Molecular insights into the effect of anionic-nonionic and cationic surfactant mixtures on interfacial properties of oil-water interface

Molecular interactions of surfactants with other chemicals in chemical flooding processes: A comprehensive review on molecular dynamics simulation studies

Due to the growing demand for fossil fuels and the transition of many oil fields into a high water-cut stage, enhanced oil recovery (EOR) techniques have become more prevalent to meet this rising demand. Among these techniques, chemical flooding stands out as an effective method, supported by numerous experimental and simulation studies. However, the complexity of a chemical slug composition under harsh reservoir conditions makes the physicochemical phenomena involved in a chemical flooding process highly intricate. To comprehensively understand the microscopic mechanisms governing the phase behavior of complex fluid systems underground, molecular dynamics (MD) simulations have been increasingly employed in recent years to investigate the molecular interactions between various chemicals involved in chemical flooding processes. In this work, we have comprehensively reviewed the recent MD studies focusing on the molecular interactions between surfactants and other chemicals in the chemical flooding processes. Based on the molecular interactions within different chemicals, various nanoscale mechanisms have been proposed to shed light on the physicochemical flow in porous media. Additionally, the MD techniques used in these studies have been summarized, and challenges in the application of MD simulations in the field of chemical flooding have been identified for improving the quality of future MD studies.

Biosurfactant-based dispersants for oil spill remediation: Salinity effects and mechanistic insights

Salinity variations, particularly in dynamic environments such as estuaries, may significantly affect the dispersion effectiveness (DE) of oil spill dispersants. While bio-based dispersants are gaining attention as alternatives to chemical dispersants, limited research exists on their adaptability to varying salinity conditions and their associated molecular mechanism and ecological impact. This study evaluated the DE of bio-based dispersants formulated with two biosurfactants (BS); Surfactin (SUC) and Rhamnolipid (RAM), either individually or in combination with Tween 80 (TWE), referred to as BS/TWE, across salinity levels of 10, 20, and 34 psu. The DE of each bio-based dispersant was compared to the chemical dispersant Corexit 9500A under various environmental conditions, including dispersant-to-oil ratios, temperature variations, and mixing energy levels simulating turbulence in natural aquatic environments. The SUC-based dispersant achieved high DE (88 %) with smallest oil droplets size around 5.08 μm at 10 psu but exhibited reduced performance at 34 psu. In contrast, the BS/TWE dispersant showed 90 % DE with droplets size of 10.45 μm at 34 psu, as a result of synergistic surfactant interactions. Molecular dynamics simulation revealed that salinity affects surfactant-water interactions, with SUC-based dispersant losing efficiency at high salinity due to lack of ion bridging, while BS/TWE dispersant remaining effective through reduced electrostatic interactions. Toxicity assessments exhibited minimal inhibitory effects of bio-based dispersants on algal growth, Dunaliella tertiolecta, highlighting their potential for environmental applications. The findings highlight the potential of these dispersants as effective, and environmentally friendly solutions for oil spill response in diverse marine environments.

Molecular Dynamics Simulation of the Effects of Anionic-Nonionic Surfactants on Interfacial Properties of the Oil-Water Interface

Surfactant oil drive is a crucially enhanced oil recovery method that improves oil recovery rates. The aggregation behavior of surfactant molecules at the oil-water interface significantly influences oil repulsion. In this study, a molecular dynamics simulation is used to investigate this repellent behavior of single and binary surfactants of alkanolamides (6501) and fatty alcohol polyoxyethylene ether sodium sulfate (AES). The oil-water interface is characterized by density distribution, interfacial thickness, radial distribution function, interfacial tension, and interfacial generation energy. The results reveal that the dodecanolamide surfactant (126501) and AES effectively reduce interfacial tension. In the binary 126501/AES system, the interfacial film thickness increases to 18.08 Å, and the diffusion coefficient increases to 0.186 Å2/ps. The radial distribution function shows that oil molecules are located 4.2 Å from the anionic head of AES, which weakens the intermolecular forces within the oil layer. In the 126501/AES system, the interfacial energy of -96.12 kJ/mol indicates a stable interface. Moreover, both the 126501/AES and tetradecanolamide surfactant (146501)/AES systems exhibit excellent resistance to metal ions. The molecular-level mechanism provides useful guidance for designing the surfactant systems for enhanced oil recovery.

Molecular dynamics simulation of synergistic behavior at the air-water interface: mixed cationic-anionic fluorocarbon-hydrocarbon surfactants

Ecological concerns surrounding conventional aqueous film-forming foam extinguishing agents, predominantly composed of long-chain fluorocarbon surfactants, have spurred the need for innovation in eco-compatible substitutes, such as short-chain fluorocarbon surfactants. Molecular dynamics simulations are a valuable tool for studying the behavior of mixed surfactant systems at the air-water interface. We have conducted molecular dynamics simulations to investigate the interfacial behavior of a mixed cationic-anionic surfactant system, including N-[3-(dimethylamino)propyl]perfluorobutanesulfonamide hydrochloride (PFB-MC) and 1-octanesulfonic acid (1-OA). The simulations explored the effects of varying PFB-MC and 1-OA ratios on aggregation and adsorption. The results indicate that the equimolar 1 : 1 ratio produced more compact aggregates at the interface and achieved the most effective reduction in surface tension and the formation of a dense interfacial film. The study highlights the competitive adsorption phenomena between surfactants and counterions at the interface, providing insights through 1D and 2D density analyses into the impact of counterbalancing ions on aggregation. An increased PFB-MC concentration resulted in decreased hydrogen bonding with water, while 1-OA showed a higher tendency for hydrogen bonding, underscoring its hydrophilicity. These findings provide valuable insights into surfactant interfacial behavior and are instrumental in the development of advanced foam extinguishing agents suitable for environmental and industrial use.

Hydrophilic Chain Length for Octylphenol Polyoxyethylene Ether Adsorption at the n-Hexadecane-Water Interface: Theoretical and Experimental Study

With the advancement of technologies for developing tight and shale reservoirs, nonionic surfactants have garnered significant attention due to their remarkable properties. The structure of these surfactants plays a crucial role in determining the characteristics of the oil-water interface, particularly influencing emulsification behavior and crude oil recovery. This study investigates the effect of varying the number of hydrophilic polar groups (n = 10, 20, 30, 50) in octylphenol polyoxyethylene ether (OP-n) on its adsorption behavior at the n-hexadecane-water interface using molecular dynamics simulation. The impact of these variations on interfacial properties was further analyzed through measurements of interfacial tension and observations of emulsion droplet morphology. The study results indicate that variations in the number of hydrophilic polar groups significantly affect interfacial properties. Increasing the number of hydrophilic polar groups led to a notable increase in the thickness of the n-hexadecane or water phase, as well as the thickness of the water or oil layer and the surfactant layer. Moreover, when the number of hydrophilic polar groups reached 20, the OP-n molecules exhibited a more curled conformation at the interface, enhancing their ability to encapsulate water and resulting in a decrease in the diffusion coefficient of the molecules in each phase. Additionally, interfacial tension was found to be positively correlated with the number of hydrophilic polar groups and remained unchanged beyond a certain emulsion diameter. This study provides a theoretical basis and reference data for optimizing surfactant structures to improve crude oil recovery.

Impact of the Hydrophobic Phase on the Interfacial Dilational Rheology of Alkoxy Carboxylate/Cetyltrimethyl Ammonium Chloride Mixtures

Despite extensive investigations on the interfacial activities of mixed anionic and cationic surfactants (Sa/c), the influence of the hydrophobic phase on their interfacial assembly and dilational rheology remains unaddressed. In this study, the interfacial dilational rheology of alkoxy carboxylate (anionic)/cetyltrimethylammonium chloride (cationic) surfactant mixtures was studied at various interfaces. The dilational modulus of Sa/c increases linearly with interfacial pressure at the interfaces of air, n-hexane/n-octane/n-hexadecane, and toluene. The limit elasticity (ε0) is similar at air and alkane interfaces but significantly decreases at the toluene interface. To explain these phenomena, all-atom molecular simulation was carried out to investigate the microscopic features of surfactants at the interface. The findings emphasize the crucial role of anionic/cationic surfactant bound pairs in regulating interfacial rheology. Sa/c tend to form large aggregates at the air/water surface. When mixed with alkanes like octane, most Sa/c remain as ion pairs. However, when toluene is employed, the coordination number between anionic and cationic surfactants sharply decreases due to π-π interactions between the toluene molecules and the phenyl groups in the anionic surfactant. This leads to a much lower interfacial modulus because interactions between oil molecules and surfactants cannot compensate for weakened interactions among anionic/cationic surfactants. These results suggest that Sa/c in this study tolerate alkanes but are not resistant to aromatics, which helps explain why Sa/c demonstrate excellent performance for the chemical enhanced oil recovery of a high-wax reservoir and further provides fundamental knowledge of their potential applications, such as gas well deliquification using foamers in the presence of condensate oil, textiles, etc.

Surfactant-Based Chemical Washing to Remediate Oil-Contaminated Soil: The State of Knowledge

As the energy demand increases, there is a significant expansion and utilization of oil resources, resulting in the inevitable occurrence of environmental pollution. Oil has been identified as a prevalent soil contaminant, posing substantial risks to the soil ecosystems. The remediation of soil contaminated with oil is a formidable undertaking. Increasing evidence shows that chemical washing, a remediation technique employing chemical reagents like surfactants to augment the solubilization, desorption, and separation of petroleum hydrocarbons in soil, proves to be an efficacious approach, but the latest advances on this topic have not been systematically reviewed. Here, we present the state of knowledge about the surfactant-based chemical washing to remediate oil-contaminated soil. Using the latest data, the present article systematically summarizes the advancements on ex situ chemical washing of oil pollution and provides a concise summary of the underlying principles. The use of various surfactants in chemical washing and the factors influencing remediation efficiency are highlighted. Based on the current research status and knowledge gaps, future perspectives are proposed to facilitate chemical washing of oil-polluted soil. This review can help recognize the application of chemical washing in the remediation of oil-polluted soil.

Effects of residual foaming agent and defoamer on defoaming-flocculation-filterpress characteristics of earth pressure balance shield muck

Foaming agents as a combination of several components are usually used as soil conditioning during earth pressure balance shield (EPBS) tunnelling. These residues in waste EPBS muck lead to a series of new challenges for in-situ recycling, i.e., foams overflow flocculation tank. This study investigates the effects of residual foaming agent components and defoamers on defoaming-flocculation-filterpress characteristics of EPBS muck using an improved flocculation and filterpress system. Residual foam height (Hf), defoaming ratio (DFR), antifoaming ratio (AFR), total suspended substance (TSS), turbidity, moisture content (MC), and zeta potential (ZP) were selected as characterization indices. The microstructure of filterpress cakes was analyzed using a scanning electron microscope. Results demonstrate that an enhancement within 0.0-1.0wt.% for sodium fatty alcohol polyoxyethylene ether sulfate (AES) and alpha olefin sulfonate (AOS) significantly reduces DFR and AFR. The MC and ZP decline, while the Hf and turbidity enhance. The combinations of nonionic surfactants alkyl polyglycoside (APG) and fatty alcohol-polyoxyethylene ether (AEO) in a concentration range of 0.0-1.0wt.% with 0.2wt.% AES causes the Hf, DFR, AFR, turbidity, and ZP to exhibit absolutely different variations. The MC with the growth in both APG and AEO presents a trend of first decreasing and then increasing. By increasing foam stabilizers sodium carboxymethyl cellulose (CMC) and guar gum (GG) within 0.02-0.10wt.%, the AFR, TSS, and ZP enhance in varying degrees, while the Hf, DFR, and MC gradually reduce. With the increase of defoamers hydroxyl silicone oil-glycerol polyoxypropylene ether (H-G) and dimethyl silicone oil-glycerol polyoxypropylene ether (D-G) within 0.002-0.010wt.%, the DFR and AFR are significantly improved, while the TSS, turbidity, MC, and ZP display varying degrees of reduction. Moreover, defoaming-flocculation-filterpress mechanisms of EPBS muck are explored to provide a useful reference for actual in-situ recycling projects.

Assessment of spilled oil dispersion affected by dispersant: Characteristic, stability, and related mechanism

Oil dispersion, a crucial process in oil transport, involves the detachment of oil droplets from slicks and their introduction into the water column, influencing subsequent oil migration and transformation. This study examines oil dispersion, considering characteristics, stability, and mechanisms, while evaluating the impact of dispersants and salinity. Results show the significant role of surfactant type in dispersants on oil dispersion characteristics, with anionic surfactants exhibiting higher sensitivity to salinity changes compared to nonionic surfactants. The dispersion efficiency varies with salinity, with anionic surfactants performing better in low salinity (<20‰) and nonionic surfactants showing superior performance at 30-35‰ salinities. Rheological analysis illustrates the breakup and coalescence of oil droplets within the shear rates of breaking waves. An increase in interfacial film rigidity impedes the coalescence of oil droplets, contributing to the dynamic stability of the oil-water hybrid system. The use of GM-2, a nonionic dispersant, results in the formation of a solid-like interface, characterized by increased elastic modulus, notably at 20‰ salinity. However, stable droplet size distribution (DSD) at 35‰ salinity for 60 h suggests droplets can remain dispersed in seawater. The enhancement of stability of oil dispersion is interpreted as the result of two mechanisms: stabilizing DSD and developing the strength of viscoelastic interfacial film. These findings offer insights into oil dispersion dynamics, highlighting the importance of surfactant selection and salinity in governing dispersion behavior, and elucidating mechanisms underlying dispersion stability.

Mechanism Analysis and Property Prediction of Extended Surfactants Based on the Respectively Optimized Force Field

Extended surfactants represent a novel class of anionic-nonionic surfactants with exceptional performance and unique application value in chemically enhanced oil recovery. Although molecular dynamics (MD) simulations can efficiently screen these surfactants, the current research is limited. Here, it is proven for the first time that existing generic force fields (GAFF and CHARMM) cannot accurately describe extended surfactants, and traditional approaches are insufficient for obtaining precise charge parameters. The concept of the respectively optimized force field (ROFF) with the purports of specialization and accuracy is proposed to construct high-accuracy models for MD simulations, and a new approach is developed to simulate the interface model. By combining the newly specialized alkane model, ROFF-based surfactant models, and the innovative simulation protocol, high accuracy and reliability can be obtained in predicting hydration free energies, minimum of area per molecule, and critical micelle concentration of extended surfactants. Key properties of the newly designed extended surfactants in conventional oil-water interfaces and oil reservoir environments are comprehensively predicted by using advanced analytical and characterization methods. Furthermore, the more rigorous mechanism underlying the special amphiphilicity of the extended surfactant is revealed, potentially offering significant improvements over previous empirical perspectives.

Study on the Behavior of Saturated Cardanol-Based Surfactants at the Crude Oil/Water Interface through Molecular Dynamics Simulations

Cardanol is a green biosurfactant with broad application prospects, which is expected to be used to enhance oil recovery (EOR). This paper designed two types of surfactants (extended and nonextended), including six kinds of nonionic and anion-nonionic surfactants. The position changes of PO and EO chains and the effects of different hydrophilic groups on the interface properties were studied with molecular dynamics simulations by constructing a model of crude oil (containing four components) and water molecules. The results of interfacial tension and solvent-accessible surface area showed that the interfacial properties of sulfate were better than those of sulfonates and nonionic surfactants. Meanwhile, the interface properties of nonextended surfactants were better than those of extended surfactants. The gyration radius (Rg) and tilt angle data demonstrated that when EO chains were located between hydrophobic groups and PO chains (nonextended surfactants), the adsorption capacity of surfactants at crude oil and water interfaces could be effectively improved. The radial distribution function of the hydrophilic group and hydrophobic group of surfactants with water molecules and four components of the crude oil molecule, respectively, explained that surfactants (8EO8POSO4) had better emulsification performance when the intermolecular interactions between crude oil and water two phases were relatively balanced. This study provides a theoretical reference for the design of oil-displacement surfactants and the mechanism analysis of emulsification properties.

Molecular Dynamics Simulation of the Effects of Complex Surfactants on Oil-Water Interaction and Aggregation Characteristics at the Interface

In response to the problem of complex interaction between oil and water in the oil-water interface, especially heavy oil and water, this study investigated the effects of complex surfactants on the interaction of two phases and their aggregation characteristics by molecular dynamics simulation. The results showed that increasing the content of sodium lauryl polyether carboxylate (AEC-9Na) was beneficial to the coordination between it and alkyl glycoside (APG-10), improved the interfacial activity, and enhanced the interfacial stability of the composite system, and the best effect was achieved when AEC-9Na:APG-10 = 8:2. The thickness of the oil and water film on the oil-water interface was irregular. When the concentration of AEC-9Na was lower than that of APG-10, the total thickness of the interfacial film (ttotal) first increased. When the content of AEC-9Na is higher, a large number of sodium ions were adsorbed near the -COO- group of AEC-9Na, which will polarize out of the hydration layer structure and attract water molecules from the second hydration layer on the heavy oil surface to the first hydration layer through electrostatic interaction. Then, the thickness of the interface film was compressed, and the interface film was reduced. When the ratio increased to 10:0, the oil and water phase competed to adsorb surfactant molecules, and the headgroup tended to lay on the interface. Moreover, the hydrophilicity of the surfactant layer was weakened, and the thickness of the water film decreased. The distribution of surfactant was looser than 8:2, the light components of heavy oil molecules (saturated and aromatic hydrocarbons) entered the gap between surfactants in large quantities, and the hydrophobic tail chain tended to be laid on the oil-water interface. The oleophilicity of the surfactant layer increased, and the thickness of the oil film remarkably increased, so the total thickness of the interface film increased again.

A Review on Interactions between Amino Acids and Surfactants as Well as Their Impact on Corrosion Inhibition

Amino acid-surfactant interactions are central to numerous studies because of their increased effectiveness in chemical, biological, household and industrial use. This review will focus on the impact and effect of the physicochemical properties, temperature, pH, and surfactant chain length of the amino acid for detailed exploration of amino acids and surfactants in aqueous medium. The impact of cosolvent on self-aggregation, critical micelle concentration (CMC), and binding affinity with other biomolecules, as well as amino acid-surfactant interactions, are the epicenters. The results show that increasing the temperature causes negative enthalpy for ionic surfactants and micellization, implying that micellization and amino acids are thermodynamically spontaneous and exothermic, accompanied by positive entropy. As these physicochemical studies are additive, the amino acid and ionic surfactant interactions provide clues on protein unfolding and denaturation under different media, which further changes with a change in physiological conditions like pH, cosolvent, chain length, and temperature. On varying the pH, the net charge of the amino acid also changes and, subsequently, the binding efficiency of the amino acids to the surfactants. The presence of cosolvent causes a lowering in the hydrophobic chain, which changes the surfactant's CMC. At a reduced CMC, the hydrophobic characteristic of amino acid-surfactant associations is amplified, leading to rapid denaturation of proteins that act as propulsion under the influence of extended chain surfactants. Amino acids are one of the most intriguing classes of chemicals that produce high inhibitory efficacy. Amino acids are also a component of proteins and therefore, found in a significant part of the human body, further making them a promising candidate as corrosion inhibitors. In this review article, authors have also focused on the collection and investigation for application of amino acid-surfactant interactions in corrosion inhibition. Various predictive studies/in silico studies are also reported by many research groups, such as density functional theory (DFT) calculations and molecular dynamics simulations to obtain tentative electronic, structural, and physiochemical characteristics like energies of the highest occupied molecular orbitals and lowest unoccupied molecular orbitals, binding energy, Gibb's free energy, electronegativity, polarizability, and entropy. In silico studies are helpful for the mechanism predictions of the process occurring on metal surfaces.