Full exploitation and utilization of the unconventional reservoirs of shale gas have become a central issue due to the increasing worldwide energy demand. Enhancing shale gas recovery by injecting CO2 is a promising technique that combines shale gas extraction and CO2 capture and storage (CCS) perfectly. In this study, a kerogen-based slit-shaped pore with a width of ∼21 Å was constructed by two kerogen matrices, and the grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulation methods were used to investigate the adsorption and diffusion properties of CH4 and CO2 in the kerogen matrix and slit nanopores and explore the displacement efficiency of the residual CH4 by CO2 in kerogen slit nanopores. The adsorption energy of CH4 and CO2 on the kerogen fragment surface and the isosteric heat of CH4 and CO2 in kerogen slit nanopores were examined to demonstrate the competitive adsorption of CO2 over CH4 in kerogen slit nanopores, and the different intensity of interactions between the CH4 and CO2 molecules with the pore surface plays a key role. An effective displacement process of the residual adsorbed CH4 by CO2 in kerogen slit nanopores was performed. The efficiency of displacement was enhanced with the increasing bulk pressure, and the sequestration amount of CO2 in kerogen slit nanopores increased at the same time. Moreover, it was found that part of CH4 adsorbed firmly inside the intrinsic pores of the kerogenmatrix was very hard to be displaced by the CO2 injection. This work demonstrates the microbehaviors of CH4 and CO2 in kerogen slit nanopores and the microscopic mechanism of the displacement of CH4 by CO2, for the purpose of providing useful guidance for enhancing shale gas extraction by injecting CO2.

In this work, grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulation methods were used to study the adsorption properties of CH4 and CO2 as single components and binary mixtures in modeled quartz nanopores (d ∼ 2 nm), of which the surface was hydroxylated to different degrees. The variation of the adsorption and molecular diffusion characteristics of CH4 and CO2 as a function of temperature and pressure were determined, and the competitive adsorption of CH4 and CO2 was investigated. As single components, both the adsorption of CH4 and CO2 in the nanopore is described well by the Langmuir model, and the diffusion capacities of the gas molecules in a non-supercritical state are much larger than that in a supercritical state. It was found that there is a tight adsorption layer of CH4 with a thickness of 3–5 Å in the nanopore, while CO2 molecules adsorb tightly as a whole phase, especially in the supercritical fluid state. In the binary mixed system, CO2 preferentially adsorbs to the nanopore surface compared to CH4 due to the strong interactions between the CO2 molecule and the hydrophilic groups on the pore surface. An obvious competitive adsorption of CO2 and CH4 occurs at certain temperature ranges (313–353 K) with increasing pressure. And the degree of surface hydroxylation has significant contributions to the adsorption selectivity of CO2 over CH4. This work provides microscopic information about adsorption properties of CH4 and CO2 in nanopores at the molecular level for the purpose of guidance towards the application of shale gas extraction by flowing CO2.

•The bulk phase aggregation and interfacial behavior of LGS were studied.•The foam stability of LGS changed with environmental conditions was investigated.•The gel could be formed in foam film and its strength influenced on the foam stability.In this paper, the bulk phase aggregation and interfacial adsorption properties as well as foam characteristics of a kind of amino acid-based surfactant sodium lauroylglutamate (LGS) under the variety of external conditions such as temperature and pH were investigated. The variation of the protonation degree of the head group of the surfactant as a function of pH was analyzed by integrating determination results of acid-base titration and FT-IR (ATR) spectra. At neutral pH, the LGS solution had the minimum surface tension and could form more stable foam. The mechanism about how the pH affects the foam stability of LGS was discussed thoroughly. TEM images of the LGS foam films and the bulk phase solutions showed that gel network association structure was formed not only in bulk phase in specific temperature range and suitable pH, but also in the foam films. The variation of the strength of the gel formed in foam film under different external conditions has obviously different influence on foam stability.

•Foamability of AES solutions was significantly enhanced by EPS.•EPS improved stability of AES foams.•EPS increased salt resistance of AES foams.•Interfacial and bulk interactions between AES and EPS were well studied.The foam properties, such as the foamability, foam stability, drainage, coalescence and bulk rheology, of aqueous solutions containing an eco-friendly exopolysaccharide (EPS) secreted by a deep-sea mesophilic bacterium, Wangia profunda SM-A87, and an anionic surfactant, sodium fatty alcohol polyoxyethylene ether sulfate (AES), were studied. Both the foamability and foam stability of the EPS/AES solutions are considerably higher than those of single AES solutions, even at very low AES concentrations, although pure EPS solutions cannot foam. The improved foamability and foam stability arise from the formation of the EPS/AES complex via hydrogen bonds at the interfaces. The synergism between the EPS and AES decreases the surface tension, increases the interfacial elasticity and water-carrying capacity, and suppresses the coalescence and collapse of the foams. The EPS/AES foams are more salt-resistant than the AES foams. This work provides not only a new eco-friendly foam with great potential for use in enhanced oil recovery and health-care products but also useful guidance for designing other environmentally friendly foam systems that exhibit high performance.

In this paper, the detailed behaviors of all the molecules, especially the interfacial array behaviors of surfactants and diffusion behaviors of gas molecules, in foam systems with different gases (N2, O2, and CO2) being used as foaming agents were investigated by combining molecular dynamics simulation and experimental approaches for the purpose of interpreting how the molecular behaviors effect the properties of the foam and find out the key factors which fundamentally determine the foam stability. Sodium dodecyl sulfate SDS was used as the foam stabilizer. The foam decay and the drainage process were determined by Foamscan. A texture analyzer (TA) was utilized to measure the stiffness and viscoelasticity of the foam films. The experimental results agreed very well with the simulation results by which how the different gas components affect the interfacial behaviors of surfactant molecules and thereby bring influence on foam properties was described.

This work reports the ordered self-assembly of nonconductive small molecules that achieved extra high conductivity, thereby stating an convenient approach for constructing a biofriendly soft material that is suitable to be used as implantable biosensors and electro-stimulated drug delivery systems. The microstructure and the conductive mechanism were investigated in detail by combining experimental methods and molecular simulation. This research demonstrated that self-assembly of amide groups with delocalized electrons into π-stacked arrays exhibited high mobilities for charge carriers. The excellent biocompatibility and processability of soft materials such as liquid crystals ensure that the system has high potential in the advance fields of biosensors and drug delivery devices.

An aqueous foam containing a zwitterionic surfactant dodecyl sulfobetaine (DSB) and an eco-friendly hyperbranched exopolysaccharide (EPS) secreted by a deep-sea mesophilic bacterium Wangia profunda SM-A87 was prepared for the first time. Compared with singular DSB solution, the EPS/DSB mixing solution exhibited better foamability and foam stability. The minimum DSB concentration (CDSB) needed to form a stable foam in the EPS/DSB solution was about one hundred times that in the DSB solution, and in a very large CDSB range, the EPS/DSB foam exhibited a better stability. The enhancement of foamability and foam stability of the complex solution arises from the hydrogen bonding and electrostatic interactions between the EPS and the DSB, and the hyperbranched structure of the EPS. The EPS/DSB foam shows great potential for application in enhanced oil recovery and health-care products.

Synergism of water-soluble hydrophobically associating polyacrylamide (HA-PAM) with nonionic surfactant laurel alkanolamide (LAA) was investigated via the rheology, fluorescence spectroscopy and dissipative particle dynamics (DPD) simulation methods. The viscosity and elasticity of HA-PAM solutions increased in a large LAA concentration range, which was principally ascribed to the crosslinking effect of LAA by aggregating to the hydrophobic chains of HA-PAM molecules as confirmed by the DPD simulation and experimental results. The crosslinking effect was enhanced in the presence of an appropriate amount of electrolyte or with increasing temperature in the studied temperature range of 20–70 °C. Thus, the LAA not only significantly enhanced the salt resistance of HA-PAM but also retarded the decrease of the viscosity and elasticity of the HA-PAM solutions at high temperature. The HA-PAM/LAA binary systems exhibit great potential for application in tertiary oil recovery of oil fields with high salinity.

The interfacial activity of surfactants is an issue that attracts considerable attention. However, most of the literature focuses on pure surfactants, and the effect of impurities on surfactant activity remains an elusive puzzle in the understanding of structure–performance relationships. In this paper, the nonionic surfactant laurel alkanolamide (LAA) synthesized by a one-step method was found to decrease the oil–water interfacial tension to ultra-low values at low concentration without co-surfactants. LAA also had surprising oil phase adaptability, making it a perfect model not only to study the theoretical mechanism for obtaining ultra-low interfacial tension, but also for distinguishing the contribution of the molecular interfacial array behaviour of surfactants and concomitants (defined as the residual reactants and by-product) on the interfacial activity. The oil–water interfacial activity of LAA with or without concomitants was investigated experimentally and theoretically using dissipative dynamic simulation (DPD). The detailed molecular array behaviour of LAA and the concomitants at the oil–water interface was investigated by molecular dynamics (MD). The concomitants were found to coexist with LAA at the oil–water interface and distribute at different positions in the interfacial layer, resulting in the high interfacial activity and oil phase adaptability. The experimental results agree well with the simulation results. We hope our strategy can provide an avenue to understand the structure–performance relationship of surfactants at the molecular level and reveal an effective approach to obtaining ultra-low interfacial tensions with different oil phases.

•The foam formed from HMPAA solution was ultra-stable.•The foam films characteristics were measured by FT-IR.•The mechanism of the foam stability was revealed by diverse techniques.Aqueous foam solely stabilized by a kind of hydrophobic modified water-soluble polymer, alkyl acrylate crosspolymer (HMPAA), was found to be extraordinary stable, no matter in static state or under disturbance, even if CO2 was used as gas agent. The high water-holding capacity of HMPAA foam film demonstrated by FoamScan and FT-IR measurement was in accordance with the low gas transmission through the foam film, which was detected by FT-IR, too. Fluorescence Microscope, TEM and Molecular Dynamic (MD) simulation were used to get information about the adsorption and array behavior of HMPAA on the gas/water interface and in foam film, it was found that the comb polymer molecules adsorbed on the interface clustered to form plat network, which covered the interface very well like a “shell”. By combining all these results, the mechanism of ultra high foam stability of HMPAA was revealed, and a novel approach to achieve long-term foam aqueous foam was proposed.Ultra-stable aqueous foam stabilized by water-soluble alkyl acrylate crosspolymer (HMPAA) was introduced. The HMPAA molecules adsorbed at the gas/water interface could interact with each other to form network structure through the hydrophobic force, and the interface could be covered very well. Besides, huge amount of water molecules was found to be strapped in the foam film and would not be drained out. Therefore the gas permeability of HMPAA foam film was low, and the coalescence of bubbles in the foam was postponed.

In this paper, ultrastable aqueous foam stabilized by a kind of flexible connecting bipolar-headed surfactant alkyl polyoxyethylene sulfate (AE3S) with coexisting Mg2+ was reported. Detailed molecular behaviors of AE3S in foam film with coexisting divalent cationic Ca2+ or Mg2+ were investigated by molecular dynamic simulation, comparing with the traditional surfactant sodium dodecyl sulfate (SDS), to find out how the microcharacter and array behavior of molecules in the foam film determined by molecular interaction effect the foam stability. It was found that the ultrastable foam film obtained by the cooperation of magnesium ions and AE3S was driven from two aspects: one is the favorable arrangement of surfactant molecules, and the other is the increase of capacity of foam films for resolutely holding water molecules deduced by a dipolar pair formed by the flexible connecting head groups of AE3S and hydrated Mg2+ via intermolecular coactions, both related to the presence of magnesium ions. Foam lamella stability measurement and foam decay method were both used to evaluate the stability of foam. Fourier transform infrared (FT-IR) was used to detect the composition variation of foam film in the drainage process; the vibration peak of OH for water molecule shifted from the 3390 cm–1 (being assigned to the bulk water integrated by hydrogen bonds) to 3685 cm–1 (being assigned to the vibration of isolated water molecules) for the ultrastable foam film after complete drainage, which agreed very well with the molecular simulation results.

•The interaction mechanism was studied at the molecular level.•The influence of hydration shell of ions was considered.•The direct ion–macromolecule interactions were very important in HMHPAM solution.How the inorganic cations (Na+, Mg2+, Ca2+, Cr3+ and Fe3+) affect the properties of partially hydrolyzed hydrophobically modified polyacrylamide (HMHPAM) in aqueous solution was studied via molecular dynamics simulations, transmission electron microscopy, dynamic light scattering, Fourier transformation infrared spectroscopy, viscometer and conductivity measurements. The hydration and dehydration states of cations and the interaction force of the hydrated cations to the charged groups of polymer have been investigated by MD, which provided a fresh look into the interactions between cationic ions and HMHPAM. It was found that the water absorbing ability of cations was not always central to the degree of the effect on properties of HMHPAM. The coordination interaction and Coulomb interaction between multivalent cations and HMHPAM were found to be overwhelmingly important which accommodate remarkable effect on the property of the polymers in solution.

A multiscale stability study of foams stabilized by sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (SDBS), and sodium polyoxyethylene alkylether sulfate (AES) was conducted, to investigate the relationship of surfactant molecular behavior and interfacial monolayer configuration of foam film to the foam film properties. Molecular dynamic (MD) simulations using a full-atom model was utilized to explore the microscopic features of the air/liquid interface layer. Several parameters such as the distribution of surfactant head groups and the order degree of surfactant hydrophobic tails were used to describe the molecular adsorption behavior. The effect of molecular structure on the nature of the foam film and the impact on the dynamic stability of wet foam is discussed. In the experimental evaluation, the SDBS foam films manifest strong stiffness and low viscoelasticity as shown by the interfacial shear rheology determination as well as texture analyzer (TA) measurement results, which agree very well with the array behavior of SDBS molecules at the air/water interface as described by the simulation results and is identified to be the reason for the poor dynamic stability. Comparing the molecular structure of SDS, SDBS, and AES, the special contributions of the linking groups such as the O atom, the phenyl group, and the EO (oxyethyl) chain to the interfacial array behavior of surfactants were characterized. It is concluded that microhardness of the foam film enhanced by rigid linking groups favors static foam stability but decreases the dynamic foam stability, while viscoelasticity of the foam film enhanced by soft linking groups increases the dynamic foam stability.

The adsorption behavior of zwitterionic surfactant dodecyl sulfobetaine (DBS) on a silica/solution interface with Ca2+, Mg2+ existing in aqueous solution is explored by atomistic molecular simulations. The interaction energy contribution of van der Waals and electrostatic potentials in the surfactants/water/silica system are respectively calculated, from which the electrical interaction can be found to play a decisive role in the adsorption tendency of DBS on the silica surface with or without inorganic ions, despite different mechanisms. The distinct decrease of energy has been found to be derived from electrical interaction when DBS adsorb on the silica surface covered by Ca2+ or Mg2+. Therefore, it can be predicted that the cationic ions combined on the negatively charged silica surface in a mineral water medium might decrease the adsorption trend of DBS on the silica surface, which has been experimentally proven by TOC measurement. Structural information of the close interface layer and the distribution of water molecules are analyzed after the complete molecular dynamics simulation using a ternary model. Ca2+ and Mg2+ combined on the silica surface can reduce the adsorption amount of DBS by preventing the direct interaction between DBS and surface, and bringing about the orientation reversal of DBS molecules to break the order of adsorption interface layer. Furthermore, changes in the status of the water spreading on the silica surface caused by the complexation of cations are also an important reason in the adsorption reduction.

The electrode-separated piezoelectric sensor (ESPS), an improved setup of quartz crystal microbalance (QCM), has been employed to investigate the adsorption behavior of nonionic surfactant Triton X-100 at the hydrophilic quartz-solution interface in mineralized water medium in situ, which contained CaCl2 0.01 mol·L−1, MgCl2 0.01 mol·L−1, NaCl 0.35 mol·L−1. In a large scale of surfactant concentration, t effects of Ca2+, Mg2+ and Na+ on the adsorption isotherm and kinetics are obviously different. In aqueous solution containing NaCl only, adsorption of Triton X-100 on quartz-solution interface is promoted, both adsorption rate and adsorption amount increase. While in mineralized water medium, multivalent positive ions Ca2+ and Mg2+ are firmly adsorbed on quartz-solution interface, result in the increasing of adsorption rate and adsorption amount at low concentration of surfactant and the peculiar desorption of surfactant at high concentration of Triton X-100. The results got by solution depletion method are in good agreement with which obtained by ESPS. The “bridge” and “separate” effect of inorganic positive ions on the adsorption and desorption mechanism of Triton X-100 at the quartz-solution interface is discussed with molecular dynamics simulations (MD), flame atomic absorption spectrometry (FAAS) and atomic force microscopy (AFM) methods.

Significant synergistic effects between sodium dodecylbenzene sulfonate (SDBS) and nonionic nonylphenol polyethylene oxyether, Triton X-100 (TX-100), at the oil/water interface have been investigated by experimental methods and computer simulation. The influences of surfactant concentration, salinity, and the ratio of the two surfactants on the interfacial tension were investigated by conventional interfacial tension methods. A dissipative particle dynamics (DPD) method was used to simulate the adsorption properties of SDBS and TX-100 at the oil/water interface. The experiment and simulation results indicate that ultralow (lower than 10−3 mN m−1) interfacial tension can be obtained at high salinity and very low surfactant concentration. Different distributions of surfactants in the interface and the bulk solution corresponding to the change of salinity have been demonstrated by simulation. Also by computer simulation, we have observed that either SDBS or TX-100 is not distributed uniformly over the interface. Rather, the interfacial layer contains large cavities between SDBS clusters filled with TX-100 clusters. This inhomogeneous distribution helps to enhancing our understanding of the synergistic interaction of the different surfactants. The simulation conclusions are consistent with the experimental results.Effect of salt on the distribution of SDBS and TX-100 on the O/W interface and in bulk solution. aHHaHH decreases and aWEaWE increases while salt concentration increases. Cyan and green beads represent head groups of SDBS and TX-100, respectively, and magenta and red beads represent tail chains of theirs. Water and oil beads are removed.

A new method for the synthesis of calcium carbonate nanoparticles using two types of foam which are separately stabilized by surfactants with opposite change has been described here. In detail, one type of foam is formed by the aqueous solution of CaCl2 which contains the anionic surfactant alkyl polyoxyethylene alcohol sulfate sodium (AES) and the other is formed by the aqueous solution of Na2CO3 and the cationic surfactant alkyl polyoxyethylene quaternary ammonium chloride (AEAC). Two types of foam contact each other in a specially designed apparatus after draining completely, then Ca2+ and CO32− entrapped by the surfactant layers at the thin borders between the foam bubbles react and result in the generation of CaCO3 nanoparticles. TEM determination showed that perfect mono-dispersed spherical nanoparticles were obtained and the size of particles can be conveniently controlled by changing the concentration of CaCl2 and Na2CO3. Also, the mechanism leading to the synthesis of CaCO3 nanoparticles was discussed in detail.

A simple and convenient method for the synthesis of ZnS nanoparticles using liquid foams as template is demonstrated in the paper. Two types of foam formed in a specially designed apparatus from aqueous solution of surfactant, respectively, containing C4H6O4Zn and Na2S contact each other after foam films drain completely and reach nanometer thickness, S2− and Zn2+ ions entrapped by the surfactant layers at the thin borders between the foam bubbles react resulting in the formation of ZnS nanoparticles. Attempts have also been made to control the size of ZnS nanoparticles via changing experimental conditions such as surfactant types, the solution concentration, etc. The effect of experimental parameters on the ZnS nanoparticles growth was studied in detail to investigate the mechanism leading to the synthesis of ZnS nanoparticles.

An apparatus containing a visual porous medium plate model and digital video recorder was employed to investigate the transportation of foam stabilized by sodium polyoxyethylene alkylether sulfate (AES), sodium dodecyl benzene sulfonate (SDBS) and TritonX-100 in porous medium. The results showed that transfiguration and fracture were the main transport manners for foam in the porous medium at high gas and liquid transfusion rate. The increase in probability of transfiguration in foam transport process corresponded to the higher flow impedance. A simple U-shape device was designed to investigate the rigidity of surfactant layer at the gas/liquid interface, and the equilibrium surface tension was assigned to be the key parameter which manifests the rigidity of surfactant interface layer. The dynamic surface tension of different surfactant system has also been measured, and the parameters gotten by Rosen model might be the measurement of dynamic elasticity of surfactant interface layer. There is consanguineous relation between the equilibrium surface activity or dynamic activity of the surfactants and the transport of the foam in the porous medium.

•Foam properties of a kind of novel anionic–nonionic surfactant AEC were studied.•AEC foam was used to remove Cd2+ from diluted solution.•The Cd2+ removal rate of AEC foam could be 99.8% under optimum conditions.•The Zeta potential and ITC were utilized to study the mechanism.In recent years, aqueous foam was known as an efficient technique with high potential on being used to remove heavy metal ions from the polluted water, not only because of the low cost, simple operation, but also ascribed to the high removal efficiency of trace heavy metal ions and would not cause secondary pollution to the environment. In this paper, the removal of Cd2+ from aqueous solution by aqueous foam stabilized by a kind of novel anionic–nonionic surfactant sodium trideceth-4 carboxylate (AEC) was investigated. The effect of conditions such as surfactant/metal ions molar ratio, surfactant concentration on the removal efficiency was studied. In large concentration range of surfactant, the removal rate was higher than 90%, and could reach up to 99.8% under the optimum conditions. The Zeta potential of gas bubbles in the AEC solutions was determined to verify the combination between the negative charged group heads of surfactant molecules and heavy metal ions, and isothermal titration calorimeter (ITC) determination was utilized to demonstrate the interaction, which helped to understand the mechanisms more clearly.