The interface structure between room temperature ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM+/PF6−) and 1-octyl-3-methylimidazolium hexafluorophosphate (OMIM+/PF6−), and the graphite (0001) surface has been studied by classical molecular dynamic simulations. It is found that the density of IL is much enhanced at the interfacial region and the density oscillation extends to ∼15 Å into the bulk with three layers. The results also demonstrate that the polar groups tend to aggregate forming a polar network, while the nonpolar groups fill up the rest of the vacancy. The imidazolium rings and the side chains preferentially lie flat at the graphite surface with the alkyl side chains of the cations elongated at the interfacial region, and the cations are closer to the graphite surface (ca. 3.6−3.7 Å) than the anions. The surface potential drop across the interface is more profound for OMIM+/PF6− than for BMIM+/PF6−, due to relatively larger local density of the anions for OMIM+/PF6− near the graphite surface.

An ethanol solution containing one or two fullerene C60 molecules was studied via molecular dynamics simulation. We found that the ethanol molecules form several solvation shells around the central fullerene molecule. Radial distribution functions (RDFs) and hydrogen-bond analyses were employed to detect the structure of the ethanol molecules in the solvation shells. The ethanol molecules in the first solvation shell tend to have their nonpolar alkyl groups exposed to the C60 surface while the polar hydroxyl groups point outward to maintain a hydrogen-bond network with a clathrate-like structure. Such orientation of the ethanol molecules in the first solvation shell modulates the orientation of the ethanol molecules in the second solvation shell to have the hydroxyl groups pointing inward. The potential of mean force (PMF) between two C60 molecules in ethanol solution showed that C60 molecules tend to aggregate in the ethanol solution. There is no ethanol molecule in the intersolute area if the distance between the centers of mass of two C60 molecules is shorter than 10.2 Å. The ethanol molecules near the intersolute area tend to have their methyl groups penetrating into the intersolute region if the distance between two C60 molecules is short, although the hydroxyl groups have smaller volume. We analyzed the dynamic properties of the ethanol molecules in different solvation shells and found that the relaxation is much slower than that of water solution of C60 molecules. In addition, the relaxation of the first solvation shell is slower than that in other solvation shells. The lifetime of the hydrogen-bond in the first solvation shell is also longer than that in other solvation shells while the reorientation of the hydrogen-bonded ethanol pair contributes little to break the hydrogen-bonds.

Atomistic simulation of defect-free single-walled carbon nanotube (SWCNT) growth is essential for the insightful understanding of the SWCNT's growth mechanism. Despite the extensive effort paid in the past two decades, the goal has not been completely achieved, due to the huge timescale discrepancy between atomistic simulation and the experimental synthesis of SWCNTs, as well as the lack of an accurate classical potential energy surface for large scale simulation. Here, we report atomistic simulations of defect-free SWCNT growth by using a new generation of carbon–metal potential and a hybrid method, in which a basin-hopping strategy is applied to facilitate the defect healing during the simulation. The simulations reveal a narrow diameter distribution and an even chiral angle distribution of the growth of SWCNTs from liquid catalyst, which is in agreement with most known experimental observations.

The effects of repulsive interaction on the electric double layer (EDL) and differential capacitance (Cd) of an ionic liquid (IL) 1-butyl-3-methyl-imidazolium hexafluorophosphate (BMIM+/PF6−) on the graphite electrode were studied by molecular dynamics (MD) simulations. The strength of repulsive interaction was studied by manually tuning the parameter lambda (λ) with λ = 1.00 for normal Lennard-Jones interaction and smaller λ for stronger repulsion between IL and the electrode. When λ changes from 1.00 to 0.25, the dependence of Cd on potential (Cd–U) curves at different repulsions is asymmetrically camel-shaped with higher Cd at the negative polarization than that at the positive due to the thinner effective thickness of EDL from the specific adsorption of BMIM+. Such a trend is opposite in the case of λ = 0.05. Apart from that, the maximum of Cd at the negative polarization monotonically decreases with increasing repulsion. On the other hand, the maximum of Cd at the positive polarization first increases with increasing repulsion, due to the more effective screening of PF6− by weakening the specific adsorption of BMIM+ as λ changes from 1.00 to 0.75, and then it decreases with increasing repulsion.

The methane uptakes of six double halogen substituted covalent organic frameworks (COFs) based on COF-102 were simulated with grand canonical Monte Carlo simulation at 298 K and pressure ranges from 1 to 80 bar. The simulation shows that COF-102-1,4-2I reaches the DOE target of 180 V(STP)/V for methane delivery. The current study highlights the correlation between the structure and the adsorption property of the double halogen substituted COF-102. In COF-102-1,4-2I, the triangle arrangement of the six I atoms around the central B3O3 ring brings close contact between I atom and B3O3 ring, and thus enhances the attraction of CH4 with high CH4 density in the vicinity above and below this region, especially in particular adsorption sites. Such favorable structural arrangement, altogether with the strongest I–CH4 attraction among the halogen substituent in this study, gives the highest isosteric heat as well as the CH4 uptakes at 298 K and 35 bar in the hypobaric region. The result in this study demonstrates that double halogen substituted COF-102 is capable of increasing CH4 uptakes for practical applications.

Based on the structures of published three-dimensional covalent organic frameworks (COF-102, COF-103, and COF-105), we developed a sequence of modified COFs by replacing some H atoms on benzene rings with other substituent groups, including –Cl, –Br, –I, –CF3, –NH2, –CN, –OCH3, and –CH3. To explore the effects of the substituents on methane storage, we studied their adsorption properties by using Grand Canonical Monte Carlo (GCMC) simulation. The methane uptakes and isosteric heat of adsorption from 0 to 100 bar were simulated at room temperature. The results indicate that in COF-102 and COF-103, all these substituents are beneficial to methane storage at low pressure, but the advantage is weakened by the volume effect at medium and high pressure. In the case of COF-105 and its derivatives, all the isotherms are linear due to their large pore volumes. Among these groups, halogen groups (–Cl, –Br, and –I) and –NH2 are the best ones to improve methane uptake, whereas –CH3 and –OCH3 are of little help. Among all the covalent organic frameworks simulated in this study, COF-102-I (156 V(STP)/V of excess uptake and 169 V(STP)/V of methane delivery), COF-102-Br (153 V(STP)/V of excess uptake and 169 V(STP)/V of methane delivery), COF-102-Cl (148 V(STP)/V of excess uptake and 165 V(STP)/V of methane delivery), and COF-102-NH2 (143 V(STP)/V of excess uptake and 160 V(STP)/V of methane delivery) are found to be the most promising adsorbents for methane uptake.

The structure and dynamics of a task-specific ionic liquid (TSIL), trihexyl(tetradecyl)phosphonium imidazolate, before and after absorbing CO2 were studied with a molecular dynamics (MD) simulation. This particular ionic liquid is one of several newly discovered azole-based TSILs for equimolar CO2 capture. Unlike other TSILs whose viscosity increases drastically upon reaction with CO2, its viscosity decreases after CO2 absorption. This unique behavior was confirmed in our MD simulation. We find that after CO2 absorption the translational dynamics of the whole system is accelerated, accompanied by an accelerated rotational dynamics of the cations. Radial distribution function and spatial distribution function analyses show that the anions become asymmetric after reaction with CO2, and this causes the imbalance of the interaction between the positive and negative regions of the ions. The interaction between the phosphorus atom of the cation and oxygen atoms of the carboxyl group on the anion is enhanced, while that between the phosphorus atom and the naked nitrogen atom of the anion is weakened. The ion-pair correlation functions further support that the weakened interaction leads to faster dissociation of cation–anion pairs, thereby causing an accelerated dynamics. Hence, the asymmetry of anions influences the dynamics of the system and affects the viscosity. This insight may help design better TSILs with decreased viscosity for CO2 capture.

The anisotropic ionic polarizabilities of two data sets of 216 cations (158 in training set and 58 in test set) and 80 anions (64 in training set and 16 in test set), which can be the components of ionic liquids (ILs), are fitted with Thole model against ab initio calculations. The isotropic atomic polarizabilities of H, B, C, N, O, F, S, Cl, P, and Br, are fitted for cations and anions, respectively, with two different smearing functions. The ab initio anisotropic ionic polarizabilities are well fitted by Thole model with a universal set of isotropic atomic polarizabilities, which are independent of their individual chemical environment. The current study also demonstrates the good transferability of Thole model to ions of different substituents, different side chain length, and different conformations.

Proton transfer (PT) via the Grotthuss mechanism in liquid imidazole (im) at 393 K is studied with molecular dynamics simulation using a reactive multistate empirical valence bond (MS-EVB) model. It is found that the proton is tightly binded to an imidazole to form an imidazolium (imH+), which is solvated in a distorted Eigen-like complex (im-imH+-im), whereas the Zundel-like complex (im-H+-im) is rare. PT occurs via an Eigen–Zundel–Eigen scenario for switching the identity of imH+ from an Eigen-like complex to another, intermediated by a Zundel-like complex. Structural and dynamical analyses demonstrate that PT in imidazole can be considered as a local event with very short spatial/temporal correlation, characterized by a few “rattling” or recurrent PT events. At long time scale, the trend of the PT correlation function may be recast with the diffusion model of reversible geminate recombination toward the power-law decay. The formation of the hydrogen bonds (HBs) for the imidazole molecules between the first and second solvation shell of imH+ is crucial to pave the PT pathway. The above features may be understood by the flexibility of the HBs in liquid imidazole, as a stable HB network is essential for the Grotthuss mechanism.

Ab initio calculations are utilized to search for transition state structures for proton transfer in the 1,2,3-triazole-triazolium complexes on the basis of optimized dimers. The result suggests six transition state structures for single proton transfer in the complexes, most of which are coplanar. The energy barriers, between different stable and transition states structures with zero point energy (ZPE) corrections, show that proton transfer occurs at room temperature with coplanar configuration that has the lowest energy. The results clearly support that reorientation gives triazole flexibility for proton transfer.

A reactive molecular dynamics simulation employing the multistate empirical valence bond (MS-EVB) methodology is reported for the hydration structure of an excess proton in a (6,6) carbon nanotube as well as for the mechanism of proton transport (PT) within the nanoconfined environment. The proton is found to be hydrated in a distorted Zundel cation (H5O2+) form within the one-dimensional, confined water chain. Proton transfer events occur via a “Zundel−Zundel” mechanism through a transient H7O3+ intermediate that differs significantly from the “Eigen−Zundel−Eigen” mechanism found in bulk water.

An electronically polarizable model has been developed for the ionic liquid (IL) 1-ethyl-3-methyl-imidazolium nitrate (EMIM+/NO3−) (Yan et al. J. Phys. Chem. B DOI:10.1021/jp9089112). Molecular dynamics simulations were then performed with both the polarizable and nonpolarizable models. Both models exhibited certain properties that are similar to a supercooled liquid behavior even though the simulations were run at 400 K (89 K above the melting point of EMIM+/NO3−). The ionic mean-squared displacement and transverse current correlation function of both models were well represented by a memory function with a fast Gaussian initial relaxation followed by the two-step exponential functions for β- and α- structural relaxations. Another feature shared by both models is the dynamic heterogeneity, which highlights the complex dynamic behavior of ILs. Apart from the overall slow dynamics, the relaxation of the H-atoms attached to the methyl group demonstrates a “free rotor” type of motion. Also, the ethyl group shows the fastest overall relaxation, due to the weak electrostatic interactions on it. Such flexibility enhances the entropic effect and thus favors the liquid state at room temperature. For the dynamical properties reported in this paper, the polarizable model consistently exhibited faster relaxations (including translational and reorientational motions), higher self-diffusion and ionic conductivity, and lower shear viscosity than the nonpolarizable model. The faster relaxations of the polarizable model result from attenuated long-range electrostatic interactions caused by enhanced screening from the polarization effect. Therefore, simulations based on the polarizable model may be analogous to simulations with the nonpolarizable model at higher temperatures. On the other hand, the enhanced intermolecular interactions for the polarizable model at short-range due to the additional charge-dipole and dipole−dipole interactions result in a red shift of the intramolecular C−H stretch spectrum and a higher degree of ion association, leading to a spectrum with enhanced conductivity across the whole frequency range. The vibrational motion associated with the intermolecular hydrogen bonding is highly IR active, highlighting the importance of hydrogen bond dynamics in ILs.

An electronically polarizable model, based on the AMBER nonpolarizable model, has been developed for the ionic liquid (IL) 1-ethyl-3-methyl-imidazolium nitrate (EMIM+/NO3−). Molecular dynamics simulation studies were then performed with both the polarizable and nonpolarizable models. These studies suggest EMIM+ cations have a strong tendency to pack with their neighboring imidazolium rings nearly parallel to each other, bridged by hydrogen bonds to NO3− anions. Polarization has two key effects, (1) additional charge−dipole and dipole−dipole interactions enhance short-range electrostatic interactions and (2) screening reduces long-range electrostatic interactions. As a result, the polarizable model exhibited enhanced hydrogen bonding compared to the nonpolarizable model, while the latter retained more ordered long-range spatial correlations than the former. Though EMIM+ has a very short nonpolar ethyl tail group, spatial heterogeneity, previously observed with long-chain ILs, was observed in this system and has been quantified using the heterogeneity order parameter. The polarizable model was slightly more heterogeneous than the nonpolarizable model. The enhanced spatial heterogeneity of the polarizable model is again attributed to the stronger short-range electrostatic interactions, which “push” the nonpolar tails away from the polar heads, leading to more aggregation and a strongly altered ionic packing pattern around NO3− as observed by a different anion−anion center-of-mass partial radial distribution function g−− (r). Interestingly, both models seemed to “remember” the crystal structure even at temperatures significantly higher (∼90 K higher) than the melting point (311 K). Along with the results on the dynamical properties reported in the accompanying paper, the current study demonstrates that electronic polarizability is significant in ionic liquid systems.

Molecular dynamics simulations have been performed on 1-ethyl-2,3-dimethyl-imidazolium hexafluorophosphate (EMMIPF6) ionic liquids (ILs) doped with different molar ratios of LiPF6 at 523.15 K and 1 bar. Ionic conductivity, self-diffusion coefficients, density, and viscosity predicted by MD simulations were found to be in good agreement with previous studies. Structural analysis shows that the Li+ cation is strongly coordinated by the F atom of the PF6− anion, and the number of F atoms coordinated with a Li+ cation in the first solvation shell is about six for all molar ratios of LiPF6/EMMIPF6 0.05, 0.15, 0.30, and 0.50. The coordination number of the PF6− anion within the first solvation shell of Li+ cation is about four, which tends to increase slightly when the salt concentration is increased. The two-dimensional radial-angular distribution study shows that the Li+−PF6− complex tends to form the C2v conformation at low salt concentration, whereas C4v conformation becomes important at higher salt concentration. It is found that the aggregation of Li+−PF6− complexes occurs in all four molar ratios, whereas ionic conductivity decreases and viscosity increases at higher salt concentration. The residence time correlation of PF6− within the first solvation shell of Li+ shows a strong memory effect. The Li+-hopping function further shows that the hopping of Li+ is strongly affected by its environment with different exchange rates of the PF6− anions for the structure diffusion, and the system of 0.5 LiPF6/EMMIPF6 molar ratio has the slowest hopping rate.

The adsorption of CO2 on TiO2 rutile (110) surface in room-temperature ionic liquid, 1-butyl-3-methyl-imidazolium hexafluorophosphate (BMIM+/PF6−), is studied by molecular dynamics simulation. Due to the strong electrostatic interactions between the O atom of CO2 with the unsaturated Ti atom of the rutile (110) surface, CO2 is adsorbed on the rutile (110) rather than dispersing in the IL bulk. At the interface, CO2 arranges itself in a highly ordered manner, with its D∞h symmetric axis parallel to the rutile (110) surface normal, and the C═O bond is elongated for the coordinating O atom. The interfacial packing pattern is CO2, PF6−, and BMIM+, in sequence, starting from the rutile (110) surface. Thus, the adsorbed CO2 molecules are confined in the narrow neighborhood adjacent to the rutile (110) surface.

The liquid structures of nonaqueous electrolytes composed of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and acetamide, with LiTFSI/acetamide molar ratios of 1:2, 1:4, and 1:6, were studied by molecular dynamics simulations. The simulations indicate that the Li+ cations prefer to be six-coordinate by the sulfonyl oxygen atoms of the TFSI− anions and the carbonyl oxygen atoms of the acetamide molecules, rather than by the most electronegative nitrogen atom of the TFSI− anion. Therefore, close Li+−TFSI− contact pairs exist in the system. The TFSI− anion prefers to provide only one of four possible oxygen atoms to coordinate to the same Li+ cation. Three conformations (cis, trans, and gauche) of the TFSI− anions were found to coexist in the liquid electrolyte. At high salt concentrations, the TFSI− anions mainly adopt the gauche conformation in order to provide more oxygen atoms to coordinate to different Li+ cations, while simultaneously reducing the repulsion among the Li+ cations. On the other hand, the fraction of TFSI− anions adopting the cis conformation is largest for the system with the molar ratio of 1:6, in which many clusters, mainly composed of the Li+ cations and the TFSI− anions, are immersed in the acetamide molecules. The size and charge distribution of clusters were also investigated. In the system with the molar ratio of 1:2, nearly all of the ions in the PBC (periodic boundary conditions) box aggregate into a bulky cluster that gradually disassembles into small clusters with decreasing salt concentration. The addition of acetamide molecules was found to effectively relax the liquid electrolyte structure, and the system with the molar ratio of 1:4 was found to exhibit a more homogeneous liquid structure than the other two electrolyte systems with molar ratios of 1:2 and 1:6.

A chemical dynamics simulation was performed to study collisions between neon (Ne) atoms and a liquid squalane (2,6,10,15,19,23-hexamethyltetracosane) surface. Ten thousand trajectories were calculated, with an incident energy of 10 kcal/mol, incident polar angle 45° with respect to the surface normal, and random azimuthal angle. The final energy distribution, angular distribution, and impact sites were determined and analyzed. The incident Ne atoms have short residence times on the surface with most atoms successfully scattering within 3−7 ps. Due to thermal fluctuations of the surface, the incident energy is dissipated efficiently, and more than 60% of the initial energy of the Ne atoms is transferred with three or more “kicks” on the surface. For in-plane scattered Ne atoms with a final polar angle of 45°, the energy transfer is 58% ± 8%, which is in good agreement with the experimental value of 60% (J. Chem. Phys. 1993, 99, 7056). A bimodal energy distribution is observed for both in-plane and out-of-plane scattering, with a much larger Boltzmann component for out-of-plane scattering as compared to in-plane scattering. The incident Ne atoms are found to primarily impinge the terminal methyl groups of the squalane molecules, and such impact probability is correlated with the interfacial structure of the squalane surface. Comparison with previous study of Ne atom scattering off a H-terminated alkyl thiol self-assembled monolayer (H-SAM) surface shows that energy transfer to squalane is less efficient than to the H-SAM, because flexible intermolecular couplings of the alkyl thiol chains of the H-SAM provide efficient dissipation channels to accommodate the incident Ne atom’s energy.

The effects of repulsive interaction on the electric double layer (EDL) and differential capacitance (Cd) of an ionic liquid (IL) 1-butyl-3-methyl-imidazolium hexafluorophosphate (BMIM+/PF6−) on the graphite electrode were studied by molecular dynamics (MD) simulations. The strength of repulsive interaction was studied by manually tuning the parameter lambda (λ) with λ = 1.00 for normal Lennard-Jones interaction and smaller λ for stronger repulsion between IL and the electrode. When λ changes from 1.00 to 0.25, the dependence of Cd on potential (Cd–U) curves at different repulsions is asymmetrically camel-shaped with higher Cd at the negative polarization than that at the positive due to the thinner effective thickness of EDL from the specific adsorption of BMIM+. Such a trend is opposite in the case of λ = 0.05. Apart from that, the maximum of Cd at the negative polarization monotonically decreases with increasing repulsion. On the other hand, the maximum of Cd at the positive polarization first increases with increasing repulsion, due to the more effective screening of PF6− by weakening the specific adsorption of BMIM+ as λ changes from 1.00 to 0.75, and then it decreases with increasing repulsion.

Atomistic simulation of defect-free single-walled carbon nanotube (SWCNT) growth is essential for the insightful understanding of the SWCNT's growth mechanism. Despite the extensive effort paid in the past two decades, the goal has not been completely achieved, due to the huge timescale discrepancy between atomistic simulation and the experimental synthesis of SWCNTs, as well as the lack of an accurate classical potential energy surface for large scale simulation. Here, we report atomistic simulations of defect-free SWCNT growth by using a new generation of carbon–metal potential and a hybrid method, in which a basin-hopping strategy is applied to facilitate the defect healing during the simulation. The simulations reveal a narrow diameter distribution and an even chiral angle distribution of the growth of SWCNTs from liquid catalyst, which is in agreement with most known experimental observations.