Enhanced Viscosity Reduction of Heavy Oil by a Synthesized Extended Anionic Surfactant: Mechanistic Exploration and Performance Evaluation

Abstract

The exploitation and transportation of heavy oil are severely hindered by its high viscosity. In this study, an extended anionic surfactant PE-54 was synthesized via sulfuric acid esterification to improve the flowability of heavy oil. The molecular structure of the surfactant (PE-54) was confirmed by Fourier transform infrared, ¹H nuclear magnetic resonance (NMR) and ¹³C NMR, elemental analysis, and gel permeation chromatography. The critical micelle concentration of PE-54 was determined to be 1.02 mmol/L. The effects of surfactant dosage, oil–water ratio, and salinity on viscosity reduction were systematically investigated. Under optimal conditions (salinity of 10,023 mg/L, temperature of 50 °C, oil/water ratio of 6:4, and dosage of 1500 mg/L, the viscosity reduction rate reached 94%). The viscosity reduction mechanism was further elucidated through particle size analysis, Zeta potential measurements, interfacial tension tests, rheological characterization, and molecular dynamics simulations (MD). PE-54 molecules rapidly adsorbed at the oil–water interface, competitively interacting with asphaltenes and resins. The surfactant formed a stable interfacial configuration, hindering the aggregation of heavy components and enhancing the interfacial stability. These synergistic effects effectively improved emulsion stability and reduced viscosity, providing theoretical and practical guidance for the development of heavy oil viscosity reduction technologies.

1. Introduction

Heavy oil, defined by its high viscosity (≥100 mPa s), density (>0.934 g/cm³), and asphaltene/resin content, is an important global energy resource estimated to account for over 50% of total hydrocarbon reserves. , However, its exploitation faces significant challenges due to inherent reservoir heterogeneity and extreme fluid properties. Under reservoir conditions, heavy oil exhibits high flow resistance, resulting in low primary recovery rates (<20%) and complex transportation/refining processes. − These technical barriers require innovative viscosity reduction strategies to improve the mobility and economic viability.

Several methods have been explored for enhancing flowability of heavy oil, including thermal techniques, dilution with light hydrocarbons, oxidation, and electromagnetic field (EMF) interventions. Although heating and dilution are effective in reducing viscosity, these methods often involve high energy consumption or solvent loss, which can increase operational costs. Oxidation methods involving metal oxide nanoparticles have shown potential in altering the molecular structure of asphaltenes and resins, thus improving the oil flow. However, these catalytic methods usually require high temperatures and specialized equipment. Similarly, EMF treatments have demonstrated the ability to modify the rheological properties of heavy oils by altering the molecular alignment. However, their application remains limited due to the need for complex setups and energy input.

In contrast, surfactant-based emulsification offers a milder, tunable, and cost-effective strategy, particularly suitable for field deployment, and has emerged as a promising technique for viscosity reduction. By forming stable oil-in-water emulsions, surfactants reduce interfacial tension (IFT) and transform interdroplet friction into lubricating water films, thereby decreasing apparent viscosity. , Conventional surfactants, however, encounter critical limitations in harsh reservoir environments. Nonionic surfactants lose efficacy at elevated temperatures due to weakened hydrogen bonding, , while anionic surfactants demonstrate poor salt tolerance. Cationic surfactants suffer from adsorption losses on negatively charged rock surfaces, , and anionic–nonionic surfactants remain constrained by synthesis complexity and cost. , These drawbacks highlight the need for advanced surfactant systems capable of maintaining stability under high salinities and temperature. Table lists the viscosity-reducing effects of different surfactants under different experimental conditions, highlighting their properties, limitations, and applicable environmental conditions.

1. Advances in Surfactant Research.

surfactant	type	surfactant concentration (mg/L)	oil viscosity (mPa·s)	salinity (mg/L)	oil–water ratio	viscosity reduction (%)	reference
AFOP-10	Anionic–nonionic	5000	3500 (50 °C)	8246	7:3	98.5
APG 1214 + LAS-30	nonionic + Anionic	3000	446 (70 °C)	10,607	3:7	97.9
SDS	anionic	10,000	3072 (60 °C)	0	5:5	86.79
SYW	anionic–nonionic	1500	1500 (50 °C)	2876	5:5	97.3
U12a	cationic	2000	27.55 (50 °C)		7:3	81.41
APVR	ampholytic	2000	1198 (60 °C)	9754	5:5	93.1

Extended surfactants, featuring ethylene oxide (EO) and propylene oxide (PO) units between hydrophilic heads and hydrophobic tails, address these challenges through gradient polarity transitions. , Their unique molecular architecture enhances interfacial activity and salt resistance, enabling ultralow IFT formation even in high-divalent cation environments. , Previous studies have emphasized the impact of EO/PO ratios on IFT, , yet the emulsification mechanisms of extended surfactants in heavy oil systems remain poorly understood. This knowledge gap limits their practical application in field-scale operations.

This study investigates the emulsification behavior of a novel sulfated extended surfactant, PE-54, designed to overcome the traditional surfactant limitations. PE-54 demonstrates robust interfacial activity and salinity resistance, achieving significant viscosity reduction under high mineralization and varying oil–water ratios. Through integrated experimental and computational methods, we characterize emulsification dynamics, droplet stabilization mechanisms, and interfacial adsorption behavior. The results provide critical insights into surfactant design for heavy oil recovery, offering a foundation for optimizing viscosity reduction strategies under challenging reservoir conditions.

2. Materials and Methods

2.1. Materials

Ethyl acetate (AR), 1,2-dichloroethane (AR), and chlorosulfonic acid (CP) were purchased from Shanghai Taitan Co., Ltd. Isooctyl polyoxypropylene–polyoxyethylene ether (90.0 wt %), sodium hydroxide (AR), ethanol (AR), petroleum ether (AR), sodium bicarbonate (99.5 wt %), calcium chloride (99.9 wt %), magnesium chloride (99.0 wt %), sodium chloride (99.0 wt %), and sodium dodecylbenzenesulfonate (SDBS, 95.0 wt %) were supplied by Shanghai Macklin Biochemical Co., Ltd. The heavy crude oil was sourced from an oilfield in Xinjiang, China. Its main physicochemical properties are summarized in Table .

2. Basic Properties of Crude Oil.

density (50 °C) (g·cm–3)	viscosity (50 °C) (mPa·s)	resin (wt %)	asphaltene (wt %)	saturates, aromatics (wt %)
0.93	1500	19.35	7.06	73.59

2.2. Surfactant Synthesis

Octyl polyoxypropylene–polyoxyethylene ether was vacuum-dried at 60 °C for 24 h. A 10 g sample was dissolved in 1,2-dichloroethane (1:1 w/w) in a 250 mL four-necked flask equipped with a mechanical stirrer, thermometer, and gas absorption apparatus. The mixture was cooled to 0 °C in an ice–water bath, and chlorosulfonic acid (2.50 g, 1.2 mol equiv) was added dropwise over 30 min. After completion of the sulfonation reaction (monitored by FTIR), the product was neutralized to pH 8 with a 1 M sodium hydroxide–ethanol solution. Unreacted intermediates were removed by column chromatography using ethyl acetate/petroleum ether (9:1, v/v) to give the surfactant C8PmEnSO₃Na (designated PE-54). The synthetic route is shown in Figure .

1.

PE-54 synthesis route.

2.3. Surfactant Characterization

Elemental analysis (C, H, S, and O) was performed using an elemental analyzer (UNICUBE, Germany), with oxygen measured in the O mode. Molecular weight and dispersity were determined by gel germination chromatography (GPC, Agilent RIDG1362A, USA). Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet iS20 spectrometer (Thermo Fisher Scientific, USA) in the range of 4000–400 cm^–1. ¹H and ¹³C nuclear magnetic resonance (NMR) spectra were acquired on a JEOL JNM-ECZ600R/S1 spectrometer (Japan) using CDCl₃ as the solvent.

2.4. Surface Tensions Measurement

The surface tension of the PE-54 solution was measured at 25 °C using a JK99B tensiometer (Shanghai Zhongchen Co., Ltd.). A 20 mL surfactant solution was added to a glass container, maintained at 25 °C. After equilibration, a platinum ring was immersed in the solution and pulled out, and the maximum surface tension was recorded. This process was repeated five times, and the average value was taken.

2.5. Emulsification and Viscosity Reduction Testing

2.5.1. Emulsion Preparation

Simulated formation water (10,023 mg/L TDS, CaCl₂-type) was formulated according to Table . A 1 wt % PE-54 solution was prepared by dissolving the surfactant in the simulated water. Heavy oil and the aqueous phase were preheated to 70 °C for 30 min. Emulsions were prepared at varying oil/water ratios using a TRE-200 homogenizer (5000 rpm, 10 min).

3. Ionic Composition of Simulated Reservoir Water (mg/L).

Cl–	HCO3 –	Na+	Ca2+	Mg2+	TDS
5863	341	3510	220	89	10,023

2.5.2. Viscosity Measurement

The viscosity of emulsions was measured using an NDJ-8ST rotational viscometer (Shanghai Yitian Scientific Instrument Co., Ltd.) at 50 °C. The viscosity reduction rate was calculated using eq :

$$f=\frac{{\mu }_0-\mu }{{\mu }_0}\times 100\%$$ 1

where μ₀ is the original oil viscosity and μ is the emulsion viscosity, mPa·s.

2.5.3. Emulsion Stability

50 mL of the prepared emulsion was transferred into a graduated test tube and maintained at a constant temperature under emulsification conditions. At set time intervals, the volume of water separated at the bottom (V _t) was recorded. The water separation rate is calculated as follows: A higher rate indicates a poorer emulsion stability.

$$\phi =\frac{V_t}{V_0}\times 100\%$$ 2

where φ is the water separation rate (%), V _t is the volume of precipitated water (mL), and V ₀ is the total water volume (mL).

2.6. Mechanistic Analysis

2.6.1. Droplet Size Distribution

Emulsion morphology was observed via a Nikon E200 POL optical microscope (Japan), and droplet size distribution was analyzed using ImageJ software.

2.6.2. Zeta Potential

The zeta potential of oil droplet surfaces was measured at 25 °C with a JS94H electrophoretic analyzer (Shanghai Zhongchen Co., Ltd.).

2.6.3. IFT

The IFT between surfactant solution and heavy oil was determined using a Kino-TX500 drop shape tensiometer (USA) at 5000 rpm and 50 ± 0.5 °C.

2.7. Rheological Properties

Rheological behavior was evaluated by using an Anton Paar MCR302 rotational rheometer with a coaxial cylinder system.

2.8. Molecular Dynamics Simulations

Molecular dynamics (MD) simulations were conducted using Materials Studio 2020 with a simulation box of 6 × 6 × 6 nm, containing oil-phase components (saturates, aromatics, resins, and asphaltenes) and water, and the model establishment method is detailed in Supporting Information 1, with its specific composition outlined in Tables S1 and S2. The system was annealed five times in the NVE ensemble from 300 to 800 K. After annealing, the system was equilibrated in the NPT ensemble at 323 K and 1 atm for 500 ps to obtain the appropriate density. Finally, the simulation was performed in the NVT ensemble with a time step of 1 fs and a simulation duration of 4000 ps, using a Nose thermostat and the COMPASS III force field.

To calculate the binding energy between PE-54 and colloid/asphalt, a bimolecular/trimolecular system was constructed under vacuum conditions, and molecular simulation steps identical to those described above were performed. The calculation formula is as follows:

$$\mathrm{\Delta }{E}_{\mathrm{int}}=\mathrm{\Delta }{E}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}-\sum _{i-1}^n\mathrm{\Delta }{E}_{\mathrm{m}\mathrm{o}\mathrm{n},i}$$ 3

where ΔE _total represents the total energy, while ΔE _mon,i represents the intermolecular interaction energy.

3. Results and Discussion

3.1. Structural Characterization

Elemental analysis (Table ) confirmed the successful synthesis of PE-54, with carbon (53.04%) and oxygen (33.41%) as the dominant elements, consistent with the presence of sulfonic acid groups and alkoxylate chains. The sulfur content (4.51%) directly reflects the introduction of hydrophilic sulfonate moieties during sulfation. GPC revealed a broad molecular weight distribution (M _n = 790, PDI = 1.2949), attributed to variable alkoxylate chain lengths, which is critical for surfactant performance in complex heavy oil systems.

4. PE-54 Elemental Composition.

C (%)	H (%)	S (%)	O (%)
53.04	9.05	4.51	33.41

Figure displays the FT-IR spectra of PE-54. The strong absorption at 1100 cm^–1 corresponds to asymmetric stretching of C–O–C ether bonds in polyether segments, while peaks at 2960 and 2861 cm^–1 denote methyl and methylene vibrations in the hydrophobic backbone. The sulfonation reaction was confirmed by the appearance of a C–O–S peak at 1249 cm^–1 and an SO stretch at 778 cm^–1. NMR analysis (Figure ) confirmed the composition of the alkoxylate chain, with calculated PO/EO ratios (p = 5, e = 4) in agreement with the GPC results (details of the calculation procedure are provided in Supporting Information 2). The deduced molecular formula is C₃₁H₆₃O₁₃NaS (Figure ).

2.

FT-IR spectra of PE-54.

3.

¹³C NMR (a) and ¹H NMR (b) spectra of PE-54.

4.

PE-54 molecular formula.

3.2. Surfactant Properties

Surface tension measurements (Figure ) demonstrated the superior interfacial activity of PE-54. The critical micelle concentration (CMC) was determined to be 1.02 mmol/L, with a corresponding γ_CMC of 30.10 mN/m, significantly lower than SDBS (CMC = 2.90 mmol/L and γ_CMC = 42.10 mN/m). Table lists the surface properties of PE-54 and SDBS, indicating the ability of PE-54 to efficiently reduce the surface energy at lower concentrations, which is a key advantage for emulsification. Detailed descriptions of the calculations and procedures used are given in Supporting Information 3, and the corresponding surface tension is the critical surface tension (γ_CMC).

5.

γ-lgc curves of PE-54 solutions at 25 °C.

5. Surface Properties of PE-54 and SDBS.

	CMC (mmol/L)	γCMC (mN/m)	Γm (10–6/(mol·m–2))	A m (Å2)
SDBS	2.44	42.10	3.44	48.29
PE-54	1.02	30.10	1.02	162.86

3.3. Measurement of Emulsification and Viscosity Reduction Properties of Heavy Oil

3.3.1. Concentration Effect

PE-54 outperformed SDBS in terms of the viscosity reduction efficiency (Figure ). At 1500 mg/L, PE-54 achieved 94% viscosity reduction, exceeding the 91.63% achieved by SDBS at 3500 mg/L. The viscosities are shown in Figure S2. This difference is due to the rapid adsorption of PE-54 at the oil–water interface, replacing asphaltenes/resins and forming a stable interfacial film. The sharp drop in viscosity at low concentrations reflects efficient IFT reduction, while the plateau at higher concentrations indicates interfacial saturation.

6.

Effect of PE-54 concentration on the viscosity reduction rate of heavy oil emulsion.

The variation in the water separation rate with time at different PE-54 dosages is observed in Figure . The results showed that the water separation rate decreased significantly as the surfactant dosage increased. At 1500 mg/L, the rate was 60.0% after 180 min, while at 2500 mg/L, it remained below 5%. This decrease indicates improved emulsion stability, which is attributed to a denser and more stable interfacial film formed at higher surfactant concentrations, preventing oil droplet reaggregation and emulsion breakage.

7.

Effect of PE-54 concentration on the water separation rate of heavy oil emulsion.

3.3.2. Salt Tolerance

PE-54 maintained more than 94% viscosity reduction even at 10,023 mg/L TDS (Figure ), demonstrating its salt resistance. The gradient polarity transition in its structure stabilizes interfacial adsorption at high salinity, counteracting surfactant aggregation. This structural advantage contributes to the improved emulsion stability and viscosity reduction performance. This is in contrast to conventional anionic surfactants, which often precipitate in divalent cation-rich brines.

8.

Effect of mineralization on the viscosity reduction rate of heavy oil emulsions.

3.3.3. Broad Applicability

PE-54 demonstrated universal efficacy in heavy oils with viscosities ranging from 500 to 5500 mPa·s (Figure ). At 1500 mg/L, the viscosity reduction exceeded 90% for all heavy oils tested, indicating adaptability to different crude oils. This is attributed to its superior interfacial activity and balanced hydrophilic–lipophilic balance (HLB) value (37.1) and the ability to stabilize the O/W emulsions even in high viscosity oil. The HLB value of PE-54 was estimated using the group contribution method as described in Supporting Information 5.

9.

Effect of PE-54 concentration on emulsification efficiency across four heavy oils.

Table shows the viscosity and asphalt content of different petroleum products. This suggests that PE-54 is effective across a wide range of viscosities and remains functional in the presence of asphaltenes. Its amphiphilic structure facilitates the dispersion of asphaltene aggregates, thereby disrupting internal associations and reducing the viscosity. In addition, asphaltenes themselves may participate in interfacial assembly due to their surface activity, further stabilizing the O/W emulsions. This dual contribution supports the broad applicability of PE-54 in asphaltene-rich crude oil systems.

6. Asphaltene Content of Heavy Oils.

	oil-1	oil-2	oil-3	oil-4
viscosity (mPa·s)	370	1750	3800	5500
asphaltne/wt %	3.36	7.06	10.02	15.36

3.3.4. Oil–Water Ratio

Figure shows the profound effect of the oil/water ratio on emulsion rheology and highlights the critical role of PE-54 in enabling phase inversion and viscosity reduction. In the absence of surfactant (blank system), emulsions have a W/O structure and are stabilized by asphaltenes and resins. As the water content increases, the volume fraction of dispersed droplets increases, reducing the interdroplet spacing and promoting close packing or weak network formation. This structural evolution leads to a significant increase in viscosity, which peaks at 40% water content due to the increased internal friction between the closely packed droplets. When the water content exceeds 40%, the system gradually changes to an O/W structure, and the viscosity drops sharply as the flow resistance decreases. In stark contrast, the addition of PE-54 fundamentally alters this behavior. PE-54 promotes phase inversion to a stable O/W emulsion at water contents >30%. This is attributed to the high hydrophilic–lipophilic balance (HLB = 37.1) of PE-54 and its preferential adsorption at the oil–water interface, which competitively displaces asphaltenes/resins and stabilizes the O/W configuration. These results highlight the unique ability of PE-54 to overcome the inherent limitations of native surfactants, driving phase inversion at lower water fractions and enabling superior viscosity reduction over a wide range of oil–water ratios.

10.

Effect of water content on viscosity reduction of heavy oil emulsion before and after additive (a) PE-54 and (b) blank.

3.4. Viscosity Reduction Mechanism of Heavy Oil Emulsification

3.4.1. Particle Size Distribution

Figure shows the droplet morphology and the size distribution analysis. Sample A displayed a typical water-in-oil (W/O) structure with an average droplet diameter (d _p) of 40.78 μm. In contrast, the addition of PE-54 induced phase inversion to an oil-in-water (O/W) structure in Sample B, reducing d _p to 18.06 μm with a narrower size distribution (10–50 μm), indicating improved uniformity and stability. The high hydrophilic–lipophilic balance (HLB = 37.1) of PE-54 promotes phase inversion and stabilizes interfacial films, effectively reducing droplet size and improving emulsion structural stability.

11.

Micrographs and particle size distribution of emulsions in different systems ((A): blank and (B): 1500 mg/L PE-54).

3.4.2. Zeta Potential

Figure shows the effect of PE-54 on the Zeta potential of the oil droplets in the emulsion. Without the surfactant, the Zeta potential of the oil droplets is −45.98 mV, mainly attributed to the natural surface-active components in crude oil. As the concentration of PE-54 increases, the absolute value of the Zeta potential gradually rises, reaching −62.88 mV at 1500 mg/L, indicating a significant increase in surface negative charge density. This change is attributed to the effective adsorption of PE-54 molecules onto the oil droplet surface, which enhances the surface charge density. The increase in charge strengthens the electrostatic repulsion between droplets, inhibits coalescence, and thereby improves the stability of the emulsion.

12.

Effect of PE-54 on the Zeta potential of oil and water.

3.4.3. Interfacial Tension

Figure shows the effect of PE-54 concentration on the oil–water IFT. At a TDS of 10,023 mg/L, without the PE-54 surfactant, the IFT was 5.66 mN/m. As the concentration of PE-54 increased, the IFT rapidly decreased to 0.238 mN/m at 1500 mg/L, a reduction of 95.80%, indicating excellent interfacial activity. As an anionic surfactant, PE-54 molecules adsorb directionally at the oil–water interface, reducing the interfacial energy. As the concentration increases, more molecules occupy the interface, further reducing the IFT. Figure also shows that at a PE-54 concentration of 1500 mg/L, the IFT gradually decreases with increasing TDS. Increasing TDS compresses the electrical bilayer and reduces electrostatic repulsion between surfactant molecules, allowing tighter molecular packing. The salt-shielding effect contributes to a lower IFT. ,

13.

Effect of PE-54 concentration on the oil–water IFT.

14.

Effect of TDS on the oil–water IFT.

3.4.4. Rheological Properties

3.4.4.1. Temperature Scanning

Figure shows the variation in viscosity of different systems with the temperature. The experimental results show that both the heavy oil and the surfactant-free emulsion are highly sensitive to temperature changes, with viscosity decreasing significantly as temperature increases. This behavior is primarily attributed to the fact that the continuous phase in both systems is oil. The increase in temperature destroys the network structure formed by asphaltenes and resin in heavy oils, resulting in a decrease in viscosity. In contrast, the emulsion system containing PE-54 has water as the continuous phase, and its viscosity shows only minor changes with increasing temperature. This suggests that the PE-54 stabilized system has better thermal stability and can maintain relatively stable rheological properties over a range of temperatures.

15.

Viscosity–temperature curves of different systems.

3.4.4.2. Frequency Scanning

Figure shows the response of storage modulus (G′) and loss modulus (G″) to oscillation frequency (0.1–180 rad/s, γ = 5%). The PE-54 system maintained low G′ and G″, indicating weak viscoelasticity and loose structural organization. Conversely, crude oil and surfactant-free emulsions showed G″ > G′ (difference >2 orders of magnitude), dominated by viscous behavior with poor elasticity and deformation resistance.

16.

Effect of frequency on G′, G″.

3.4.4.3. Shear Rate

Figure shows shear rate (0.1–1000 s^–1) effects on apparent viscosity. The surfactant-free emulsion exhibited a 98.38% viscosity reduction, indicating strong shear-thinning pseudoplastic behavior. This suggests that the system exhibits typical characteristics of a non-Newtonian pseudoplastic fluid with flow properties highly sensitive to shear rate. The PE-54-stabilized emulsion showed Newtonian-like viscosity stability, reflecting its continuous water phase and robust structural integrity under shear.

17.

(a) Effect of shear rate on oil–water emulsion; (b) Rheological fitting of no surfactant emulsion systems.

To quantitatively characterize the non-Newtonian behavior of surfactant-free emulsions, a segmented fitting approach was applied. The Carreau model was used for the low-shear-rate region (<100^–1) to describe initial high viscosity and shear sensitivity, while the Cross model fitted the high-shear-rate region (>100^–1), reflecting viscosity stabilization and shear thinning. These models are commonly used for rheological analysis of emulsion-like fluids. Detailed fitting procedures, model parameters, and goodness-of-fit data are provided in Supporting Information 6.

3.4.5. MD Simulations

As shown in Figure S3, during the 4000 ps simulation process, the system energy remained stable with minimal fluctuations, indicating that the system had reached equilibrium and that the simulation results converged well.

3.4.5.1. Density Distribution

Figure shows the dynamic distribution of surfactants, resins, and asphaltenes at the oil–water interface during the simulation. In the PE-54 system, surfactant molecules rapidly adsorb to the interface within 1000 ps, forming an initial interfacial layer. As the simulation progresses to 2500 ps, some resins and asphaltenes gradually migrate to the interface and coadsorb with PE-54 to form a multicomponent interfacial structure. This behavior indicates strong interfacial affinity and synergistic effects that significantly improve interfacial stability. Figure shows the relative concentration distributions of resins and asphaltenes at 0 and 4000 ps in both the PE-54 and surfactant-free systems. At 4000 ps, it is evident that some resins and asphaltenes have migrated to the oil–water interface. In comparison to the nonasphaltic system, the movement trend of the asphaltic material remains consistent. The presence of PE-54 effectively promotes the migration of these heavy components to the interface while inhibiting their accumulation in the oil phase. This demonstrates that the surfactant plays a critical role in improving the dispersion of heavy components and stabilizing the interface, ultimately improving the overall stability of the emulsion.

18.

MD simulation snapshots of blank systems (a–d), PE-54 (a′–d′), and asphalt-free systems ((a″–d″) (blue: PE-54, green: Resin, and red: Asphalt).

19.

Concentration distribution of resin and asphaltene at 0 ps (a-c) and 4000 ps (a′–c′).

Figure (a–a′, b–b′) shows the relative concentration distribution of some representative components of the PE-54 surfactant at 0 and 4000 ps. The hydrophobic C₈ chains are mainly located in the oil phase and at the interface, while the sulfonate (SO₃ ^–) headgroups penetrate into the aqueous phase, and the EO/PO segments partially align along the interface. Figure a″, b″ visually demonstrates this “stepped” trans-interfacial configuration, indicating that the PE-54 molecules are ordered at the interface, which facilitates the reduction of IFT and enhances the interaction with the recombinant components. Additionally, we observe that the relative concentration distribution of representative PE-54 molecular components in the deasphalted system aligns with the molecular conformation in the PE-54 system. This indicates that under the current modeling conditions, variations in asphaltene content exert limited influence on the interfacial behavior of PE-54.

20.

Relative concentration distribution of representative surfactant components in the PE-54 system (a–a″) and the nonasphaltene system (b–b′). Orientation distribution configuration at the oil–water interface.

3.4.5.2. Radial Distribution Function

Figure shows the radial distribution function (RDF) between asphaltene/asphaltene, asphaltene/resin, and resin/resin. In the surfactant-free system, the first RDF peak reaches 22.33 at a position of 3.55 Å, indicating short-range aggregation via π–π stacking. The addition of PE-54 reduces the peak intensity to 19.26 and shifts the peak to 4.85 Å, reflecting weakened aggregation due to competitive surfactant adsorption and steric hindrance by hydrophobic chains.

21.

RDF of atoms in asphaltic resin centers.

To investigate the interfacial behavior of PE-54, the RDF between its sulfonate group (SO₃ ^–) and hydrogen atoms in water was calculated. As shown in Figure , a sharp peak at 1.38 Å with a g(r) of 9.64 indicates strong hydrogen bonding. These interactions enhance the hydrophilicity and interfacial adsorption of the surfactant, promoting molecular alignment at the oil–water interface, reducing IFT, and facilitating emulsion formation. The resulting hydration layer also prevents oil droplet coalescence, improving the emulsion stability.

22.

RDF of H in water and SO₃ ^–of PE-54.

3.4.5.3. Interaction Energy

To further investigate the molecular mechanism of emulsification, we calculated the interaction energies between PE-54 and representative asphaltene and resin molecules were calculated. Resin 2, which exhibited the strongest individual binding with PE-54, was selected for the mixed system. The results (Table ) show that PE-54 exhibits a stronger affinity for asphaltenes, indicating a clear preference for adsorption. This preferential binding helps to disrupt asphaltene aggregation, weaken internal cohesion, and facilitate the formation and stabilization of the interfacial film, thereby enhancing both the viscosity reduction and emulsification performance.

7. PE-54 Adhesive/Asphalt Bonding Energy.

model combination	ΔE int (kcal/mol)
PE-Asphalt 1	–92.39
PE-Asphalt 2	–74.15
PE-Resin 1	–48.60
PE-Resin 2	–69.81
PE-Resin 3	–47.2
PE-2*Asphalt	–152.39
PE-Asphalt 1-Resin 2	–136.45

3.4.5.4. Limitation

While MD simulation provides valuable molecular-level insights, it has inherent limitations. The 4000 ps simulation time may not capture long-term behaviors in complex emulsions. The small simulation box (<10 nm) cannot reflect large-scale phenomena like droplet aggregation or phase separation. In addition, real crude oil contains thousands of components, but simulations typically use simplified models and omit heteroatoms (e.g., V, Ni), possibly underestimating asphaltene–surfactant interactions.

3.4.6. Mechanism

Figure illustrates the mechanism of PE-54 emulsion stabilization. PE-54 molecules rapidly adsorb at the oil–water interface, competing with asphaltenes and resins to reduce IFT and induce W/O to O/W phase inversion, thereby improving fluidity. At the interface, PE-54 molecules adopt a stable “step-like” adsorption configuration in which the hydrophobic chains are embedded in the oil phase, the hydrophilic headgroups extend into the aqueous phase, and the intermediate polyether chains align along the interface, forming a stable interfacial film. The SO₃ ^– groups form hydrogen bonds with hydrogen atoms in water, enhancing the molecule’s affinity for the interface and maintaining the integrity of the interfacial film, thereby preventing PE-54 from desorbing. Molecular simulations revealed that PE-54 exhibits a stronger affinity toward asphaltenes, and this preferential adsorption contributes to disrupting aggregates and enhancing interfacial stability, thereby improving emulsification and viscosity reduction. At the same time, the enhanced zeta potential due to PE-54 adsorption increases the electrostatic repulsion between droplets, suppressing coalescence and improving stability.

23.

Mechanism study on viscosity reduction of PE-54.

4. Conclusion

An extended surfactant (PE-54, C₃₁H₆₃O₁₃NaS) with a CMC of 1.02 mmol/L at 25 °C was synthesized and characterized. Under optimized conditions, PE-54 achieved 94% viscosity reduction in heavy oil emulsions with >90% efficiency in various crude oils. PE-54 reduced the oil–water IFT by 95.80% to 0.238 mN/m and increased the zeta potential by −62.88 mV, enhancing electrostatic repulsion. PE-54 induced the heavy crude oil emulsion from water-in-oil (W/O) to oil-in-water (O/W) and converted the shear-thinning pseudoplastic flow to Newtonian behavior, significantly reducing the apparent viscosity, viscous modulus (G″), and elastic modulus (G′). Mechanistic investigations revealed that PE-54 rapidly adsorbs at the oil–water interface and effectively disrupts asphaltene and resin aggregates through competitive adsorption and steric hindrance. Binding energy calculations demonstrated stronger interactions between PE-54 and asphaltenes than with resins, supporting its preferential interfacial adsorption and its role in aggregate dispersion. Additionally, RDF analysis identified strong hydrogen bonding between sulfonate headgroups and water molecules, reinforcing interfacial film stability. RDF results also indicated suppressed π–π stacking among aromatic species, suggesting reduced interfacial energy and improved emulsion robustness. These combined interfacial and molecular-scale effects underline the broad applicability of PE-54 in heavy oil emulsification and viscosity reduction.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c03136.

• Construction of the simulation system and molecular composition in MD simulation; molecular structure characterization; CMC; viscosity of PE-54 and SDBS at different additive doses; HLB; non-Newtonian fluid fitting model; and MD simulation system energy evolution trajectory diagram (PDF)

The authors declare no competing financial interest.