•The self-assembly of SDBS on nanosized graphene surface had been simulated.•Morphology with SDBS adsorption on multilayer graphene nanosheets was exhibited.•The interaction between graphene sheets encapsulated by SDBS micelles had been computed.Self-assembly of surfactant molecules on the graphene surface is of great importance for better preparation and application of graphene nanoparticles. Herein, classical molecular dynamics (MD) simulation has been performed to study the adsorption self-assembly of sodium dodecylbenzene sulfonate (SDBS) on nanosized graphene sheets. The effect of surface coverage on the self-assembly structure has been thoroughly investigated by quantifying various interfacial properties. In particular, we probed unique supramolecular morphology with SDBS adsorption on multilayer graphene nanosheets. Under higher surfactant concentration, multilayered micelles can be formed on the graphene surfaces. We also simulated the potential of mean force (PMF) between two nanoscale graphene sheets covered by SDBS surfactants in order to understand the interaction mechanism in the SDBS-based graphene dispersion and stabilization. This result verifies that the SDBS surfactants have ability to disperse and stabilize graphenes.

The interfacial force for exfoliating a graphene monolayer from graphite in solvent media was studied by restrained molecular dynamics simulations. Three solvents, N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), and water, were considered. The interfacial structures show that NMP and DMSO have a stronger affinity with graphene surfaces. In the solvent media, there exists an inherent attractive force hindering the exfoliation, which is almost exclusively determined by the interaction between graphene sheets. Along the perpendicular exfoliation direction (relative to the graphene plane), the initial exfoliation is less dependent upon solvent conditions, and the subsequent exfoliation can be facilitated by the solvent-induced interaction. However, along the shift direction parallel to the graphene plane, the organic solvent provides a favorable driving force to assist the exfoliation, whereas water offers obstructing effect. The parallel shift of graphene requires less external power than the perpendicular shift for our simulated systems. The confined solvent molecules between graphene sheets play an important role in exfoliating and stabilizing graphene in solvent media. This result provides a microscopic understanding of the function of solvent-induced interaction in the solvent exfoliation of graphene.

•The wettability of solid surface can be modified with solvent.•Water droplet can be separated from hydrophobic surface by solvent.•Result suggests that aqueous contaminant on surfaces might be removed by dense solvent.In this work, molecular dynamics simulations were performed to investigate the wetting behavior of solid surfaces in the presence of model solvent. Three kinds of solid surfaces were considered with different hydrophilicities. We simulated the microscopic contact angles of water droplet on the solid surfaces by analyzing the phase boundary locus at the water/solvent/solid interfaces. It was observed that the contact angle generally increases with the surrounding solvent density. This behavior is qualitatively consistent with the available limited experimental observation. However, at very high solvent density, the water droplet was found to separate with the hydrophobic surface. This simulation study provides direct microscopic evidence for the effect of solvent on the wettability of a solid substrate. The wetting phenomenon was interpreted in terms of free energy analysis. It was shown that the wetting behavior is determined by a competition effect between the water–solvent interaction and the surface–water interaction.

The interaction between surfactant-coated graphenes plays a critical role in the performance of surfactant-stabilized graphene dispersion. Herein, we quantified the interaction by simulating the potential of mean force (PMF) between two graphenes encapsulated in sodium dodecyl sulfate (SDS) surfactant micelles. It is observed that adsorbed SDS surfactants produce a long-range free energy barrier, hindering the aggregation of graphenes. Both increasing surfactant coverage and introducing electrolyte (CaCl2) can lead to an enhanced repulsive nature of PMF. Through splitting the total PMF into various contributions, the precise interaction mechanism of graphenes in aqueous SDS environment has been demonstrated. Furthermore, our result reveals the role of electrolyte ions in the modifying the interaction between the SDS/graphene assemblies, which cannot be accounted for by the traditional Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. This result might show a possible microscopic evidence or explanation on the recently reported experiments. Additionally, a further analysis for SDS self-assembly morphology on graphene surface was used to explain the molecular origin of the electrolyte-induced structure transformation. The salt bridges formed between electrolyte cations and surfactants anions are confirmed to cause the structure change in the SDS/graphene assembly. This work provides a correlation analysis between the supramolecular self-assembly nanostructure and the interfacial interaction.

Self-assembly of amphiphilic molecules on the surfaces of nanoscale materials has an important application in a variety of nanotechnology. Here, we report a coarse-grained molecular dynamics simulation on the structure and morphology of the nonionic surfactant, n-alkyl poly(ethylene oxide) (PEO), adsorbed on planar graphene nanostructures. The effects of concentration, surfactant structure, and size of graphene sheet are explored. Because of the finite dimension effect, various morphological hemimicelles can be formed on nanoscale graphene surfaces, which is somewhat different from the self-assembly structures on infinite carbon surfaces. The aggregate morphology is highly dependent on the concentration, the chain lengths, and the size of graphene nanosheets. For the nonionic surfactant, the PEO headgroups show strong dispersion interaction with the carbon surface, leading to a side edge adsorption behavior. This simulation provides insight into the supramolecular self-assembly nanostructures and the adsorption mechanism for the nonionic surfactants aggregated on graphene nanostructures, which could be exploited to guide fabrication of graphene-based nanocomposites.

In this work, molecular dynamics simulations were performed to study the process and mechanism of fullerene (C60) encapsulation into single-walled carbon nanotube (SWNT) in three kinds of solvents: supercritical CO2 (scCO2), CS2, and water. It was demonstrated that a C60 molecule could spontaneously insert into the SWNT in scCO2, which is in good agreement with the experimental observation. However, no encapsulation of C60 was observed in CS2 and water during the simulation period. We also, for the first time, simulated the PMF (potential of mean force) profile along the C60 inserting path. The occurring free energy barrier near the entrance of the SWNT could hinder the insertion of C60, which is determined by the interaction strength between solvent and C60, as well as the solvation structures around C60. The C60–SWNT interaction provides the driving force in the filling, whereas the solvent-induced force instead offers an obstructing effect. It is revealed that both the solvent-induced free energy barrier and the change of free energy cooperatively determine whether C60 can fill a SWNT. Our simulation result provides an insightful understanding of the role of solvents on the encapsulation of molecules into SWNTs, which is important for further development of low-temperature nanotube filling techniques.

All-atomic molecular dynamics simulations have been performed to study the interfacial structural and dynamical properties of passivated gold nanoparticles in supercritical carbon dioxide (scCO2). Simulations were conducted for a 55-atom gold nanocore with thiolated perfluoropolyether as the packing ligands. The effect of solvent density and surface coverage on the structural and dynamical properties of the self-assembly monolayer (SAM) has been discussed. The simulation results demonstrate that the interface between nanoparticle and scCO2 solvent shows a depletion region due to the preclusion of SAM. The presence of scCO2 solvent around the passivated Au nanoparticle can lead to an enhanced extension of the surface SAM. Under full coverage, the structure and conformation of SAM are insensitive to the density change of scCO2 fluid. This simulation results clarify the microscopic solvation mechanism of passivated nanoparticles in supercritical fluid medium and is expected to be helpful in understanding the scCO2-based nanoparticle dispersion behavior.Graphical abstractMolecular simulations have been performed to study the interfacial structural and dynamical properties of gold nanoparticles, passivated by thiolated perfluoropolyether, in supercritical carbon dioxide.Research highlights► The solvation behavior of passivated gold nanoparticles in supercritical carbon dioxide was simulated. ► The presence of scCO2 solvent around the nanoparticle can lead to an enhanced extension of the surface self-assembly monolayer. ► The effect of solvent density and surface coverage on the structures and properties of the self-assembly monolayer has been discussed.

In this work, Monte Carlo simulations have been carried out to investigate the swelling stability and interlayer structures of alkylammonium-modified montmorillonite both in vacuum and in supercritical CO2 (scCO2) fluid. In the vacuum (dry) condition, the stable spacing for this kind of organoclay was determined based on the energy minimum. In the stable spacing, the corresponding interlayer structure of dry organoclay is the monolayer arrangement with the intercalated surfactant chains lying parallel to the silicate surface. In scCO2 fluid medium, the normal pressures within the organoclay gallery and the swelling free energy have been obtained from Gibbs ensemble Monte Carlo simulation. The mechanically and thermodynamically stable spacings of the organoclay have been determined. As compared with the case in vacuum, the simulation shows that the swelling of the organoclay is thermodynamically favorable in the environment of scCO2 fluid. The interlayer structure and conformation have been used to analyze the mechanism of swelling. The headgroups of surfactant cations are distributed close to the clay surfaces. The presence of CO2 molecules within the clay gallery can cause a specific steric arrangement of the long-chain alkylammonium cations.

Here we report a larger-scale atomic-level molecular dynamics (MD) simulation for the self-assembly of sodium dodecyl sulfate (SDS) surfactant on single-walled carbon nanotube (SWNT) surfaces and the interaction between supramolecular SDS/SWNT aggregates. We make an effort to address several important problems in regard to carbon nanotube dispersion/separation. At first, the simulation provides comprehensive direct evidence for SDS self-assembly structures on carbon nanotube surfaces, which can help to clarify the relevant debate over the exact adsorption structure. We also, for the first time, simulated the potential of mean force (PMF) between two SWNTs embedded in SDS surfactant micelles. A novel unified PMF approach has been applied to reveal various cooperative interactions between the SDS/SWNT aggregates, which is different from the previous electrostatic repulsion explanation. The unique role of sodium ions revealed here provides a new microscopic understanding of the recent experiments in the electrolyte tuning of the interfacial forces on the selective fractionation of SDS surrounding SWNTs.

A coarse-grained (CG) model for perfluorocarbons (PFCs) with arbitrary chain length has been established in this work. The construction of the CG model is based on the all-atomic (AA) simulation results and the thermodynamics experimental data. The intramolecular parameters are obtained through reproducing the intramolecular bond and angle distributions from AA simulations. The intermolecular parameters are determined by fitting the experimental bulk densities and surface tensions. Comparison between the CG and AA simulations has confirmed that the CG model can reasonably represent the structure behavior of PFCs. Furthermore, the CG model has been also extended to simulate the vapor−liquid phase equilibria for the mixture of PFCs and carbon dioxide by using the Gibbs ensemble Monte Carlo (GEMC) method. As compared with the experimental data, the GEMC simulation based on the CG model satisfactorily reproduces the mixture phase equilibria. The developed CG model will be useful in simulating the self-assembly of PFC-based surfactants/blockpolymers in supercritical carbon dioxide fluid.

Molecular dynamics simulations were performed to investigate the structural properties of aqueous reverse micelles of AOK (aerosol-octyl-ketone), a non-fluorinated hydrocarbon surfactant with two symmetrical carbonyl groups in the tail, in supercritical CO2 fluid. The result for the first time demonstrates the atomic-level structural picture for the formation of nanodomain in water-in-CO2 microemulsion based on the AOK surfactant, which is in good agreement with the recent experiment. The dynamics process shows that the self-assembly of the reverse micelle takes a relatively short period of time (∼4 ns) and remains stable over 50 ns. For comparison, the simulation for the AOK/scCO2 system has also been performed and an irregular and sparse reverse micelle structure can be formed in the absence of water molecules. The radial density profiles, the pair radial distribution functions and the orientation distribution have been calculated to investigate the interfacial structure properties. The carbonyl groups in the surfactant tails may enhance the solvation of the AOK surfactant, thereby affording to stabilize the reverse micelle.

We used molecular dynamics simulation to demonstrate the microscopic wetting behavior of two solid model surfaces for the first time. Hydrophilic and hydrophobic features were modeled in a dense CO2 fluid environment under various densities. The water droplet loses contact with the surface under the influence of higher density CO2 fluids on the hydrophobic surface. For the hydrophilic surface, no separation between the water droplet and the surface was observed. However, the contact angle of the water droplet on the hydrophilic surface was found to increase with the fluid density. The effect of dense CO2 fluid on the surface wettability can be interpreted in terms of enhanced interactions from the surrounding CO2 molecules.

Molecular dynamics (MD) simulations have been used to investigate the melting processes of 55-atom and 561-atom Pt–Au nanoalloys with random (RD) and core-shell (CS) orderings. The simulation results show that the Pt–Au CS nanoalloys have higher thermal and structural (including geometrical shape and chemical ordering) stability than the RD ones with the same size and composition. For all the CS nanoalloys studied, their geometric shape and chemical ordering are preserved well before the complete melting transition occurs and their premelting corresponds to the surface melting. In the RD ordering cases, nevertheless, obvious shape distortion and chemical order transformation are observed during the premelting stage. The nature of premelting of the 55-atom RD nanoalloy is not surface melting but dynamic coexistence melting. Additionally, the melting behavior of the RD nanoalloys is found to depend on the particle size. Several separated ordering transformation stages associated with the mutual conversion of different geometrical structures can be observed in the smaller 55-atom particle instead of the 561-atom case. These results suggest that different atomic orderings of nanoalloys can lead to distinctive melting features.

A modified non-local free energy density functional theory (NDFT) model, with the consideration of the nonadditivity term of solid-fluid and fluid-fluid interactions and finite pore wall thickness (≈2 layers), was developed to model the confined fluid mixtures in slit pore. This improved NDFT approach, combining with the pore size distribution (PSD) analysis of adsorbent material can be applied to predicting the adsorption equilibria of high-pressure gas mixtures on activated carbon. Compared with the conventional NDFT method, this new approach partly improves the correlation performance of adsorption equilibrium for pure species and increases the reliability of the PSD analysis. For the mixtures, CH4/N2 and CO2/N2, a relatively improved performance has been observed for the adsorption equilibrium prediction of the mixtures under high-pressure conditions, especially for the weakly adsorbed species.

The adsorption free-energy of surfactant on solid surfaces has been calculated by molecular dynamics (MD) simulation for a model surfactant/solvent system. The umbrella-sampling with the weight histogram analysis method (WHAM) was applied. The entropic and enthalpic contributions to the full potential of mean force (PMF) were obtained to evaluate the detailed thermodynamics of surfactant adsorption in solid/liquid interfaces. Although we observed that this surfactant adsorption process is driven mainly by a favorable enthalpy change, a highly unfavorable entropic contribution still existed. By decomposing the free energy (including its entropic and enthalpic components) into the solvent-induced contribution and the surfactant−wall term, the effect of surface and solvent on the adsorption free-energy has been distinguished. The contribution to the PMF from the surface effect is thermodynamically favorable, whereas the solvent term displays an obviously unfavorable component with a monotonic increase as the surfactant approaches to the surface. The impact of various interactions from the surfaces (both solvent-philic and solvent-phobic) and the solvent on the adsorption PMF of surfactant has been compared and discussed. Compared to the solvent-philic surface, the solvent-phobic surface generates more stable site for the surfactant adsorption. However, the full PMF profile for the solvent-phobic system shows a clear positive maximum value at the bulk-interface transition region, which leads to a considerable long-range free-energy barrier to the surfactant adsorption. These results have been analyzed in terms of the local interfacial structures. In summary, this comprehensive study is expected to reveal the microscopic interaction mechanisms determining the surfactant adsorption on solid surfaces.

The structural and dynamical properties of the supercritical CO2 fluid confined in the slit nanopores with the hydroxylated and silylated amorphous silica surfaces have been studied using molecular dynamics (MD) simulation. The amorphous bulk silica was obtained by a melt-quench MD simulation technique and the modified silica surfaces were artificially created by the attachment of hydrogen (−OH model) and trimethysilane (−Si(CH3)3 model) to the nonbridging oxygen atoms on the silica surfaces. The VdW interaction potential between the CO2 molecule and the hydroxylated silica surface was determined based on the ab initio quantum mechanics (QM) computation. The adsorption potential distributions of CO2 on the two modified silica surfaces were examined in order to evaluate the different surface interaction characteristics. The density profiles, the radial distribution functions, as well as the interfacial dynamics properties (self-diffusion coefficients and residence time) for the confined supercritical CO2 fluid have been simulated. It is demonstrated that the hydroxylated silica surface gives a stronger confining effect on the supercritical CO2 fluid as compared with the silylated surface. The remarkable impact on the supercritical CO2 fluid from the hydroxylated silica surface can be attributed to the H-bonding interaction between CO2 molecules and surface silanol groups. The analysis of the vibrational density of states of the confined supercritical CO2 fluid reveals the phenomena of the spectral shifts and the Fermi resonance in compaison with the bands in unconfined supercritical CO2. This spectrum behavior is associated with the enhanced interaction from the functional groups on silica surfaces.

Using the grand canonical Monte Carlo (GCMC) simulation technique, the effect of the heterogeneous surfaces on the confined behavior of model fluid has been studied in amorphous silica solid. The amorphous bulk silica was obtained by a melt-quench technique using molecular dynamics simulation and the heterogeneous surface was created by fracturing the bulk structure with relaxation. The adsorption isotherms and structure properties of a Lennard–Jones model fluid in slit-like silica pores with the amorphous structure surfaces have been investigated for different pore widths and temperatures. The minimum potential distribution of the fluid–solid interaction for the amorphous surface was obtained in order to characterize the surface energy heterogeneity. The ordered surface from the β-cristobalite solid has also studied for comparison. The amorphous surface gives a stronger adsorption affinity for the probe particle due to its surface distortion structure. The different adsorption and structure behaviors in the two silica pores were compared and discussed.

Molecular dynamics (MD) simulations have been used to investigate the melting processes of 55-atom and 561-atom Pt–Au nanoalloys with random (RD) and core-shell (CS) orderings. The simulation results show that the Pt–Au CS nanoalloys have higher thermal and structural (including geometrical shape and chemical ordering) stability than the RD ones with the same size and composition. For all the CS nanoalloys studied, their geometric shape and chemical ordering are preserved well before the complete melting transition occurs and their premelting corresponds to the surface melting. In the RD ordering cases, nevertheless, obvious shape distortion and chemical order transformation are observed during the premelting stage. The nature of premelting of the 55-atom RD nanoalloy is not surface melting but dynamic coexistence melting. Additionally, the melting behavior of the RD nanoalloys is found to depend on the particle size. Several separated ordering transformation stages associated with the mutual conversion of different geometrical structures can be observed in the smaller 55-atom particle instead of the 561-atom case. These results suggest that different atomic orderings of nanoalloys can lead to distinctive melting features.

In this work, Monte Carlo simulations have been carried out to investigate the swelling stability and interlayer structures of alkylammonium-modified montmorillonite both in vacuum and in supercritical CO2 (scCO2) fluid. In the vacuum (dry) condition, the stable spacing for this kind of organoclay was determined based on the energy minimum. In the stable spacing, the corresponding interlayer structure of dry organoclay is the monolayer arrangement with the intercalated surfactant chains lying parallel to the silicate surface. In scCO2 fluid medium, the normal pressures within the organoclay gallery and the swelling free energy have been obtained from Gibbs ensemble Monte Carlo simulation. The mechanically and thermodynamically stable spacings of the organoclay have been determined. As compared with the case in vacuum, the simulation shows that the swelling of the organoclay is thermodynamically favorable in the environment of scCO2 fluid. The interlayer structure and conformation have been used to analyze the mechanism of swelling. The headgroups of surfactant cations are distributed close to the clay surfaces. The presence of CO2 molecules within the clay gallery can cause a specific steric arrangement of the long-chain alkylammonium cations.