Investigation of interfacial and structural properties of CTAB at the oil/water interface using dissipative particle dynamics simulations

Promoting Cross-Link Reaction of Polymers by the Matrix-Filler Interface Effect: Role of Coupling Agents and Intermediate Linkers

The matrix-filler interface effect plays an important role in determining the structural stability and mechanical properties of polymer composite materials, which remain ambiguous and need to be studied. The network-forming dynamics of poly(3,3-bis (azidomethyl) oxetane-tetrahydrofuran) (PBT) at the ammonium perchlorate (AP) surface was studied by using atomistic molecular dynamics simulation, considering the additives of curing agent toluene diisocyanate (TDI), cross-linker trimethylolpropane (TMP), and coupling agent triethanolamine (TEA). The presence of the AP surface promotes chain cross-link reaction, which is attributed to the increased production of intermediate linkers formed by TDI, TMP, and TEA. The intermediate linker has three reactive sites that can react with PBT main chains to form a cross-linked structure. Owing to the strong interaction with the AP surface, the coupling agent TEA plays a dominant role in forming the intermediate linker. At the early stage of network forming (reaction ratio r < 30%), the AP surface adsorbs TEA, which leads to a maximum contact density to PBT. As r increases to 60%, the density of intermediate linkers near the AP surface reaches a maximum value. Consequently, the chain cross-link reactions between the intermediate linker and PBT main chains are enhanced as r > 60%. This work explains the micromechanism of the promotion of chain cross-link reaction by the interface effect and provides important insights on designing polymer materials with high mechanical properties.

Dissipative Particle Dynamics-Based Simulation of the Effect of Asphaltene Structure on Oil-Water Interface Properties

Asphaltenes are the main substances that stabilize emulsions during the production, processing, and transport of crude oil. The purpose of this research is to investigate the process of asphaltenes forming interfacial films at the oil-water interface by means of dissipative particle dynamics (DPD) and the effect of asphaltenes of different structures on the oil-water interface during the formation of interfacial film. It is demonstrated that the thickness of the interfacial film formed at the oil-water interface gradually increases as the asphaltene concentration rises and the amount of asphaltene adsorbed at the oil-water interface gradually multiplies. Both the number and type of heteroatoms in asphaltenes affect the interfacial behavior of asphaltenes. The interface activity of asphaltenes can be enhanced by increasing the number of heteroatoms in the asphaltene, and the type of heteroatom affects as well the interfacial activity of the asphaltene as it affects the aggregation behavior of the asphaltene in the system. As the number of asphaltene aromatic rings increases, the oil-water interfacial tension (IFT) trends down gradually, while the effect of alkyl side chains on the reduction of IFT of asphaltenes is different, and asphaltenes with medium length alkyl side chains can reduce IFT more efficiently.

Novel Anionic-Nonionic Surfactant Based on Water-Solid Interfacial Wettability Control for Residual Oil Development

Irreversible colloidal asphaltene adsorption layers are formed on formation rock surfaces due to long-term contact with crude oil, and large amounts of crude oil adhere to these oil-wet layers to form residual oil films. This oil film is difficult to peel off due to the strong oil-solid interface effect, which seriously restricts further improvement in oil recovery. In this paper, the novel anionic-nonionic surfactant sodium laurate ethanolamide sulfonate (HLDEA) exhibiting strong wetting control was synthesized by introducing sulfonic acid groups into the nonionic surfactant laurate diethanolamide (LDEA) molecule through the Williamson etherification reaction. The introduction of the sulfonic acid groups greatly improved the salt tolerance and the absolute value of the zeta potential of the sand particles. The experimental results showed that HLDEA altered the wettability of the rock surface from oleophilic to strongly hydrophilic, and the underwater contact angle increased substantially from 54.7 to 155.9°. In addition, compared with LDEA, HLDEA exhibited excellent salt tolerance and enhanced oil recovery performance (the oil recovery was improved by 19.24% at 2.6 × 10⁴ mg/L salinity). Based on nanomechanical experimental results, HLDEA was efficiently adsorbed on the core surfaces and regulated microwetting. Moreover, HLDEA effectively reduced the adhesion force between the alkane chains and the core surface, which facilitated residual oil stripping and oil displacement. This new anionic-nonionic surfactant affording great oil-solid interface wetting control has practical significance for the efficient development of residual oil.

Comprehensive review of the interfacial behavior of water/oil/surfactant systems using dissipative particle dynamics simulation

A comprehensive understanding of interfacial behavior in water/oil/surfactant systems is critical to evaluating the performance of emulsions in various industries, specifically in the oil and gas industry. To gain fundamental knowledge regarding this interfacial behavior, atomistic methods, e.g., molecular dynamics (MD) simulation, can be employed; however, MD simulation cannot handle phenomena that require more than a million atoms. The coarse-grained mesoscale methods were introduced to resolve this issue. One of the most effective mesoscale coarse-grained approaches for simulating colloidal systems is dissipative particle dynamics (DPD), which bridges the gap between macroscopic time and length scales and molecular-scale simulation. This work reviews the fundamentals of DPD simulation and its progress on colloids and interface systems, especially surfactant/water/oil mixtures. The effects of temperature, salt content, a water/oil ratio, a shear rate, and a type of surfactant on the interfacial behavior in water/oil/surfactant systems using DPD simulation are evaluated. In addition, the obtained results are also investigated through the lens of the chemistry of surfactants and emulsions. The outcome of this comprehensive review demonstrates the importance of DPD simulation in various processes with a focus on the colloidal and interfacial behavior of surfactants at water-oil interfaces.

Janus Nanoparticle and Surfactant Effects on Oil Drop Migration in Water under Shear

The effects of surface-active nanoparticles and surfactants on the behavior of oil-water interfaces have implications for a variety of industrial processes related to multiphase flows including separation processes, enhanced oil recovery, and environmental remediation. In this work, the migration of an oil droplet in shear flow is investigated with the presence of surface-active molecules and nanoparticles at the oil-water interface. Pure oil (heptadecane) in water and oil with the presence of Janus nanoparticles (JPs) and/or octaethylene glycol monododecyl ether, a nonionic surfactant, were examined using coarse-grained computations. The shear flow field was created utilizing a Couette flow, where the top wall of a channel moved with a specified velocity and the bottom wall was kept stationary. The dissipative particle dynamics (DPD) method was applied. The oil drop was placed on the stationary wall, and its displacement was recorded over time. When surfactants were added at the oil-water interface, the slip of the water over the oil drop was reduced, leading to a larger displacement of the drop. Moreover, surfactant molecules tended to concentrate toward the rear side of the oil drop rather than the front as the drop moved in the flow field. The presence of only JPs on the oil-water interface resulted in slower droplet migration. In the presence of both JPs and surfactants, the effect of JPs on the oil-surfactant-water system was investigated by changing the number of JPs on the drop surface while keeping the concentration of the surfactant constant. Under the same shear rate, the droplet's migration speed increased in the presence of both surfactants and JPs compared to the case of bare oil. The JPs appeared to follow a repeated pattern of motion while residing close to the solid substrate-oil drop contact line. These findings elucidate the contribution of both surfactants and JPs on oil drop displacement for enhanced oil recovery or remediation of an oil-contaminated subsurface.

Designing anisotropic inorganic nanocapsules via self-assembly of polymer-like ultrathin Au nanowires

Anisotropic assembly of nanomaterials into hollow structures is an attractive technique in biomedicine and biosensing. Commonly used polymer materials are easy to assemble yet it is hard to form anisotropic morphologies. Here in this work, we successfully prepared a novel gold nanocapsule with an anisotropic ellipsoidal shape and cavity structure by the self-assembly of ultrathin Au nanowires. The assembly mechanism is further studied by tuning the assembly conditions such as nanowire concentration, solvent composition, and temperature. It is found that the controlling forces of the nanowire assembly process are mainly the symmetric interfacial tension and the asymmetric nanowire deformation potential, which contribute together to result in anisotropic nanocapsules. Finally, the obtained Au nanocapsules were used as nanocarriers to load pyrene as a model drug, showing great drug loading ability and pH-responsive drug release behavior. We believe that this unique anisotropic assembly product will bring new insights into nanostructure design and soft matter research.

Predicting nanoemulsion formulation and studying the synergism mechanism between surfactant and cosurfactant: A combined computational and experimental approach

Nanoemulsion (NE) is a dosage form widely used in pharmaceutical, food, agrochemical, cosmetics, and personal care industries. NE systems are usually formulated through trial and error via numerous semi-empirical experiments. Moreover, the complex interaction mechanisms between the formulation surfactant and cosurfactant are difficult to understand. Dissipative particle dynamics (DPD) may be helpful in solving these formulation problems. Silibinin is a flavonolignan isolated from milk thistle, which has demonstrated antioxidant and antimicrobial effects. For this project, silibinin-loaded nanoemulsion (SBNE) was formulated by DPD, including surfactant and cosurfactant screening, pseudo-ternary phase construction, and SBNE characterization, all of which were verified by experimentation. Most importantly, this work shows that DPD can be adopted to explore the synergetic mechanisms between the surfactant and cosurfactant, including emulsification efficiency, distance, angle, arrangement, and order parameter. Additional verification experiments on the antioxidant and antimicrobial applications of simulation-designed SBNE were also carried out and confirmed DPD-predicted results. As such, predicting NE formulation by DPD has been proven to be feasible. For SBNE, the addition of PEG400 cosurfactant stretches the Cremophor RH40 surfactant molecules and assists in a more orderly arrangement. An enhanced interfacial thickness in SBNE could be attributed to the stretched hydrophilic head group and the decreased angle between the molecular axis and interface normal. These DPD and experimentally-verified results indicated that a proper cosurfactant will enhance the interfacial thickness, decrease the consumption of surfactant, and benefit NE formation. This new computationally applied knowledge should facilitate optimizing, designing, and understanding NE formulation more rationally and scientifically.

Potential dependence of the ionic structure at the ionic liquid/water interface studied using MD simulation

The structure at the electrochemical liquid/liquid interface between water (W) and trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide, a hydrophobic ionic liquid (IL), was studied using molecular dynamics (MD) simulation in which the interfacial potential difference was controlled. On the IL side of the IL/W interface, ionic multilayers were found in the number density distribution of IL ions whereas monolayer-thick charge accumulation was found in the charge density distribution. This suggests that the potential screening is completed within the first ionic layer and the effect of overlayers on the potential is marginal. The W side of the interface showed the diffuse electric double layer as expected, and unexpectedly unveiled a density depletion layer, indicating that the IL surface is hydrophobic enough to be repelled by water. The IL ions in the first ionic layer showed anisotropic orientation even at the potential of zero charge, in which the polar moieties were oriented to the W side and the non-polar moieties preferred parallel orientation to the interface. When an electric field is applied across the interface so that the IL ions are more accumulated, the non-polar moieties changed the parallel preference to more oriented to the IL side due to the dipolar nature of the IL ions. The ionic orientations at the IL/W interface were compared with those at other two IL interfaces, the vacuum and graphene interfaces of the IL. The parallel preference of the non-polar moieties was similar at the IL/graphene interface but different from the perpendicular orientation toward the vacuum side at the IL/vacuum interface. The comparison suggests that water behaves like a wall that repels IL ions like a solid electrode.

Spotlight onto surfactant-steam-bitumen interfacial behavior via molecular dynamics simulation

Heavy oil and bitumen play a vital role in the global energy supply, and to unlock such resources, thermal methods, e.g., steam injection, are applied. To improve the performance of these methods, different additives, such as air, solvents, and chemicals, can be used. As a subset of chemicals, surfactants are one of the potential additives for steam-based bitumen recovery methods. Molecular interactions between surfactant/steam/bitumen have not been addressed in the literature. This paper investigates molecular interactions between anionic surfactants, steam, and bitumen in high-temperature and high-pressure conditions. For this purpose, a real Athabasca oil sand composition is employed to assess the phase behavior of surfactant/steam/bitumen under in-situ steam-based bitumen recovery. Two different asphaltene architectures, archipelago and Island, are used to examine the effect of asphaltene type on bitumen's interfacial behavior. The influence of having sulfur heteroatoms in a resin structure and a benzene ring's effect in an anionic surfactant structure on surfactant-steam-bitumen interactions are investigated systematically. The outputs are supported by different analyses, including radial distribution functions (RDFs), mean squared displacement (MSD), radius of gyration, self-diffusion coefficient, solvent accessible surface area (SASA), interfacial thickness, and interaction energies. According to MD outputs, adding surfactant molecules to the steam phase improved the interaction energy between steam and bitumen. Moreover, surfactants can significantly improve steam emulsification capability by decreasing the interfacial tension (IFT) between bitumen and the steam phase. Asphaltene architecture has a considerable effect on the interfacial behavior in such systems. This study provides a better and more in-depth understanding of surfactant-steam-bitumen systems and spotlights the interactions between bitumen fractions and surfactant molecules under thermal recovery conditions.

Closed-Form Coexistence Equation for Phase Separation of Polymeric Mixtures in Dissipative Particle Dynamics

To date, no extensive study of the phase diagram for binary fluid mixtures in dissipative particle dynamics (DPD) has been published. This is especially pertinent for newer parameterization schemes where the self-self interaction, or the effective volume, of different particle types is varied. This work presents an exhaustive study of the parameter space concerning DPD particles with soft interaction potentials. Moreover, we propose a closed-form coexistence equation or binodal curve that is inspired by the Flory-Huggins model. This equation describes the phase diagram of all binary mixtures made up out of monomers, homopolymers, and the mixtures thereof when self-self interactions are varied. The mean absolute percentage error (MAPE) of the equation on simulated data, including validation simulations, is 1.02%. The equation can a priori predict the phase separation of mixtures using only DPD interaction parameters. The proposed coexistence equation can therefore be used to directly validate interaction parameters resulting from novel parameterization schemes, including coarse graining and equations of state, without the need for additional simulations. Finally, it is shown that the choice of bond potential markedly influences phase behavior.

Cationic zinc (II) phthalocyanine nanoemulsions for photodynamic inactivation of resistant bacterial strains

BACKGROUND

The growing emergence of microbial resistance to antibiotics represents a worldwide challenge. Antimicrobial photodynamic inactivation (aPDI) has been introduced as an alternative technique, especially when combined with nanotechnology. Therefore, this study was designed to investigate the therapeutic merits of combined aPDI and nanoemulsion in infections caused by resistant bacterial strains.

METHODS

Cationic zinc (II) phthalocyanine nanoemulsions (ZnPc-NE) were prepared using isopropyl myristate (IPM) as oil phase, egg phosphatidylcholine (egg PC) as emulsifier, and N-cetyl-N,N,N-trimethyl ammonium bromide (CTAB). Nanoemulsions were characterized for particle size, polydispersity, zeta potential, viscosity, and skin deposition. The in-vitro aPDI was investigated on human resistant pathogens; gram-positive methicillin-resistant Staphylococcus aureus (MRSA) and gram-negative Multidrug-resistant strain of Escherichia coli (MDR E. coli), under different experimental conditions. In addition, in-vivo model of abrasion wound infected by MDR E. coli was induced in rats to investigate the therapeutic potential of the selected formulation.

RESULTS

It was evident that the selected ZnPc formulation (20 % IPM, 2 % egg PC and 0.5 % CTAB) displayed a particle size of 209.9 nm, zeta potential +73.1 mV, and 23.66 % deposition of ZnPc in skin layers. Furthermore, the selected formulation combined with light achieved almost 100 % eradication of the two bacterial strains, with superior bacterial load reduction and wound healing propertiesin-vivo, compared to either the nanoemulsion formulation or laser alone.

CONCLUSION

ZnPc nanoemulsion improved antimicrobial photodynamic therapy in inactivating resistant bacterial infections and provided a promising therapeutic means of treating serious infections, and hence could be applied in diseases caused by other bacterial strains.

Synthesis of Guerbet Amine-Oxide for Enhanced Oil Recovery

Guerbet amine-oxide was synthesized by using Guerbet acid, N,N′-diethyl-1,3-propanediamine and hydrogen peroxide. Its structure was confirmed by FT-IR spectra and mass spectra. The critical micelle concentration (CMC) and the interfacial tension between the crude oil and water were measured. The results showed that the CMC of the Guerbet amine-oxide is 2.14 × 10−5 mol/L and the interfacial tension could be lowered to ultralow levels. The core flood test showed that the surfactant and polymer binary system could increase oil recovery by 24.7% above the water flood.

Molecular Dynamics Simulation Insight into Interfacial Stability and Fluidity Properties of Microemulsions

Although the interfacial properties of microemulsions have been extensively studied in both experimental and simulation research studies, the molecular mechanisms of stability and fluidity about microemulsion are still poorly understood. Herein, we report a molecular dynamics simulation study to elaborate the motion of an emulsion droplet involving dichain surfactant Aerosol OT (AOT) and its dynamics evolution at the oil-water interface. By varying the concentrations of AOT, we show that the interfacial thickness and emulsification rate display a piecewise change as the interfacial coverage increases and the W/O emulsion is more stable than the O/W one while O/W emulsion presents better fluidity. In addition, the dispersed system combined with water/AOT/n-heptane tends to form a W/O microemulsion instead of an O/W microemulsion due to the structural collapse of the latter. This work provides a molecular understanding of microemulsion interfacial stability and fluidity.

Takes Two to Tango: Choreography of the Coadsorption of CTAB and Hexanol at the Oil-Water Interface

Mixed surfactant systems at the oil-water interface play a vital role in applications ranging widely from drug delivery to oil-spill remediation. Synergistic mixtures are superior emulsifiers and more effective at modifying surface tension than either component alone. Mixtures of surfactants with dissimilar polar head groups are of particular interest because of the additional degree of control they offer. The interplay of hydrophobic and electrostatic effects in these systems is not well understood, in part because of the difficulty in examining their behavior at the buried oil-water interface where they reside. Here, surface-specific vibrational sum frequency spectroscopy is utilized in combination with surface tensiometry and computational methods to probe the cooperative molecular interactions between a cationic surfactant cetyltrimethylammonium bromide (CTAB) and a nonionic alcohol (1-hexanol) that induce the two initially reluctant surfactants to coadsorb synergistically at the interface. A careful deuteration study of CTAB reveals that hexanol cooperates with CTAB such that both molecules preferentially orient at the interface for sufficiently large enough concentrations of hexanol. This work's methodology is unique and serves as a guide for future explorations of macroscopic properties in these complex systems. Results from this work also provide valuable insights into how interfacial ordering impacts surface tensiometry measurements for nonionic surfactants.

Dynamics of Surfactant Clustering at Interfaces and Its Influence on the Interfacial Tension: Atomistic Simulation of a Sodium Hexadecane-Benzene Sulfonate-Tetradecane-Water System

The process of equilibration of the tetradecane-water interface in the presence of sodium hexadecane-benzene sulfonate is studied using intensive atomistic molecular dynamics simulations. Starting as an initial point with all of the surfactants at the interface, it is obtained that the equilibration time of the interface (several microseconds) is orders of magnitude higher than previously reported simulated times. There is strong evidence that this slow equilibration process is due to the aggregation of surfactants molecules on the interface. To determine this fact, temporal evolution of interfacial tension and interfacial formation energy are studied and their temporal variations are correlated with cluster formation. To study cluster evolution, the mean cluster size and the probability that a molecule of surfactant chosen at random is free are obtained as a function of time. Cluster size distribution is estimated, and it is observed that some of the molecules remain free, whereas the rest agglomerate. Additionally, the temporal evolution of the interfacial thickness and the structure of the surfactant molecules on the interface are studied. It is observed how this structure depends on whether the molecules agglomerate or not.

Alpha-linolenic acid-loaded oil/water microemulsion: Effects of phase behaviour simulation and environmental stress on phase stabilizing and anti-oxidation capacity

α-Linolenic acid (ALA)-loaded microemulsion (ME) was prepared from isoamyl acetate, polyoxyethylene ether 35 (EL-35), ethanol and water. The dynamic phase behaviour was simulated using dissipative particle dynamics (DPD), which showed that spherical ME was formed at water/oil ratios of 1:9 and 9:1, while a lamellar structure with distinctive water-course and oil layer appeared at ratios of 3:7, 5:5, and 7:3. Phase stabilizing and anti-oxidation effect of environmental stresses on ALA-loaded microemulsion were investigated. Results showed that the ME region was large and had good environmental tolerance. Subsequently, the investigation of anti-oxidation stability revealed that more than 60% ALA of ALA-loaded ME could be protected from oxidation under environmental stresses. Furthermore, ALA-loaded ME was applied in aqueous-based foods. The transparency, precipitate, stratification and phase separation were used to evaluate influence of ME on product properties, confirming great feasibility and stability of ALA-loaded ME for practical applications.

Interfacial Properties and Monolayer Collapse of Alkyl Benzenesulfonate Surfactant Monolayers at the Decane-Water Interface from Molecular Dynamics Simulations

The molecular structure of a surfactant molecule is known to have a great effect on the interfacial properties and the type of nanostructures formed. In this work, we have performed molecular dynamics simulations on six isomers of an alkyl benzenesulfonate surfactant to investigate the effect of the degree and position of aromatic substitution on the interfacial properties and on the collapse of the surfactant monolayer at a decane-water interface. The surface pressure of the monolayers was shown to increase with increasing surface coverage, until some of the monolayers become mechanically unstable and form large undulations. Shifting the primary alkyl chain of the surfactant from the para to the meta position was found to significantly affect the orientation of the surfactant head groups, while the attachment position of the benzene ring along the primary alkyl chain plays a greater role in the orientation of the surfactant tails. In general, to the extent considered in this work, our results suggest that additional alkyl substitution and meta substitution of the primary alkyl chains increase both the effectiveness and efficiency of the surfactants, and accelerate the onset of monolayer collapse. The interface was found to consist of an inner Helmholtz layer of partially dehydrated counterions in contact with the surfactant head groups, an outer Helmholtz layer of hydrated counterions, and a diffuse layer. The di- and trisubstituted surfactants formed nearly spherical swollen micelles encapsulating pure decane, which effectively solubilizes decane in water as a microemulsion. The monosubstituted surfactants formed elongated buds that protrude from the interface, but did not detach from the monolayer. To our knowledge, the role of aromatic substitution on interfacial properties has not been investigated by molecular simulations previously. The results from this work could provide insights to design improved surfactants by exploiting aromatic substitution to encapsulate material for drug delivery and other applications.

Mesoscopic probes in asphaltenes nanoaggregate structure: from perpendicular to paralleled orientation at the water-in-oil emulsions interface

The asphaltenes nanoaggregate structures at water-in-oil emulsions interface were majorly mediated by the molecular weights of the asphaltenes as changed from perpendicular to parallel orientation at the interface with increasing molecular weight.

Amphiphilic nanosheet self-assembly at the water/oil interface: computer simulations

The self-assembly of amphiphilic Janus triangular-plates at the water/oil interface is simulated for the first time.