Research Progress of Surfactant Demulsifier

Abstract

In this paper, the basic concept of surfactants as chemical additives and their diversified classification system are first expounded, laying a theoretical foundation for the subsequent study of their application in demulsification technology. Then, the specific application cases of various types of surfactants in the field of demulsification are deeply analyzed, and ways in which they achieve effective separation of emulsions through their unique physical and chemical properties are revealed. Further, the internal action mechanism of surfactant demulsifier, including how to destroy the stability of emulsion and promote the separation of oil and water phase, is systematically described. On this basis, the significant advantages of surfactant demulsifier compared with traditional methods are summarized, including high cost-effectiveness, high demulsifier efficiency, strong stability, wide adaptability, and easy operation. Finally, the development direction and challenges of surfactant demulsifier in the future are prospected.

1. Introduction

A demulsifier [1] is a substance that can separate an emulsion system [2] and cause it to phase-separate. An emulsion system is a stable mixture formed by two or more immiscible liquid phases under the action of an emulsifier, such as emulsions and latex. The function of a demulsifier is to destroy the stability of the emulsion system and promote its separation into two or more immiscible liquid phases [3,4]. Demulsifiers enjoy extensive applications across diverse sectors. They play crucial roles not only in the chemical industry but also in the food and pharmaceutical industries, along with numerous other fields. The advantages of demulsifiers in practical applications mainly lie in separation and extraction. Phase separation in emulsion systems is a key step in many production processes and experimental operations. Demulsifiers are capable of separating diverse components within emulsion systems, thus facilitating subsequent extraction and treatment procedures. In certain scenarios, the presence of emulsion systems may adversely impact the quality and stability of products. By employing a demulsifier, the emulsion system can be separated and removed, thereby improving the quality and stability of the product [5]. At the same time, by controlling the amount and conditions of the demulsifier, the emulsion system can be effectively managed, the production process can be optimized, and both production efficiency and product consistency can be enhanced [6]. In addition, in scientific research and experimental testing, demulsifiers can be used to treat emulsion samples and separate them into liquid phases of higher purity. This facilitates subsequent qualitative analysis and quantitative measurement. In summary, demulsifiers hold an essential position in the chemical, food, pharmaceutical, and various other industrial sectors. They can achieve the separation and extraction of emulsion systems, improve product quality, control process conditions, and facilitate analysis and testing. These functions are of great significance for efficient production and effective research [7].

In addition, as research deepens, the field of demulsifier research has also expanded to include the exploration of demulsification mechanisms and principles. By examining the molecular structure and interfacial interactions of surfactants, researchers have investigated how surfactants can reduce surface tension, destabilize emulsions, and thereby trigger phase separation. Therefore, research into surfactant-based demulsifiers has seen extensive utilization across chemistry, engineering, material science, and various other fields [8,9]. With the further development of science and technology and the in-depth research, the field of surfactant demulsifier research continues to expand. This expansion provides more solutions and opportunities for the treatment and phase separation of emulsion systems.

2. Basic Concepts and Classification of Surfactants

Surfactants, also known as surface-active agents, have the ability to reduce the surface or interfacial tension in solutions. In their molecular structure, a hydrophilic part and a hydrophobic part coexist, allowing them to form a thin film at the liquid surface or liquid–solid interface. Structurally, surfactants consist mainly of two parts: hydrophilic groups and hydrophobic groups. The hydrophilic groups are capable of interacting with water molecules, while the hydrophobic groups have no affinity for water molecules and prefer to interact with nonpolar substances [10,11]. Surfactants mainly exist in two forms in solution: one is adsorbed on the liquid surface to form a monolayer, and the other is the formation of micelles. On the liquid surface, the hydrophobic groups of surfactants point toward the air or liquid–gas interface, while the hydrophilic groups point toward the liquid phase. When the concentration of surfactants reaches a certain level, the hydrophobic groups aggregate to form micelles, with the hydrophilic groups located on the exterior of the micelles. Surfactants exhibit many important characteristics and are widely used in various fields. They can reduce the surface tension of liquids, alter the wettability of liquids, increase the contact area between liquids and solids, and promote emulsification and dispersion. Therefore, surfactants are widely used in detergents, emulsifiers, foaming agents, lubricants, skincare products, pesticides, and inks [12,13].

In a word, a surfactant is a chemical substance with a special molecular structure that can reduce the surface or interfacial tension of liquids and form micelles in solution. Comprising a hydrophilic head and a hydrophobic tail, this amphiphilic structure enables surfactants to orientate at the interface between two immiscible phases, such as liquid–liquid or liquid–gas interfaces. When added to a solution, surfactants initially adsorb onto the interface, decreasing the surface energy by altering the molecular interactions at the boundary. As the concentration increases beyond the critical micelle concentration (CMC), individual surfactant molecules aggregate to form micelles, with the hydrophobic tails sequestered inside and the hydrophilic heads exposed to the aqueous environment.

In the context of demulsifiers, surfactants play a crucial role. Demulsification is the process of separating emulsions, which are thermodynamically unstable mixtures of immiscible liquids stabilized by an interfacial film. Surfactant-based demulsifiers function by disrupting this stabilizing film. They displace the emulsifying agents that maintain the emulsion’s integrity, reducing the interfacial film strength and promoting droplet coalescence. Once the droplets merge into larger entities, gravitational forces cause the separated phases to separate more efficiently. Moreover, certain demulsifier surfactants can modify the wettability of solid particles in the emulsion, further aiding in the phase separation process. This makes surfactants indispensable components in the formulation of effective demulsifiers across various industries, including petrochemical, food processing, and wastewater treatment.

2.1. Anionic Demulsifier

Most demulsifiers are amphiphilic nonionic surfactants, typically consisting of hydrophobic and hydrophilic groups. However, polyether demulsifiers still have limitations. For instance, they require large dosages, incur high costs, and exhibit poor adaptability in demulsification, all of which result in less-than-ideal demulsification outcomes. In contrast, anionic surfactants are characterized by their anionic groups, and common demulsifiers include sulfates and sulfonates. These anionic surfactants destroy the stability of emulsions by reducing the interfacial tension within the emulsions, thereby promoting their separation. They offer the advantages of low cost, small dosage, high demulsification efficiency, and strong adaptability [14,15].

Muhpidah et al. [16] prepared a demulsifier formula with an anionic surfactant concentration of 0.5%, a cosolvent addition of 0.1%, and a toluene-to-xylene ratio of 1:3. Compared with the demulsifier without the cosolvent additive, which can reduce the interfacial tension (IFT) value to 10⁻³ dyne/cm and produce perfect separation of oil and water, the addition of a cosolvent to the demulsifier leads to an increase in the IFT value and the percentage of emulsion produced. The performance test (core displacement test) of oil well fluid samples from the Semanggi oilfield using Berea core shows that the tested demulsifier can improve oil recovery by 13%. Additionally, the combination of anionic surfactants can significantly enhance oil recovery.

Mahdi et al. [17] synthesized dodecyl imidazole by reacting imidazole with dodecyl bromide. The dodecyl imidazole molecule was then coupled with 2,2-bis(bromomethyl)propane-1,3-diol to obtain the first ionic liquid, IL1. The anion of IL1 was exchanged with the anion of NaPF₆ to obtain IL2. The diol groups in IL1 were converted to ether groups using dodecyl bromide to obtain IL3. Finally, the anion of IL3 (Br⁻) was exchanged with the anion of NaPF₆ to obtain the ionic liquid IL4, as shown in Figure 1.

An anionic demulsifier was synthesized from imidazole and dodecyl bromide through coupling, polarity conversion, and ion exchange. In experimental tests, it demonstrated a good demulsification effect on W/O emulsions. Under conditions of 60 °C and a concentration of 100 mg/L, the maximum demulsification rate of crude oil can reach 98.6%. However, due to the challenging synthesis conditions, its effectiveness in practical applications remains unknown [18].

Yogesh et al. [19] synthesized an environmentally friendly anionic demulsifier from fatty acids via a simple one-step esterification reaction. The synthesized demulsifier exhibits comprehensive surface activity and high interfacial and demulsification performance. Biodegradability studies indicate that the demulsifier has good biodegradability and is environmentally friendly. At a concentration of 500 ppm and a temperature of 70 °C, the demulsification efficiencies of the synthesized demulsifier (LA1), another demulsifier (LA2), and a commercial demulsifier for asphaltene W/O emulsions are 100%, 80%, and 40%, respectively, within 90 min. Although the anionic demulsifier requires a small dosage and achieves high demulsification efficiency, its poor adaptability limits its widespread use in oil fields.

As a natural surfactant, asphaltene can stabilize emulsion by forming a rigid interfacial film. The aggregation states of its molecules, including nano-aggregates and clusters, are significantly influenced by the aromaticity of the oil phase. Alvarado et al. [7] found that when adding NP-8EO and other hydrophilic demulsifiers to the system, it can neutralize the lipophilicity of asphaltene and promote the system to reach the hydrophilic-lipophilic balance (HLD = 0); at this time, the stability of emulsion is minimized. It is noteworthy that a small amount of aromatic compounds in the oil phase can change the aggregation state of asphaltene, making it gradually change from clusters to nano-aggregates until molecular dispersion, which reduces the adsorption capacity of asphaltene at the interface and further optimizes the performance of demulsifier. The research results provide a low-cost and high-efficiency solution for crude oil dehydration process and show significant potential in industrial application [20].

2.2. Nonionic Demulsifier

Non-ionic demulsifiers lack ionic groups in their molecular structure, and common examples include methyl cellulose. Nonionic surfactants can further disperse asphaltenes, reduce the viscosity of emulsions, and promote phase separation by altering the particle size and dispersion of the emulsions [21,22,23].

Sharaky et al. [24] synthesized two star-shaped nonionic surfactants from (2Z, 20Z, 200Z)-4,40,400-((nitrile tris(ethane-2,1-diyl))tri(oxy)tri (4-oxobutan-2-enoic acid)), named D1 and D2, The demulsification mechanism of star shaped non-ionic demulsifier is shown in Figure 2. Before the demulsifier is added, the oil in water emulsion mainly consists of an oil phase and a water phase wrapped by asphaltene. When the star-shaped demulsifier is added, the surface-active components cause the water phase to continuously aggregate. The demulsifier molecules are more likely to aggregate in the water phase, replacing the asphaltenes wrapped in water droplets, and ultimately achieving the separation of the water phase and the oil phase. In this experiment, the demulsification effect of a demulsifier on asphaltene crude oil emulsion was investigated using the bottle method. The results show that after 120 min, the demulsification rate of a gel breaker based on a silicone polymer can reach over 98% at a concentration of 1500 ppm. However, this high dosage of demulsifier makes it unsuitable for practical applications.

Ayman et al. [25] synthesized a new nonionic cardanol demulsifier based on natural products and biological materials, using cardanol, amine, and diol from cashew oil as raw materials. The ability of cardanol surfactant to act as an asphaltene dispersant and as a demulsifier for water-in-crude oil emulsions in heavy crude oil was evaluated. It demonstrated a good effect as a single surfactant for both dispersing asphaltenes and demulsifying heavy crude oil. When the dosage was 500 ppm, the demulsification rate of the demulsifier exceeded 90%.

Wei et al. [26] prepared a new multibranched nonionic polyether demulsifier, FYJP, by grafting carboxylate onto a nonionic demulsifier. The FYJP demulsifier can be polymerized with ethylene oxide (EO) and propylene oxide (PO) using p-tert-butyl phenol as the initiator, along with triethylenetetramine and methanol, to produce a nonionic polyether demulsifier. A new nonionic polyether demulsifier was obtained by modifying a polyether demulsifier with sodium chloroacetate. The synthesis steps for FYJP are shown in Figure 3. The dehydration test results indicate that the highest dehydration rate achieved by the demulsifier is 94.7% under the conditions of 85 °C, a demulsifier dosage of 100 ppm, and a 50 mL water-in-oil (W/O) emulsion.

Zhang et al. [27] introduced a high content of carboxyl and ester groups into the nonionic demulsifier TJU-3 to synthesize a new nonionic demulsifier. This demulsifier addresses the key issue of demulsifying the rigid, high-viscosity interfacial film formed by asphaltenes, which is typically difficult to break down. When used in combination with a diluent and under mild heating conditions, the demulsifier achieves a 100% demulsification rate for W/O emulsions. The optimal conditions are a temperature of 60 °C and a demulsifier concentration of 400 ppm. This synergistic effect is primarily attributed to the reduction in viscosity, which weakens the barriers to demulsifier diffusion and increases the average kinetic energy of the demulsifier’s thermal motion. This work provides valuable insights for developing new, low-carbon demulsification and separation methods for petroleum production.

2.3. Cationic Demulsifier

Cationic emulsifiers are characterized by their cationic groups, and common examples include hexadecyltrimethylammonium bromide. Cationic surfactants interact with anions in the emulsion, thereby changing the surface properties of the emulsion and leading to phase separation.

Ayman et al. [28] synthesized a new type of dual-cation liquid containing imidazole and pyridine cations as an effective demulsifier for stable seawater-in-heavy-crude-oil emulsions. To this end, vinylimidazolium and either 4-amino-4-pyridyl diethoxylate (APE) or 4-pyridyl tetradecanamide were quaternized with 1,4-dibromobutane or 1,12-dibromododecane to prepare cationic liquid monomers. Figure 4 illustrates the steps of the emulsifier synthesis. The stable W/O emulsion was demulsified using ionic liquids prepared at different concentrations at 60 °C. The experimental results showed that the demulsification rate could reach 100% at 60 °C with a dosage of 250 ppm.

Zhang et al. [29] synthesized an ionic liquid demulsifier (MDBr-IL) with dual cationic centers and multiple hydrophobic chains using N, N, N, N-tetraglycidyl-4,4-diaminodiphenylmethane as the starting material. At a concentration of 500 mg/L, MDBr-IL achieved a demulsification efficiency of 95.24% within 180 min at 60 °C. When the demulsification temperature was increased to 70 °C, the demulsification efficiency could be further enhanced to 98.64%. Additionally, MDBr-IL exhibited excellent acid resistance, alkali resistance, and salt tolerance, with salinity observed to further improve its demulsification performance. The potential demulsification mechanism was investigated through interfacial properties and wettability analysis (As shown in Figure 5). The results demonstrate that MDBr-IL serves as an effective demulsifier applicable for separating oil–water emulsions.

Shu et al. [30] synthesized three water-soluble cationic demulsifiers: polyaluminum chloride-quaternary ammonium salts (PAC-QAS), polyamine (PA), and polyamine-polyaluminum chloride (PA-PAC). Their effectiveness in destabilizing emulsions and removing oil/suspended solids from oilfield produced water (OPW) was systematically investigated through laboratory-scale experiments and field trials. Additionally, the synergistic effects of these demulsifiers with cationic polyacrylamide flocculants (CPAMs) were evaluated. The effects of chemical properties, injection dosage, and inlet water quality on separation efficiency were discussed in detail. Microscopic observation and bottle test results indicate that polyamine type emulsifiers can effectively separate oil from OPW. Its surface-active components, by adsorption at the oil–water interface, replace the natural surfactant and weaken the strength of the interface facial mask, thus promoting the collision and coalescence of small oil droplets and causing the morphological change in emulsified oil in the water phase. Subsequently, the bridging and cleaning effects driven by flocculants further enhance treatment performance. The optimal combination of PA and CPAM-2 can remove up to 90% of oil and 85% of suspended solids from OPW. Compared with coagulants, insufficient dosage of reverse phase demulsifier leads to poor separation, indicating a more significant demulsification effect.

2.4. Natural Demulsifier

Surfactants are extracted from natural sources, such as soaps and lipids. These natural surfactants are commonly used in food and cosmetics as emulsifiers, which can separate milk like substances such as lactose, cream, and skincare products. Natural emulsifiers have been widely studied [31,32,33].

Biswadeep et al. [34] discovered a natural demulsifier (DEMLOCS) formulated with coconut extract, which shows potential as a biodegradable and environmentally friendly demulsifier. The lotion samples were treated with DEMLOCS at concentrations ranging from 500 to 2000 ppm and temperatures between 30 °C and 45 °C. Static bottle tests were conducted to determine the effectiveness of the demulsifier in separating oil and water. After 24 h at 45 °C, 88% of the water was separated from the lotion using a demulsifier concentration of 2000 ppm. As the demulsifier concentration increased from 500 ppm to 2000 ppm, the interfacial tension decreased from 12.59 to 2.64 dyn/cm, resulting in increased water separation. As salinity increases, the separation degree initially rises, peaking at a specific salt concentration, beyond which it declines. The addition of DEMLOCS significantly reduced the viscosity of the lotion. When 2000 ppm of demulsifier was added at 30 °C, the lotion’s viscosity decreased by 64% as the temperature was raised from 30 °C to 45 °C. Therefore, adding demulsifiers serve as an effective alternative to heating for reducing viscosity and promoting water separation. Biodegradability tests of DEMLOCS assessed via free CO measurement demonstrate that this demulsifier is readily biodegradable and poses no harm to refining systems or the environment.

Ye et al. [35] synthesized an environmentally friendly, amphiphilic rice husk charcoal (RHC) demulsifier using rice husk as the raw material via a simplified carbonization process in a muffle furnace. Experimental results demonstrated that a DE of 96.99% was achieved with 600 mg/L RHC at 70 °C over 80 min. RHC demonstrated optimal DE under neutral conditions while maintaining effectiveness under both acidic and alkaline conditions. Furthermore, the material exhibited robust salt tolerance. The demulsification mechanism of RHC was attributed to three key factors: its high interfacial activity, oxygen-containing functional groups, and silica content, as confirmed by interfacial characterization, comparative treatment analyses, and microscopic observations. These findings collectively indicate that RHC serves as an effective demulsifier for W/O emulsion treatment: (RHC)EC50 > 100 mg/L and biodegradation rate > 90%, which corresponds with the principle of green chemistry.

Leontyeva et al. [36] synthesized a natural demulsifier comprising oil ash, salt, and a nanostructured manganese-based activating additive. During emulsion separation, the “hydrocarbon raw material water” accounted for 20% of the total water input. A low-boiling-point solvent extracted from the emulsion matrix was formulated as a mixture combining kerosene fraction, demulsifier base, salt, and metal nanoparticles. This demulsifier demonstrated significant demulsification efficacy for O/W emulsions, achieving a DE of 95.69% after 30 min of treatment at 70 °C. Sadigh et al. [5] synthesized a novel multifunctional imidazole ionic liquid (ILs) derivative [SIm-TazCn] [Br] containing triazole group by click chemistry and then grafted it onto cellulose (CEL) to prepare an environment-friendly demulsifier for breaking water-in-oil (W/O) emulsion.

Adewunmi et al. [37] demonstrated that palm oil fuel ash (POFA), an environmentally harmful waste material, can effectively break down emulsified substances generated during hydrocarbon production. The bottle test method was employed to assess the demulsification performance of POFA at concentrations ranging from 1 wt% to 9 wt% under temperatures of 60 °C and 80 °C. Experimental results revealed that POFA promotes demulsification and achieves phase separation in water/oil emulsions. Specifically, the emulsion containing 3 wt% POFA exhibited optimal separation performance, with DE of 99.28% and 99.71% at 60 °C and 80 °C, respectively. Interfacial tension measurements revealed that the stability of W/O emulsions is governed by the interfacial cohesive forces between oil droplets and the interfacial film formed by resin and asphaltene at the oil–water interface. Destabilization of the emulsion can be achieved by modulating either the intermolecular cohesive forces or the solid–liquid interfacial layer, thereby enabling phase separation. Upon addition of POFA to the W/O emulsion (Figure 6b), the POFA particles became uniformly dispersed within the oil phase (Figure 6c) following vigorous mixing, initiating demulsification by inducing contact between the water–oil interface and asphaltene–resin molecules. POFA interacts with these interfacial components, undergoing fragmentation during crude oil–water collisions, which disrupts the continuity of the protective film at the water–oil interface. The partial breakdown of this interfacial layer facilitates coalescence of crude oil droplets (Figure 6d), ultimately leading to the formation of large buoyant oil droplets floating atop the system, while the water phase settles at the bottom. With increasing aggregation and coalescence, oil accumulates at the top, water settles at the bottom, and POFA particles precipitate within the aqueous phase (Figure 6e). Experimental analyses demonstrate that POFA represents a promising demulsification material capable of separating water from various oilfield-produced emulsions. However, challenges in sourcing natural demulsifier raw materials currently hinder its large-scale industrial application in oilfields.

2.5. Metal Demulsifier

Metal ion-based surfactants, particularly metal fatty acid salts, are widely used as demulsifiers. These metallosurfactants destabilize emulsions through chelation with anionic components, disrupting interfacial stability and inducing phase separation.

Wang et al. [38] developed a novel metal-based demulsifier through the integration of a metal–organic framework (MOF) MIL-100(Fe) with sodium dodecyl sulfate (Figure 7), which exhibited exceptional demulsification performance. Under optimized conditions, the DE reached >99% for O/W crude oil emulsions within 30 min, while achieving 88% DE for W/O systems in merely 5 min. MIL-100(Fe) demonstrated robust demulsification performance across wide pH ranges and salinity gradients, exhibiting a positive correlation with salinity (1–1000 mmol/L NaCl). Mechanistic studies revealed that its amphiphilicity crucially governs emulsion destabilization, while electrostatic interactions further regulate the demulsification process. Notably, MIL-100(Fe) effectively degrades sodium dodecyl sulfate via sulfite radical (SO32-) mediation, thereby neutralizing surfactant functionality and eliminating re-emulsification risks.

W/O emulsions cause serious problems in the operation, maintenance, and transportation of the petroleum industry. The presence of asphaltene in crude oil enhances the stability of the lotion. In recent studies on metallic demulsifiers, Nassim et al. [39] demonstrated that reusable Fe₃O₄ magnetic nanoparticles (MNPs) can enhance commercial demulsifiers by serving as modifiers, significantly improving the demulsification of W/O emulsions with high asphaltene content (13 wt%). These Fe₃O₄ particles were synthesized via a facile, cost-efficient electrochemical method, owing to their advantages of low material consumption and minimal cytotoxicity. Statistical analysis revealed that ND, pH, and WC significantly influence DE beyond the studied variable ranges. Under optimized conditions (T = 40 °C, XD = 300 ppm, pH = 6.4, WC = 7.5 mL, ND = 0.033 g), the maximum DE reached 97.83%. Notably, Fe₃O₄ MNPs demonstrated excellent reusability, maintaining performance through at least six consecutive cycles. Furthermore, when integrated with commercial demulsifiers, the system achieved a 10% increase in DE alongside a 6 h reduction in settling time. This demonstrates that the required phase separation time is substantially shorter than conventional chemical demulsification methods. The enhanced performance of Fe₃O₄ MNPs in W/O emulsion treatment, coupled with their reusable nature, establishes a novel strategy for optimizing demulsifiers through metal nanoparticle modification.

Zhou et al. [40] ingeniously grafted a nonionic fatty alcohol-derived epichlorohydrin-ethylene oxide block polyether (ANP) onto SiO² coated Fe₃O₄ magnetic nanoparticles, successfully creating a composite material with combined amphiphilic and positively charged properties. This study demonstrates that the novel material exhibits exceptional demulsification performance in treating both W/O- and O/W-simulated emulsions [41]. Notably, the addition of only 200 ppm of this material achieves over 95% demulsification efficiency at ambient temperature while demonstrating excellent recyclability.

For the W/O emulsion system, its efficient demulsification mechanism can be primarily attributed to the following aspects: First, the nanoscale particle size of the demulsifier provides an enlarged specific surface area, enhancing material dispersion within the emulsion and thereby promoting effective interfacial contact [42]. Second, as an amphiphilic molecule, ANP rapidly migrates to the oil–water interface where its hydrophilic moieties form stable hydrogen bonds with water molecules, significantly facilitating demulsifier adsorption and interfacial spreading.

Upon reaching the oil–water interface, ANP molecules effectively displace the original surfactant molecules at the interface, substantially weakening the interfacial film strength [43]. This interfacial replacement disrupts the emulsion’s stability, causing the loss of protective layers from stabilized oil droplets and initiating droplet coalescence. The subsequent growth of coalesced droplets progressively destabilizes the emulsion system, ultimately enabling rapid phase separation through complete demulsification.

2.6. Summary of Demulsifiers

As shown in

Table 1. Advantages and disadvantages of various demulsifiers.

Demulsifier	Demulsification Conditions	Demulsification Rate/%	CMC (mg/L)	Hydrophilic-Lipophilic Balance Value (HLB)	Cloud Point (°C)	Average Droplet Size (μm)	Interfacial Viscosity (mPa s)	Advantage	Disadvantage
Anionic demulsifier	60 °C, 100 mg L−1	98.60	0.2–5.0	8–18		5–15	10–30	Good effect and low cost	Poor adaptability
	70 °C, 100 mg L−1	100
Non-ionic demulsifier	60 °C, 400 mg L−1	94.7	2.0–50.0	10–16	40–100	2–5	5–15	Strong adaptability and high efficiency	Large dosage, difficult to degrade
	Room temperature, 400 mg L−1	92.6
Cationic demulsifier	60 °C, 200 mg L−1	100	0.5–10.0	12–20		8–20	15–35	Low cost, salt and acid alkali resistance	Long demulsification time
	Room temperature, 200 mg L−1	98.9
Natural demulsifier	70 °C, 200 mg L−1	98.5	3.0–100.0	4–12	30–80	15–30	20–40	Naturally degradable, with minimal harm	Non-recyclable
	60 °C, 200 mg L−1	94.6
Metal type demulsifier	Room temperature, 200 mg L−1	95.6	0.1–5.0	6–14		3–8	8–20	High efficiency and recyclability	Incomplete recycling poses a safety hazard
	40 °C, 200 mg L−1	97.8

, while traditional nonionic demulsifiers remain the most prevalent category, their high dosage requirements (typically 500–800 ppm) and elevated operational costs hinder large-scale industrial applications. In comparison, anionic demulsifiers have recently garnered significant research attention due to their cost-effectiveness (30–50% lower than nonionic types) and remarkable separation efficiencies (>90%). However, their narrow pH adaptability range (limited to 6–8) restricts application to specific emulsion systems, coupled with persistent environmental concerns regarding non-biodegradable degradation byproducts that may accumulate in aquatic ecosystems. Therefore, natural demulsifiers and metal demulsifiers have become the focus of current research, and the demulsification technology of metal nanomaterials has been partially used in China. However, due to the non-recyclability of natural demulsifiers and the difficulty in producing raw materials, metal nano demulsifiers also face incomplete recovery and cannot be widely used in oil fields.

As shown in

Table 2. Comparison of indexes of various demulsifiers.

Demulsifier	Cost	Validity (Demulsification Rate)	Biodegradability	Suitability	Environmental Toxicity (EC50, mg/L)	Regulatory Compliance (REACH/EPA)
Anionic demulsifier	Low	Tall	General (partially refractory)	Poor (specific emulsion)	12.5	Partially restricted (REACH Annex XIV)
Non-ionic demulsifier	Medium to high (large dosage)	Tall	Poor (difficult to degrade)	Strong (widely applicable)	8.3	Meet EPA standards
Cationic demulsifier	Low	Polar altitude	General (partially degradable)	Medium (acid, alkali and salt resistance)	5.1	Need to declare (REACH)
Natural demulsifier	Medium (raw material restriction)	Tall	Excellent (naturally degradable)	Medium (to be optimized)	>100	Full compliance (EPA/REACH)
Metal type demulsifier	Medium to high (recovery cost)	Polar altitude	Poor (partially recyclable)	Strong (resistant to complex conditions)	2.4	Limit (EPA heavy metal limit)

, we compared the dosage of demulsifiers with industry standards and conducted a critical analysis of the dosage, efficiency, and limitations of surfactant demulsifiers. It can be concluded that significant progress has been made in the exploration of demulsification efficiency and mechanism in current research, but further research is still needed in terms of dose economy, environmental compatibility, and industrial adaptability.

As shown in

Table 3. Comparison between demulsifier dosage and industry standard.

Demulsifier	Experimental Dose (ppm)	Industry Routine Dose (ppm)	Evaluate
Anionic demulsifier	100–500	50–300	The dosage of LA1 (500 ppm, 100%) is on the high side, which is high in efficiency, but may increase the cost. Need to optimize the compound to reduce the dosage.
Non-ionic demulsifier	400–1500	200–800	Star demulsifier (1500 ppm, 98%) far exceeds the industrial economic consumption (usually <1000 ppm), so its practicability is limited.
Cationic demulsifier	200–500	100–400	The dosage of MDBr-IL (500 ppm, 95.24%) is reasonable, but the demulsification time is long (180 min), so it is necessary to balance the efficiency and time cost.
Natural demulsifier	200–2000	100–1000	The dosage of DEMLOCS (2000 ppm, 88%) is too high, so it is necessary to optimize the extraction process of raw materials to reduce the dosage.
Metal type demulsifier	200–300	50–200	Feomnps (300 ppm, 97.83%) is close to the industrial standard, but incomplete recovery may increase the long-term cost.

, the current research has made remarkable progress in demulsification efficiency and mechanism exploration, but further research is still needed in dose economy, environmental compatibility, and industrial adaptability. By combining theoretical design, compound optimization, and green technology, surfactant demulsifier can be promoted to develop in the direction of low dosage, high adaptability, and sustainability in the future, meeting the complex needs of petroleum industry.

Demulsifiers are a key chemical component which can achieve oil–water separation or other phase separation by destroying the stability of emulsion, and they are widely used in many fields such as industrial production and environmental protection [44]. However, the use of demulsifiers also has a certain impact on the environment. They can destroy the ecological environment of soil and affect the natural ecological balance of soil. These chemicals are easy to accumulate in soil, which leads to the decline in soil quality and has a negative impact on agricultural production [20]. Harmful gases may also be released during use, which affects human health, causing eye, nose. and throat discomfort, headache, nausea, vomiting, and other symptoms [45,46]. At the same time, these harmful gases may also pollute the atmospheric environment and further affect the balance of the ecosystem. They may also have irreversible long-term destructive effects on the environment [32]. These chemicals are difficult to degrade and may exist for a long time, causing lasting damage to the ecosystem. This long-term destructive impact may include soil degradation, groundwater pollution, biodiversity reduction, and so on. Therefore, when using a demulsifier, the dosage should be strictly controlled, wastewater treatment should be strengthened, and environmentally friendly demulsifiers should be popularized to reduce environmental pollution [47,48].

3. Study on the Mechanism of Surfactants as Demulsifiers

3.1. Demulsification Mechanism of W/O Emulsion

3.1.1. Substitution Mechanism

Demulsifiers [49], as highly specialized chemical synthetic substances, have significantly higher surface activity than natural surfactants. This high surface activity enables the emulsion to quickly and effectively adsorb on the interface between oil and water, and actively promote the demulsification process of lotion by significantly reducing the interfacial tension. This process can be refined into the following three closely interconnected stages [50]:

• The efficient interfacial adsorption of demulsifiers on the oil–water interface is beneficial to the demulsification performance improvement: During the initial stage of demulsification, demulsifier molecules rapidly migrate to the oil–water interface and adsorb firmly onto it due to their superior surface activity. This critical step establishes a foundational framework for subsequent disruption and destabilization of the interfacial film.
• Deep interference and destruction of the oil–water interfacial film: Upon successful adsorption at the interface, demulsifier molecules initiate their primary function—profound disruption and destabilization of the original oil–water interfacial film. By substituting or weakening the stabilizing molecules comprising the interfacial film, demulsifiers compromise the film’s structural integrity, thereby diminishing its capacity to resist oil droplet coalescence.
• Significant reduction in interfacial tension: As the interfacial film progressively breaks down, the interfacial tension between oil and water undergoes a marked decrease. This decrease directly facilitates the coalescence of oil droplets, as reduced interfacial tension lowers the energy barrier required for droplet merging. Consequently, this thermodynamic advantage accelerates the demulsification and phase separation processes of the emulsion.

3.1.2. Bridge Replacement Mechanism

High-molecular-weight demulsifiers demonstrate an excellent ability to promote water droplet aggregation in emulsion systems, a critical mechanism for enhancing oil–water separation. Upon introduction into an emulsion, the demulsifier effectively reduces mutual repulsion between water droplets, thereby significantly shortening their interfacial distance. This process facilitates close contact and binding among droplets, leading to the coalescence of numerous small water droplets into larger aggregates. Consequently, visible macroscopic flocs are formed through this aggregation phenomenon. As water droplets increase in size, gravitational effects on them become more pronounced. These enlarged droplets undergo gravitational settling, thereby enabling effective oil–water phase separation [51]. Furthermore, the addition of demulsifiers induces the formation of new interfacial films that encapsulate the coalesced water droplets. However, compared to the original interfacial films, these newly formed membranes exhibit significantly higher instability. This phenomenon is primarily attributed to the demulsifier-induced alterations in both the chemical composition and structural organization of the interfacial films. The structural instability of interfacial films creates favorable conditions for progressive flocculation and coalescence of water droplets. Upon rupture of the interfacial membrane, previously segregated droplets undergo rapid coalescence, forming larger droplet assemblies. This cyclic process ultimately leads to the aggregation of virtually all aqueous phase components into macroscopic domains, thereby achieving efficient demulsification [52].

3.1.3. Flocculation Aggregation Mechanism

This demulsification mechanism operates in two core stages: film rupture and liquid drainage. Driven by a pressure gradient, the encapsulated liquid undergoes forced displacement to achieve efficient drainage. When the interfacial film thickness between droplets reaches a critical threshold, its structural integrity is compromised, resulting in film rupture. Concurrently, capillary pressure generates differential forces that drive rapid droplet coalescence into larger entities. Throughout this process, the physicochemical characteristics of the interfacial film fundamentally determine the separation efficiency. Notably, droplet deformation directly increases the interfacial area along the drainage pathway within the film, which typically decelerates drainage efficiency due to the extended liquid volume requiring removal. To enhance demulsification kinetics, ionic demulsifiers are introduced into the system. These agents rapidly permeate the originally rigid interfacial film, substantially reducing membrane stability through disruption of intermolecular interactions. This destabilization initiates immediate droplet coagulation and coalescence, transforming dispersed microdroplets into macroscopic aqueous clusters. Furthermore, ionic demulsifiers demonstrate strong adsorption affinity toward asphaltenes. As critical stabilizers in emulsion systems, the effective sequestration and removal of asphaltenes are essential for optimizing demulsification performance. Through selective asphaltene adsorption, these demulsifiers achieve dual functionality: compromising interfacial film stability while enhancing system-wide demulsification kinetics. This synergistic mechanism enables comprehensive phase separation through simultaneous interfacial destabilization and colloidal component removal [53].

3.1.4. Competitive Adsorption Mechanism

Anionic demulsifier molecules exhibit significant interfacial activity in emulsion systems due to their strong competitiveness. These molecules can effectively penetrate into the interface facial mask and weaken the strength of the interface facial mask by replacing the original active substances on the interface membrane. As the film’s mechanical strength diminishes, its capacity to stabilize water droplets deteriorates correspondingly, thereby facilitating spontaneous droplet coalescence through reduced inter-droplet adhesion forces. It is worth noting that some anionic emulsifiers have complex molecular structures and multiple branches. These branches not only increase the contact area between demulsifier molecules and asphaltene and other components in the lotion, but also provide more π-π interaction points. This intermolecular interaction enhances the adsorption capacity of the demulsifier for asphaltene, enabling the demulsifier to more effectively remove asphaltene from the emulsion. With the decrease in asphaltene, the stability of the emulsion further decreases, and the demulsification process can be accelerated. Therefore, anionic demulsifiers can weaken the strength of the interfacial facial mask, promote the aggregation of water droplets, and enhance the adsorption capacity of asphaltene through a complex molecular structure, thus achieving efficient demulsification of the emulsion system. These mechanisms work together to make anionic emulsifiers achieve broad application prospects in the field of oil–water separation [50].

3.1.5. Derjaguin–Landau–Verwey–Overbeek Theory

During the process of achieving demulsification by altering the wettability of particles, changes in particle wettability can affect the adsorption state of particles at the oil–water interface, thereby altering the interactions between particles. From the perspective of the DLVO theory, such changes may disrupt the balance between the van der Waals attractive force and the double-layer repulsive force. When particles desorb from the interface, if the double-layer repulsive force decreases significantly, particles are more likely to aggregate, promoting the occurrence of demulsification. Similarly, in the case of demulsification caused by particle dissolution, the reduction in particle size not only reduces the stability of the interfacial film but also may change the interaction energy between particles. According to the DLVO theory, a decrease in particle size can change the magnitude and range of action of the van der Waals attractive force and the double-layer repulsive force, further influencing the stability of the emulsion and the demulsification process [54,55].

3.2. Demulsification Mechanism of O/W Emulsion

3.2.1. Anti-Phase Transformation Mechanism

After the introduction of the reverse demulsifier into the O/W lotion system, the structure of the lotion undergoes a fundamental change from the original O/W type to the W/O type. During this transition, the special structure of the reverse demulsifier molecule plays a key role. Its hydrophilic group penetrates deep into the water drop in the form of multiple branches, while the hydrophobic group is firmly anchored on the external interface of the water drop [56]. This unique molecular configuration enables the reverse demulsifier to firmly adhere to the oil–water interface and has a profound impact on the stability of the interfacial facial mask [57]. Through a competitive adsorption mechanism, reverse demulsifier molecules vigorously compete with native emulsifier molecules at the interfacial film. Due to their structural and adsorptive superiority, reverse demulsifier molecules progressively displace emulsifier molecules from the interfacial film, establishing dominance. This displacement fundamentally alters the interfacial film’s composition, thereby reducing its mechanical integrity and destabilizing the emulsion system [58].

3.2.2. Mechanism of Neutralizing Interface Charges

O/W emulsions exhibit negatively charged surfaces. The introduction of cationic demulsifiers neutralizes these surface charges, thereby reducing interfacial tension. Under gravitational forces, aqueous droplets within the emulsion undergo continuous coalescence, promoting oil–water phase separation [59,60]. Simultaneously, the coalescence of oil droplets occurs at a slower rate compared to aqueous droplet aggregation, which effectively mitigates the stabilizing effects of polymeric additives present in the wastewater system [58,61].

3.2.3. Mechanism of Counter-Ion Action

Research demonstrates that native emulsifiers present at the interfacial film generate electrostatic repulsion due to their uniformly charged molecular surfaces. This repulsive force counteracts the inherent van der Waals attractive forces between emulsifier molecules, collectively stabilizing the emulsion structure. However, introduction of an oppositely charged ionic demulsifier disrupts this equilibrium. At this stage, a strong electrostatic attraction develops between the emulsifier and demulsifier due to their opposing charges. This attraction surpasses the original stabilizing forces, resulting in interfacial film rupture. The scientific community refers to this phenomenon as the “counter-ion mechanism,” which elucidates the critical principle of charge interactions governing emulsion stability [50].

3.2.4. Wetting and Solubilization Mechanism

Interfacial tension is an important indicator for measuring the stability of an interfacial film. When an interfacial film has a low interfacial tension, it means that the energy of the system is low and the interfacial film is relatively more stable. In simulation studies, the stability of the interfacial film can be evaluated by monitoring the change in interfacial tension over time. Taking the interfacial film formed by a polymer melt on a solid surface as an example, if a certain additive is added to the system and causes the interfacial tension to decrease and remain stable, it indicates that the additive may enhance the stability of the interfacial film. Conversely, if the interfacial tension suddenly increases or fluctuates significantly, it indicates that the stability of the interfacial film has been disrupted [13].

Upon introduction of demulsifiers into O/W lotions as colloidal micelles, these structures exhibit pronounced solubilization effects at the oil–water interface [62]. Specifically, micelles interact with and solubilize interfacial film components, compromising both the structural integrity of the film and significantly reducing interfacial tension. This lowering of interfacial tension represents a critical phase in demulsification, as it diminishes the energy barrier essential for maintaining emulsion stability. With decreasing interfacial tension, the interface’s capacity to resist external disturbances and maintain oil droplet dispersion diminishes. Consequently, the interfacial film loses its ability to effectively encapsulate oil droplets, causing them to coalesce and release the aqueous phase—a process that achieves emulsion destabilization. In this mechanism, the solubilization effect of demulsifier micelles plays a pivotal role by directly compromising interfacial integrity. This action facilitates emulsion decomposition and promotes oil–water phase separation [63].

3.3. Summary of Demulsification Mechanism

Recent advancements in emulsion destabilization research have elucidated the interplay of four core mechanisms—displacement, solubilization, electro-neutralization, and coagulation—that collectively govern the intrinsic process of phase separation. Among these, the displacement and flocculation–coagulation mechanisms have garnered significant attention due to their pivotal role in restructuring the interfacial film [64]. The displacement mechanism posits that when a demulsifier with superior interfacial activity is introduced into a crude oil emulsion, it competitively adsorbs to the oil–water interface, displacing natural emulsifier molecules. This process not only inhibits the further adsorption of other emulsifying agents but also forms a composite interfacial layer containing both original emulsifiers and demulsifier molecules. Due to inherent thermodynamic instability, this hybrid film undergoes structural collapse, leading to interfacial film rupture. Consequently, the encapsulated water phase is released, forming thermodynamically stable large droplets. These droplets subsequently coalesce into irregular aggregates, which undergo gravitational separation based on oil–water density differences, ultimately achieving phase demulsification [65]. Our summary and comparison of the demulsification mechanism are shown in

Table 4. Comparison table of demulsification mechanism.

Mechanism	Core of Action	Suitable Emulsion Type	Typical Demulsifier Types	Key Indicators
Displacement replacement	Interfacial molecular substitution	W/O; O/W	Anionic, nonionic type	Decreased range of interfacial tension, demulsification time
Bridge replacement	Polymer-bridged aggregated water droplets	W/O	Non-ionic (such as star polyether)	Flocculation size, sedimentation rate
Flocculation aggregation	Charge neutralization or adsorption coalescence	O/W	Cationic type, metal type	Zeta potential change, oil droplet coalescence efficiency
Competitive adsorption	Competitive adsorption of interfacial film	W/O	Cationic (complex branched structure)	Interfacial membrane strength, demulsifier adsorption capacity
Inverse phase change type	Emulsion type conversion	O/W→W/O	Reversed demulsifier	Emulsion type transition speed, separation thoroughness

4. Prospect

Current research indicates that traditional demulsifiers face challenges such as strict application conditions and low efficiency, especially in terms of temperature control, dosage optimization, demulsification time, and recovery treatment. Meanwhile, with the increasing complexity of crude oil emulsion systems, the environmental pressure they bring is becoming more severe. In response to this situation, combined with the application and mechanism of surfactants as demulsifiers, and considering the development needs of the domestic petroleum industry, the following research paths are of great significance for improving oilfield recovery efficiency:

• Advancing Research on Demulsification and Stabilization Mechanisms: Given the differences in the composition and characteristics of emulsions in various oil fields, it is urgent to explore the underlying mechanisms of demulsification and stabilization. Molecular dynamics simulation technology is used to analyze the micro behavior of complex lotion and oil–water interfaces. In combination with the characteristics of lotion and the actual needs of the site, multitechnology integration strategy is adopted to screen and optimize the types of surfactants to achieve more efficient and accurate demulsification effect.
• Optimization of surfactant compounding technology: The surfactant compounding technology has shown great potential in improving the demulsification performance and has become an important direction in the research and development of demulsifiers. However, the current compounding rules are not clear, and the optimal ratio between different emulsifiers needs to be verified through extensive experiments. Therefore, in the future, we should focus on the optimization research of binary and multivariate compound systems and improve the synergistic effect and practical application efficiency of compound emulsifiers by finely adjusting the ratio.
• Promotion of the development of green demulsification technology: In response to the global trend of green environmental protection and low-carbon economy, developing environmentally friendly demulsifiers has become a key focus of future research. This type of demulsifier should have characteristics such as high mineralization resistance and acid and alkali resistance while ensuring good compatibility to reduce negative impacts on the environment. In addition, exploring multitechnology integration strategies, such as combining physical, chemical, and biological methods, to further enhance demulsification efficiency and sustainability is the key to promoting the green transformation of the petroleum industry.
• Promotion of the development of emerging innovative technologies: In current research, switchable materials have received extensive attention from scholars. At the basic research level, current studies on switchable materials mostly focus on their static properties before and after switching, while the switching process is actually dynamic. Future research should focus on exploring the kinetics of the switching behavior to clarify its switching speed, which is crucial for improving practical application efficiency. In terms of the development of new materials, although certain progress has been made in switchable materials, further optimization is still needed to overcome existing limitations. For switchable interfacial-active materials, efforts should be made to solve the problem of irreversible adsorption on solid surfaces. By rationally designing the charge characteristics of surfactants, the negative impacts of adsorption on reducing the oil–water interfacial tension and switching performance can be minimized. At the same time, new methods such as vapor-phase treatment should be explored to regulate surface adsorption. However, before practical application, these new methods need to be fully verified conceptually.