Interfacial properties and salt tolerance of carboxylated nonylphenol ethoxylate surfactants

Interaction, Surface Activity, and Application of Mixed Systems of Alcohol Ether Sulfate Anionic Surfactants with Multiple Ethylene Oxide Groups and Gemini Quaternary Ammonium Surfactant

The surface activities and application properties for the mixtures of cationic surfactants tetramethylene-1,4-bis[N,N-bis(hydroxypropyl)-hexa/decyloxypropylammonium] bromide (GC10-P) and tetramethylene-1,4-bis[N,N-bis(hydroxyethyl)-hexa/decyloxypropylammonium] bromide (GC10-E) and anionic surfactant isomeric sodium fatty alcohol ether sulfates (iso-AE9S) were investigated using both the tensiometry and the conductometry. The interaction parameters and thermodynamic micellization parameters of GC10-P/iso-AE9S and GC10-E/iso-AE9S mixtures were evaluated by Clint-Rubingh and Motomura theoretical models. When the mole fraction of α1 for GC10-P/iso-AE9S mixed system was 0.2, the critical micelle concentration (CMC) reached a minimum of 1.61 × 10-4 mol/L, and the minimum critical micelle concentration of the GC10-E/iso-AE9S mixed system is 2.67 × 10-5 mol/L at α1 = 0.6. The CMC value of the mixed system is 1-2 orders of magnitude lower than that of any single component. The results indicate that the synergistic effects of the investigated mixed systems (evaluated by βm) are in order of GC10-P/iso-AE9S < GC10-E/iso-AE9S, with maximum βm values of -17.98 and -9.78, respectively. The change in zeta potential indicates that the poly(ethylene oxide) chain has weakened the charge density of the hydrophilic headgroup of the anionic surfactant. The interfacial tension at the oil-water interface in the mixed system of anionic/cationic surfactants is lower than that of any single component, exhibiting a higher interfacial activity. The mixed system exhibits a decreased contact angle and superior wetting ability over any single component, and it also enhances foam performance, emulsification performance, and degreasing performance.

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.

Dilational Rheology of Extended Surfactants at the Air/Water and Oil/Water Interfaces: Effect of Propylene Oxide Group Numbers

In this paper, the interfacial properties of extended surfactants with different oxypropylene (PO) groups were comprehensively investigated by using interfacial dilational rheology. The differences in molecular orientation, spatial configuration, and relaxation process were compared at the gas-water interface and oil-water interface. The influences of the PO groups on the interface viscoelasticity were analyzed, providing important theoretical support for the wide application of extended surfactants. Experimental results show that the lower number of PO groups in extended surfactants does not cause differences in their presence states on the interface; however, once it increases, the longer PO segment will spiral up in the direction perpendicular to the interface, forming a spatial configuration like a thin cylinder. Compared with air, the PO group has better solubility in the oil phase. The chain segment can still maintain a helical extension from the beginning to the end as a result. However, the upper layer of the thin cylinder will collapse to a certain extent at the surface. Moreover, the orientation of the hydrophobic side has a dynamic process of "tilting to upright" with the increase of adsorption amount or in response to interfacial dilation and compression. The increase of PO number or the insertion of oil molecules has little effect on dilational modulus, and the interfacial film strength is generally relatively low. That is to say, the better emulsifying and solubilizing ability of PO-containing extended surfactants may be more attributed to the matching steric effect at interface or better packing action in bulk phase than to higher film strength.

Binding of Ca2+ Ions to Alkylbenzene Sulfonates: Micelle Formation, Second Critical Concentration and Precipitation

Anionic surfactants, such as sodium linear alkylbenzene sulfonates (NaLAS), are utilized in various fields, including industry, household, and agriculture. The efficiency of their use in aqueous environments is significantly affected by the presence of cations, Ca²⁺ and Mg²⁺ in particular, as they can decrease the concentration of the surfactant due to precipitation. To understand cation-sulfonate interactions better, we study both NaLAS colloidal solutions in the presence of CaCl₂ and precipitates forming at higher salt concentrations. Upon addition of CaCl₂, we find the surface tension and critical micelle concentration of NaLAS to decrease significantly, in line with earlier findings for alkylbenzylsulfonates in the presence of divalent cations. Strikingly, an increase in the surface tension is discernible above 0.6 g L^-1 NaLAS, accompanied by the decrease of apparent micelle sizes, which in turn gives rise to transparent systems. Thus, there appears to be a second critical concentration indicating another micellar equilibrium. Furthermore, the maximum salt tolerance of the surfactant is 0.1 g L^-1 Ca²⁺, above which rapid precipitation occurs yielding sparingly soluble CaLAS₂∙2H₂O.

Salting-Up of Surfactants at the Surface of Saline Water as Detected by Tensiometry and SFG and Supported by Molecular Dynamics Simulation

Surfactant adsorption at the air-water interface is critical to many industrial processes but its dependence on salt ions is still poorly understood. Here, we investigate the adsorption of sodium dodecanoate onto the air-water interface using model saline waters of Li⁺ or Cs⁺ at pH values 8 and 11. Both cations enhance the surfactant adsorption, as expected, but their largest effects on the adsorption also depend on pH. Specifically, surface tension measurements, sum-frequency generation spectroscopy, and microelectrophoresis show that small (hard) Li⁺ enhances the surfactant adsorption more than large (soft) Cs⁺ at pH 11. This effect is fully reversed at pH 8. We argue that this salting-up (increasing adsorption) reversal is attributable to the conversion of the neutralized carboxylic (-COOH) headgroup at pH 8 into the charged carboxylate (-COO^-) headgroup at pH 11, which, respectively, interact with Cs⁺ and Li⁺ favorably. Molecular dynamics simulation shows that the affinity of Cs⁺ to the interface is decreased and eventually overtaken by Li⁺ as the carboxylic groups are deprotonated. This study highlights the importance of the charge and size of salt ions in selecting surfactants and electrolytes for industrial applications.

Molecular dynamics simulation study of adsorption of anionic–nonionic surfactants at oil/water interfaces

Four anionic–nonionic surfactants with the same headgroups and different units of oxygen ethyl (EO) and oxygen propyl (PO) were adopted to investigate the influence on oil/water interfacial tensions in this article. Molecular dynamics (MD) simulations were conducted to study the interfacial property of the four surfactants. Four parameters were proposed to reveal the effecting mechanism of molecular structure on interfacial tension, which included the interfacial thickness, order parameter of the hydrophobic chain, radial distribution function, and the solvent accessible surface area. In addition, the electrostatic potential of the four surfactants was calculated. The research results indicated that the interface facial mask formed by the surfactants, which contained three EO or three PO units was more stable, and it was easier for the surfactants of six EO or six PO units to form a microemulsion at higher concentrations. The adsorption mechanism of the anionic–nonionic surfactant systems at the oil/water interfaces was supplemented at a molecular level, which provided fundamental guidance for an in-depth understanding of the optimal selection of the surfactants in enhancing oil recovery.

Four anionic–nonionic surfactants with the same headgroups and different units of oxygen ethyl (EO) and oxygen propyl (PO) were adopted to investigate the influence on oil/water interfacial tensions in this article.

Effects of Micellization Behavior on the Interfacial Adsorption in Binary Anionic/Nonionic Surfactant Systems: A Molecular Simulation Study

A surfactant interfacial adsorption process is highly associated with its micellization behaviors in the water phase, which is of great fundamental and practical significance in enhanced oil recovery. In this paper, the typical anionic surfactant 1-dodecanesulfonic acid sodium (DAS) and nonionic surfactants octylphenol polyoxyethylene ether-n (OP-n, n = 1, 5, and 10) are introduced to investigate their micellization behavior and interfacial adsorption process via molecular dynamics simulation. Number density profiles reveal that the additional OP5 molecules in the water phase generate the mixed micelle with DAS molecules and greatly promote its interfacial adsorption. Interaction energy calculation is employed to confirm the interaction of anionic/nonionic surfactants in the mixed micelle. Then, the radial distribution function, solvent-accessible surface area, and solvation free energy are calculated to further explore and verify the adsorption mechanism of the mixed micelle. It is found that the nonionic surfactant obviously decreases the hydrophilicity of the mixed micelle in the water phase, which should be responsible for its intensive tendency of the interfacial adsorption.