Equilibrium Interfacial Tension Research Articles

Influence of resins, asphaltenes and petroleum acid interactions on water/model oil interfacial properties and emulsion stability

The presence of natural lipophilic emulsifiers, such as asphaltenes (A), resin (R) and petroleum acids (PA) in crude oil, plays a crucial role in stabilizing the W/O emulsions. This study primarily focuses on elucidating the influence of their interactions on the interfacial properties of the water/model oil system, as well as its emulsion stability. First, equilibrium interfacial tension (IFT), interfacial dilational modulus (IDM) and phase angle are measured. Then, emulsion stability experiments are performed. Finally, microscopic images of the emulsions were captured to evaluate droplet size and dispersity. Results show that the addition of petroleum acid to both asphaltene-only and resin-only system reduces IFT and dilational modulus, weakening the strength of the interfacial film. The inhibitory effect of petroleum acid on resins’ adsorption at the interface is more pronounced compared to asphaltenes’. A low proportion of resin and petroleum acid (A/R ≥ 5, A/PA ≥ 7) enhances emulsion stability by co-adsorbing with asphaltenes at the oil–water interface. Conversely, a high proportion of resin and petroleum acid leads to a decrease in IDM and emulsion stability. For asphaltene/petroleum acid–water system, when A/PA ≤ 5, the presence of petroleum acid decreases the droplet size but enhances droplet dispersion while decreasing emulsion stability. It can be inferred that the presence of petroleum acids contributes more to the formation rather than stabilization of emulsions. This work deepens insights into the emulsion behavior of natural surfactants, providing theoretical guidance for the formation of stable emulsions underground and efficient demulsification above ground.

Influence of nitrogen cushion gas in 3-phase surface phenomena for hydrogen storage in gas condensate reservoirs

Recent research has primarily focused on 2-phase systems for hydrogen storage in porous media. However, the impact of cushion gases mixed with hydrocarbon fluids, such as n-octane, in depleted condensate-type reservoirs remains poorly understood. This study addresses the gap by evaluating the effects of pressure (500–3000 psi), temperature (30–70 °C), and NaCl brine salinities (2–20 wt%) by measuring the contact angles (CAs) between n-octane and substrates (for Quartz and Wolf camp shale) in brine, and interfacial tensions (IFTs) between brine and n-octane while injecting pure H2 gas (base case) and a mixture of 60% H2 and 40% cushion gas (30% N2 + 5% CH4 + 5% CO2). The CA results revealed that pressure had a relatively minor impact on the system with pure H2 injection, compared to temperature and salinity. However, when 40% cushion gas was added, the CA increased with pressure and salinity but decreased with rising temperature. It was observed that CAs for pure H2 gas were lower than those for the H2 + cushion gas mixture. Also, the equilibrium IFTs showed a consistent reduction with increasing pressure and temperature, while they increased with higher salinity. Comparing the IFTs of the 3-phase (H2 + cushion)/n-octane/brine system to those of the 2-phase (H2 + cushion)/brine system revealed that n-octane could effectively reduce the system's IFT by approximately 33%. Overall, this study provides valuable data for hydrogen storage in depleted gas condensate reservoirs containing hydrocarbon fluids like n-octane.

Adsorption behavior of asphaltene aggregates generated by self-association at the oil/water interface

The formation of emulsions is undesirable yet unavoidable in petroleum production. Researchers suggest that asphaltenes adsorb at the oil/water interface, forming a protective film that stabilizes emulsions. This study investigates how asphaltene aggregates, formed through intermolecular interactions, affect emulsion stability and the properties of the oil/water interface. First, the effects of asphaltene concentrations, components, and temperatures on emulsion stability and the association state of asphaltenes were explored using the bottle test method and dynamic light scattering, respectively. The promotion of asphaltene aggregation by higher asphaltene concentrations and liquid paraffin content was confirmed by comparing the average size of aggregates, while higher temperatures were found to inhibit aggregation. Higher temperatures destabilize emulsions containing asphaltenes by increasing water droplet collisions, whereas higher asphaltene concentrations and liquid paraffin content stabilize emulsions. Subsequently, the effects of these conditions on expansion modulus were thoroughly investigated using the pendant drop technology. Pendant drop experiments demonstrate that increasing asphaltene concentration, along with the introduction of liquid paraffin, enhances asphaltene aggregation, thereby reinforcing the structural stability of the interfacial film. Spinning drop experiments were conducted to obtain dynamic interfacial tension (IFT) and analyze the adsorption behavior of aggregates at the oil/water interface. The adsorption process of asphaltenes driven by intermolecular interactions, occurs in three regimes: a short-term diffusion-controlled process, a transition process, and a long-term low-speed descent process, referred to as Regimes I, II, and III. The diffusion coefficient in Regime I increases with temperature, while the equilibrium IFT in Regime III decreases with temperature. In Regime II, adsorbed asphaltene aggregates from Regime I tend to prevent further adsorption, reducing the rate of dynamic IFT decrease. In Regime III, dynamic IFT decreases slowly, primarily due to the continued adsorption of asphaltene aggregates into the sublayer and their subsequent reconfiguration. These findings have significant implications for improving the understanding of oil/water interface properties and emulsion behavior in oil production, transportation, and refining systems.

Adsorption behavior of non-ionic demulsifiers at the oil/water interface stabilized by asphaltenes: Experiments, adsorption kinetics, and mechanisms

Formation of emulsions containing asphaltenes is both undesirable and unavoidable in petroleum production. Chemical demulsifiers, such as non-ionic surfactants, have long been applied as the cheapest and most effective method of breaking emulsions by altering the oil/water interfacial properties. In this study, the association state of the asphaltenes, the dehydration rate, and the interfacial rheology under various demulsifier concentrations were used to analyze the contributions of non-ionic demulsifiers to the breaking of the asphaltene-containing emulsions. Additionally, the dynamic interfacial tension (DIFT) data over time were used to investigate the adsorption kinetics of the asphaltenes and the demulsifier at the oil/water interface. Results indicate that liquid paraffin enhanced the association of the asphaltene molecules, while the non-ionic demulsifier rendered asphaltene aggregate sizes smaller. The addition of the demulsifiers reduced the interfacial strength, leading to an unstable interface. The oil-soluble demulsifiers reduced IFT more effectively than the water-soluble ones, due to their faster migration rate and low interfacial tension gradient to the oil–water interface. The adsorption process of the asphaltenes, influenced by the demulsifier concentration, could be divided into three stages: a short-term diffusion-controlled process, a transition process, and a slow descent process (referred to as Regimes I, II, and III). In Regime I, the diffusion coefficient of asphaltenes increased with the demulsifier concentration, indicating that the demulsifier aided the dispersing of the asphaltenes in the solution. The equilibrium IFT and the maximum surface excess concentration of the asphaltenes in the long-term adsorption process decreased with the demulsifier concentration, indicating that the asphaltene-containing emulsions generally became unstable after adding the demulsifiers. Our results offer valuable insights into the adsorption kinetics and the mechanisms of alterations by the asphaltenes and demulsifiers of the oil/water interface, with implications for various interfacial phenomena (e.g., emulsion stability) involving surfactants in oil production, transportation, and processing.

Interfacial Rheological Investigation of Modified Silica Nanoparticles with Different Alkyl Chain Lengths at the n-Octane/Water Interface.

The interfacial dilational rheology of silica nanoparticles (NPs) directly reflects the relationship between surface structure and interfacial behaviors in NPs, which has attracted significant attention in various industrial fields. In this work, modified silica nanoparticles (MNPs) with various alkyl chain lengths were synthesized and systematically characterized using Fourier transform infrared spectra, Zeta potential, and water contact angle measurements. It was found that the MNPs were successfully fabricated with similar degrees of modification. Subsequently, the interfacial behaviors of the MNPs in an n-octane/water system were investigated through interfacial dilational rheological experiments. The length of the modified alkyl chain dominated the hydrophilic-lipophile balance and the interfacial activity of the MNPs, evaluated by the equilibrium interfacial tension (IFT) variation and dilational elasticity modulus. In the large amplitude compression experiment, the balance between the electrostatic repulsion and interfacial activity in the MNPs was responsible for their ordered interfacial arrangement. The MNPs with the hexyl alkyl chain (M6C) presented the optimal amphipathy and could partly overcome the repulsion, causing a dramatic change in surface pressure. This was further confirmed by the variations in IFT and dilational elasticity during the compression path. The study provides novel insights into the interfacial rheology and interactions of functionally modified NPs.

Electric-field induced phase separation and dielectric breakdown in leaky dielectric mixtures: Thermodynamics and kinetics.

Dielectric materials form the foundation of many electronic devices. When connected to a circuit, these materials undergo changes in microscopic morphology, such as the demixing of dielectric mixtures through phase separation and dielectric breakdown, resulting in the formation of micro-filaments. Consequently, the macroscopic properties and lifespan of the devices are significantly altered. To comprehend the physical mechanisms behind it, we conducted a systematic investigation of the thermodynamics of multicomponent leaky dielectric materials. Beginning with the total energy functional, we derived expressions for the binodal composition, spinodal composition, and critical points. Furthermore, we constructed and validated theoretical phase diagrams for the binary leaky dielectric mixture, incorporating three crucial freedoms: composition, temperature, and electric field strength. In addition, we analyzed the equilibrium interfacial tension impacted by the electric field and studied the dynamic aspects of dielectric materials, examining two morphological transformations: electrostriction and dielectric breakdowns. Our analysis unveiled a connection between these dynamic phenomena and the electric field-induced interfacial instability. The present work is expected to be supportive of future research on multicomponent dielectric materials by offering a comprehensive understanding of their thermodynamic and kinetic behaviors.

The effect of heat treatment and high‐pressure homogenization on the dispersibility and interfacial behavior of faba bean protein isolate and concentrate

AbstractThe effect of hydrothermal treatments, high‐pressure homogenization (HPH) and their combined effects on faba bean protein isolate (FBI) and concentrate (FBC) dispersions was investigated. With HPH, a significant reduction in particle size was observed for FBI but not for FBC. For FBI, dispersion stability under accelerated gravitation was significantly improved by both hydrothermal and HPH. For FBC, hydrothermal treatment had a negative effect on dispersion stability unless it was combined with HPH. The protein dispersibility of FBI increased, and FBC decreased with increasing temperature. Upon HPH, the protein dispersibility of FBI dramatically improved, making it almost completely soluble. All FBI dispersions had a higher surface charge compared to FBC. SDS PAGE profile showed strong protein crosslinking in the original FBI, leading to a loss of legumin and vicilin bands, which were visible in a more native state of FBC. Physical modifications did not significantly affect the viscosities except for a significant increase in FBC heated at 75°C due to starch gelatinization. The equilibrium interfacial tension of the modified FBI was significantly lower than the FBC. Analysis of dynamic interfacial adsorption showed that FBI had a much faster initial diffusion (beyond detection limit) towards the interface than FBC. Physical modifications improved the surface hydrophobicity of the proteins, leading to faster interfacial adsorption. Overall, FBI dispersions had improved functional properties compared to FBC, and HPH had a more significant effect than hydrothermal treatments. Physically modified FBI could be a promising emulsifier in various food applications.

Performance and mechanism of organic solid-phase sediment composite plugging agent

The surface pipeline sediments of the Yingmai and Hade oil fields mainly contain substances such as asphalt and paraffin. The formation of sediment can increase the backpressure during pipeline transportation, and in severe cases, it can also lead to pipeline blockage. In order to solve this on-site situation, indoor experiments were conducted in the early stage, and an efficient organic unblocking system LXW was optimized and formulated. Based on this system, this article selects the wire mesh filtration method to evaluate the unblocking performance of LXW and on-site unblocking agents in the study and comparison of three evaluation methods. At the same time, based on the analysis of the chemical composition, microstructure, and melting point of organic sediments, from the perspectives of safety, environmental protection, and economic factors, further compares the performance and universality of the three unblocking systems, optimizes the process parameters of the LXW unblocking system, explores its stability and impact on the demulsification and dehydration of crude oil emulsions, and evaluates its safety performance. The LXW unblocking system has a more efficient unblocking effect and better universality, which can solve the problem of multiple block blockages on site. In addition, compared with existing unblocking agents, it has better economic benefits and is in line with the characteristics of low cost on site. On this basis, the dissolution and dispersion mechanism of the blockage system on sediment was studied through laser scattering system and fluorescence microscope, and its microscopic mechanism was further verified. At the same time, the oil-water interfacial tension under the action of the unblocking system was studied, and it was found that adding the agent can significantly reduce the equilibrium interfacial tension, which is of great significance for increasing production and injection.

Ionic Strength Effect in the Equilibrium and Rheological Behavior of an Amphiphilic Triblock Copolymer at the Air/Solution Interface

This study investigates the effect of an inert salt (NaCl) on the equilibrium interfacial tension and dilatational modulus of Pluronic F-68 copolymer, a triblock copolymer consisting of two terminal blocks of poly(ethylene oxide) and a less hydrophilic central block of poly(propylene oxide). Interfacial tension measurements were carried out using a surface force balance and a drop shape tensiometer, while rheological measurements were carried out in two different frequency ranges. This involved the use of the oscillatory barrier/droplet method and electrocapillary wave measurements, complemented by an appropriate theoretical framework. This work aimed to elucidate the influence of NaCl on the interfacial behavior of Gibbs monolayers of Pluronic F-68. In addition, this study highlights some of the technical and theoretical limitations associated with obtaining reliable dilatational rheological data at high frequencies (<1 kHz) using electrocapillary wave measurements. The results provide valuable insights into the interplay between salt presence and interfacial properties of Pluronic F-68 and highlight the challenges of obtaining accurate dilatational rheological data under specific measurement conditions.

Molecular dynamics simulation of surfactant reducing MMP between CH4 and n-decane

Reinjecting produced methane offers cost-efficiency and environmental benefits for enhances oil recovery. High minimum miscibility pressure (MMP) in methane-oil systems poses a challenge. To overcome this, researchers are increasingly focusing on using surfactants to reduce MMP, thus enhancing the effectiveness of methane injections for oil recovery. This study investigated the impact of pressure and temperature on the equilibrium interfacial tension of the CH4+n-decane system using molecular dynamics simulations and the vanishing interfacial tension technique. The primary goal was to assess the potential of surfactants in lowering MMP. Among four tested surfactants, ME-6 exhibited the most promise by reducing MMP by 14.10% at 373 K. Key findings include that the addition of ME-6 enriching CH4 at the interface, enhancing its solubility in n-decane, improving n-decane diffusion capacity, CH4 weakens n-decane interactions and strengthens its own interaction with n-decane. As the difference in interactions of n-decane with ME-6's ends decreases, the system trends towards a mixed phase. This research sets the stage for broader applications of mixed-phase methane injection in reservoirs, with the potential for reduced gas flaring and environmental benefits.

A comprehensive study on the effect of rock corrosion and CO2 on oil-water interfacial tension during CO2 injection in heavy oil reservoirs

This study is focused on addressing issues related to the interaction mechanism between reservoir rocks and fluids during CO2 injection in heavy oil reservoirs. The results indicate that the carbonated water formed after high-pressure CO2 injection is causing corrosion in the core structure. Furthermore, the average corrosion rate increases from 0.358% as pressure increases from 8 MPa to 16 MPa to 0.742%. The core slices exhibit corrosion pits and cracks on their surface, as well as surface particles flaking and dissolving. The dynamic IFT between decyl trimethyl ammonium bromide solution and crude oil exhibited slower stabilization and less change without CO2 injection. However, with increasing pressure post CO2 injection, the dynamic IFT decreased at a more rapid rate. Furthermore, positive correlation between equilibrium IFT and temperature was observed with low-pressure CO2. In contrast, the surfactant played a more dominant role in reducing IFT with high-pressure CO2. The findings of this study can help for better understanding of the mechanism of microscopic action on reservoir rock and oil-water interface after CO2 injection, and offer theoretical basis and reference for CO2 injection in heavy oil reservoirs to enhance the recovery rate.

Model selection for dynamic interfacial tension of dead crude oil/brine to estimate pressure and temperature effects on the equilibrium tension

Interfacial tension (IFT) measurements using pendant drop techniques are typically time-dependent and embed the underlying physics that can be exploited. A physics-based modeling approach of the time-varying IFT is chosen to estimate the equilibrium IFT and to identify which phenomena are leading the dynamic. This approach is both consistent and reproducible, hence advantageous for routine research and industrial applications because typical approaches are especially challenging when the techniques rely on long-time measurements or detailed characterization of phase composition and transport properties. The approach is applied to the Brazilian pre-salt dead crude oil/brine system when the equilibrium between the phases and between bulk and interface concentrations are not assured initially. Selected models based on a different physical phenomenon, mainly focused on diffusion and sorption, and with a particular time scale behavior are tested on measured data. The model which better fits the data in its full-time range should point out which phenomenon is dominant. The results indicate that the model with t time scale seems preferable for this oil/brine system, which is associated with a diffusion-controlled process in multiple regimes, for instance, bulk-to-interface diffusion of naturally present interface chemicals and their rearrangement at the interface. The effect of pressure (14.7 psi–4000 psi) and temperature (22 °C–60 °C) variations on the equilibrium IFT of the Brazilian pre-salt reservoir dead crude oil/brine at various conditions is characterized and discussed using the consistent physics-based estimate for the equilibrium IFT. The results show that the equilibrium IFT does not change significantly with pressure and increases with temperature, further supporting the identified presence of naturally present interface chemicals in the transient IFT measurements and the brine ionic valence influence. Furthermore, the wettability of carbonate rock samples with different mineralogical compositions to the dead crude oil/formation water is assessed using adhesion tension which embeds both IFT and contact angle contributions. Pressure changes do not affect wettability considerably, while the effects of temperature increase show that every sample tended to be wetter to its respective fluid affinity, following the IFT temperature behavior.

Bottlebrush Polymers at Liquid Interfaces: Assembly Dynamics, Mechanical Properties, and All-Liquid Printed Constructs.

Bottlebrush polymer surfactants (BPSs), formed by the interfacial interactions between bottlebrush polymers (BPs) with poly(acrylic acid) side chains dissolved in an aqueous phase and amine-functionalized ligands dissolved in the oil phase, assemble and bind strongly to the fluid-fluid interface. The ratio between NBB (backbone degree of polymerization) and NSC (side chain degree of polymerization) defines the initial assembly kinetics, interface packing efficiency, and stress relaxation. The equilibrium interfacial tension (γ) increases when NBB < NSC, but decreases when NBB ≫ NSC, correlating to a pronounced change in the effective shape of the BPs from being spherical to worm-like structures. The apparent surface coverage (ASC), i.e., the interfacial packing efficiency, decreases as NBB increases. The dripping-to-jetting transition of an injected polymer solution, as well as fluorescence recovery after photobleaching experiments, revealed faster initial assembly kinetics for BPs with higher NBB. Euler buckling of BPS assemblies with different NBB values was used to characterize the stress relaxation behavior and bending modulus. The stress relaxation behavior was directly related to the ASC, reflecting the strong influence of macromolecular shape on packing efficiency. The bending modulus of BPSs decreases for NBB < NSC, but increased when NBB ≫ NSC, showing the effect of molecular architecture and multisite anchoring. All-liquid printed constructs with lower NBB BPs yielded more stable structured liquids, underscoring the importance of macromolecular packing efficiency at fluid interfaces. Overall, this work elucidates fundamental relationships between nanoscopic structures and macroscopic properties associated with various bottlebrush polymer architectures, which translate to the stabilization of all-fluidic printed constructs.

Alginate-Chitosan Microgel Particles, Water–Oil Interfacial Layers, and Emulsion Stabilization

In this work, alginate-chitosan microgel particles were formed at different pH levels with the aim of using them as viscoelastic interfacial layers, which confer emulsion stability to food systems. The particles’ size and structural characteristics were determined using laser diffraction, confocal laser microscopy (CLSM), and time-domain nuclear magnetic resonance (TD-NMR). The pH affected the microgel characteristics, with larger particles formed at lower pH levels. T2 relaxation measurements with TD-NMR did not reveal differences in the mobility within the particles for the different pH levels, which could have been related to the more or less swollen structure. The rate of adsorption of the particles at the sunflower oil–water interface differed between particles formed at different pH levels, but the equilibrium interfacial tension of all systems was similar. Higher interfacial dilatational viscoelasticity was obtained for the systems at lower pH (3, 4, 5), with G’ reaching 13.6 mN/m (0.1 Hz) at pH 3. The interfacial rheological regime transitioned from a linear elastic regime at lower pH to a linear but more viscoelastic one at higher pH. The thicker, highly elastic interfacial layer at low pH, in combination with the higher charges expected at lower pH, was related to its performance during emulsification and the performance of the emulsion during storage. As revealed by laser diffraction and CLSM, the droplet sizes of emulsions formed at pH 6 and 7 were significantly larger and increased in size during 1 week of storage. CLSM examination of the emulsions revealed bridging flocculation with the higher pH. Nevertheless, all emulsions formed with microgel systems presented macroscopic volumetric stability for periods exceeding 1 week at 25 °C. A potential application of the present systems could be in the formation of stable, low-fat dressings without the addition of any emulsifier, allowing, at the same time, the release of the bioactive compounds for which such particles are known.

Effect of surfactants and interfacial rheology on equilibrium drop size in turbulent liquid-liquid dispersions

Surfactants are often required to stabilize liquid-liquid dispersions produced by high shear mixers. Due to the high power input, drops are deformed rapidly in the dispersion zone so the dynamic interfacial properties governed by the surfactant adsorption rate can have a significant impact on the resulting drop size. The objective of this work is to develop a fundamental link between surfactant adsorption dynamics, interfacial properties, and turbulent emulsification processes. To ensure constant bulk surfactant concentration during dispersion, equilibrium drop size for dilute dispersions produced by a batch rotor-stator mixer were studied. Silicone oils of various viscosity were dispersed in aqueous nonionic surfactant solutions. Drop size distributions (DSD) were measured via a video microscopy/automated image analysis technique. Equilibrium interfacial tension was measured via a pendant drop technique, as a function of surfactant concentration. The dynamic surface tension was similarly measured. The data were fit to the Langmuir adsorption isotherm and the long-times approximation to the Ward – Tordai equation and the adsorption parameters and surfactant diffusivities so obtained were employed, with an estimate of the drop deformation timescale, to estimate the surface dilational modulus (Esd) or Marangoni stress acting on the drop’ surface due to interfacial tension gradients. Trends observed in the measured equilibrium mean drop size and DSD are explained in terms of the interfacial and rheological properties. Below the CMC, Esd peaks and the drop size increases with bulk surfactant concentration, despite a decrease in equilibrium interfacial tension. Above the CMC, Marangoni stresses are small but the presence of the surfactant still modifies the rheology of the interface, increasing the effective viscosity of the drops. A comprehensive set of mechanistic models for drop size in turbulent flows, based on Kolmogorov-Hinze theory, was developed and modified to partially account for the effect of surfactants via an appropriately defined effective viscosity. The approach allows integration of surfactant phenomena into widely accepted correlations for drop size in surfactant free systems.

Baru oil (Dipteryx alata vog.) applied in the formation of O/W nanoemulsions: A study of physical-chemical, rheological and interfacial properties

The oil extracted from baru (Dipteryx alata Vog.) seeds is in bioactive compounds and it presents potential to be used in food and cosmetic industries. Therefore, this study aims to provide insights into the stability of baru oil-in-water (O/W) nanoemulsions. For this purpose, the effects of the ionic strength (0, 100 and 200 mM), pH (6, 7 and 8), and storage time (28 days) on the kinetic stability of these colloidal dispersions were evaluated. The nanoemulsions were characterized in terms of interfacial properties, rheology, zeta potential (ζ), average droplet diameter, polydispersity index (PDI), microstructure, and creaming index. In general, for samples, the equilibrium interfacial tension ranged from 1.21 to 3.4 mN.m−1, and the interfacial layer presented an elastic behavior with low dilatational viscoelasticity. Results show that the nanoemulsions present a Newtonian flow behavior, with a viscosity ranging from 1.99 to 2.39 mPa.s. The nanoemulsions presented an average diameter of 237–315 nm with a low polydispersity index (<0.39), and a ζ-potential ranging from 39.4 to 50.3 mV after 28 days of storage at 25 °C. The results obtained for the ζ-potential suggest strong electrostatic repulsions between the droplets, which is an indicative of relative kinetic stability. In fact, macroscopically, all the nanoemulsions were relatively stable after 28 days of storage, except the nanoemulsions added with NaCl. Nanoemulsions produced with baru oil present a great potential to be used in the food, cosmetic, and pharmaceutical industries.

Experimental and modelling study of the interfacial tension of (n-decane + carbon dioxide + water) in the three phase region

The interfacial tensions (IFTs) between hydrocarbon-, water- and CO2-rich phases are important in the processes of carbonated water injection for enhanced oil recovery and carbon geological storage. In this work, the IFTs between decane-rich and water-rich phases, in the presence of the third CO2-rich phase, were measured by the pendant drop method and modelled with the density gradient theory at temperatures from 298.15 K to 353.15 K and at pressures up to the critical point pressure of (CO2 + decane) (maximum 11 MPa at 353 K). The dynamic IFT decreased over time due to CO2 adsorption on the interface and mass transfer into the decane-rich drop until equilibrium was reached. As expected, the equilibrium IFTs were observed to decrease with increasing pressure. At low pressures, the equilibrium IFTs decreased with increasing temperature while, at high pressures, the reverse was observed. By comparing the IFTs of ternary (decane + CO2 + H2O) system with those of binary (decane + H2O) system, it was found that CO2 could reduce system IFT, for example by 15.2 mN/m at T = 333 K and p = 10 MPa. The extent of reduction depends on the solubility of CO2 in the liquid phases and the adsorption amount at the interface. Furthermore, the equilibrium IFT was found to be linearly dependent on the CO2 concentration in the water-rich phase isothermally, and an empirical relation was developed with an average absolute deviation of 0.3 mN/m. Density gradient theory coupled with the volume translated CPA equation of state was found to provide an accurate description of the IFTs with an average absolute deviation of 0.8 mN/m, proving its capability of predicting IFTs of (alkane + CO2 + water) in the three-phase region. The density profile in the interfacial region is also demonstrated. There is an enrichment of CO2 molecules at the interface and the enrichment is more pronounced with increasing pressure or decreasing temperature.

Soy protein isolate-citrus pectin-gallic acid ternary composite high internal phase Pickering emulsion for delivery of β-carotene: Physicochemical, structural and digestive properties

The structure properties, stability and β-carotene slow-release mechanism of soybean protein isolate-citrus pectin-gallic acid complex (SPI-CP-GA) stabilized high-internal phase Pickering emulsion (HIPPE) were investigated. The results showed that compared with the SPI-CP binary complex, the turbidity of the SPI-CP-GA ternary complex increased from 2.174 ± 0.001 to 3.027 ± 0.001, the surface wettability was increased, the infrared peaks was blue-shifted, changed from hydrophilic to hydrophobic, and the equilibrium interfacial tension of particles increased from 10.77 ± 0.02 mN/m to 13.46 ± 0.03 mN/m, the complex was more stable. When the GA was 2.0 mg/mL, the encapsulation efficiency of β-carotene was higher. With increased GA concentration and oil phase volume fraction (φ), the apparent viscosity and viscoelastic behavior of HIPPE performed well, forming a stable gel network structure. After 30 days of storage, there was no oil separation in the sample group with GA concentration of 2.0 mg/mL and φ = 0.7, and the stability was strong. After gastrointestinal digestion, the particle size of the HIPPE decreased from 13.51 ± 0.86 μm to 7.70 ± 0.68 μm, the free fatty acid (FFA) release rate was 22.03%, and the bioaccessibility of β-carotene was 6.67 ± 0.19%, and the sustained-release effect was obvious. These results indicated that the SPI-CP-GA ternary complex is a potential stabilizer for HIPPE, and providing theoretical guidance for the design of protein-polysaccharide-polyphenol stabilized HIPPE.

Effects of Pressure and Depressurization Rate of Dissolved CO2 on the Foaming Characteristics of Crude Oil

With the CO2 flooding technique implemented, much CO2 gas is dissolved in the produced fluid. A large amount of CO2 escapes from crude oil due to continual depressurization in the surface pipeline system, leading to massive foam. In this paper, the effects of dissolved-CO2 pressure and depressurization rate on foaming characteristics were investigated by depressurization. To explore the mechanism in depth, CO2 solubility and interfacial characteristics were considered. The results indicated that foamability increases as the pressure increases and increases initially and then decreases with an increase in the rate. Further experimental results showed that when the pressure increases from 1.5 to 3.0 MPa at 30 °C, equilibrium interfacial tension decreases only by 12.78%, while CO2 solubility increases by 81.73%, indicating that CO2 solubility plays a dominant role in foamability. Additionally, it is inferred that when the rate is lower than 18 L/min, it increases at which point the gas that is originally dissolved inside the crude oil transforms to free gas. When it is increased above 18 L/min, the decrease in the solubility and enhancement of disturbance appear. These may be reasons why foamability decreases. Meanwhile, foam stability, the decay degree of foam height over time, was measured immediately after the foam height reached maximum. It was found that foam stability decreases when the pressure increases and increases initially and then decreases when the rate elevates. Further experimental results indicated that when the pressure increases from 1.5 to 3.0 MPa at 30 °C, the bulk viscosity and interfacial dilational modulus decrease by 23.45 and 38.58%, respectively. It is suggested that the role of interfacial viscoelasticity in foam stability is superior to that of viscosity. Additionally, it was inferred the increased rate below 18 L/min makes bubble size smaller, while the weaker structure of the foam film and higher surface energy make foam stability worse with the rate increased above 18 L/min.

Effect of double chain anionic surfactant on the dynamic interfacial tensions of betaine solutions

To clarify the effect of the anionic surfactant with double alkyl chains on the interfacial tension (IFT) of betaine solutions, the IFT values of the sodium dialkyl sulfosuccinates (AOT) mixed with different betaines (alkyl sulphobetaine (ASB) and benzyl substituted alkyl sulphobetaine (BSB)) against n-alkanes and crude oil were detected by the spinning drop method. Additionally, molecular dynamics (MD) simulation was employed to verify the interaction mechanism between AOT and different betaine molecules. The results demonstrate that AOT molecules reach hydrophilic-lipophilic balance and form a relatively compacted interfacial film at the dodecane-water interface. Owing to the flexible double alkyl chains, AOT molecules possess a robust self-regulating ability at the oil–water interface. It is the self-regulating ability of AOT molecules and the electrostatic attraction between betaines and AOT molecules that achieve superior synergistic effects between AOT and betaine molecules with different molecular sizes. Besides, mixed adsorption and hydrophilic-lipophilic balance (HLB) reduce the IFT values of mixed systems containing AOT and different betaines effectively. The equilibrium IFT of the AOT-ASB system and the AOT-BSB system can be reduced to ultralow vales of 5.7 × 10−3 mN/m and 8.2 × 10−3 mN/m, respectively. At the crude oil–water interface, active components in crude oil compete with AOT molecules, which results in a significant increase in IFT values for AOT solution alone. Despite the different molecular sizes of ASB and BSB molecules, the flexible double alkyl chains of AOT molecules enable AOT to have synergistic effects with both ASB and BSB molecules at crude oil–water interface. Ultralow IFTs against crude oil can be obtained at optimized ratio of AOT to betaines, which is meaningful for the application of mixed solutions containing betaines and anionic surfactants in enhanced oil recovery.