Interfacial Tension Behavior Research Articles

Radial basis function modeling approach to prognosticate the interfacial tension CO2/Aquifer Brine

Geological carbon sequestration (GCS) is a process in which anthropogenic CO2 is stored by capturing and injecting CO2 into deep saline aquifers. In this process, interfacial tension (IFT) between CO2 and aquifer brine is an important parameter that greatly affects the efficiency of process. Literature review shows that there are various reported trends in IFT behavior between brine and CO2 at elevated temperatures and pressures. This discrepancy in reports is due to different experimental conditions of reported works. The purpose of this study is to develop a computer-based modeling algorithm to predict the IFT between CO2 and brine at different temperatures, pressures, and brine salinities. Comparison between outcomes of the developed model and experimental data proved the accuracy and robustness of the developed intelligent model.

A hard sphere fluid with quantum correction in nanospherical pores: A DFT study

In this paper, the density functional theory is employed, drawing upon the effective diameter method, to investigate the effects of quantum correction on the structure and such thermodynamic properties of quantum-mechanical hard sphere fluids confined in nano-scale spherical cavities as excess adsorption, interfacial tension, solvation force, and local normal pressure. Results indicate that quantum correction performed on the modified fundamental measure theory yields accurate results for hard sphere fluids. It is also found that the repulsion between quantum-mechanical hard sphere molecules before they come into contact play an important role in determining the structure and thermodynamic properties of the confined fluid. The periodic oscillation in the density profile becomes more obvious and the order of layers in the pore increases as a result of increased quantum effect, λ⁎. Increasing the values of λ⁎ is found to lead not only to increased excess adsorption of the fluid confined in pores but also to drastic changes in the structure of the fluid molecules, thereby increasing the interfacial tension. Another finding of the present study involves the failure of the quantum correction to change either the sign or the qualitative behavior of interfacial tension and excess adsorption with curvature. In the quantum mechanical fluid, pressure arises from collisions without direct contact, that is, from inter-particle repulsions. It is observed that the quantum-mechanical repulsions become great, the periodic oscillation in the pressure profile becomes more obvious, and pressure grows larger at extremely low temperatures and high densities. Investigation of the quantum mechanical molecules escaping repulsion revealed their greater tendency for accumulation at the pore wall; hence, the higher normal pressure and wall pressure at the pore wall.

Low salinity water–Surfactant–CO 2 EOR

Coreflood, interfacial tension (IFT), contact angle, and phase behavior measurements were performed to investigate the viability of a hybrid of low-salinity water, surfactant, and CO2 flood enhanced oil recovery (EOR) process. Low-permeability carbonate reservoir cores were aged for eight weeks at reservoir temperature and pressure. Coreflood and contact angle between oil droplets and core surface measurements were performed. Additional contact angle measurements on sandstone and shale cores were also performed. The coreflood sequences were seawater flood, followed by low-salinity water flood, followed by surfactant floods until residual oil saturations to each flooding sequences and finally CO2 injection. Coreflood in low-permeability carbonate cores show that the hybrid EOR process produces incremental oil up to twenty-five percent beyond seawater flooding. Contact angle measurements on carbonate, sandstone and shale cores indicate that wettability alteration and IFT decrease are the main oil-mobilizing mechanisms in the hybrid EOR process. The hybrid EOR process mobilizes part of the residual oil because: (i) low-salinity brine improves wettability towards hydrophilic condition favorable for surfactant flooding; (ii) surfactant in low-salinity water solubilizes some of the remaining oil as Winsor type II− microemulsion and lowers IFT between oil and water; and (iii) CO2 will follow surfactant to mobilize more of the remaining oil in the wettability-improved condition.

Experimental and modeling investigation of dynamic interfacial tension of asphaltenic–acidic crude oil/aqueous phase containing different ions

Abstract In this way, after experimental measurement of interfacial tension, different models including mono-exponential decay, dynamic adsorption models and empirical equation are used to correlate this time-dependent behavior of interfacial tension (IFT). During the modeling approach, the induction, adsorption, equilibrium, and meso-equilibrium times as well as diffusivity of surface active components known as natural surfactant including asphaltene and resin from crude oil to the interface are obtained. In addition, the surface excess concentration of surface active components at the interface and Gibbs adsorption isotherm are utilized to analyze the measured dynamic IFTs. Finally, the mechanisms of crude oil/aqueous solution IFT including (a) the activity of surface-active components and (b) surface excess concentration of them at fluid/fluid interface are proposed and discussed in details.

Modeling of Pressure Dependence of Interfacial Tension Behaviors of Supercritical CO2 + Crude Oil Systems Using a Basic Parachor Expression

Parachor based expressions (basic and mechanistic) are often used to model the experimentally observed pressure dependence of interfacial tension (IFT) behaviors of complex supercritical carbon dioxide (sc-CO2) and crude oil mixtures at elevated temperatures. However, such modeling requires various input data (e.g. compositions and densities of the equilibrium liquid and vapor phases, and molecular weights and diffusion coefficients for various components present in the system). In the absence of measured data, often phase behavior packages are used for obtaining these input data for performing calculations. Very few researchers have used experimentally measured input data for performing parachor based modeling of the experimental IFT behaviors of sc-CO2 and crude oil systems that are of particular interest to CO2 injection in porous media based enhanced oil recovery (EOR) operations.This study presents the results of parachor based modeling performed to predict pressure dependence of IFT behaviors of a complex sc-CO2 and crude oil system for which experimentally measured data is available in public domain. Though parachor model based on calculated IFT behaviors shows significant deviation from the measured behaviors in high IFT region, difference between the calculated and the experimental behaviors appears to vanish in low IFT region. These observations suggest that basic parachor expression based calculated IFT behaviors in low IFT region follow the experimental IFT behaviors more closely.An analysis of published studies (basic and mechanistic parachor expressions based on modeling of pressure dependence of IFT behaviors of both standard and complex sc-CO2 and crude oil systems) and the results of this study reinforce the need of better description of gas-oil interactions for robust modeling of pressure dependence of IFT behavior of these complex systems.

Experimental assessment of a lysine derivative surfactant for enhanced oil recovery in carbonate rocks: Mechanistic and core displacement analysis

Abstract Enhanced oil recovery (EOR) from carbonate reservoirs is of great challenge due to the complex geology that originated from high rock heterogeneity and viscous fingering phenomenon during a displacement process. One of the highly applicable EOR approaches is the utilization of surfactant especially for the aims of foam generation, wettability alteration, emulsion stability and well stimulation in petroleum industry. The dominant mechanisms of surfactant for increasing oil production are wettability alteration of reservoir rock and interfacial tension (IFT) reduction of oil-water system resulting in higher sweep efficiency by diminishing the adverse capillary forces existing in the porous media and easier flow of the residual oil toward the producing wells. In the present study, (S) 2-amino-6-dodecanamidohexanoic acid as an amino acid-based surfactant, is proposed in order to evaluate an EOR application. Firstly, the pH, density and viscosity of the surfactant solutions were measured regarding the impact of salt concentration. Afterwards, IFT and wettability tests were conducted by means of pendent drop and sessile drop methods, respectively. The impact of salt concentration was also examined on the IFT behavior. Moreover, the recovery potential of the proposed amino acid-based surfactant via core displacement test was compared with a traditional brine injection process. Consequently, it is found out that increasing salt concentration has an increasing effect on both IFT trend and the values of critical micelle concentration. Additionally, change in carbonate rock wettability from oil-wet to neutral-wet was also observed. Finally, it is proved that surfactant flooding can improve the oil recovery factor more than the time at which water injection is used.

Main Controlling Factor of Polymer-Surfactant Flooding to Improve Recovery in Heterogeneous Reservoir

This study aims to analyze the influence of viscosity and interfacial tension (IFT) on the recovery in heterogeneous reservoir and determines the main controlling factors of the polymer-surfactant (SP) flooding. The influence of the salinity and shearing action on the polymer viscosity and effects of the surfactant concentration on the IFT and emulsion behavior between chemical agent and oil were studied through the static and flooding experiments. The results show that increasing the concentration of polymer GF-11 (HPAM) can reduce the influence of the salinity and GF-11 has high shear-resistance property. In the condition of the Jilin Oilfield, the oil/water IFT can reach 10−3 mN/m when the surfactant concentration is 0.3 wt%. The lower the IFT is, the easier the emulsion of SP and oil is formed. Seven flooding experiments are conducted with the SP system. The results show that the recovery can be improved for 5.02%–15.98% under the synergistic effect of the polymer and surfactant. In the heterogeneous reservoir, the contribution of oil recovery is less than that of the sweep volume.

How to Attain Ultralow Interfacial Tension and Three‐Phase Behavior with a Surfactant Formulation for Enhanced Oil Recovery: a Review—Part 3. Practical Procedures to Optimize the Laboratory Research According to the Current State of the Art in Surfactant Mixing

AbstractThe minimum interfacial tension to be reached in enhanced oil recovery by surfactant flooding implies the attainment of a so‐called optimum formulation. Part 1 of the present review showed that this formulation may be described as a numerical correlation between the involved variables defining the oil, the water, the surfactant and the temperature. Since it is unlikely to find a single surfactant matching the crude/brine/T/P system characteristic of a reservoir, a mixture of at least two surfactant species is always used. The scan technique method to test the mixing requires about ten interfacial tension or phase behavior experiments and results in a single data. Hence, the scan experiments have to be repeated many times to find a minimum tension which is low enough, e.g. 0.001 mN/m, for the given crude oil‐brine system. Part 2 of this review has shown that there are many formulation variables and thus too many possibilities to easily choose experimental conditions. Since there is no simple method to select two or more surfactant species, the choice is made from partial experience or intuition, and sometimes at random. The laboratory time and cost to reach an appropriate optimum formulation is often excessive. Part 3 of this review shows that by cleverly using a three‐surfactant mixture, the experimental work to attain a very low interfacial tension for a given reservoir case can be considerably reduced. It is a matter of using the available information along a proper sequential step by step path toward the optimum.

Effect of Emulsion Breakers on Interfacial Tension Behavior of Heavy Oil–Water Systems

Bottle testing for emulsion breakers (EBs) in steam-assisted gravity drainage (SAGD) operations is a standard practice; however, it has been observed that, in some cases, EBs recommended via bottle tests can fail during field trials as a result of conditions being disparate, such as higher temperatures and pressures in the field that cannot be safely or easily replicated in the lab. To make better EB recommendations, an optimized methodology based on interfacial rheology measurements was developed. Compressibility tests performed at different temperatures (80–120 °C) were used to compare effect of EBs on parameters such as initial compressibility, initial interfacial tension, phase transition ratio, and drop crumpling or detachment ratio (ratio of the compressed surface area to the original surface area at which the interface starts to collapse or at which the drop detaches). Furthermore, the addition of the EB to the oil phase versus aqueous phase was investigated. The effects of EBs at the oil/brine int...

The comparison study of IFT and consumption behaviors between organic alkali and inorganic alkali

Alkali plays an important role in alkali/surfactant/polymer (ASP) technology. The mechanisms of alkali were reviewed in this paper and the advantages were significant. In this study, three main steps were conducted. Firstly, ethanolamine was selected from four organic alkalies (ethanolamine, diethanolamine, triethanolamine and triethylamine) by interfacial tension (IFT) tests. Then the selected organic alkali, ethanolamine, was compared with inorganic alkali (sodium carbonate) by IFT performance. The results showed that ethanolamine had low IFT like inorganic alkali and it has lower IFT than sodium carbonate when concentration was higher than 0.8%. Secondly, alkali consumption experiment was conducted to compare organic alkali and inorganic alkali. The results indicated that ethanolamine had lower consumption than inorganic alkali, which maintained the efficiency of ASP flooding. Finally, the concentrations of silicon and aluminum in solution were measured to evaluate the reservoir damage caused by alkali. The results found that ethanolamine system had the lower concentrations of silicon and aluminum than inorganic alkali, which means it caused less reservoir damage. Thus, ethanolamine may be a potential alkali which can replace inorganic alkali in ASP technology.

Synthesis and evaluation of physicochemical properties of anionic polymeric surfactant derived from Jatropha oil for application in enhanced oil recovery

In this study an attention has been paid to synthesize polymer grafted anionic surfactant (PMES) derived from nonedible vegetable oil (Jatropha) for its application in enhanced oil recovery (EOR). The polymeric surfactant was prepared by reacting acrylamide monomer with methyl ester sulfonate (MES) synthesized from Jatropha oil by free radical polymerization mechanism. Polymeric surfactant with properties of both surfactant and polymer can control the mobility ratio and reduce interfacial tension (IFT), which are desirable for EOR. The synthesized polymeric surfactant was characterized through FTIR, 1H NMR, FESEM, EDX, TGA, DLS analysis. The effectiveness of PMES for chemically enhanced oil recovery process was investigated by measurement of physiochemical properties of its aqueous solution viz. reduction of IFT, wettability alteration and rheological behaviour. Rheological studies shows shear thinning behaviour with apparent viscosity comparable to conventional polymers. The IFT between crude oil and aqueous PMES solution at its critical micelle concentration (CMC) was observed as 2.74mN/m, which was further reduced to 0.37mN/m on addition of 2.5wt% NaCl. Core flooding experiments were conducted in sandpack system, to study the EOR efficiency using the synthesized polymeric surfactant and more than 26% additional recovery was observed after usual water flooding. Higher recoveries at higher temperature were observed because of swelling of crude oil and lowering of IFT.

Wettability modification, interfacial tension and adsorption characteristics of a new surfactant: Implications for enhanced oil recovery

This paper concerns with the interfacial tension (IFT), wettability modification and adsorption behavior of a new plant-based surface active agent, Zizyphus Spina Christi, onto sandstone minerals which has been rarely attended in the available literature. Both kinetics and equilibrium adsorption data were obtained from batch mode tests. It was revealed that Freundlich isotherms matched better fit to the equilibrium data which implied that multilayer coverage of Zizyphus Spina Christi onto the sandstone particle surfaces was more likely to occur. Analysis of experimental kinetic data based on intraparticle diffusion model disclosed that the intraparticle diffusion mechanism is not the only rate limiting step and the boundary layer diffusion or surface adsorption also contributes to the rate-controlling step. The reservoir rock wettability modification was evaluated on sandstone-oil phase−water system by change of surfactant concentration, time and solution salinity. The results of this work help toward a clearer understanding the impact of various influencing parameters on the efficiency of a surfactant flooding project.

Effect of Different Surfactants on the Interfacial Behavior of the n-Hexane-Water System in the Presence of Silica Nanoparticles.

This paper presents the effect of negatively charged silica nanoparticles (NPs) on the interfacial tension of the n-hexane-water system at variable concentrations of four different surfactants, viz., an anionic surfactant, sodium dodecyl sulfate (SDS), a cationic surfactant, cetyltrimethylammonium bromide (CTAB), and two nonionic surfactants, Tween 20 and Triton X-100 (TX-100). The presence of negatively charged silica nanoparticles is found to have a different effect depending on the type of surfactant. In the case of ionic surfactants, SDS and CTAB, silica NPs reduce the interfacial tension of the system. On the contrary, for nonionic surfactants, Tween 20 and TX-100, silica NPs increase the interfacial tension. The increasing/decreasing nature of the interfacial tension in the presence of NPs is well supported by the calculated surface excess concentrations. The diffusion kinetic control (DKC) and statistical rate theory (SRT) models are used to understand the behavior of dynamic interfacial tension of the surfactant-NP-oil-water system. The DKC model is found to describe the studied surfactant-NP-oil-water systems more aptly.

Effects of the confinement on wall pressure, interfacial tension, and excess adsorption at the nanocylindrical wall

In this article, a hard sphere fluid confined between two homocentric cylinders which formed a bicylindrical pore has been investigated. Our aim is investigation of the effect of an outer cylinder on the values of wall pressure, interfacial tension, and excess adsorption of fluid at a convex nanocylindrical wall, inner wall. To investigate this effect, the modified fundamental measure theory (MFMT) used. To do so at first we presented a general solution for weighted density integrals in cylindrical coordinate which is applicable for infinite and infinite lengths of cylindrical pore, bicylindrical pore, cylindrical wall, and even truncated cone. In the second step, the wall pressures, interfacial tensions, and excess adsorptions at a convex nanocylindrical wall are obtained for confined fluids in bicylindrical pores and compared with those values for bulk fluids. Our results showed that confinement leads to an oscillatory behavior for wall pressure, interfacial tension, and excess adsorption of the fluid at the wall. The reason for these oscillations lies in structural changes that occur for a fluid as a result of confinement. Variation of the type of interaction of inner wall with fluid from hard to attractive one can reverse the behavior of interfacial tension and adsorption versus size. Also in some cases, it changes their signs while the change in the type of interaction of the outer wall only leads to an increase in amplitudes of the oscillations.

Influence of spray drying and dispersing agent on surface and dissolution properties of griseofulvin micro and nanocrystals

The purpose for the current research is to compare and evaluate physiochemical properties of spray-dried (SD) microcrystals (MCs), nanocrystals (NCs), and nanocrystals with a dispersion agent (NCm) from a poorly soluble compound. The characterization was carried out by performing size and surface analysis, interfacial tension (at particle moisture interface), and in-vitro drug dissolution rate experiments. Nanosuspensions were prepared by media milling and were spray-dried. The SD powders that were obtained were characterized morphologically using scanning electron microscopy (SEM), polarized light microscopy (PLM), and Flowchem. Solid-state characterization was performed using X-ray powder diffraction (XRPD), Fourier transfer infrared spectroscopy (FT-IR), and differential scanning calorimetry (DSC) for the identification of the crystalline nature of all the SD powders. The powders were characterized for their redispersion tendency in the water and in pH 1.2. Significant differences in redispersion were noted for both the NCs in both dissolution media. The interfacial tension for particle moisture interface was determined by applying the BET (Braunauer–Emmett–Teller) equation to the vapor sorption data. No significant reduction in the interfacial tension was observed between MCs and NCs; however, a significant reduction in the interfacial tension was observed for NCm at both 25 °C and 35 °C temperatures. The difference in interfacial tension and redispersion behavior can be attributed to a difference in the wetting tendency for all the SD powders. The dissolution studies were carried out under sink and under non-sink conditions. The non-sink dissolution approach was found suitable for quantification of the dissolution rate enhancement, and also for providing the rank order to the SD formulations.

Effects of a tiny sphere inserted in the center of a nano-spherical pore on the interfacial tension and surface adsorption of fluids confined in nanospherical pores

The density functional theory is used to investigate the effects of a tiny sphere (TS) inserted in the center of a nanospherical pore (SP) on the interfacial tension and surface adsorption behavior of confined fluids. Results show that the size and type (hard or attractive) of the inserted TS play important roles in the fluid interfacial tension. The perturbation created in the structure of the confined fluid due to the insertion of a hard TS is not capable of changing the positive sign of the interfacial tension although it does give rise to changes in the interfacial tension and surface adsorption behavior of the pore depending on its size. If, on the other hand, an attractive TS is inserted, the sign may become negative or positive depending on the TS-fluid interactions and TS size. Insertion of a hard TS in the SP, however, causes the interfacial tension to exhibit a positive deviation from its value for the fluid in the SP while this deviation will be negative if an attractive TS is inserted. Investigation of surface adsorption and interfacial tension reveals that, for large SPs, fluid interfacial tension in each thermodynamic state has a linear relationship with adsorption when a TS is inserted. This is while this linear relationship fails for small SPs. Also, it is shown that the locus of deviation from linearity, H, is shifted to higher values of H or Rs as a result of increasing TS radius and bulk density. Finally, it is found that the type (hard or attractive) of the fluid and the inserted TS, however, cannot change the locus of the break point.

Modeling of Pressure Dependence of Interfacial Tension Behaviors of a Complex Supercritical CO2 + Live Crude Oil System Using a Basic Parachor Expression

Parachor based basic expressions are often used to model the experimentally observed pressure dependence of interfacial tension (IFT) behaviors of complex supercritical CO2 + crude oil (dead or live) mixtures at elevated temperatures. However, such modeling requires compositions and densities of the equilibrium liquid and vapor phases and molecular weights of various components present in the system. In the absence of measured data, often phase behavior packages are used for obtaining these input data for performing calculations. Very few researchers have used experimentally measured input data for performing parachor based modeling of the experimental IFT behaviors of complex supercritical CO2 + live crude oil systems. This study presents the results of a basic parachor model based modeling of pressure dependence of IFT behaviors of a complex supercritical CO2 + live crude oil system for which experimentally measured input data is available in public domain. Though the calculated IFT behaviors showed significant deviation from the experimentally measured behaviors at a given pressure, calculated IFT versus pressure trends appeared to follow the experimental observed trends closely. Also, despite a large variation in the compositions of feed supercritical CO2 + live crude oil mixtures, both trends (calculated and experimental) converged rapidly to attain a near zero IFT condition at more or less same pressure.

Physicochemical, interfacial and emulsifying properties of a non-conventional exudate gum (Prosopis alba) in comparison with gum arabic

Physicochemical and emulsifying properties of a novel exudate gum from Prosopis alba (G) were assessed in comparison with the well-known arabic gum (GA).Compositional analysis, intrinsic viscosity and structural characterization as well as gum emulsifying properties (evaluated in terms of mean droplet size distribution of o/w emulsions, ζ-potential, polidispersity and creaming indexes), were performed and related to the dynamic interfacial tension and rheological behavior of the films formed at oil/water interface. Both gums closely resemble in composition with carbohydrates representing the higher fraction (62 and 65% d.b. for G and GA, respectively). The major difference was observed in the protein content which was higher in P. alba gum and largely explains its better interfacial properties. FTIR analysis further supported that the gums share essentially a similar chemical nature.P. alba gum was able to stabilize emulsions better than GA based on its lower droplet size distributions, higher ζ-potential, higher interfacial film elasticity and lower values of the polidispersity and creaming indexes during storage. Furthermore, the formation of stronger viscoelastic films at the oil/water interface as well as the charge distribution of the adsorbed G fractions could contribute to understand the higher stability of G emulsions.Present results provide the first approach to the composition of P. alba gum and evidence its similar or even better emulsifying properties than GA.

Experimental Investigation on the Effect of Asphaltene Types on the Interfacial Tension of CO2–Hydrocarbon Systems

Interfacial tension (IFT) is known as the critical parameter affecting the efficiency of CO2 flooding during the enhanced oil recovery (EOR) process. Besides, the asphaltene precipitation phenomenon is reported as the most significant problem during CO2 injection into asphaltenic oil reservoirs. Accordingly, it is important to examine the effect of asphaltene precipitation on the IFT behavior of the oil–CO2 system at reservoir conditions. The main objective of this research work is to study of the effect of asphaltene and its type on the IFT behavior of the oil–CO2 system. The IFT between pure CO2 and a model oil both with and without asphaltene was measured using an axisymmetric drop shape analysis (ADSA) technique over a wide range of pressures and a constant temperature of 323 K. The asphaltene particles used for the work were precipitated and separated from three different crude oil samples, each having different physical properties. The model oil, consisted of 50 vol % n-heptane and 50 vol % toluene ...

Surfactant (in situ)–Surfactant (Synthetic) Interaction in Na2CO3/Surfactant/Acidic Oil Systems for Enhanced Oil Recovery: Its Contribution to Dynamic Interfacial Tension Behavior

The effect of synthetic surfactant molecular structure on the dynamic interfacial tension (DIFT) behavior in Na2CO3/surfactant/crude oil was investigated. Three surfactants, a nonionic (iC17(EO)13), an alcohol propoxy sulfate (C14–15(PO)8SO4), and sodium dodecyl sulfate (SDS) were considered in this study. Sodium tripolyphosphate (STPP) was added to ensure complete compatibility between brine and Na2CO3. In Na2CO3/iC17(EO)13/oil and Na2CO3/C14–15(PO)8SO4/oil systems, a strong synergistic effect for lowering the dynamic interfacial tension was observed, in which the dynamic IFT are initially reduced to ultralow transient minima in the range 1.1 × 10−3–6.6 × 10−3 mNm−1 followed by an increment to a practically similar equilibrium value of 0.22 mNm−1 independent of Na2CO3 concentration (for iC17(EO)13) and to decreasing equilibrium values with increasing alkali concentrations (for C14–15(PO)8SO4). The observed difference in the equilibrium IFT for the two systems suggest that in both systems, the mixed interfacial film is efficient in reducing the dynamic interfacial tension to ultralow transient minima (~10−3 mNm−1) but the mixed film soap-iC17(EO)13 is much less efficient than the mixed film soap-C14–15(PO)8SO4 in resisting soap diffusion from the interface to the bulk phases. In both systems, the synergism was attributed, in part, to the intermolecular and intramolecular ion–dipole interactions between the soap molecules and the synthetic surfactant as well as to some shielding effect of the electrostatic repulsion between the carboxylate groups by the nearby ethylene oxide (13 EO) and propylene oxide (8 PO) groups in the mixed interfacial monolayer. SDS surfactant showed a much lower synergism relative to iC17(EO)13 and C14–15(PO)8SO4, probably due to the absence of ion–dipole interactions and shielding effect in the mixed interfacial layer at the oil–water interface.