideal for encapsulating bioactive components such as cells, the oil phase can interfere with straightforward presentation of crosslinker molecules. Several approaches have been developed to induce in-emulsion gelation by exploiting the triggered generation or release of crosslinker molecules. However, these methods typically rely on photo- or acid-based reactions that are detrimental to cell survival and functioning. In this work, we demonstrate the diffusion-based supplementation of small molecules for the in-emulsion gelation of multiple tyramine-functionalized polymers via enzymatic crosslinking using a H₂O₂/oil nanoemulsion. This strategy is compatible with various emulsification techniques, thereby readily supporting the formation of monodisperse hydrogel particles spanning multiple length scales ranging from the nano- to the millimeter. As proof of principle, we leveraged droplet microfluidics in combination with the cytocompatible nature of enzymatic crosslinking to engineer hollow cell-laden hydrogel microcapsules that support the formation of viable and functional 3D microtissues. The straightforward, universal, and cytocompatible nature of nanoemulsion-induced enzymatic crosslinking facilitates its rapid and widespread use in numerous food, pharma, and life science applications.

Bioinspired Diatomite Membrane with Selective Superwettability for Oil/Water Separation

Membranes with selective superwettability for oil/water separation have received significant attention during the past decades. Hierarchical structures and surface roughness are believed to improve the oil repellency and the stability of Cassie-Baxter state. Diatoms, unicellular photosynthetic algae, possess sophisticated skeletal shells (called frustules) which are made of hydrated silica. Motivated by the hierarchical micro- and nanoscale features of diatom, we fabricate a hierarchical diatomite membrane which consists of aligned micro-sized channels by the freeze casting process. The fine nano-porous structures of frustules are well preserved after the post sintering process. The bioinspired diatomite membrane performs both underwater superoleophobicity and superhydrophobicity under various oils. Additionally, we demonstrate the highly efficient oil/water separation capabililty of the membranes in various harsh environments. The water flux can be further adjusted by tuning the cooling rates. The eco-friendly and robust bioinspired membranes produced by the simple, cost-effective freeze casting method can be potentially applied for large scale and efficient oil/water separation.

Extraction of Oil from an Aqueous Emulsion by Coupling Thermal Swing with a Capillary Pump

Separation of oil from water is an area of increasing interest because of the ever-increasing emphasis on reducing discharge of oily wastewater streams and for managing accidental oil spills. While several methods to separate oil from water are available, the current methods often require elaborate processing steps and/or have low extraction rates. Here, we report two simple and potentially inexpensive methods of separating oil from aqueous emulsions. The first method employs hydrophobized glass wool in a pressure-driven capillary pump, while the second method employs novel zeolite pellets the exterior surface of which is hydrophobic. These pellets selectively absorb oil from an aqueous emulsion, which can subsequently be recovered using thermal swing with hot fluid at a temperature far below the boiling point of the oil. Separation of oil with a very high yield (ca. 97%) appears possible using a combination of the two methods.

A new nano-engineered hierarchical membrane for concurrent removal of surfactant and oil from oil-in-water nanoemulsion

Surfactant stabilized oil-in-water nanoemulsions pose a severe threat to both the environment and human health. Recent development of membrane filtration technology has enabled efficient oil removal from oil/water nanoemulsion, however, the concurrent removal of surfactant and oil remains unsolved because the existing filtration membranes still suffer from low surfactant removal rate and serious surfactant-induced fouling issue. In this study, to realize the concurrent removal of surfactant and oil from nanoemulsion, a novel hierarchically-structured membrane is designed with a nanostructured selective layer on top of a microstructured support layer. The physical and chemical properties of the overall membrane, including wettability, surface roughness, electric charge, thickness and structures, are delicately tailored through a nano-engineered fabrication process, that is, graphene oxide (GO) nanosheet assisted phase inversion coupled with surface functionalization. Compared with the membrane fabricated by conventional phase inversion, this novel membrane has four times higher water flux, significantly higher rejections of both oil (~99.9%) and surfactant (as high as 93.5%), and two thirds lower fouling ratio when treating surfactant stabilized oil-in-water nanoemulsion. Due to its excellent performances and facile fabrication process, this nano-engineered membrane is expected to have wide practical applications in the oil/water separation fields of environmental protection and water purification.

A pH-responsive superwetting nanostructured copper mesh film for separating both water-in-oil and oil-in-water emulsions

A new pH-responsive nanostructured copper mesh film was reported for the bidirectional separation of emulsified oil/water mixtures.

A facile approach to silica-modified polysulfone microfiltration membranes for oil-in-water emulsion separation

A facile strategy to prepare silica-modified membranes with superhydrophilicity and underwater superoleophobicity was developed. These hybrid membranes can be applied in oil/water separation with high filtration efficiency and pressure endurance.

Hierarchy in inorganic membranes

Inorganic membranes usually possess an unsymmetrical cross section showing a hierarchical structure: the ZIF-8 separation layer is deposited on a graded titania support.

Adsorption of Anionic or Cationic Surfactants in Polyanionic Brushes and Its Effect on Brush Swelling and Fouling Resistance during Emulsion Filtration

Atom transfer radical polymerization of ionic monomers from membrane surfaces yields polyelectrolyte brushes that swell in water and repel oil droplets to resist fouling during filtration of oil-in-water emulsions. However, surfactant adsorption to polyelectrolyte brushes may overcome this fouling resistance. This work examines adsorption of cationic and anionic surfactants in polyanionic brushes and the effect of these surfactants on emulsion filtration. In situ ellipsometry with films on flat surfaces shows that brushes composed of poly(3-sulfopropyl methacrylate salts) (pSPMK) swell 280% in water and do not adsorb sodium dodecyl sulfate (SDS). pSPMK-modified microfiltration membranes reject >99.9% of the oil from SDS-stabilized submicron emulsions, and the specific flux through these modified membranes is comparable to that through NF270 nanofiltration membranes. Moreover, the brush-modified membranes show no decline in flux over a 12 h filtration, whereas the flux through NF270 membranes decreases by 98.7%. In contrast, pSPMK brushes adsorb large quantities of cetyltrimethylammonium bromide (CTAB), and at low chain densities the brushes collapse in the presence of this cationic surfactant. Filtration of CTAB-stabilized emulsions through pSPMK-modified membranes gives minimal oil rejection, presumably due to the brush collapse. Thus, the fouling resistance of polyelectrolyte brush-modified membranes clearly depends on the surfactant composition in a particular emulsion.

Inorganic-Organic Thiol-ene Coated Mesh for Oil/Water Separation

A highly efficient mesh for oil/water separation was fabricated by using a superhydrophobic and superoleophilic coating of thiol-ene hybrid, consisting of pentaerythritol tetra(3-mercaptopropionate) (PETMP), 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (TMTVSi), and hydrophobic fumed silica nanoparticles, via a simple two-step fabrication process. Spray deposition and UV curing photopolymerization were sequentially performed, during which solvent evaporation provides microscale roughness while nanoparticle aggregation forms nanoscale roughness. The hierarchical morphologies were stabilized after UV curing photopolymerization. High contact angle (>150°) and low roll-off angle (<5°) were achieved due to the multiscale roughness structure of the hierarchical morphologies. These coatings also have excellent chemical resistance, as well as temperature and pH stability, after curing.

Photothermal-Responsive Single-Walled Carbon Nanotube-Based Ultrathin Membranes for On/Off Switchable Separation of Oil-in-Water Nanoemulsions

Oil-contaminated wastewater threatens our environment and health, especially that stabilized by surfactants. Conventional separation protocols become invalid for those surfactant-stabilized nanoemulsions due to their nanometer-sized droplets and extremely high stability. In this paper, photothermal-responsive ultrathin Au nanorods/poly(N-isopropylacrylamide-co-acrylamide) cohybrid single-walled carbon nanotube (SWCNT) nanoporous membranes are constructed. Such membranes are capable of separating oil-in-water nanoemulsions with a maximum flux up to 35 890 m(2)·h(-1)·bar(-1) because they feature hydrophilicity, underwater oleophobicity, and nanometer pore sizes. It is remarkable that the permeation flux can be simply modulated by light illumination during the process of separation, due to the incorporation of thermal-responsive copolymers and Au nanorods. Meanwhile, it shows ultrahigh separation efficiency (>99.99%) and desired antifouling and recyclability properties. We anticipate that our ultrathin photothermal-responsive SWCNT-based membranes provide potential for the generation of point-of-use water treatment devices.

Template-particle stabilized bicontinuous emulsion yielding controlled assembly of hierarchical high-flux filtration membranes

A novel solvent-evaporation-based process that exploits template-particle stabilized bicontinuous emulsions for the formation of previously unreached membrane morphologies is reported in this article. Porous membranes have a wide range of applications spanning from water filtration, pharmaceutical purification, and battery separators to scaffolds for tissue engineering. Different situations require different membrane morphologies including various pore sizes and pore gradients. However, most of the previously reported membrane preparation procedures are restricted to specific morphologies and morphology alterations require an extensive optimization process. The tertiary system presented in this article, which consists of a poly(ether sulfone)/dimethylacetamide (PES/DMAc) solution, glycerol, and ZnO-nanoparticles, allows simple and exact tuning of pore diameters ranging from sub-20 nm, up to 100 nm. At the same time, the pore size gradient is controlled from 0 up to 840%/μm yielding extreme asymmetry. In addition to structural analysis, water flux rates of over 5600 L m(-2) h(-1) are measured for membranes retaining 45 nm silica beads.

Conditions for spontaneous oil–water separation with oil–water separators

A theory is proposed for the selection of the nature of the separator for spontaneous oil–water separation from oil-in-water and water-in-oil systems.