Interaction Mechanism of Oil-in-Water Emulsions with Asphaltenes Determined Using Droplet Probe AFM

Probing the interactions between asphaltenes and a PEO-PPO demulsifier at oil-water interface: Effect of temperature

HYPOTHESIS

Asphaltenes are primary stabilizers in water-in-oil (W/O) emulsions that cause corrosion and fouling issues. In oil sands industry, oil/water separation processes are generally conducted at high temperatures. A high temperature is expected to impact the interactions between asphaltenes and emulsion breakers (EBs), consequently influencing demulsification performance.

EXPERIMENTS

The adsorption and interactions of asphaltenes and a PEO-PPO type EB (Pluronic F68) at the oil-water interface were investigated at various temperatures, using tensiometer, quartz crystal microbalance with energy dissipation (QCM-D), and atomic force microscopy (AFM). The effect of temperature on EB's demulsification performance was explored through bottle tests. Additionally, demulsification mechanisms were studied using direct force measurements with the droplet probe AFM technique.

FINDINGS

Dynamic interfacial tension and QCM-D results demonstrate that the PEO-PPO type EB exhibits higher interfacial activity than asphaltenes and can disrupt rigid asphaltene films at the oil-water interfaces. Elevated temperatures accelerate the displacement of adsorbed asphaltenes by EB molecules, leading to sparse interfacial films, rapid droplet coalescence, and improved demulsification efficiency (supported by AFM and bottle test results). This work provides valuable insights into interfacial interactions between asphaltenes and EB at different temperatures, enhancing the understanding of demulsification mechanisms and offering useful implications for the development of efficient EBs to enhance oil/water separation performance.

Probing the interfacial behaviors of interfacially active and non-active asphaltenes and their impact on emulsion stability

HYPOTHESIS

Asphaltenes subfractions with distinct interfacial behaviors may play different roles in stabilizing oil-water emulsions.

EXPERIMENTS

In this work, whole asphaltenes were separated into interfacially active asphaltenes (IAA) and interfacially non-active asphaltenes (INAA). Employing advanced nanomechanical techniques, we have explored the compositions, morphologies, sizes, adsorption, and interfacial behaviors of IAA and INAA.

FINDINGS

IAA exhibits a high and unevenly distributed oxygen content, distinguishing it from INAA. In toluene, the diameters of IAA and INAA are about 60 nm and 6 nm, respectively. When adsorbed irreversibly on mica surfaces, the thickness of the IAA and INAA film was measured at ∼5.5 nm or 1 nm, respectively; while in a toluene solution, the film thickness reached ∼46 nm and 3.1 nm for IAA and INAA, respectively. IAA demonstrates superior interfacial activity, and elastic/viscous moduli compared to INAA at the water-toluene interface. Quantified surface force measurements reveal that IAA stabilizes water droplets in toluene at a concentration of only 10 mg/L, while INAA requires a higher concentration of 100 mg/L. This work provides the first comprehensive investigation into the adsorption and interfacial behaviors of asphaltene subfractions and provides useful insights into the asphaltenes-stabilization mechanism of emulsions.

Liquid-liquid and gas-liquid dispersions in electrochemistry: concepts, applications and perspectives

Electrochemistry plays a pivotal role in a vast number of domains spanning from sensing and manufacturing to energy storage, environmental conservation, and healthcare. Electrochemical applications encompassing gaseous or organic substrates encounter shortcomings ascribed to high mass transfer/internal resistances and low solubility in aqueous electrolytes, resulting in high overpotentials. In practice, strong acids and expensive organic electrolytes are required to promote charge transfer in electrochemical cells, resulting in a high carbon footprint. Liquid-liquid (L-L) and gas-liquid (G-L) dispersions involve the dispersion of a nano/micro gas or liquid into a continuous liquid phase such as micelles, (macro)emulsions, microemulsions, and microfoams stabilised by surface-active agents such as surfactants and colloidal particles. These dispersions hold promise in addressing the drawbacks of electrochemical reactions by fostering the interfacial surface area between immiscible reagents and mass transfer of electroactive organic and gas reactants and products from/to the bulk to/from the electrode surface. This tutorial review provides a taxonomy of liquid-liquid and gas-liquid dispersions for applications in electrochemistry, with emphasis on their assets and challenges in industrially relevant reactions for fine chemistry and depollution.

Investigation on the Adhesion Force between Tetrabutylammonium Bromide Hydrate Particles Using Atomic Force Microscopy

This work investigates the adhesion force between tetrabutylammonium bromide (TBAB) hydrate particles dispersed in decane at different temperatures and TBAB concentrations using an atomic force microscopy. The thickness of the quasi-liquid layer (QLL) on the surface of the hydrate particles is calculated based on an adhesion force model. The results of force measurements indicate that the adhesion force between the hydrate particles increases with increasing temperature when TBAB concentration is 30 wt %. The increment of adhesion force between particles could be due to the increase in the QLL thickness on the particle surfaces. Furthermore, the force results also reveal that the adhesion force between hydrate particles(ice) at 253 K decreases when TBAB concentration increases from 0 to 30 wt %. The calculation indicates that QLL on the surface of formed hydrate particles becomes thinner at higher TBAB concentrations, which could be due to the conversion rate of water to hydrate within particles. The thickness of QLL is directly influenced by the temperature and TBAB concentration, which contributes to the adhesion force between hydrate particles.

Food emulsions stabilized by proteins and emulsifiers: A review of the mechanistic explorations

The stability of food emulsions is the basis for other properties. During their production and processing, emulsions tend to become unstable due to their thermodynamic instability, and it is usually necessary to add emulsifiers and proteins to stabilize emulsions. It becomes crucial to study the intrinsic mechanisms of emulsifiers and proteins and their joint stabilization of food emulsions. This paper summarizes the research on intrinsic mechanisms of food emulsions stabilized by emulsifiers and proteins in recent years. The destabilization and stabilization of emulsions are related to the added surfactants. The properties, type, and concentration of emulsifiers determine the stability of emulsions, and the emulsifiers can be classified into different types (e.g., ionic or nonionic, solid or liquid) according to their properties and sources. The physicochemical properties of proteins (e.g., spatial conformation, hydrophobicity) and the composition of proteins can also determine the stability of emulsions, and emulsions stabilized by emulsifiers and proteins together not only depend on these factors but also have a great relationship with the mutual combination and competition between the two. The instability and stability of emulsions are related to factors such as interfacial interaction forces, the rheological nature of the interface, and the added surfactant.

Optical Tweezers-Based Measurements of Colloidal Forces between Asphaltene Thin Films: Effect of Ultrasonication

Oil production and processing often involve the treatment of water-in-oil emulsions stabilized by asphaltenes. The asphaltenes adsorb irreversibly at the water-oil interface and, by self-association at the interface, form a viscoelastic film that stabilizes the emulsions mechanically and sterically. Hydrophobic forces associated with these films may also contribute to the emulsion stability. A key step in treating these emulsions is to weaken the asphaltene film at the interface, and one way to do so is with ultrasonic treatment. The effect of ultrasonic waves on the interactions between asphaltene films was investigated at a silica-water interface using optical tweezers. Silica microparticles were aged in asphaltene solutions to form asphaltene coatings on their surfaces. The particles were dispersed in water, and interparticle force measurements were performed with optical tweezers to capture the steric force and hydrophobic force contributions. The asphaltene coating thickness and hydrophobic coefficient (a factor resembling the strength of the hydrophobic interaction) were obtained from fitting these forces. The effect of ultrasonication on the thickness of the asphaltene films on the surfaces of the particles was investigated. No change in the hydrophobic coefficient was observed upon changing the interfacial asphaltene concentration. The asphaltene film thickness increased with the concentration of the asphaltene solution and aging time. After treatment of the dispersion with ultrasonic waves for different durations (between 5 and 40 min), a significant reduction in the coating thickness was observed. This reduction was confirmed by thermogravimetric analysis (TGA) measurements. It is hypothesized that cavitation at the interface removed part of the surface layer of asphaltenes from the coated particles. Based on these findings, we proved that a low-power ultrasound field can effectively break asphaltene-stabilized water-in-oil emulsions.

Zeta potential of crude oil in aqueous solution

Despite the broad range of interest and applications, controls on the surface charge of crude oil in aqueous solution remain poorly understood. The primary data source to understand the surface charge on crude oil comprises measurements of zeta potential on individual drops or emulsions obtained using the electrophoretic method (EPM). Here we (i) collate and review previous measurements of zeta potential on crude oil, (ii) compare and contrast the results, and (iii) report new measurements of zeta potential on crude oil wetting films and layers relevant to oil-saturated porous media, obtained using the streaming potential method (SPM). Results show that the zeta potential depends on electrolyte pH and the concentration of divalent ions Ca2+ and Mg2+. Lower pH and higher concentration of these divalent ions yields more positive zeta potential. The isoelectric point (IEP) in simple NaCl electrolytes lies in the pH range 3-5. The IEP in simple CaCl2 and MgCl2 electrolytes can be expressed as pCa or pMg, respectively, and lies in the range 0-1. Close to the IEP, the zeta potential varies linearly with pH, pCa or pMg, suggesting simple Nernstian behaviour of the crude oil surface. The sensitivity of the zeta potential to pH, pCa and pMg decreases with increasing total ionic strength. The impact of pH, pCa and pMg on zeta potential varies significantly across different crude oils and differs from non-polar hydrocarbons. The potential for other multivalent ions to modify crude oil zeta potential has not been tested. Data for crude oil wetting films and layers, obtained using the SPM and strongly oil-wet porous substrates in which the solid surfaces are coated with the crude oil of interest, are comparable to those obtained using emulsions and the EPM, suggesting that the controls on zeta potential on crude oil are the same irrespective of whether the oil forms droplets or wetting layers. The literature data reviewed here, along with new measured data, provide important insight into the effect of pH, and the concentration of divalent ions, on the zeta potential of crude oil in aqueous solution. They demonstrate relationships between ion concentration and zeta potential that are observed irrespective of crude oil composition. They also show that the crude oil composition plays a role, yet no consistent trends are observed between zeta potential and commonly measured bulk oil properties, possibly because bulk properties do not reflect the concentrations of interfacially active species in crude oil that may impact the development of surface charge. Moreover, data are extremely scarce for complex, high ionic strength electrolytes or at elevated temperature. The data reviewed and reported here have broad relevance to many engineering and industrial activities involving crude oil.

Surface interaction mechanisms of air bubbles, asphaltenes and oil drops in aqueous solutions with implications for interfacial engineering processes

HYPOTHESIS

Surface interactions of bubbles and oil with interface-active species like asphaltenes influence many interfacial phenomena in various engineering processes. It holds both fundamental and practical significance to quantitatively characterize these interactions.

EXPERIMENTS

The surface forces of air bubbles, asphaltenes and asphaltenes-toluene droplets in various aqueous solutions have been quantified using an integrated thin film drainage apparatus and an atomic force microscope coupled with bubble probe. The effects of asphaltenes concentration, pH, salinity, Ca²⁺ ions and surfactants have been examined.

FINDINGS

Hydrophobic interaction drives attachment of bubbles and asphaltenes surfaces or oil droplets under high salinity condition. Increasing asphaltenes concentration in oil droplets enhances their hydrophobic attraction with bubbles due to strengthened asphaltenes adsorption and aggregation at oil-water interface. Increasing pH weakens the hydrophobic interaction as oil surfaces become more negatively charged and less hydrophobic. Under low salinity condition, strong electrical double layer and van der Waals repulsion inhibits the bubble-oil droplet contact. Introducing Ca²⁺ ions and surfactants leads to strong steric repulsion, preventing bubble-oil contact. This research has advanced our mechanistic understanding of how bubbles and oil droplets interact in aqueous systems and offers useful insights to modulate such interactions in oil production, water treatment and other interfacial processes.

Effect of Oil-Soluble/Water-Soluble Surfactants on the Stability of Water-in-Oil Systems, an Atomic Force Microscopy Study

The stabilization mechanism of water-in-oil (W/O) emulsions has been studied by measuring the interactions between two water droplets in n-tetradecane using atomic force microscopy. The effects of water-soluble surfactants (SDS/CTAB/Tween 80), an oil-soluble surfactant (Span 20), and the coexistence of the water and oil-soluble surfactants on the stability of water droplets in oil were investigated separately. It is found that the addition of oil-soluble surfactants (Span 20) prevents the coalescence of water droplets in oil. To discuss the role of an oil-soluble surfactant, we analyzed the force curve by applying the theoretical model. The results demonstrate that the oil-soluble surfactant (Span 20) stabilizes dispersed droplets by adsorbing onto the interface and forming a relatively tighter layer with the increase in surfactant concentration, which hinders film rupture. This behavior of the surfactant could also be properly characterized by steric hindrance. A further step was taken by introducing another water-soluble surfactant. It is found that the addition of either SDS or CTAB into the water phase is futile in inducing droplet coalescence in the presence of Span 20. In contrast, Tween 80 was found to be effective in destabilizing water droplets, which could be due to the competitive adsorption between Tween 80 and Span 20 at the interface. By characterizing the interfacial adsorption of Tween 80 and Span 20 with a theoretical adsorption isotherm model, the result indicates that interface replacement would result in a loose adsorption layer that is insufficient to hinder droplet coalescence. Our study provides an intriguing understanding of the role of surfactants in the stabilization and destabilization of water-in-oil emulsions.

Probing interaction forces associated with calcite scaling in aqueous solutions by atomic force microscopy

The prevention of calcite aggregation and scaling remains a challenging problem in aqueous based systems and environmental science. Decades of research studies have proposed microscopic mechanisms of aggregation control, but experiments at the nanoscale and molecular level are rarely conducted. Here we show that the nanoscale topographic features of calcite during its aggregation depend significantly on the intermolecular and surface forces involved in this process. By measuring the forces between a calcite or silica particle and a calcite surface in aqueous solutions using atomic force microscopy, we found that higher solution pH and inhibitor concentration and lower salinity resulted in a system of stronger repulsion and weaker adhesion, which is favorable for reducing the possibility of calcite aggregation and surface deposition. Conflicting roles of Mg²⁺ in calcite aggregation prevention, being positive in acidic pH and negative in alkaline pH, were also observed. The nanoscale structural changes of calcite, visualized by atomic force microscopy or scanning electron microscopy, indicated a size dependence of aggregated and deposited calcite crystals on the calcite-calcite and calcite-silica interactions, respectively. The generalized framework of the calcite aggregation mechanism achieved in this work can be extended to other types of systems and provides a basis for investigating the anti-aggregation strategy of calcite from industrial and environmental perspectives.

Novel polymer nanoparticles with core-shell structure for breaking asphaltenes-stabilized W/O and O/W emulsions

HYPOTHESIS

The removal of stable water-in-oil (W/O) or oil-in-water (O/W) emulsions has been a challenging issue in chemical and oil industry for decades. Traditional demulsifiers were generally designed specifically for treating either W/O or O/W emulsions. A demulsifier that is effective for treating both types of emulsions will be highly desired.

EXPERIMENTS

Novel polymer nanoparticles (PBM@PDM) was synthesized as a demulsifier for treating both W/O and O/W emulsions prepared by toluene, water, and asphaltenes. The morphology and chemical composition of synthesized PBM@PDM were characterized. Demulsification performance and interaction mechanisms including interfacial tension, interfacial pressure, surface charge properties and surface forces were systematically studied.

FINDINGS

PBM@PDM could immediately prompt the coalescence of water droplets upon addition and effectively release the water in asphaltenes-stabilized W/O emulsion. In addition, PBM@PDM successfully destabilized asphaltenes-stabilized O/W emulsion. Not only could PBM@PDM substitute the asphaltenes adsorbed at the water-toluene interface, but they could also dominate the water-toluene interfacial pressure in competition with asphaltenes. The steric repulsion between interfacial asphaltene films could be suppressed in the presence of PBM@PDM. Surface charges significantly influenced the stability of asphaltenes-stabilized O/W emulsion. This work provides useful insights into the interaction mechanisms of asphaltene-stabilized W/O and O/W emulsions.

Rational Design of a Zr-MOF@Curli-Polyelectrolyte Hybrid Membrane toward Efficient Chemical Protection, Moisture Permeation, and Catalytic Detoxification

Developing high-performance protective materials is important for soldiers and civilians who are exposed to the atmosphere of highly toxic chemical warfare agents (CWAs). Polyelectrolyte membranes are promising candidates with excellent chemical resistance and moisture permeability, but they cannot efficiently degrade CWAs. Here, we design and prepare a hybrid membrane through in situ growth of catalytically active zirconium-based metal-organic frameworks (Zr-MOFs) on a polyelectrolyte membrane mediated by biofilm-inspired curli nanofibers (CNFs). Superior to the bare polyelectrolyte membrane, the prepared MOF-808@CNF-PQ hybrid membrane exhibits improved rejection of the nerve agent simulant dimethyl methyl phosphonate (DMMP) vapor and permeation of the water vapor by 113 and 45%, respectively. The water/DMMP selectivity of the hybrid membrane reaches 498.6, approximately 13 times that of the commercial polyelectrolyte membrane Nafion 117. In addition, the hybrid membrane possesses appreciable catalytic activity for the hydrolysis of the nerve agent simulant dimethyl 4-nitrophenyl phosphate (DMNP) with a half-life of ∼38 min. Nanomechanical characterization results based on atomic force microscopy (AFM) techniques demonstrate the critical role of CNFs in mediating Zr-MOF nucleation and the dominant effect of electrostatic interactions on self-assembly of CNFs on polyelectrolyte base. It is also confirmed that the Zr-MOF toppings serve as the key components in physically adsorbing and chemically degrading the DMNP molecules through multiple strong intermolecular interactions. Our work offers a rational strategy to develop advanced membranes toward efficient chemical protection, moisture permeation, and catalytic detoxification against CWAs.

Charge-Dipole Attraction as a Surface Interaction between Water Droplets Immersed in Organic Phases

The dynamic behavior of emulsion droplets during their interactions with one another or with solid surfaces plays a paramount role in their ultimate stability in various applications. While the interaction of oil droplets through a surrounding aqueous phase is well understood, recent studies on the interaction of water droplets through a surrounding pure organic phase showed the presence of an unexplained attraction between water droplets at relatively long ranges. In this research study, we propose fixed-surface-charge-bulk-dipole attraction as a new interaction force between water-in-oil droplets and then derive an equation for its disjoining pressure. The behavior of water droplets in the presence and absence of this charge-dipole interaction was numerically quantified using the Stokes-Reynolds-Young-Laplace model and compared to the experimental data. Numerically calculated net force curves are in excellent agreement with experimental data from the literature when charge-dipole attraction is included, while they deviate in its absence. In addition, the water droplet and thin oil film profiles in the presence and absence of charge-dipole attraction were calculated and compared. This research indicates that charge-dipole attraction can adequately explain the mysterious force observed in some studies, which demonstrates its unexplored potential to capture the physical properties and dynamic behavior of water droplets in organic phases with useful implications to unravel unidentified interactions between emulsion droplets in different industries.

Interfacial Adhesion between Fatty Acid Collectors and Hydrophilic Surfaces: Implications for Low-Rank Coal Flotation

Fatty acids, which are enriched in vegetable oil, have attracted much attention in low-rank coal flotation because of their unique chemical structure. In this study, density functional theory calculations, molecular dynamics simulations, and atomic force microscopy were employed to investigate the adsorption structure and forces between collectors and hydrophilic surfaces. The results show that fatty acids can be easily adsorbed onto surfaces through hydrogen bonds, and can cover the oxygen sites. The existence of hydration film on hydrophilic surfaces prevented nonpolar molecules from being able to adsorb, while polar fatty acids could adsorb and expel water molecules. The adhesion force between the RCOOH-terminated probe and the surface appeared in the retraction process, which differed significantly from that of the RCH₃-terminated probe, indicating that polar fatty acids are more suitable as flotation collectors for low-rank coal than nonpolar hydrocarbon oil. The simulation and AFM test revealed the mechanisms of polar fatty acids, and can provide guidance for low-rank coal flotation applications.

Magnetic Hyperbranched Molecular Materials for Treatment of Oily Sewage Containing Polymer in Oilfield Compound Flooding

With the continuous improvement in oilfield development and the application of tertiary oil recovery technology, the water content of oilfield-produced fluids has gradually increased, and a large number of oilfield sewage with complex components has also been produced after oil–water separation, and effective treatment is urgently needed. ASP flooding sewage contains alkali, various surfactants, polymers, microemulsion oil droplets, and solid impurities, which are difficult to be effectively treated by traditional water treatment agents and methods. In this study, aminopropyl triethoxysilane (APTES) was used to modify the nano-Fe3O4 coated with tetraethyl silicate (TEOS). The product was used as the ferromagnetic nano-core for the iterative reaction of Michael addition and ester amidation to synthesize a magnetic hyperbranched polyamide amine, and its performance in the treatment of ASP flooding wastewater was evaluated experimentally. For the preparation of APTES-modified Fe3O4@SiO2 (FOSN) product, TEOS was coated over Fe3O4 in an ethanol aqueous solution environment and then APTES was added dropwise. The first-generation branched product (1-FSMN) was obtained by the reaction of FOSN and methyl acrylate graft product (FOSN-M) with ethylenediamine, and the highest yield was 93.7%. The highest yield of the second-generation branched product (2-FSMN) was 91.6%. In this study, a composite flooding wastewater sample from a block in the Bohai oilfield was taken. The suspended solids content was 143 mg/L, the oil content was 921.09 mg/L, the turbidity was 135 NTU, and the zeta potential was −47 mV. The third-generation hyperbranched polymer (3-FSMN) and its quaternary ammonium salt (3-FSMN-Q) performed best in the appropriate dosage range, with the highest oil removal rate of 97%, suspended solid removal rate of 90.3%, turbidity reduction rate of 86.6% and zeta potential reduction rate of 88%. For 3-FSMN and its quaternary ammonium salt, the gravity/magnetic PAC compound treatment experiment was carried out. In the settlement time of only 5 min, 3-FSMN/PAC and 3-FSMN-Q/PAC can achieve the maximum oil removal rate of 87.1% and suspended solids removal rate of 87.3% for polymer containing wastewater from ASP flooding, and 86.3 and 86.0% for 120 mg/L. Its treatment capacity was much better than that of common treatment agent combination (CPAM/PAC).

Effect of sodium citrate on asphaltene film at the oil-water interface

HYPOTHESIS

Sodium citrate (Na₃Cit) has been proven to improve the oil sands extraction recovery, but its mechanism is still unclear. Here we hypothesize that the presence of Na₃Cit affects the asphaltene behaviour at the oil-water interface, which enhances oil-water separation and, thereby, heavy oil recovery.

EXPERIMENTS

Na₃Cit-asphaltene interaction was first investigated on their interfacial shear rheology at one heptol-water interface. Na₃Cit-asphaltene interaction was further revealed by measuring the interaction forces between two heptol-water interfaces using the atomic force microscopy droplet technique combined with the Stokes-Reynolds-Young-Laplace (SRYL) model. Interfacial properties were further illustrated through interfacial tension, zeta potential, Langmuir trough, and FE-SEM.

FINDINGS

Na₃Cit was found to weaken the strength of the asphaltene film at the heptol-water interface. Moreover, Na₃Cit could diminish the adhesion forces observed between two asphaltene-in-heptol droplets in high salinity solutions. Besides, Na₃Cit also made the asphaltene-in-heptol droplet more negatively charged. These results collectively suggest that Na₃Cit-asphaltene interaction results in a looser and more elastic asphaltene interfacial network with the slow formation and reduces the adhesion between two interfaces, all of which are most likely the consequence of increased electrostatic repulsion between asphaltene interfacial nanoaggregates. Our study provided new understandings of Na₃Cit-asphaltene interactions at the interface.

Study on the Effect of Nanoparticle Used in Nano-Fluid Flooding on Droplet-Interface Electro-Coalescence

Nano-fluid flooding is a new method capable of improving oil recovery; however, nanoparticles (NPs) significantly affect electric dehydration, which has rarely been investigated. The effect of silica (SiO₂) NPs on the droplet–interface coalescence was investigated using a high-speed digital camera under an electric field. The droplet experienced a fall, coalescence, and secondary droplet formation. The results revealed that the oil–water interfacial tension and water conductivity changed because of the SiO₂ NPs. The decrease of interfacial tension facilitated droplet deformation during the falling process. However, with the increase of particle concentration, the formed particle film inhibited the droplet deformation degree. Droplet and interface are connected by a liquid bridge during coalescence, and the NP concentration also resulted in the shape of this liquid bridge changing. The increase of NP concentration inhibited the horizontal contraction of the liquid bridge while promoting vertical collapse. As a result, it did not facilitate secondary droplet formation. Moreover, the droplet falling velocity decreased, while the rising velocity of the secondary droplet increased. Additionally, the inverse calculation of the force balance equation showed that the charge of the secondary droplet also increased. This is attributed to nanoparticle accumulation, which resulted in charge accumulation on the top of the droplet.

Probing the Interactions between Pickering Emulsion Droplets Stabilized with pH-Responsive Nanoparticles

The presence and adsorption of particles at the oil/water interface play a critical role in stabilizing Pickering emulsions and affecting their bulk behavior. For water-in-oil (W/O) and oil-in-water (O/W) Pickering emulsions with pH-responsive nanoparticles, their interaction forces and stabilization mechanisms at the nanoscale have not been reported. Herein, the Pickering emulsions formed by oil/water mixtures under different pH values with bilayer oleic acid-coated Fe₃O₄ nanoparticles (Fe₃O₄@2OA NPs) were characterized using microscopy imaging and zeta potential and interfacial tension (IFT) measurements. The interaction forces between formed emulsion droplets were quantified using an atomic force microscope (AFM) drop probe technique. A W/O emulsion formed at pH 2 and 4 is mainly stabilized by the steric barrier formation of confined particle layers (with Fe₃O₄@2OA NPs and aggregates). At pH 9 and 11, an O/W emulsion is formed, and its stabilization mechanism is mainly due to relatively low IFT, strong electrostatic repulsion due to carboxyl groups, and steric repulsion from confined nanoparticles and aggregates, leading to a stable confined thin water film. Increasing the maximum loading force and dwelling time enhances the confinement of Fe₃O₄@2OA particles and aggregates at the oil/water interface. This work provides useful insights into the interaction and stabilization mechanisms of Pickering emulsions with stimuli-responsive interface-active particles.

Asphaltene Aggregation in Aqueous Solution Using Different Water Models: A Classical Molecular Dynamics Study

The aggregation behavior of asphaltene in aqueous solution is systematically investigated based on a classical molecular dynamics study. In this work, a novel approach is adopted in order to investigate the structural and dynamical properties of the asphaltene nanoaggregates using different water models. The end-to-end distance of the asphaltene molecule is probed in order to understand the aggregation behavior in aqueous solution. The accuracy of different water models, that is, simple point charge, TIP4P-D, and TIP5P, is thoroughly investigated. In order to probe the dynamical properties of the asphaltene nanoaggregates, the transport coefficients, namely, diffusion coefficient and shear viscosity, are computed. The obtained results highlight the importance of using the appropriate water model in order to accurately study the aggregation behavior of asphaltene in aqueous solution.

Interactions between CO2-Responsive Switchable Emulsion Droplets Determined by Using Optical Tweezers

CO₂-responsive switchable emulsions have been of great interest in some industrial processes where the stability of the emulsion is only required temporarily, such as oil transport, drug delivery, and fossil fuel production. The good understanding of the stability and instability mechanism is vital to the switchable behavior between emulsification and demulsification. Herein, a novel approach was developed to determine the interactions between two switchable emulsion droplets directly by a dual-laser optical tweezers instrument. The repulsive force between a couple of tetradecane droplets occurs to increase progressively with the increasing concentration of switchable surfactant in solutions. However, the repulsive force appears to decrease progressively in turn when the switchable surfactant concentration is far higher than the critical micelle concentration (CMC). Moreover, the depletion effect starts to emerge in the higher surfactant concentration which is attributed to the switchable surfactant micelles generated in solutions. In addition, according to the measurements of interaction forces, a mechanism of the switchable behavior is well proposed, which is established by the principle of self-assembly/detachment of the switchable surfactant, resulting in the weakening and re-enhancing of the electrostatic double-layer (EDL) repulsive forces between tetradecane droplets, upon selective introduction and removal of CO₂. Based on this work, a novel perspective was provided to study the switchable emulsion, which can contribute instructive messages for the understanding of stability and instability mechanisms of switchable emulsions.

In Situ Measurements of Interactions between Switchable Surface-Active Colloid Particles Using Optical Tweezers

Switchable surface-active colloid particles are critical to the preparation of switchable Pickering emulsions, which are widely involved in multitudinous fundamental and practical fields, such as biomedical, food products, and spinning cosmetics. The stability of switchable surface-active particles relies on the full understanding of interaction forces between individual colloid particles quantitatively. In this work, a dual-laser optical tweezers instrument was applied to measure the interaction forces between silica particles coated with a common cationic surfactant (cetyltrimethylammonium bromide, CTAB) in water, and all of the measured forces can be well fitted with the theoretical model derived from the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. It was revealed that the minimum surface distance to engender the interaction forces between silica particles was closer progressively with the increase of CTAB concentrations, suggesting that the introduction of CTAB molecules in the solution thinned the electric double layer. In addition, the minimum surface distance between surface-inactive silica particles further decreased compared to surface-active states, although the ζ-potential has returned to the initial value of bare silica in pure water when the molecular ratio of 1:1 anionic surfactant (sodium dodecyl sulfate, SDS) was added into the solution to switch the surface-active silica particles to surface-inactive states. Our results provide a considerate methodology for quantifying the interaction forces and investigating the switchable behaviors of CTAB molecules from the adsorption to desorption at the particle-water interfaces, which provide vital foresights into the stabilization mechanism of switchable surface-active colloid particles and the further development of switchable Pickering emulsions.

CO2/N2-responsive oil-in-water emulsions using a novel switchable surfactant

HYPOTHESIS

Recently, switchable or stimuli-responsive emulsions have attracted much research interest in many industrial fields. In this work, a novel CO₂/N₂-responsive surfactant was designed and developed to facilitate the formation of switchable oil-in-water (O/W) emulsions with fast switching characteristics between a stable emulsion and separate phases upon alternatively bubbling CO₂ and N₂.

EXPERIMENTS

The novel CO₂/N₂-responsive surfactant was facilely prepared by mixing an anionic fatty acid (oleic acid) and a cationic amine (1,3-Bis (aminopropyl) tetramethyldisiloxane) at a 1:1 molecular ratio, which was assembled based on electrostatic interactions. The structure and properties of the novel CO₂/N₂-responsive switchable surfactant were investigated by Fourier-transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance (¹H NMR) spectroscopy, and interfacial tensions.

FINDINGS

The developed surfactant shows an excellent interfacial activity at the oil/water interface, which can significantly reduce the dosage of the switchable surfactant compared with previous CO₂/N₂-responsive surfactants. The dynamic interfacial tension of n-decane and aqueous phase decreased from 45 mN m^-1 to 5 mN m^-1 within 100 s with the addition of 0.2 mM surfactant. In this work, a low concentration of the novel switchable surfactant (e.g., 20.0 mM) can realize reversible emulsification and demulsification in an emulsion system as compared with the high dosage (e.g., ~150 mM) in previous reports, which will bring huge economic benefits in industrial applications in the future. Moreover, this work expands the family of ion-pair surfactants to small amino-functionalized molecules beyond Jeffamine D-230, which promotes the development of simple and switchable ion-pair surfactant. It is found that the O/W emulsions stabilized by the switchable surfactant show excellent stability, which can be stored for over 60 days at room temperature without any obvious change. Interestingly, the stable O/W emulsion is completely demulsified upon bubbling CO₂ for 30 s and can be easily re-emulsified to the initial state after purging N₂ at 60 °C within 10 min, which demonstrates a rapid and highly efficient switching behavior. The reversible emulsification and demulsification process is ascribed to the reversible assembly and disassembly of the switchable surfactant, which is induced by the removal and purge of CO₂.

Recent advances of characterization techniques for the formation, physical properties and stability of Pickering emulsion

Recently, there have been increasing demand for the application of Pickering emulsions in various industries due to its combined advantage in terms of cost, quality and sustainability. This review aims to provide a complete overview of the available methodology for the physical characterization of emulsions that are stabilized by solid particles (known as Pickering emulsion). Current approaches and techniques for the analysis of the formation and properties of the Pickering emulsion were outlined along with the expected results of these methods on the emulsions. Besides, the application of modelling techniques has also been elaborated for the effective characterization of Pickering emulsions. Additionally, approaches to assess the stability of Pickering emulsions against physical deformation such as coalescence and gravitational separation were reviewed. Potential future developments of these characterization techniques were also briefly discussed. This review can act as a guide to researchers to better understand the standard procedures of Pickering emulsion assessment and the advanced methods available to date to study these emulsions, down to the minute details.

Surface Interactions between Water-in-Oil Emulsions with Asphaltenes and Electroless Nickel-Phosphorus Coating

Surface interactions between emulsion drops and substrate surfaces play an important role in many phenomena in industrial processes, such as fouling issues in oil production. Investigating the interaction forces between the water-in-oil emulsion drops with interfacially adsorbed asphaltenes and various substrates is of fundamental and practical importance in understanding the fouling mechanisms and developing efficient antifouling strategies. In this work, the surface interactions between water drops with asphaltenes and Fe substrates with or without an electroless nickel-phosphorus (EN) coating in organic media have been directly quantified using the atomic force microscope drop probe technique. The effects of asphaltene concentration, organic solvent type, aging time, contact time, and loading force were investigated. The results demonstrated that the adhesion between water drops and the substrates was enhanced with higher asphaltene concentration, better organic solvent to asphaltenes, longer aging time, longer contact time, and stronger loading force, which was due to the growing amount and conformational change of asphaltenes adsorbed at the water/oil interface. Meanwhile, the adhesion between the water drop and the EN substrate was much weaker than that with the Fe substrate. The bulk fouling tests also showed that EN coating had a very good antifouling performance, which was in consistence with the force measurement results. Our work sheds light on the fundamental understanding of emulsion-related fouling mechanisms in the oil industry and provides useful information for developing new coatings with antifouling performances.

In Situ Measurement of Depletion Caused by SDBS Micelles on the Surface of Silica Particles Using Optical Tweezers

Dual-trap optical tweezers have been used to directly measure the interaction forces between two silica particles upon controlling the concentration of the ionic surfactant sodium dodecylbenzenesulfonate (SDBS). By capturing two silica particles in one spot optical trap and one linear optical trap and controlling the linear trap to bring one particle to approach another sufficiently closer, the interaction forces between these two particles can be measured as the separation distance changes. Results showed that with increasing concentrations of SDBS, the interaction force between the two silica particles emerges at closer surface distance between two silica particles. Only repulsive force exists between silica particles below the critical micelle concentration (cmc) of SDBS and it could be well-fitted using the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. However, the depletion attraction force appears above the cmc of SDBS which is induced by the generation of SDBS micelles. By in situ measurement of the interaction force between two silica particles in the presence of different concentrations of SDBS, the depletion force can be quantitatively calculated.

Separation Mechanism and Construction of Surfaces with Special Wettability for Oil/Water Separation

Oil leakage and the discharge of oil/water mixtures by domestic and industrial consumers have caused not only severe environmental pollution and a threat to all species in the ecosystem but also a huge waste of precious resources. Therefore, the separation of oil/water mixtures, especially stable emulsion, has become an urgent global issue. Recently, materials containing a special wettability feature for oil and water have drawn immense attention because of their potential applications for oil/water separation application. In this paper, we systematically summarize the fundamental theories, separation mechanism, design strategies, and recent developments in materials with special wettability for separating stratified and emulsified oil/water mixtures. The related wetting theories that unveil the physical underlying mechanism of the oil/water separation mechanism are proposed, and the practical design criteria for oil/water separation materials are provided. Guided by the fundamental design criteria, various porous materials with special wettability characteristics, including those which are superhydrophilic/underwater superoleophobic, superhydrophobic/superoleophilic, and superhydrophilic/in-air superoleophobic, are systemically analyzed. These superwetting materials are widely employed to separate oil/water mixtures: from stratified oil/water to emulsified ones. In addition, the materials that implement the demulsification of emulsified oil/water mixtures via the ingenious design of the multiscale surface morphology and construction of special wettability are also discussed. In each section, we introduce the design ideas, base materials, preparation methods, and representative works in detail. Finally, the conclusions and challenges for the oil/water separation research field are discussed in depth.