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. 2026 Sep 28;11(40):62280–62289. doi: 10.1021/acsomega.6c10608

Synthesis of Interface-Targeting Polyether Ester Demulsifiers and Investigation of the Mechanism of Rapid Demulsification of O/W Emulsions at Room Temperature

Xiaojing Yao †, Jiangli Lin †,*, Ruijia Yang ‡,*, Xiuling Zhu †, Xiangyun Wei §, Shishan Zhou †, Bohang Cang †, Wenhao Zhang †, Jiaolong He ‡, Zhen Su †
PMCID: PMC13670752  PMID: 42889987

Abstract

This study addresses the problems of strong temperature dependence, low demulsification efficiency, and long treatment time for oil-in-water (O/W) emulsions in the petroleum industry. Using polyethylene glycol/polyethylene glycol monomethyl ether (PEG/mPEG) as the hydrophilic polyether backbone, a novel amphiphilic polyether ester–amide demulsifier PEOSu-C18 was designed and synthesized by succinic acid esterification and terminal modification with octadecyl amide to introduce hydrophobic octadecyl-amide segments at polymer chain ends. The linear polyether-ester backbone supplies interfacial adsorption capacity, while terminal long-chain octadecyl hydrophobic segments provide effective interfacial displacement and hydrophobic interactions. Demulsifier dosage, temperature, time, and pH were systematically studied for oil-removal performance. PEOSu-C18 achieved an oil-removal efficiency above 98% within 30 min at a low dosage of 100 mg/L. It maintained excellent performance over a wide temperature range (30–60 °C) and possessed favorable salt tolerance, which can cut the energy and time costs of traditional demulsification. PEOSu-C18 was characterized via FTIR and 1H NMR. Its demulsification mechanism was explored by dynamic interfacial tension, Zeta-potential, and optical microscopy. Results reveal that PEOSu-C18 rapidly migrates to the oil–water interface, competes with and displaces negatively charged kaolinite particles through steric hindrance and hydrophobic interaction, breaks the rigid interfacial film, and accelerates oil-droplet coalescence. This work provides an efficient, rapid, and energy-saving demulsifier with promising prospects for complex oilfield-produced-fluid treatment.

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1. Introduction

With the rapid advancement of industrialization, sectors such as oil extraction, metal processing, and food manufacturing generate large volumes of oily wastewater. Annual emissions exceed 10 billion tons, making oily wastewater one of the largest sources of industrial wastewater in China; oil-in-water (O/W) emulsions account for most of the difficult-to-treat fraction. Large-scale discharge of this wastewater exacerbates water pollution and further aggravates freshwater shortages. Therefore, the development and application of efficient oil–water separation technologies have become urgent priorities in water pollution control. ,, Natural or added surfactants in industrial wastewater readily interact with oil and water phases to form complex, highly stable O/W emulsions, which significantly increase treatment difficulty.,, O/W emulsified wastewater is the most common emulsion type. It carries a negative surface charge and, under the stabilizing effects of substances such as surfactants, forms a resilient film at the oil–water interface, , In actual oilfield production, kaolinthe most widely distributed clay mineral in sandstone reservoirscan form a rigid Pickering solid-particle interfacial film in a surfactant-free system, thereby increasing emulsion stability. These two factors make such emulsified wastewater difficult to demulsify and render oil–water separation highly challenging. Conventional physical methods (such as gravitational settling and centrifugation) and simple chemical methods (such as acidification and salting out) often have limitations, including low treatment efficiency, high energy consumption, a tendency to cause secondary pollution, and difficulty in disrupting the stable interfacial film.

Against this backdrop, chemical demulsification using specialized polymeric demulsifiers , has emerged as a mainstream technology because of its high efficiency and relatively simple operation. An ideal demulsifier should rapidly migrate to the oil–water interface, displace the original stabilizers through competitive adsorption, and promote the coalescence and flotation of oil droplets. However, commercial products and demulsifiers reported in the literature still face substantial challenges when they are used to treat low-concentration, highly stable industrial O/W emulsions. Some demulsifier production processes pose safety risks, and many demulsification processes require long treatment times, high dosages, and elevated temperatures. These requirements substantially increase treatment costs and cause unnecessary resource consumption. ,

Polyether demulsifiers have favorable biodegradability, accessible raw materials, and clear economic advantages, which make them promising candidates for oily wastewater treatment. However, current demulsifiers still have shortcomings such as inadequate low-temperature performance and weak interfacial interactions. These limitations restrict their applicability and make it difficult to meet the practical treatment requirements of the complex oilfield systems. Researchers have therefore developed high-performance polyether demulsifiers through molecular structural modification and the regulation of interfacial properties. Zhou et al. used dilatational rheology testing to show that the interfacial activity of polyether demulsifiers is closely related to molecular structure. Linear structures exhibit a stronger interfacial adsorption capacity, whereas branched structures are more effective at displacing naturally active components at the interface. An et al. used proanthocyanidins as the core material. By adjusting the branched structure, number of aromatic rings, and hydrophilic–lipophilic balance, the modified product achieved improved surface activity and elastic modulus. Its interfacial performance and asphaltene removal capacity were also substantially strengthened, enabling efficient demulsification and dehydration of low-temperature-aged crude oil emulsions. Yan et al. optimized the hydrophilic–lipophilic balance by adjusting the proportion of hydrophobic chains, thereby disrupting the dense interfacial film of asphaltenes and reducing the interfacial elastic modulus. Furthermore, Xu et al. graft-modified PVDF membranes with hyperbranched phenylamine resin-blocked polyether demulsifiers. By using the synergistic effect of polyether-driven interfacial demulsification and membrane separation, they effectively disrupted the oil–water interfacial film and achieved excellent oil–water separation, further confirming the broad application prospects of polyether materials. Although polyether demulsifiers have shown good potential for oily wastewater treatment, molecular structures that balance strong interfacial adsorption, efficient interfacial displacement, and excellent low-temperature adaptability remain scarce.

Accordingly, this study describes the design and synthesis of a novel polyether demulsifier that combines the strong interfacial adsorption of a linear structure with the high interfacial displacement capacity of a branched structure. Hydrophobic long chains were introduced into the molecular structure to precisely regulate the amphiphilic balance. This design significantly improves the material’s adaptability to low-temperature conditions and increases its demulsification rate, ultimately enabling the efficient and rapid demulsification of complex O/W emulsions at low temperatures. By evaluating the effects of dosage, temperature, time, and system pH on demulsification performance and using microscopic interfacial characterization techniques, this study clarifies the structure–activity relationships among the molecular structure, interfacial behavior, and demulsification efficiency. The findings provide new insights and technical guidance for the efficient treatment of low-temperature oil-containing wastewater from oil fields, the optimization of on-site processes, and industrial applications.

2. Experimental Section

2.1. Materials

All chemicals used in the experiments were of an analytical grade or higher. Polyethylene glycol (PEG-10000), polyethylene glycol monomethyl ether (Mpeg-750), succinic acid, and octadecylamine were purchased from Beijing Bailing Wei Technology Co., Ltd. p-Toluenesulfonic acid was supplied by Shanghai Shanpu Chemical Co., Ltd. 4-Dimethylaminopyridine (DMAP) was purchased from Aladdin Biochemical Technology Co., Ltd. N,N-Dimethylformamide (DMF) was purchased from Tianjin Zhiyuan Chemical Reagent Co., Ltd.

2.2. Synthesis of PEOSu-C18

Succinic acid (0.295 g, 2.50 mmol), polyethylene glycol monomethyl ether (0.5 g, 0.50 mmol, M n ≈ 1000), polyethylene glycol (22.5 g, 2.25 mmol, M n = 10,000), and p-toluenesulfonic acid (0.28 g, 1.63 mmol) were added to a three-neck flask. The mixture was stirred at 120 °C for 2.5 h, then the temperature was raised to 140 °C, and the reaction continued for a further 1 h to carry out the first-step esterification reaction. After cooling slightly, the mixture was transferred to a vacuum apparatus and subjected to a polycondensation reaction for 5.5 h at 165 °C and 0.085 MPa, yielding the intermediate copolymer PEOSu. The purified intermediate was then dissolved in 15 mL of anhydrous DMF; under a nitrogen atmosphere, octadecylamine (0.143 g, 0.531 mmol) and DMAP (0.04 g, 0.327 mmol) were added, and the mixture was reacted at 80 °C for 6 h. The resulting product was precipitated with ethyl acetate and dried under vacuum at 40 °C for 48 h to yield the final product, PEOSu-C18.

2.3. Preparation of an O/W Emulsion

To reproduce field emulsions stabilized by formation clay particles, we adopted kaolin as the sole stabilizer to construct Pickering model emulsions. A kaolin-stabilized diesel O/W model emulsion was prepared to simulate oilfield-produced water. Briefly, 40 mL of diesel oil and 0.6 g of kaolin powder were mixed with 360 mL of deionized water. The suspension was homogenized via a high-speed shear emulsifier at 10,000 rpm for 30 min. The resultant emulsion remained homogeneous without visible oil–water stratification after standing for 48 h, confirming the high stability for subsequent demulsification tests.

2.4. Demulsification Performance and Oil Removal Quantification

The demulsification performance was evaluated using the static bottle test, with the residual oil concentration determined at a wavelength of 273 nm using a UV–visible spectrophotometer (UV-1780). Oil-removal efficiency and the transmittance of the aqueous phase after treatment were used as quantitative evaluation criteria. Each set of experiments was repeated three times to ensure the reliability of the data. The formula for calculating oil-removal efficiency is as follows

DE=C0−CC0×100%

In the equation, DE (%) represents the demulsification efficiency; C 0 and C represent the initial oil concentration in the original emulsion and the residual oil concentration in the aqueous phase, respectively (mg/L).

2.5. Characterization

The functional groups of PEOSu-C18 were analyzed over the 4000–400 cm–1 spectral range using a Fourier transform infrared spectrometer (FTIR, Bruker INVENIO, Germany). The 1H NMR spectrum of synthesized PEOSu-C18 was recorded using a nuclear magnetic resonance spectrometer (Bruker AVANCE NEO 600), with deuterated chloroform as the solvent and an operating frequency of 400 MHz. The surface morphology and composition of the samples were quantitatively evaluated using a scanning electron microscope (SEM, Hitachi SU8600, Japan) coupled with an energy-dispersive spectrometer (EDS) at an acceleration voltage of 15 kV. The average molecular weight and molecular weight distribution of PEOSu-C18 were determined using gel permeation chromatography (GPC, Hitachi Chromaster), with chromatographically pure tetrahydrofuran as the eluent. The surface tension of the demulsifier solution was measured at room temperature using a surface tension meter (K100, KRUSS, Germany) with the hanging film method. The three-phase contact angle (TCA) was determined using an optical contact angle analyzer (Dataphysics OCA25, Germany). The ζ-potential of PEOSu-C18 was measured using a Malvern laser particle size analyzer (Malvern S3500, U.K.). Finally, the demulsification process was observed using an optical microscope (Olympus CH20, Japan).

3. Results and Discussion

3.1. Structural Characterization

Figure shows the surface morphology and elemental composition of PEOSu and modified PEOSu-C18. Figure (a1–a3),(b1–b3), and (c1–c4) present the characterization results for PEOSu and PEOSu-C18 and the elemental distribution maps of PEOSu-C18. Scanning electron microscopy (SEM) images show that the surface of unmodified PEOSu Figure (a1, a2) is relatively smooth and structurally dense, with a flat block-like morphology and no obvious wrinkles or agglomeration. In contrast, after hydrophobic modification with octadecylamine, the surface morphology of PEOSu-C18 Figure (b1, b2) changed significantly. The surface became rough and uneven, with distinct layered stacking and wrinkled structures. This morphological change results from changes in intermolecular forces after the grafting of hydrophobic long-chain groups, confirming that the modification process significantly affects the material surface structure. The EDS spectra Figure (a3, b3) indicate that unmodified PEOSu contains only three characteristic elements: carbon (C), oxygen (O), and sulfur (S). In contrast, the EDS spectrum of PEOSu-C18 shows distinct nitrogen (N) peaks in addition to these elements. Elemental mapping Figure (c1–c4) further shows that C, O, S, and N are uniformly distributed on the surface of PEOSu-C18. These results confirm that nitrogen-containing hydrophobic octadecylamine long chains were successfully grafted onto the PEOSu backbone and that the target modification reaction was completed.

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Scanning electron microscope images and energy-dispersive X-ray spectroscopy (EDS) maps of PEOSu (a) and PEOSu-C18 (b), as well as elemental distribution maps (c).

Figure shows the Fourier transform infrared (FTIR) and nuclear magnetic resonance (1H NMR) characterization results. Figure (a) compares the FTIR spectra of PEOSu and PEOSu-C18, whereas Figure (b) presents their respective 1H NMR spectra. For PEOSu (red curve), typical −CH2 stretching vibration peaks, ester CO peaks, and polyether chain peaks appeared at ∼2920 cm–1, ∼1730 cm–1, and ∼1100 cm–1, respectively. These signals indicate that the succinic acid-modified polyether backbone was successfully constructed. In the spectrum of PEOSu-C18 (blue curve) after octadecylamine grafting, the aforementioned peaks were retained, and a new N–H stretching vibration peak appeared at ∼3300 cm–1 . In addition, the intensity of the −CH2– peak increased significantly, and the shape of the CO peak changed, confirming that the hydrophobic long chain of octadecylamine was successfully grafted onto the PEOSu molecular chain via amidation and that the target amide-bond structure was formed.

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FTIR spectra (a) and 1H NMR spectra (b) of PEOSu and PEOSu-C18.

Figure 2­(b) presents a comparative analysis of the hydrogen NMR spectra of the intermediate PEOSu and the target demulsifier PEOSu-C18. For the intermediate PEOSu, the strong signal peak b at δ ≈ 3.63 ppm is attributed to the −O–CH2 protons on the polyether backbone; peak a at δ ≈ 3.59 ppm corresponds to the methoxy end-group (H3C–O−); peak c at δ ≈ 2.65 ppm corresponds to the methylene protons of the succinic acid segment (−CO–CH2CH2–CO). After the amide grafting reaction with octadecylamine to form PEOSu-C18, new signals appear and show a marked increase. The strong signal d at δ ≈ 1.27 ppm demonstrates that a large number of fatty-chain methylene groups (−CH2)16 from octadecylamine have been successfully introduced into the polymer molecule. The signal e at δ ≈ 0.87 ppm is contributed by the terminal methyl group (−CH3) of the grafted C18 long alkyl chain. Additionally, the characteristic peak c of the succinic acid segment shifts slightly to δ ≈ 2.59 ppm, confirming the successful amidation. The characteristic peaks of the polyether main chain (peak b at δ ≈ 3.61 ppm) and the methoxy end-group (peak a at δ ≈ 3.59 ppm) remain unchanged, indicating that the backbone structure of the PEOSu parent copolymer remained stable during the reaction.

To verify the changes in molecular weight and molecular weight distribution before and after modification, PEOSu and PEOSu-C18 were characterized by GPC, and the results are shown in the table. The number-average molecular weight (M n) of PEOSu was 2125 g/mol, the weight-average molecular weight (M w) was 3179 g/mol, and the polydispersity index (PDI) was 1.50. After octadecylamine graft modification, the M n of PEOSu-C18 increased to 3519 g/mol, the M w increased to 4385 g/mol, the retention time decreased from 9.80 to 9.68 min, and the peak molecular weight (M p) increased from 4512 g/mol to 5682 g/mol. The significant increase in molecular weight and the decrease in retention time directly confirm that the terminal carboxyl group of PEOSu reacted with the amino group of octadecylamine to form a stable amide bond, thereby effectively extending the molecular chains of the target product. At the same time, the PDI of PEOSu-C18 decreased from 1.50 to 1.24, and the molecular weight distribution became narrower. These results indicate that fewer side reactions and chain scission events occurred during grafting and that the product had good uniformity, providing a basis for the stable emulsion-breaking performance in subsequent tests.

3.2. Demulsification Performance Evaluation of PEOSu-C18

This study systematically investigated the effects of demulsifier dosage, system pH, ambient temperature, and reaction time on demulsification and oil removal from oily wastewater. To guarantee experimental comparability, only one target variable was altered in each test group while all other operational parameters were kept constant. The fixed baseline conditions for each series are specified as follows. Oil-removal efficiency and the transmittance of the treated aqueous phase were used as the main evaluation indicators. The influence of each process parameter was identified, and the optimal operating conditions were determined. Figure a illustrates the effect of demulsifier concentration on demulsification performance. Under fixed conditions of pH = 7, a reaction temperature of 60 °C, and a standing time of 30 min, only the dosage gradient of PEOSu-C18 (10–500 mg/L) was varied; as the dosage increased, the oil-removal efficiency rose gradually and remained at a high level. The transmittance of the aqueous phase also improved markedly with increasing concentration and reached its maximum at 100 mg/L. When the concentration exceeded 100 mg/L, excess demulsifier tended to induce secondary interfacial emulsification and steric hindrance effects, causing a rapid decrease in aqueous-phase transmittance and a slight decrease in oil-removal efficiency. These results indicate that the optimal demulsifier dosage is 100 mg/L. Excessive dosing does not enhance treatment performance; instead, it reduces separation efficiency and increases chemical costs. Figure b illustrates the effect of ambient temperature on demulsification performance. The demulsifier exhibits excellent temperature adaptability over a wide range of 30–70 °C, with oil-removal efficiency consistently maintained above 90%. Under low-temperature conditions, the demulsifier remains highly effective in disrupting the oil–water interfacial film and achieves rapid oil–water separation, with aqueous-phase transmittance approaching 50%. This performance is significantly better than that of many conventional demulsifiers, which often show insufficient activity and a sharp decrease in treatment efficiency at low temperatures. These results indicate that this demulsifier combines excellent low-temperature performance with stability at medium and high temperatures. It is therefore suitable for a broad range of temperature conditions, with an optimal treatment temperature of 60 °C. It should be noted that PEG-based polymers inherently exhibit a cloud point effect under elevated temperature and high salinity. , Within the conditions tested in this study, this effect did not adversely affect demulsification, yet it should be considered for applications under harsher conditions. Figure c illustrates the effect of system pH on demulsification efficiency. Fixed dosage of 100 mg/L, 60 °C, 30 min. Under acidic conditions (pH = 2–6), the amino groups on the PEOSu-C18 molecular chains are fully protonated, giving the demulsifier an overall positive charge. This enables it to efficiently neutralize the negative charges on the surfaces of oil droplets and kaolin particles, resulting in excellent interfacial adsorption capacity; the oil-removal efficiency of the system is consistently maintained at over 90%; In contrast, in a weakly alkaline system at around pH = 8, the low concentration of hydroxide ions in the solution partially inhibits the protonation of the amino groups, while failing to completely deprotonate the carboxyl groups on the main chain. The molecules thus exhibit a weak bivalent state, which significantly weakens the electrostatic driving force for the demulsifier’s migration toward the oil–water interface. When the system is in the strongly alkaline range (pH = 10–12), a large concentration of hydroxide ions promotes the complete deprotonation of the carboxyl groups in PEOSu-C18, causing the molecule’s surface to carry a substantial negative charge. The negatively charged demulsifier can then restructure the interfacial film through electrostatic repulsion and polymer bridging; consequently, as the pH increases, demulsifying activity gradually recovers, and both oil-removal efficiency and light transmittance in the aqueous phase improve simultaneously. The above results indicate that PEOSu-C18 possesses excellent demulsifying capabilities in both acidic and strongly alkaline aqueous systems. Figure d illustrates the effect of reaction time on demulsification efficiency and light transmittance. During the initial reaction stage (5–30 min), oil-removal efficiency and aqueous-phase transmittance increased rapidly in an approximately exponential manner. Within 30 min, oil-removal efficiency reached nearly 100% and aqueous-phase transmittance exceeded 95%, indicating completion of most oil–water separation. Between 60 and 90 min, only a slight increase in aqueous-phase transmittance was observed, ultimately approaching 98%. No significant further performance improvement occurred, indicating that the demulsification process was essentially complete within 30 min. Salt tolerance tests (Figure e) indicate that PEOSu-C18 maintains a high oil- removal efficiency even under high-salt conditions, demonstrating outstanding salt tolerance; comparative tests with the commercial demulsifiers P-84 and L-64 (Figure f) further confirm that the oil–water separation capacity and effluent purification performance of the self-prepared demulsifier are significantly superior to those of the commercially available samples. In summary, the optimal demulsification conditions for the PEOSu-C18 demulsifier developed in this study were a demulsifier dosage of 100 mg/L, an acidic or strongly alkaline environment, a temperature of 60 °C, and a reaction time of 30–60 min. Under these conditions, the demulsifier achieved highly efficient demulsification and oil removal from oily wastewater. The treated aqueous phase has a light transmittance of nearly 98%, demonstrating potential for industrial applications due to its high efficiency, rapid processing, strong salt tolerance, and excellent adaptability to various operating conditions.

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Oil-removal efficiency and aqueous-phase transmittance of PEOSu-C18 under varied conditions: (a) demulsifier concentration; (b) temperature; (c) initial pH; (d) settling time; (e) salinity; and (f) performance comparison with commercial demulsifiers.

3.3. Analysis of Interfacial Properties

3.3.1. Determination of Zeta-Potential

The Zeta-potential of PEOSu-C18 under different pH conditions is shown in Figure . Its surface charge changes systematically with environmental pH. As the pH increases, theZeta-potential gradually decreases. In an acidic environment, basic sites, such as amino groups on the molecular chain, are protonated, resulting in a positively charged surface. A higher H+ concentration increases the degree of protonation and the surface positive charge density, thereby increasing the Zeta-potential. In contrast, under alkaline conditions, acidic sites such as carboxyl groups on the molecular chains are deprotonated, whereas amino-group protonation is inhibited. As a result, the surface gradually becomes negatively charged, and the negative charge density increases. Under acidic conditions, positively charged PEOSu-C18 can neutralize the surface charge of negatively charged oil droplets, weakening the electrostatic repulsion between droplets and promoting droplet coalescence. Under alkaline conditions, the increased negative charge may enhance the interfacial rearrangement and polymer bridging, thereby promoting droplet aggregation. This pH-responsive charge behavior provides a theoretical basis for applying PEOSu-C18 in demulsification systems at different pH values.

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Zeta-potential of PEOSu-C18 at different pH values.

3.3.2. Determination of Interfacial Tension

The dynamic interfacial tension (IFT) of the water-diesel system was measured using the hanging film method at different PEOSu-C18 concentrations, and the mechanism by which the demulsifier acts at the oil–water interface containing kaolin particles was investigated. The results are shown in Figure . As shown in Figure a, the interfacial tension gradually decreases with increasing PEOSu-C18 concentration. In the absence of a demulsifier, the IFT stabilizes at approximately 30 mN/m. When the concentration is increased to 0.04 g/L, the IFT rapidly decreases to approximately 10 mN/m, corresponding to a reduction of more than 65%. The time-dependent IFT shows a “rapid decrease followed by stabilization” pattern. This behavior indicates that PEOSu-C18 molecules can rapidly migrate to the oil–water interface, occupy interfacial vacancies, and form a saturated adsorption layer, ultimately maintaining the IFT at a stable level. Figure b compares the IFT changes in the pure water-diesel system with and without PEOSu-C18. Without a demulsifier, the water-diesel interfacial tension stabilizes at approximately 30 mN/m. After PEOSu-C18 is added, the IFT decreases sharply to approximately 14 mN/m within a short time and remains stable, corresponding to a reduction of more than 50%. This result further verifies the high efficiency of PEOSu-C18 in reducing the oil–water interfacial tension. As illustrated in the mechanism diagram in Figure c, kaolin particles at the original oil–water interface form a dense interfacial film by adsorbing onto the surface of oil droplets, thereby preventing droplet coalescence. After PEOSu-C18 is added to the system, it rapidly migrates to the oil–water interface because of its stronger interfacial adsorption affinity. It displaces kaolin particles and destabilizes the interfacial film, causing the oil droplets to lose their interfacial protection and coalesce. These results indicate that PEOSu-C18 has excellent interfacial migration and interfacial film-breaking capabilities. It can effectively reduce oil–water interfacial tension and displace natural interfacial emulsifiers, thereby providing direct interfacial chemical evidence for its efficient demulsification performance. ,

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IFT images of PEOSu-C18 at different concentrations (a) and during the interfacial substitution process (b), along with a schematic diagram (c).

3.3.3. Determination of the Three-Phase Contact Angle

To investigate the wetting behavior and interfacial adsorption capacity of PEOSu-C18 at the oil–water interface, this study measured changes in the three-phase contact angle before and after modification using kaolin-modified glass slides as the substrate. As shown in Figure , the contact angle of the unmodified kaolin surface was 127.6°, indicating strong hydrophobicity. After treatment with the PEOSu-C18 solution, the surface contact angle decreased significantly to 50.2°, indicating a transition from hydrophobicity to amphiphilicity. These results show that PEOSu-C18 can effectively adsorb onto and modify the kaolin surface. The combined action of its hydrophilic polyether segments and hydrophobic long chains significantly improves surface wettability. This change in wettability indicates that PEOSu-C18 has excellent interfacial activity and can disrupt stable adsorption at the oil-solid interface through wetting displacement, thereby weakening the interfacial film and promoting oil-droplet coalescence. In addition, the contact angle approaching amphiphilic equilibrium suggests that PEOSu-C18 is more readily oriented at the oil–water interface, where it exerts bridging and demulsifying effects. This provides direct interfacial-behavior evidence for its efficient demulsification performance.

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Schematic diagram showing changes in the contact angles of diesel–water mixtures and demulsifiers before and after modification with PEOSu-C18.

4. Demulsification Mechanism

To systematically elucidate how the composite demulsifier breaks O/W emulsions, we propose a mechanistic model based on microscopic observations and interfacial-property analysis. In the initial emulsion, the oil phase is dispersed in the aqueous phase as small droplets. Kaolin particles (negatively charged) are stably adsorbed at the oil–water interface through steric hindrance and weak electrostatic repulsion, forming a dense particle-stabilized interfacial film that provides long-term emulsion stability, as illustrated in Figure (e). This interfacial film suppresses oil-droplet coalescence and maintains the emulsion stability. After PEOSu-C18 is added, it rapidly migrates to the oil–water interface because of its amphiphilic structure (Figure (a)). Because the surface activity of PEOSu-C18 is much higher than that of kaolin particles, PEOSu-C18 displaces the kaolin particles through competitive adsorption. The flexible polyethylene glycol (PEG) chains provide steric hindrance, preventing the kaolin particles from readsorbing. The long hydrophobic octadecylamine tail penetrates deeply into the oil phase, enhancing the molecular anchoring effect at the interface and further strengthening the displacement effect (Figure (b)). Once the interfacial film is disrupted, the PEG chains of PEOSu-C18 swell in water and accelerate the coalescence of small droplets into larger droplets through a bridging effect. In this process, small droplets are captured by PEG chains associated with larger droplets, further promoting demulsification (Figure (c)) and forming larger aggregates. As the flocs grow, the probability of droplet collisions increases significantly, further triggering oil-droplet coalescence (Figure (d)). Small oil droplets are engulfed by and merge with larger droplets, forming oil-phase droplets with larger diameters. As coalescence continues, the large oil droplets rise rapidly under the buoyancy. A continuous aqueous phase ultimately forms at the bottom of the system, with a clear oil phase above, thereby achieving phase separation between the oil and aqueous phases. After demulsification is complete, only trace amounts of small oil droplets remain in the aqueous phase (Figure (b)), providing direct evidence of the excellent demulsification efficiency of the composite demulsifier. This mechanism overcomes the limitations of conventional electrostatic attraction-based demulsification. The synergistic effects of hydrophobic interactions and steric displacement efficiently disrupt the interfacial film of kaolin-containing O/W emulsions, providing a theoretical basis for demulsification of complex emulsion systems.

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Schematic diagram of the Demulsification mechanism of PEOSu‑C18 for kaolin‑stabilized O/W emulsion. (a‑d) Schematic of demulsification process; (e, f) Optical micrographs of emulsion before and after demulsification.mechanism.

5. Conclusions

In response to the high-temperature dependence and prolonged reaction times associated with O/W emulsion demulsification in the petroleum industry, this study designed and synthesized a novel polyether ester demulsifier (PEOSu-C18). Hydrophobic octadecylamine chains and strong anionic groups were introduced onto a PEG backbone, effectively improving molecular interfacial activity and low-temperature responsiveness and reducing the dependence of traditional demulsifiers on high-temperature conditions. PEOSu-C18 achieved highly efficient demulsification of kaolin-containing O/W emulsions. The demulsifier achieved an oil removal rate of >98% and a light transmittance of >95% within 30 min. Furthermore, the demulsification efficiency of PEOSu-C18 showed exceptional tolerance to temperature fluctuations. It remained stable at over 98% within the ambient-to-moderate temperature range of 30–60 °C, and it maintained high-efficiency demulsification even at 30 °C. Comprehensive analysis of interfacial tension, contact angle measurements, and microscopic characterization confirmed that PEOSu-C18 does not rely solely on electrostatic adsorption. Instead, it rapidly migrates to the oil–water interface because of its excellent surface activity and hydrophobicity. Through steric hindrance and competitive adsorption, it effectively displaces kaolin particles and disrupts the stable interfacial film that they form, thereby promoting droplet coalescence and ultimately achieving oil–water separation. Compared with conventional demulsifiers, PEOSu-C18 demonstrates outstanding speed, efficiency, and energy-saving potential. These properties reduce the reliance of traditional processes on high temperatures and long reaction times, directly lowering the energy consumption and time costs associated with industrial demulsification. In summary, PEOSu-C18 is a novel demulsifier that combines technological advancement with economic viability and is expected to promote the upgrading of petroleum-industry demulsification processes toward high efficiency, energy savings, and low cost (Table ).

1. Molecular Weight and Distribution Parameters of PEOSu and PEOSu-C18.

Materials Retention time (min) Peak molecular weight (M p) g/mol number-average molecular weight (M n)g/mol weight-average molecular weight (M w)g/mol Polydispersity index (PDI)
PEOSu 9.80 4512 2125 3179 1.50
PEOSu-C18 9.68 5682 3519 4385 1.24
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Acknowledgments

This work was supported by the Key Research and Development Projects of the Xinjiang Uyghur Autonomous Region (2023B01012-1)

X.Y.: Writing–original draft, methodology, investigation, formal analysis, data curation. J.L.: Writing–review and editing, conceptualization, funding acquisition. R.Y.: Preprocess samples and verify the manuscript format. X.Z.: Investigation, formal analysis X.W.: Methodology, conceptualization. S.Z.: Writing–review and editing. B.C.: Investigation, data curation. W.H.Z.: Project administration, investigation. J.H.: Supervision, methodology, investigation. Z.S.: Funding acquisition.

The authors declare no competing financial interest.

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