At short times, interfacial tension depends on experimental geometry because surfactant transport to the interface depends on the mass-transfer conditions.A predictive description therefore requires more than a dynamic tension curve or a fitted adsorption constant: interfacial thermodynamics, diffusion, and adsorption kinetics must be identified separately. Here, we combine equilibrium and diffusion measurements with a microfluidic EDGE tensiometer that provides a nearly stationary interface and controlled micrometer-scale transport. Equilibrium properties and diffusion are determined independently, leaving adsorption kinetics as the key unknown. Dynamic tension is then calculated using a nonequilibrium thermodynamic description, without assuming instantaneous equilibrium between the adsorbed layer and the subsurface solution. For the nonionic surfactant C_10E_8, equilibrium thermodynamics and transport are independently constrained, and a single intrinsic adsorption rate constant describes several concentrations. We extend the framework to SDS by including electrostatic interactions and subsurface-concentration dynamics, capturing transient depletion and replenishment. Once thermodynamic, transport, and kinetic parameters are identified, the model predicts dynamic interfacial tension beyond the geometry and conditions used to determine them. The microfluidic EDGE tensiometer thus provides both a reliable short-time tensiometry method and a quantitative framework for identifying the physical mechanisms governing surfactant mass transfer at interfaces.

Water-in-water (W/W) emulsions exhibit extremely low interfacial tensions, making their stabilization challenging using conventional surfactants. In this study, Pickering stabilization of polyethylene glycol (PEG)/MgSO₄ aqueous two-phase emulsions was investigated using silica nanoparticles with different surface properties. Silanol quantification by ¹H NMR and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy confirmed differences in silanol content, while contact angle measurements between the two equilibrated phases revealed differences in particle affinity for each phase. Emulsions with controlled phase volume fractions were prepared along a selected tie line. Stable MgSO₄-in-PEG emulsions were obtained, with the highest stability observed for particles with lower silanol content and a stronger affinity for the PEG-rich phase, and with high PEG-rich phase volume fractions. The stabilization strategy was further evaluated in a polymer-polymer W/W system based on polyethylene oxide (PEO) and dextran, where silica particles with lower silanol content again provided enhanced stabilization of emulsions. Higher stability was obtained at high PEO phase fractions, highlighting the importance of particle surface properties and phase volume fraction in W/W emulsion stabilization. Finally, the influence of temperature (60–85 °C) and acidic conditions (pH ≈ 1.65) on emulsion stability was investigated. The optimized system remained stable under conditions representative of biphasic catalytic processes, highlighting its potential for catalytic applications.

The recovery of biobutanol from highly diluted aqueous fermentation broth usually suffers from intensive energy consumption. In this study, we developed a Pickering emulsion system stabilized by silica nanoparticles for the rapid and efficient recovery of low concentrations of butanol (<20 g L-1) from fermentation broth in the form of esters. Each droplet in the emulsion system serves as a microreactor for enzymatic esterification of butanol in the water phase, and the ester product is spontaneously extracted to the oil phase, thereby promoting the esterification reaction. The system offers a significantly larger interfacial area and a 2-5 times improvement in reaction rate compared to the biphasic system. Under optimal conditions, the conversion and extraction of butanol from the fermentation broth into butyl butyrate achieved a yield of 79% in the presence of a Pickering emulsion. This study presents a sustainable and efficient approach for the recovery of biobased butanol.

In this study, dissipative particle dynamics (DPD) simulations were employed to determine the critical micelle concentration (CMC) of perfluoroalkyl and polyfluoroalkyl substances (PFAS) in ionic aqueous solutions. This approach provides precise CMC data for PFAS surfactants in the presence of various ionic species, thereby addressing a gap in the current literature. Additionally, this study contributes to the development of open-source molecular force fields for charged perfluorinated compounds, which are currently limited. These models incorporate hydration free energy values obtained from density functional theory (DFT) and account for ionic interactions through a well-established linear relationship. Hydrophobic interactions between the surfactant tail and water were fine-tuned to match the CMC of chosen surfactants. Then, the DPD models successfully predicted CMC values for a diverse range of surfactants, including those based on hydrocarbons and PFAS, demonstrating the ability to represent realistic salinities encountered in natural waters. Experimental validation of the methodology was conducted using sodium n-nonyl sulfate (SNS) and sodium n-dodecyl sulfate (SDS) via interfacial tension measurements, confirming the accurate representation of the CMC changes with salinity. This study enhances our understanding of the behavior of PFAS surfactants in ionic aqueous solutions and provides a valuable tool for predicting CMC values in complex environmental systems.

We present a study of moderately stable dilute emulsions. These emulsions are models for water contaminated by traces of oil encountered in many water treatment situations. The purification of water and the elimination of oil rely on the emulsion stability. Despite actively being studied, the topic of emulsion stability is still far from being fully understood. In particular, it is still unclear whether experimental methods accessing different length scales lead to the same conclusions. In the study presented in this paper, we have used different methods to characterize the emulsions, such as centrifugation and simple bottle tests, as well as investigations of the collision of single macroscopic oil drops at an oil-water interface. We studied different emulsions containing added polymer or surfactant. In the case of added polymer, centrifugation and single drop experiments led to opposite trends in stability when the polymer concentration is varied. In the case of added surfactant, both centrifugation and single drop experiments show a maximum stability when the surfactant concentration is increased, whereas bottle tests show a monotonous increase in stability. We propose tentative interpretations of these unexpected observations. The apparent contradictions are due to the fact that different methods require different drop sizes or different drop concentrations. The puzzling decrease in emulsion stability at a higher surfactant concentration observed with some methods, however, remains unclear. This coalescence study illustrates the fact that different results can be obtained when different experimental methods are used. It is therefore advisable not to rely on a single method, especially in the case of emulsions of limited stability for reasons explained in the paper.

Surfactant mass transport towards an interface plays a critical role during formation of emulsions, foams and in industrial processes where two immiscible phases coexist. The understanding of these mechanisms as experimentally observed by dynamic interfacial tension measurements, is crucial. In this review, theoretical models describing both equilibrated systems and surfactant kinetics are covered. Experimental results from the literature are analysed based on the nature of surfactants and the tensiometry methods used. The innovative microfluidic techniques that have become available to study both diffusion and adsorption mechanisms during surfactant mass transport are discussed and compared with classical methods. This review focuses on surfactant transport during formation of droplets or bubbles; stabilisation of dispersed systems is not discussed here.

A Pickering emulsion is a dispersion of two immiscible liquids stabilized by solid particles anchored at the liquid-liquid interface. Nowadays, ionic liquid-containing Pickering emulsions are playing a more and more important role in catalysis to immobilize organic acid or base (dissolved in ionic liquids) within emulsion droplets to tune the reactivity of these catalysts and increase the selectivity. In this study, some ionic liquids commonly used for catalytic applications have been selected, characterized from a physico-chemical point of view (density, viscosity, interfacial tension, contact angle) and used to manufacture ionic liquid-in-heptane emulsions (IL/H). The main parameters studied are the influence of the nature of the cation (1-butyl-3-methylimidazolium cation [BMI] and N-butyl-N-methylpyrrolidinium cation [BMPyrr]) and of the anion (bis(trifluoromethylsulfonyl)imide anion [NTf2], hexafluorophosphate [PF6], tetrafluoroborate [BF4], trifluoromethanesulfonate [CF3SO3]), as well as the quantity of silica nanoparticles and the volume fraction of the dispersed phase. The type of emulsion, the size of the drops and the stability to coalescence were systematically determined and analyzed according to the physico-chemical properties of the systems. It is shown that it is possible to manufacture stable IL/H emulsions up to a volume fraction of 30–40% depending on the systems. The main parameter to take into account is the contact angle of the nanoparticles towards the ionic liquid used. This study provides a solid basis for the development of stable emulsions that can be further encapsulated for the implementation of continuous catalytic reactions in a two-phase system.

In this paper, an optical technique based on fluorescence intensity is applied and calibrated to simultaneously measure film thickness (up to 8 mm) and interfacial velocity of the liquid film flow. Thus, thin-film flows at intermediate and low Reynolds number (13 < Re < 290) are experimentally studied with special attention given to the spatial variation, the frequency of the waves, and interfacial liquid velocity associated with global visual observation. Two fluids (water-ethanol mixture) characterized by low surface tension (a = 35 and 50 mN/m) and three fluids (water-glycerin mixture) with high viscosity (mu = 5, 10, and 15 mPa.s) are used to investigate the influence of physico-chemical properties on thin-film flow on flat and corrugated plate topologies. First, the effect of physico-chemical properties on the global variation of waves shape and instability evolution has been investigated. Different work fluids were used in the same experimental conditions on a flat plate. Results showed that decreasing surface tension has a stabilizing effect on the flow, dampening the capillary waves that should otherwise arise. At high viscosity, solitary waves with high thickness followed by an undisturbed thin-film with a constant thickness appear. The impact of liquid flow rate and inclination (0 = 10 degrees and 20 degrees) was also studied. It shows that at higher inclination the film thickness decreases and then increases as the liquid flow rate increases. Next, the impact of counter-current gas was studied and we demonstrated that the amplitude of the main waves significantly increases even at low gas velocity while the effect on the velocity of the wave starts to be significant at a counter-current gas velocity of 3 - 4 m/s. Finally, the effect of the corrugation of the solid plate at a high inclination angle (0 = 60 degrees) shows the onset of the breaking waves phenomenon, especially for fluids with low surface tension.

Surfactant flooding is one of the Chemical Enhanced Oil Recovery (cEOR) methods used to meet the growing demand for oil. It consists in injecting an aqueous formulation containing surfactants, whose performance, both in terms of incremental oil production and surfactant adsorption, is assessed in the laboratory with coreflood tests. Currently, coreflood effluents are collected in tubes throughout the experiment and the analyses are performed offline. Surfactants are measured in the aqueous phase by Hyamine assay or by liquid chromatography. It is noteworthy that these analyses may be difficult to carry out since the effluents may contain stable emulsions. Moreover, it is unclear whether all surfactants are in the aqueous phase, or whether they are partly trapped in the oil phase. To overcome these difficulties and quantify surfactants reliably in coreflood effluents, we have developed an online experimental setup that includes: a dilution “millifluidic” chip to transfer the surfactants in the aqueous phase, a “microfluidic” membrane-based separation device to separate oil from the aqueous phase, and an online UV–visible spectrometer to measure the surfactant concentration. This setup was successfully validated with model fluid mixtures and was evaluated on real systems. Finally, it was tested under representative conditions of coreflood experiments. The obtained results proved the efficiency of the setup to facilitate the quantification of the surfactants in the effluents which clearly improves the accuracy of the measurements.

In order to increase oil recovery, new techniques were developed such as chemical Enhanced Oil Recovery (EOR). The water injected in the reservoir tends to form and stabilize emulsions of oil droplets in produced water. That makes the separation of oil from the aqueous phase more difficult. The main goal of this work is to achieve a better understanding of emulsion destabilization in the presence of brine. In this work, different methods were used with emulsions and macroscopic drops to better understand oil droplet behavior. The experimental results are based on the determination of the zeta potential, the evolution of the kinetics of light transmission, the critical force of coalescence in centrifugation, and the rupture time of the aqueous thin film between an oil drop and a water/oil interface. These studies allowed the characterization of the impact of salinity on emulsion destabilization. It was observed that the negative charges at the oil droplets surface, due to migration of indigenous amphiphilic species from crude oil to the interface, are reduced by the presence of salts because salts decrease the adsorption of charged natural surfactants at the oil-water interface. The electrostatic repulsion between oil droplets is thus reduced, promoting flocculation and coalescence.

Chemical flooding, one of the Enhanced Oil Recovery (EOR) techniques used to increase oil production, consists in the injection in the well of an aqueous formulation containing various chemical additives such as surfactants. However, its performance can be significantly altered by the loss of surfactants in reservoir rocks. More precisely, surfactant loss due to adsorption on the reservoir rock may have a non-negligible impact on the efficiency of the injected formulation. In this article, we considered the adsorption of a mixture of two anionic surfactants having an important industrial relevance for EOR applications. Adsorption was studied on silica, representative of reservoir rocks such as sandstone, by combining Quartz Crystal Microbalance with Dissipation monitoring (QCM-D) and neutron reflectivity experiments. A preliminary characterization of the surfactant mixture solution demonstrated the formation of unilamellar vesicles in the bulk. Whereas an adsorbed layer was measured with the single AOT vesicles, but not with the SDBS micelles, we observed that mixing both anionic surfactants change the adsorption phenomenon. Indeed, non-negligible adsorption was measured for the mixture even at concentrations where only slight adsorption had been observed with the individual surfactants. This suggests that the structure of the aggregates formed in the bulk has a non-negligible impact on the adsorption. We note that the addition of salt tends to enhance the adsorption by screening the repulsive interactions between both negatively charged surfactants and silica at neutral pH. The present work provides new insights into the description of the adsorption of a mixture of surfactants of same nature in unfavorable conditions.

HYPOTHESES:The performance of bicontinuous microemulsions is usually assessed on the characteristics of the middle phase at equilibrium. However, applied to Enhanced Oil Recovery, such an evaluation would not be representative of the structure and composition of fluids in reservoir rocks. Studies on the properties of non-equilibrated microemulsions are still needed to better understand the formation of such complex systems, in particular to optimize input parameters of process simulation tools. EXPERIMENTS:For this purpose, we monitored the formation of a microemulsion from contact with the oil to equilibrium when no mixing or convection is provided. Non-destructive methods such as Nuclear Magnetic Resonance, Micro-Computed Tomography, Dynamic Light Scattering and Small Angle X-ray scattering were used to extract the compositions, phase thicknesses, dynamics and structures of the system over time. FINDING:We found that the system gets structured into several layers over time that include the transient presence of an oriented semi-crystalline phase. The growth of the bicontinuous middle phase results from a progressive reorganization of the liquid crystal. The compositional and structural gradients, observed along the sample height, are correlated and linked to the corresponding structures of the phase diagram of the quaternary system. Equilibrium is reached after the total transfer of the liquid crystal into the bicontinuous phase.

In the context of enhanced oil recovery or soil remediation, we study the role of interactions between polymers and surfactants on the injectivity of formulations containing mixtures of polymers and surfactants. We show that contrary to the first intuition, the formation of aggregates in polymers surfactants formulations is not necessarily a hindrance to the injection of these formulations into pores. It is important above all to compare the size of aggregates according to the applied shear rate and the pore size to find the formulations that may induce clogging. We highlight a new positive and unexpected phenomenon. The small aggregates that do not lead to clogging ensure the transport of the surfactant vesicles in the porous medium and limit the adsorption of the latter.

A soil decontamination or enhanced oil recovery procedure typically requires the injection of a surfactant solution to reduce interfacial tension and promote oil recovery at the pore level, followed by the injection of a polymer solution to avoid the creation of preferential pathways and perform a homogeneous sweep of the reservoir. It is well known that polymers and surfactants interact with each other to form aggregates if they are of opposite charge or due to depletion interactions. To date, it is recommended to use polymers and surfactants that do not interact with each other to avoid clogging the wells. We show here that this precaution is not necessary and that in some cases, the use of interacting polymer and surfactant systems can even be an advantage for oil recovery. Contrary to previous studies, we injected previously mixed and homogeneous formulations into the porous medium and not a sequence of surfactant and polymer plugs. In addition to a total recovery of the oil in place, this strategy allows to limit the adsorption of the surfactant in the pore.

Applied to Enhanced Oil Recovery, microemulsions are valuable systems for extracting the crude oil trapped by capillary forces in the porous reservoir rocks. The performances of the injected formulations are often assessed by quantifying oil composition in model systems that contain relatively high amount of surfactant/co-surfactant. Recently, the question of representativity of such systems was raised because kinetics aspects and complexity of crude were neglected in model systems and are likely to impact the process efficiency. The current quantification techniques limit the characterization of representative model systems as they are destructive, time consuming and not often applicable to dark or opaque systems. In the original aim to provide a quantitative kinetic study of such microemulsions, we propose a high resolution T1-weighted imaging technique to have access to 1D-composition profiles of co-surfactant, oil and brine in Winsor I, Winsor III and Winsor II microemulsions. The analysis is carried out on model systems at equilibrium for proof of concept. Results are correlated with X-Ray Micro-CT experiments to provide better interpretations and assess the method accuracy. We provide conditions of validity of the developed NMR method and discuss its potential limitations. To a larger extent, the method could be of interest to other applications that use similar systems.

The structure of adsorbed surfactant layers at the equilibrium state has already been investigated using various experimental techniques. However, the comprehension of the formation of structural intermediates in nonequilibrium states and the resulting adsorption kinetics still remain a challenging task. The temporal characterization of these intermediate structures provides further understanding of the layer structure at equilibrium and of the main interactions involved in the adsorption process. In this article, we studied the adsorption kinetics of AOT vesicles on silica at different pHs at ambient temperature. The AOT vesicles were formed in a brine solution. Quartz crystal microbalance with dissipation monitoring (QCM-D) was used to obtain information on the kinetics of surfactant adsorption and on the structure of the adsorbed layer at the equilibrium state. Additionally, neutron reflectivity experiments were performed to provide a detailed description of the mean surfactant concentration profile normal to the surface at equilibrium. Results suggest that vesicles in the bulk influence the adsorption mechanisms. In acidic conditions, after a time-dependent structural rearrangement step, followed by the rupture of initially adsorbed vesicles, the formation of a bilayer was observed. At an intermediate and basic pH, in spite of the electrostatic repulsion between the negatively charged surfactants and silica, results demonstrated the existence of an adsorbed layer composed of AOT vesicles. Vesicles are more or less closely packed depending on the pH of the solution. Results show a non-negligible influence of NaCl addition at pH values where adsorption is initially inhibited. Vesicle adsorption at the intermediate and basic pH is probably due to the combination of attractive van der Waals interactions promoted in high ionic strength systems and the formation of hydrogen bonds. Interpretation of adsorption kinetics gave insight into adsorption mechanisms in an electrostatic repulsion environment.

The thermal and oxidation stability of fatty acids methyl esters (FAME) is arousing attention in the transport industry, since they are the main components present in biodiesel products used in the market. Low FAME stability can induce easy fuel degradation and produce oxidation products that can form sticky deposit causing serious malfunctioning and failures of engine and turbines components. We have focused the present work on the study of fuel oxidation process and the characterization of oxidation products in order to identify the main levers to avoid deposit formation. Soy and Rapeseed biodiesels were oxidized using an autoclave Parr reactor and characterized by FTIR, density and viscosity measurements. After oxidation, two different liquid phases were clearly observed. These two phases tend to form complex oil-oil emulsions after remixing as evidenced by optical microscopy. The separation behavior of the different liquid phases remixed after oxidation were studied using Multiple Light Scattering (Turbiscan (TM)). A comparison was made between the chemical functions of deposit obtained in the liquid phase after demixing (sedimented phase) and the solid deposit obtained on hot metallic surfaces. Results showed a that a complex oil-oil dispersion seems to form during the oxidation process. The phase separation rate of the oil-oil emulsified systems formed from oxidized fuels seems strongly related to the differences of polarity (e.g. oxygenates content) of both sedimented and supernatant phases. The understanding of this sedimentation or "demixing" process leading to deposit can be a key feature to develop strategies to prevent deposit formation in real systems.

From May 29 to June 1, 2017, IFP Energies nouvelles has organized the “JCAT48”, Calorimetry and Thermal Analysis Days, under the auspices of the French Association of Calorimetry and Thermal Analysis (AFCAT). Calorimetry and thermal analysis are universally recognized techniques for the characterization of fluids and materials in Chemistry, Physics and Biology. They make it possible to have access to the evolution of matter with time, temperature or pressure. The key topic for this new edition was “thermal analysis and calorimetry techniques applied to the characterization of materials and fluids for energy”. Progress in the characterization of these systems is a major challenge for the development of new processes that will ensure the energy transition. The techniques of thermal analysis and calorimetry, which can be easily coupled with many other characterization techniques, are a powerful tool for accessing the physicochemical, thermophysical, thermodynamic and chemical kinetics properties of various systems. Among the main fluids of interest, mention can be made of petroleum products (oil, gas, fuels), those resulting from the conversion of biomass (bio-oils and biofuels), as well as colloidal systems (emulsions, microemulsions, hydrate suspensions, etc.). The materials concerned are catalysts, zeolites, MOFs, adsorbents, native biomass, rocks, amorphous and semicrystalline polymers, composites, organic deposits, etc. Applications range from energy production and storage (thermal or chemical) to engine applications as to CO2 capture and storage. In this special issue of the OGST journal, we have grouped together a few articles highlighting the importance of thermodynamic and physicochemical data acquisition in characterizing and predicting the behavior of complex systems of interest for new energy applications. The first article entitled “Characterization of Complex Crude Oil Microemulsions-DSC Contribution” by [1] presents how Differential Scanning Calorimetry (DSC) can be used to provide information on the physico-chemical composition of complex microemulsions (water and oil content, salinity, etc.) and on their morphology (continuous phase, dispersed phase, etc.). This type of information is of prime importance to model the behavior of crude oil microemulsions formed during surfactant flooding, a chemical enhanced oil recovery (EOR) process which consists in injecting optimized formulations of surfactants in a reservoir in order to remobilize the residual oil trapped in the pores of the rock. The next one, “DSC (Differential Scanning Calorimetry) used to follow the evolution of W/O emulsions versus time on ground and in space in the ISS” by [2] gives some new insight on the evolution of W/O emulsions versus time in microgravity conditions. This study was performed in the framework of the FASES (Fundamental and Applied Studies on Emulsion Stability) project sponsored by the European Space Agency (ESA). The objective was to use calorimetry to investigate the stability of W/O emulsions under purely diffusive conditions (no gravity driven effects) and to compare it with the stability of similar emulsions observed on ground. For that purpose, two identical calorimeters were built by Airbus (formerly EADS) with the support of the French company SETARAM: The Flight Model (FM) located onboard the International Space Station (ISS) and the Engineering Model (EM) located at the TELESPAZIO Centre in Naples. The article entitled “Excess/deviation properties of binary mixtures of 2,5-dimethylfuran with furfuryl alcohol, methyl isobutyl ketone, 1-butanol and 2-butanol at temperature range of (293.15–323.15) K” by [3] gives some new thermodynamic properties of binary mixtures of fluids representative of new fuels derived from biomass. These thermodynamic properties were used to interpret the molecular interactions among the different liquid components. The values of excess/deviation functions have been * Corresponding author: christine.dalmazzone@ifpen.fr