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Research Progress in Development and Application of High-Efficiency Demulsifiers for Complex Crude Oil Emulsions
What is a demulsifier?
Demulsifier is a specialized chemical or material designed to break down stable crude oil emulsions (such as W/O, O/W, or multiple emulsions) formed during oil production, transportation, or processing. These emulsions—stabilized by interfacially active substances (e.g., asphaltenes, resins, wax crystals) and factors like interfacial tension, viscoelastic interfacial films, and interfacial charge—can reduce production efficiency, cause equipment corrosion, and lead to environmental pollution.
Demulsifiers function by disrupting the stability of emulsion systems through mechanisms including displacing natural surfactants at the oil-water interface, neutralizing interfacial charges, bridging dispersed droplets to promote coalescence, or altering the phase properties of emulsions. Common types include polymer demulsifiers (e.g., polyethers, polyquaternary ammonium salts), biomass-based demulsifiers (derived from lignin, tannic acid, lotus leaves, etc.), ionic liquid demulsifiers, and nanomaterial demulsifiers (e.g., magnetic nanoparticles, carbon-based materials).
The core goal of demulsifiers is to achieve efficient oil-water separation, thereby improving oilfield production efficiency, reducing operational costs, and mitigating environmental risks. High-performance demulsifiers are typically characterized by strong universality, high demulsification efficiency, low dosage, environmental friendliness, and tolerance to harsh conditions (e.g., high temperature, high salinity).
Abstract
Large quantities of complex crude oil emulsions (W/O, O/W, or multiple emulsions) are commonly generated during oil production, leading to significantly reduced production efficiency and severe environmental issues. Current demulsifiers suffer from limitations such as low demulsification efficiency, poor universality, high cost, and insufficient safety. Thus, the development of high-efficiency, universal, green, and safe demulsifiers remains a focus of attention. Based on an analysis of the characteristics and stabilization mechanisms of crude oil emulsions, this review summarizes the research progress of demulsifiers for different crude oil emulsions, focusing on the characteristics, application scopes, and performance of polymer demulsifiers, biomass-based demulsifiers, ionic liquid demulsifiers, and nanomaterial demulsifiers. The demulsification mechanisms of corresponding demulsifiers are specifically analyzed, and research methods for these mechanisms are outlined. Finally, the review prospects the development of high-efficiency demulsifiers for complex crude oil emulsions and proposes future trends in demulsification technology, aiming to provide references for the application of demulsification technology in complex crude oil emulsions (W/O, O/W, or multiple emulsions).
1. Types and Stabilization Mechanisms of Crude Oil Emulsions
1.1 Basic Types of Crude Oil Emulsions
Crude oil emulsions formed during oil production are metastable systems in the presence of surfactant molecules, amphiphilic polymers, or solid particles. Emulsions are classified into stable, metastable, entrained, and unstable types based on density, viscosity, saturation, asphaltenes, resins, and fine solid indices. Each type has unique characteristics and cannot be converted to other types once formed; the relative balance of hydrophilicity and lipophilicity is the most critical parameter determining emulsion type.
Oil-in-Water (O/W) emulsions are produced by highly hydrophilic molecules.
Water-in-Oil (W/O) emulsions form in the presence of hydrophobic molecules, commonly encountered in heavy oil transportation, asphalt foam processing, and desalination before refining. Asphaltenes, waxes, resins, solids, naphthenes, aliphatics, and aromatic acids act as natural emulsifiers to form thin films, stabilizing dispersed droplets and preventing coalescence.
Broadly, complex emulsions include intractable O/W, W/O emulsions, and multiple emulsions (W/O/W, O/W/O) from oil production (e.g., aged oil at offshore platform terminals and highly emulsified oil). Narrowly, complex emulsions refer to multiple emulsions with a unique "two-film, three-phase" multi-compartment structure, forming a highly dispersed, heterogeneous system with uneven particle sizes:
W/O/W emulsions: The dispersed phase is a W/O emulsion; the inner aqueous phase (W₁) and outer aqueous phase (W₂) are miscible due to similar polarity.
O/W/O emulsions: O/W emulsion droplets are re-dispersed in an outer oil phase, forming a system with oil as both the outer and inner phases. The dual interface films (O/W and W/O) divide the system into three immiscible regions (oil/water/oil), enabling encapsulation and controlled release of oil-soluble active ingredients for enhanced stability.
In practical production, ASP (alkaline-surfactant-polymer) flooding produced fluids often contain coexisting W/O, O/W, and O/W/O emulsions with high stability.
1.2 Stabilization Mechanisms of Crude Oil Emulsions
The formation of Pickering emulsions greatly enhances the stability of crude oil emulsions. Thermodynamically unstable due to excess interfacial energy, emulsions can maintain kinetic stability for long periods. Key factors influencing stability include:
Oil-water interfacial tension: Higher interfacial energy leads to poorer emulsion stability.
Oil-water interfacial film: Emulsifiers in emulsions not only reduce interfacial tension but also strongly adsorb at the oil-water interface to form a viscoelastic interfacial film.
Interfacial charge: Ionization of molecules at the interface during Brownian motion of droplets leads to the formation of an electric double layer in the continuous phase. Repulsive forces between overlapping double layers prevent droplet coalescence, stabilizing the emulsion.
Notably, interfacially active substances such as asphaltenes, resins, naphthenic acids, wax crystals, and amphiphilic solids significantly affect interface properties. Asphaltenes (IAA), a mixture of polycyclic aromatic rings, saturated substituents, and polar groups (e.g., amines, hydroxyls, carboxyls, sulfur-containing functional groups), are the most polar interfacially active substances in crude oil. They accumulate at the oil-water interface via π-π stacking and hydrogen bonding, forming a viscoelastic interfacial film that hinders water droplet coalescence. Additionally, interfacially active substances induce additional steric and electric double layer repulsion between oil droplets in the aqueous phase, further stabilizing the emulsion. Increasing the number and length of aliphatic chains reduces asphaltene polarity, enhancing their dispersibility at the interface, rapid migration from the emulsion to the interface, and adsorption—though adsorbed asphaltenes cannot persist stably under external forces, ultimately causing interface instability. Targeted demulsifiers can be designed based on emulsion stabilization mechanisms, asphaltene polarity, and dispersibility to achieve efficient and precise demulsification.
2. Performance and Application Progress of Demulsifiers
2.1 Polymer Demulsifiers
2.1.1 Polyether Demulsifiers
Traditional demulsifiers, mostly positive-phase polyether demulsifiers for W/O emulsions, have played a crucial role in oilfields. Compound AP series polyether demulsifiers have been successfully applied in offshore oilfields, achieving a dehydration rate of 90%, ensuring normal production and meeting on-site needs. For oil-containing sludge systems, DS-1, a demulsifier for dirty oil with high mechanical impurity content, was developed by compounding DS series polyether demulsifiers with reverse demulsifiers. Laboratory evaluations showed that adding 0.4% DS-1 to dirty oil (31% water content in the upper layer of the settling tank and 6.7% centrifugal sediment) from Daqing Oilfield’s Bei’er United Station resulted in a dehydration rate of 97% and a top oil water content of 0.6% after 4.5 hours at 50°C. However, traditional polymer demulsifiers have poor universality, with some being toxic and non-degradable.
Table 1 Application of Traditional Polyether Demulsifiers
| Demulsifier | Applicable Oil-Water System | Dehydration Efficiency (DE)/% | Optimal Demulsification Conditions | Advantages | Disadvantages |
|---|---|---|---|---|---|
| BP Series | Shengli Oilfield | >90 | 50°C, 3h | Good dehydration effect | High requirements for molecular weight and temperature; insignificant effect if not met |
| AP Series | Bohai Oilfield, East China Sea Oilfield | 90 | 1.25% water dispersion system, 25mg/L dosage | Fast dehydration rate, sharp oil-water interface, clear dehydrated water | Environmental pollution |
| SD Demulsifier | Tahe Oilfield heavy crude oil | 89 | 100mg/L dosage, 70°C | ||
| DS-06/DL11 (positive-reverse compound) | Hohhot Refinery oil-containing sludge | 95 | 180mg/L dosage, 30:10 ratio | Strong specificity, significant industrial application effect | |
| DS-1 (compounded) | Daqing Oilfield Bei’er United Station dirty oil with high mechanical impurities | 97 | 4.5h, 50°C |
2.1.2 Polyquaternary Ammonium Salt Demulsifiers
Compared to positive-phase polyether demulsifiers, quaternary ammonium salt demulsifiers and some dendritic macromolecular demulsifiers are widely used as reverse demulsifiers for O/W systems. Polyquaternary ammonium salts have been shown to exhibit significantly superior demulsification performance compared to latex polymers in laboratory and field tests. To address the difficulties in dehydrating O/W produced fluids and turbid water quality in Nanyang Oilfield, oligomeric quaternary ammonium salt MD-50 was synthesized, showing excellent compatibility with demulsifier AE-932. Compared to polyether or polyamine demulsifiers, oligomeric quaternary ammonium salts accelerate oil droplet coalescence in polymer-flooding O/W emulsions, achieving a dehydration rate of 96.5%. A series of reverse demulsifiers fabricated by combining colloidal silica with various structures (e.g., starch, dithiocarbamate, latex-dispersed polymers) have also exhibited excellent performance in field tests, with clear oil-water separation interfaces and transparent water.
2.1.3 Dendritic Macromolecular Demulsifiers
Highly branched structures and numerous terminal groups endow dendritic polymers with good water solubility and excellent interfacial activity, playing a key role in demulsification performance. Three dendritic polymers synthesized using 1,3-propanediamine, diethylenetriamine (TETA), and triethylenetetramine (DETA) as cores showed that TETA-based dendritic polymers achieved the best demulsification efficiency at relatively low dosages. However, complex synthesis steps and strict intermediate monitoring limit their application in oilfield chemistry. Two polyamine-type dendritic polymer demulsifiers with large hydrophobic segments and strong interactions with interfacially active substances have been synthesized, but demulsification results indicated unclear oil-water interfaces and turbid water, limiting practical applicability.
In recent field applications for O/W systems, oligomeric quaternary ammonium salt modification and compounding have demonstrated superior oil-water separation, with clear interfaces, transparent water, and a low demulsification temperature of 55°C (no high-temperature operation required). In contrast, dendritic polymers have strict system requirements, complex synthesis under high temperature and pressure, and narrow applicability. Despite high oil removal rates, they exhibit unclear separation interfaces and yellowish water, often requiring combination with other chemicals or technologies for optimal results.
2.1.4 Copolymer Demulsifiers
Classic copolymer demulsifiers are mainly EO/PO block copolymers, modified via chain extension, grafting, crosslinking, ionization, or compounding. PO/EO block copolymers are primarily used for W/O emulsions, with the optimal EO/PO ratio being critical for practical application. Sequential block copolymers outperform reverse block copolymers in demulsification. BP series demulsifiers applied to 1:1 oil-water emulsions from Shengli Oilfield achieved a dehydration rate of over 95% with BP64 and over 90% with other models. Demulsification capacity largely depends on molecular weight; increased branching accelerates interfacial instability and diffusion in crude oil. Polyol hyperbranched copolymers and terpolymers have exhibited excellent demulsification performance in O/W and W/O/W systems, while two series of random copolymers (alkyl and alkyl acrylate carboxylate monomers) also play an important role in demulsifying complex oil-water systems.
2.2 Biomass-Based Demulsifiers
Biomass is the most abundant source for manufacturing sustainable polymers and renewable chemicals. Using lignin as a substitute for petroleum-derived chemicals in aromatic polymers is an economically viable strategy. In recent years, biomass materials such as cellulose, anthocyanins, tannic acid, lignin, cardanol, lotus leaves, rice husks, and seaweed have been applied in oilfields, showing good demulsification performance for aged oil at offshore platforms. Amid the "dual carbon" goal, biomass-based demulsifiers hold great potential due to their practicality, green properties, high efficiency, and relatively simple synthesis.
Table 2 Demulsification Performance Comparison of Biomass-Based Demulsifiers
| Biomass Raw Material | Demulsifier | Applicable Emulsion Type | DE/% | Optimal Demulsification Conditions | Advantages | Disadvantages |
|---|---|---|---|---|---|---|
| Anthocyanins | PC9015 | Bohai Oilfield aged oil emulsion | 84.6 | 70°C, 2h, 100mg/L | Good dehydration performance, high practicality; suitable for crude oil and aged oil at offshore platforms | Severe wall adhesion |
| Epigallocatechin Gallate | EGA603 | Bohai Oilfield aged crude oil emulsion | 94.3 | 70°C, 100min, 100mg/L | Clear oil-water interface, slight wall adhesion; natural polyphenol-based, economical and green | Slight wall adhesion |
| Tannic Acid | TAPA9920 | South China Sea Oilfield aged oil emulsion | 97.9 | 70°C, 40min, 100mg/L | Replaces petroleum-derived nonylphenol surfactants | Complex synthesis |
| Cardanol | DECA | Arabian heavy crude oil | 100 | 60°C, 40min, 100mg/L | Complex synthesis, still in laboratory stage | |
| Lignin | Lignin-silica high-efficiency membrane | O/W system | 98.6 | Complete contact with filter paper; separation completes when colored oil is fully filtered | Significant effect, simple separation | Laboratory stage |
| Lotus Leaves | HLLF | Changqing Oilfield | 88.17 | 1000mg/L, 70°C, 90min | Clear oil-water interface, transparent water | Narrow applicability, higher dosage than similar products |
| Rice Husks | RHC | W/O system | 96.99 | 600mg/L, 70°C, 80min | Wide pH range, good salt tolerance | Narrow applicability, higher dosage than similar products |
| Seed Extract | DEMLOCS | Aged oil emulsion | 88 | 45°C, 24h, 2000mg/L | Moderate temperature, simple hydrothermal synthesis |
2.2.1 Phenol-Amine Modified Biomass-Based Demulsifiers
In the past five years, a series of phenol-amine modified demulsifiers derived from natural polyphenols have been developed for aged oil demulsification at offshore platforms. Natural polyphenols such as tannic acid, anthocyanins, epigallocatechin gallate, and cardanol—widely present in plants (e.g., oak, tea, unripe fruits, Chinese walnuts)—have garnered attention for their green properties and practicality. From the perspective of structure and cost, hyperbranched aromatic demulsifiers are proven effective and economical for crude oil demulsification. Their high interfacial activity and fully substituted benzene ring hydrogen atoms facilitate hyperbranched structure formation, enabling multiple extensions toward water droplets and enhancing affinity with asphaltenes in the oil-water film.
Anthocyanin-based (PC9015) and tannic acid-based (TAPA) demulsifiers synthesized using proanthocyanidins and tannic acid (poly-aromatic, hyperbranched structures) as raw materials achieved a dehydration rate of 84.6% and 97.9% within 45 minutes, respectively, when applied to aged oil emulsions at offshore platforms. A similar demulsifier (EGA) prepared from epigallocatechin gallate achieved a dehydration rate of 94.3% for aged oil emulsions at 80°C for 40 minutes. Cardanol (derived from natural cashew oil) modified with polyamines and ethylene glycol to synthesize ionic surfactant DECA—a green alternative to petroleum-derived nonylphenol surfactants—achieved complete dehydration of aged oil emulsions at 60°C for 40 minutes with 100mg/L dosage. While significant progress has been made in modifying anthocyanins and tannic acid, synthesis challenges remain to be addressed.
2.2.2 Lignin-Based Demulsifiers
Lignin exhibits high reactivity due to hydroxyl and aromatic functional groups in its backbone. Chemical modification of lignin—targeting aromatic rings, phenolic, and aliphatic groups—enables the synthesis of high-performance bio-based phenolic resin polymers, offering an economical and environmentally friendly approach to producing renewable resins. Using lignin as a substitute for petroleum-derived chemicals in aromatic polymers is a promising strategy.
Cotton surfaces modified with a beeswax-lignin mixture have been used to prepare superhydrophobic and superoleophilic biomass-based porous materials, showing good absorption for heavy oil. Lignin nanoparticles synthesized by adjusting pH via solvent transfer were used to develop polymer microcapsules for emulsified oil removal from aqueous environments with a 94.4% oil removal rate. A low-cost, high-efficiency membrane based on filter paper, lignin, and silica achieved an oil-water separation efficiency of 98.6%. Oleic acid lignin nanoparticles (OLNPs) with long-term stability under acidic (pH=2) and alkaline (pH=12) conditions have also been synthesized. Due to their tunable surface properties, lignin-derived polymer nanoparticles hold broad prospects for oilfield demulsification, though current research remains in the laboratory stage.
2.2.3 Micro-Nano Biomass-Based Demulsifiers
A micro-nano biomass-based demulsifier (HLLF) prepared from natural lotus leaves via a simple hydrothermal method without additional chemicals achieved a dehydration rate of 88% at 1000mg/L and 70°C for 90 minutes. Rice husk carbon demulsifier (RHC) developed from rice husks achieved a dehydration rate of 96.99% for W/O emulsions at 70°C for 80 minutes with 600μg/L dosage, featuring a wide pH range and good salt tolerance. Both micro-nano biomass-based demulsifiers feature simple synthesis and high demulsification efficiency.
2.3 Ionic Liquid Demulsifiers
Ionic liquids (ILs) have attracted significant attention as effective demulsifiers due to their non-flammability, high thermal stability, and recyclability. While ILs are potential alternatives to commercial demulsifiers, most are toxic and poorly or non-biodegradable. Given their infinite structural combinations, computational modeling may facilitate the design of eco-friendly ILs. Future efforts should focus on developing low-toxicity, low-cost, and low-viscosity ILs to enhance demulsification efficiency.
2.3.1 Basic Structure of Ionic Liquids
Ionic liquids consist of asymmetric organic cations (e.g., ammonium, imidazolium, pyridinium, phosphonium) and inorganic anions (e.g., [BF₄]⁻, [AlCl₄]⁻, [PF₆]⁻) or halide ions. Their properties can be tuned by modifying cation-anion combinations. Cation type and alkyl chain length influence demulsification effectiveness: larger cation volumes increase polarizability, enhancing demulsification efficiency. For anions, hydrophobicity, hydrophilicity, and size are key factors. Due to high costs, ILs are often compounded with surfactants to reduce total costs.
2.3.2 Performance Characteristics of Ionic Liquids
The following table compares the performance of ionic liquids and traditional demulsifiers:
Table 3 Performance Comparison Between Ionic Liquids and Traditional Demulsifiers
| Performance | Ionic Liquids | Traditional Demulsifiers (Surfactants) |
|---|---|---|
| Interfacial Tension | Surfactant ionic liquids can reduce interfacial tension to 10 mN/m; improving surface activity and considering salinity synergy is necessary | Can reduce IFT to 10⁻²~10⁰ mN/m, effective for oil-water separation |
| Viscosity | Tunable via branching modifications | Cannot achieve high viscosity in aqueous solutions |
| Stability | Thermally stable at high temperatures; no decomposition during long-term storage | Unstable at high temperatures |
| Toxicity | Low toxicity, environmentally friendly | Some alkylphenol surfactants are highly toxic and non-degradable |
| Recyclability | Recyclable via multiple methods (liquid-liquid extraction, distillation, etc.) | Non-recyclable |
Key advantages of ILs include high tolerance to harsh conditions (e.g., high salinity, high temperature) and effective reduction of interfacial tension, dependence on critical micelle concentration (CMC), and recyclability via various methods, which is essential for on-site applications to reduce costs.
2.3.3 Application of Ionic Liquid Demulsifiers
Compared to traditional demulsifiers, ionic liquids offer higher stability and simpler synthesis. ILs derived from alginic acid, rosin acid, and waste plastics have recently been applied to heavy oil demulsification, achieving up to 100% efficiency. Modification of recyclable magnetic nanomaterials with ILs enables recyclability. Despite successful oilfield applications, ILs face limitations in interfacial tension reduction, toxicity, and separation, requiring further improvements.
Table 4 Application Effects of Ionic Liquid Demulsifiers
| Ionic Liquid | Cation | Anion | Emulsion Type | Dosage/mg·L⁻¹ | DE/% | Interfacial Tension Reduction | Conclusion |
|---|---|---|---|---|---|---|---|
| mim NTf₂ (n=10, 12, 14) | Imidazolium | Bis(trifluoromethylsulfonyl)imide | SW/O | 100~3500 | 93.6~100 | 77~95 | For hydrophilic ILs, increased dosage and alkyl chain length cause aggregation, leading to poor demulsification and increased IFT |
| mim PF₆ (n=10, 12, 14) | Imidazolium | Hexafluorophosphate | SW/O | 500~3500 | 71.25~86.25 | 54~81 | |
| mim Cl (n=10, 12, 14) | Imidazolium | Chloride | SW/O | 500~3500 | 76.25~93.75 | 64~80 | |
| TNPy | Aminopyridinium | Bromide | W/O | 250~1000 | 100 | 24 | Alkyl chain length, dosage, and RSN value affect demulsification activity; nonyl and tetradecyl chain TCPyILs outperform hexadecyl counterparts; 100% efficiency for W/O emulsions (50/50, 30/70, 10/90 v/v) even at low concentrations |
| TTPy | Aminopyridinium | Bromide | W/O | 250~1000 | 100 | 26.5 | |
| TCPy | Aminopyridinium | Bromide | W/O | 500 | 100 | 26 | |
| Seaweed-based ILs | Ammonium | Bromide | SW/O | 1000 | 100 | BH-ALG and BN-ALG exhibit good interfacial tension reduction; BH-ALG has higher surface activity due to longer alkyl chains | |
| Rosin acid (AA)-based ILs | Imidazolium | Iodide | SW/O | 250~1000 | 100 | 100% dehydration rate for 90:10 crude oil-brine emulsions; enhanced hydrophobicity from polyglycolide rings | |
| Waste plastic-derived ILs | Ammonium | Chloride | W/O | 1000 | 94 | 34.5 | BHET from PET glycolysis reacts with thionyl chloride to form BCET, which is quaternized with HEOD/DOAD to produce amphiphilic GILs (HEOD-IL, DOAD-IL); HEOD-IL has a lower CMC; alkyl chains facilitate diffusion to replace rigid asphaltene films |
| IL-modified magnetic nanocomposites | Vinylimidazolium | Bromide | W/O | 1000 | 99.9±0.3 | 18±0.1 | 99.9±0.3% dehydration rate at 70°C for 2h; interfacial tension reduced from 20.0±0.1 to 1.9±0.1 mN/m; recyclable via external magnet for 4 cycles (89% efficiency) |
2.4 Nanomaterial Demulsifiers
2.4.1 Magnetic Nanoparticle Demulsifiers
In recent years, nanomaterials have been used for oil-containing wastewater treatment due to their high active site density, modifiability, high surface energy, and controllable properties. Magnetic nanoparticles (MNPs) modified for oil-water separation have broad application prospects. Pure Fe₃O₄ MNPs are highly hydrophilic but lack abundant surface groups; surface modification with organic molecules improves stability, dispersibility, and flexibility. Fe₃O₄-based demulsifiers show potential for treating emulsified oil (W/O, W/O/W emulsions) in acidic/neutral oil-containing wastewater.
Hydrophilic HFG prepared by reducing functional groups (FG) with hydrazine hydrate, followed by solvothermal synthesis of magnetically separable HFG-Fe₃O₄ demulsifiers, exhibited good demulsification performance at pH 2.0~6.0 and maintained high efficiency after 4 cycles. MR-GO with tunable wettability and surface charge via surface functional groups (e.g., amino, carboxyl, hydroxyl) achieved 99.7% demulsification efficiency for O/W emulsions at pH 6.0 and over 91.0% after 6 cycles. Fe₃O₄-PEI prepared by grafting polyethyleneimine (PEI) onto magnetic demulsifiers maintained a crude oil transmittance of 98.3% after 2h of sedimentation at 600μg/L (acidic conditions) and over 90% after 10 magnetic recycling cycles. Magnetic carbon nanomaterial demulsifiers synthesized by combining Fe₃O₄ with graphene oxide (GO), carbon nanotubes (CNTs), or carbon nitride nanosheets (CNNs) achieved optimal dehydration in 5 minutes at 25°C (800μg/L); Fe₃O₄/GO maintained stable performance with only a 0.08% increase in residual water content after 8 cycles. However, large-scale oilfield application of MNPs faces challenges: MNPs may remain suspended in oil after separation, increasing operational costs. Controlling MNP shape and size to ensure rapid sedimentation is critical for practical use.
2.4.2 Carbon-Based Demulsifiers
Carbon-based materials (e.g., carbon black, graphene) modified for demulsification have shown promising results. Functionalized carbon black (F-CB) nanoparticles synthesized by covalently grafting polyvinyl alcohol (PVA) onto carbon black (CB) achieved 99.9% demulsification efficiency for oil-containing wastewater. PVA modification tunes CB surface wettability, promoting oil droplet coalescence and migration to the oil-water interface.
Modified graphene demulsifiers, primarily used as reverse demulsifiers for O/W systems, exhibit good performance over a wide pH range. Functionalized fluorinated graphene (FG) materials (UFG, AFG, HFG) have been prepared for oil separation from wastewater across a broad pH spectrum. Amino-functionalized graphene oxide (GO-A) synthesized via Hummer’s method enables rapid separation of O/W crude oil emulsions. Novel Janus graphene oxide nanosheets have been developed via asymmetric chemical modification of GO with n-octylamine.
Recyclability can be achieved by grafting with magnetic nanoparticles, but high costs and agglomeration limit on-site oilfield application. Future research should focus on surface modification and dispersion technologies to develop low-cost nanoparticle dispersions for large-scale use.
3. Demulsification Mechanisms
The widely accepted demulsification mechanism is the displacement mechanism, supplemented by phase inversion, bridging replacement, wetting-solubilization, interfacial charge neutralization, and the latest "lock-and-key" theory. Practical demulsification often combines flocculation-coalescence with other mechanisms. Correlating emulsion stabilization and demulsification mechanisms, and integrating experiments, theoretical calculations, and multi-scale simulations, helps clarify the formation and demulsification mechanisms of oil-water interface double films.
3.1 Demulsification Mechanisms for W/O Emulsions
3.1.1 Displacement Mechanism (EO/PO Copolymer Demulsifiers)
Chemical demulsifiers exhibit higher surface activity than natural surfactants, enabling preferential adsorption at the oil-water interface, significant reduction of interfacial tension, displacement of film-forming substances, and emulsion breakdown. Demulsification involves three stages: demulsifier adsorption at the interface, interfacial film destruction, and enhanced film drainage by reducing interfacial tension gradients.
3.1.2 Bridging Replacement Mechanism (Micro-Nano Biomass-Based Demulsifiers)
High-molecular-weight demulsifiers aggregate dispersed water droplets in crude oil emulsions (a reversible flocculation process). Aggregated droplets coalesce into larger droplets, which settle under gravity for separation. For example, HLLF (lotus leaf) or RHC (rice husk) demulsifiers adsorb, bridge, displace, or destroy the interfacial film to facilitate oil-water separation. Droplets approach, coalesce into large aggregates, and form thermodynamically unstable new interfacial films, leading to flocculation, coalescence, and demulsification.
3.1.3 Flocculation-Coalescence Mechanism (Gemini Ionic Liquid Demulsifiers)
Interfacial adsorption occurs in three stages: diffusion-controlled stage (rapid initial IFT reduction), steric hindrance stage (slow IFT reduction), and reconfiguration stage (negligible IFT reduction or plateau). During demulsification, ionic liquid molecules rapidly migrate to the interface, penetrate the rigid interfacial film, reduce stability, and induce water droplet flocculation-coalescence. Efficient demulsifiers require rapid diffusion to the interface and competitive adsorption to displace asphaltenes.
3.1.4 Competitive Adsorption Mechanism (Biomass-Based Demulsifiers)
Tannic acid-based demulsifiers exhibit strong competitiveness, displacing interfacially active substances in the film, weakening film strength, and promoting droplet coalescence. Greater reduction in interfacial film viscoelasticity correlates with better demulsification performance. Increased branching and aromaticity enhance π-π interactions between asphaltenes and demulsifier molecules, improving efficiency.
3.2 Demulsification Mechanisms for O/W Emulsions
3.2.1 Phase Inversion Mechanism (Dendritic Reverse Demulsifiers)
Adding reverse demulsifiers to O/W emulsions reacts with natural surfactants to form inclusion complexes, inducing phase inversion (e.g., W/O to O/W). During inversion, the aqueous phase settles under gravity to achieve demulsification. Hydrophilic groups extend into water droplets via multiple branch points, while hydrophobic groups remain around droplets; demulsifier molecules compete with asphaltenes for interfacial adsorption, displacing emulsifiers, weakening film strength, and facilitating droplet aggregation and sedimentation.
3.2.2 Interfacial Charge Neutralization Mechanism (Oligomeric Quaternary Ammonium Salt Demulsifiers)
Oil-water emulsions carry negative surface charges. Cationic demulsifiers neutralize these charges, reducing inter-droplet repulsion. Oligomeric quaternary ammonium salts destabilize and flocculate oil droplets via charge neutralization, promoting coalescence into large droplets and oil-water separation. Coalescence lags behind flocculation, effectively overcoming polymer inhibition in wastewater.
3.2.3 Counterion Mechanism (Ionic Liquid Demulsifiers)
Naturally occurring emulsifiers on the interfacial film carry the same charge, generating electrostatic repulsion; van der Waals forces between emulsifier molecules stabilize the emulsion. Adding oppositely charged ionic liquid demulsifiers induces electrostatic attraction, breaking the interfacial film. PILs adsorb on the film, reduce interfacial barriers, and trap separated water droplets in their network; electro-neutralization between PILs and asphaltene films destroys the protective layer, enabling oil droplet aggregation.
3.2.4 Wetting-Solubilization Mechanism (Nanoparticle Demulsifiers)
Demulsifiers exist as micelles that solubilize the oil-water interfacial film, leading to film destruction and demulsification. Highly wettable materials enable efficient "oil-water separation" for crude oil emulsions. Wettability-tunable separation offers advantages over traditional methods, including high efficiency and recyclability.
3.3 Analysis Methods for Demulsification Mechanisms
Atomic Force Microscopy (AFM) and Surface Force Apparatus (SFA): AFM quantifies interactions between water droplets in O/W/W/O emulsions (with/without asphaltenes); SFA measures surface forces (e.g., van der Waals, electric double layer, hydration, hydrophobic, steric, cation-π, anion-π forces) to analyze the role of interfacially active substances.
Interfacial Tension (IFT) Measurement: IFT, a key indicator of emulsion stability, is calculated using the Young-Laplace equation:
ΔP = -γ(1/R₁ + 1/R₂)
where ΔP = interfacial pressure difference; γ = interfacial tension; R₁ = concave surface curvature radius; R₂ = convex surface curvature radius.
Interfacial Rheology Measurement: Interfacial rheology reflects adsorption-desorption kinetics of interfacially active substances during emulsification, expressed by dilation modulus (ε):
ε = dγ/dlnA
Resistance to interfacial deformation is characterized by elastic modulus (G′, storage modulus) and viscous modulus (G″, loss modulus):
G′ = cos(δ)(τ₀/γ₀)
G″ = sin(δ)(τ₀/γ₀)
Molecular Dynamics Simulation: The Interface Formation Energy (IFE) module in simulation software screens demulsifiers by evaluating their ability to reduce total energy. Neural Network Analysis (NNA) and Genetic Function Approximation (GFA) predict demulsification performance and analyze mechanisms.
4. Conclusion
Traditional demulsifiers have narrow applicability and strict temperature requirements, often requiring compounding for expanded use. Modified polyether demulsifiers, widely used for W/O emulsions, face challenges in synthesis and practical application. Ionic liquids derived from alginic acid, rosin acid, or waste plastics exhibit high efficiency for O/W emulsions but need improvements in toxicity, recyclability, and separation; their green credentials require further verification. Modified lignin, leveraging its unique structure, shows potential for W/O emulsion demulsification. Biomass-derived demulsifiers perform excellently for offshore platform W/O emulsions, aligning with economic and green development goals. Nanoparticle demulsifiers are limited by high costs, poor recyclability, agglomeration, and dispersion issues; developing low-cost dispersions and optimizing magnetic nanoparticle size/shape for rapid sedimentation is critical for reducing post-treatment costs.
Demulsification mechanisms for complex oil-water systems often require comprehensive analysis, as single mechanisms cannot fully explain the process. Future research should focus on correlating emulsion stabilization and demulsification mechanisms, analyzing specific components, and combining multiple characterization methods to study dynamic changes at the oil-water interface.
Oxidation-based demulsification is a promising technology, with advanced oxidation technologies enabling green and efficient processes. However, current oxidation demulsifiers have limitations: ClO₂ exhibits significant efficiency but poor safety; ferrate introduces metal ions, increasing water treatment pressure; HNO₃ has limited oxidizable substrates; H₂O₂ requires combination with other technologies for complex emulsions. Molecular oxidation technologies suffer from low oxidation potential, high reagent consumption, and environmental pressure. Photocatalytic and electrocatalytic oxidation based on free radicals offer high efficiency and no reagent pollution but require solutions for targeted interfacial action of nanophotocatalysts and energy consumption reduction in electrochemical devices.