Studies of structural, dynamical, and interfacial properties of 1-alkyl-3-methylimidazolium iodide ionic liquids by molecular dynamics simulation

Bulk and surface properties of the ionic liquids 1-alkyl-3-methyl-imidazolium iodides ([C(n)mim]I) were simulated by classical molecular dynamics using all atom non-polarizable force field (n = 4, butyl; 6, hexyl; 8, octyl). The structure of ionic liquids were initially optimized by density functional theory and atomic charges obtained by CHELPG method. Reduction of partial atomic charges (by 20% for simulation of density and surface tension, and by 10% for viscosity) found to improve the accuracy, while a non-polarizable force field was applied. Additionally, the simulation ensembles approach the equilibrium faster when the charge reduction is applied. By these refined force field parameters, simulated surface tensions in the range of 323-393 k are quite in agreement with the experiments. Simulation of temperature dependent surface tension of [C(4)mim]I well beyond room temperature (up to 700 K) permits prediction of the critical temperature in agreement with that predicted from experimental surface tension data. Simulated densities in the range of 298-450 K for the three ionic liquids are within 0.8% of the experimental data. Structural properties for [C(4)mim]I were found to be in agreement with the results of Car-Parrinello molecular dynamics simulation we performed, which indicates a rather well-structured cation-anion interaction and occurs essentially through the imidazolium ring cation. Diffusion coefficient changes with alkyl chain length in the order of [C(8)mim]I > [C(6)mim]I > [C(4)mim]I for the cation and the anion. Formation of a dense domain in subsurface region is quite evident, and progressively becomes denser as the alkyl chain length increases. Bivariate orientational analysis was used to determine the average orientation of molecule in ionic liquids surface, subsurface, and bulk regions. Dynamic bisector-wise and side-wise movement of the imodazolium ring cation in the surface region can be deduced from the bivariate maps. Atom-atom density profile and bivariate analysis indicate that the imidazolium cation takes a spoon like configuration in the surface region and the tilt of alkyl group is a function length of alkyl chain exposing as linear as possible to the vapor phase.     

A systematic molecular simulation study of ionic liquid surfaces using intrinsic analysis methods

In this paper, we apply novel intrinsic analysis methods, coupled with bivariate orientation analysis, to obtain a detailed picture of the molecular-level structure of ionic liquid surfaces. We observe pronounced layering at the interface, alternating non-polar with ionic regions. The outermost regions of the surface are populated by alkyl chains, which are followed by a dense and tightly packed layer formed of oppositely charged ionic moieties. We then systematically change the cation chain length, the anion size, the temperature and the molecular model, to examine the effect of each of these parameters on the interfacial structure. Increasing the cation chain length promotes orientations in which the chain is pointing into the vapor, thus increasing the coverage of the surface with alkyl groups. Larger anions promote a disruption of the dense ionic layer, increasing the orientational freedom of cations and increasing the amount of free space. The temperature had a relatively small effect on the surface structure, while the effect of the choice of molecular model was clearly significant, particularly on the orientational preferences at the interface. Our study demonstrates the usefulness of molecular simulation methods in the design of ionic liquids to suit particular applications.     

Deblurred observation of the molecular structure of an oil-water interface

We simulated the interface between liquid water and a stationary phase of tethered n-C18 alkyl chains at a thermodynamic state of low pressure and water vapor-liquid coexistence. The interfacial water (oxygen atom) density profile so obtained is compared with a precisely defined proximal density of water molecules (oxygen atoms) conditional on the alkyl chain configurations. Though the conventional interfacial density profile takes a traditional monotonic form, the proximal radial distribution of oxygen atoms around a specific methyl (methylene) group closely resembles that for a solitary methane solute in liquid water. Moreover, this proximal radial distribution function is sufficient to accurately reconstruct the water oxygen density profile of the oil-water interface. These observations provide an alternative interpretation to collective drying or vaporization interpretations of commonly observed oil-water interfacial profiles for which water penetration into the interfacial region plays a role.     

Molecular dynamics simulations of ionic liquid-vapour interfaces: effect of cation symmetry on structure at the interface

The influence of alkyl chain symmetry of the imidazolium cation on the structure and properties of the ionic liquid-vapour interface has been addressed through molecular dynamics simulations. The anion chosen is bis(trifluoromethylsulfonyl)imide (NTf(2)). Profiles of number densities, orientation of cations, charge density, electrostatic potential, and surface tension have been obtained. At the interface, both cations and anions were present, and the alkyl chains of the former preferred to orient out into the vapour phase. A large fraction of cations preferred to be oriented with their ring-normal parallel to the surface and alkyl chains perpendicular to it. These orientational preferences are reduced in ionic liquids with symmetric cations. Although the charge densities at the interface were largely negative, an additional small positive charge density has been observed for systems with longer alkyl chains. The electrostatic potential difference developed between the liquid and the vapour phases were positive and decreased with increasing length of the alkyl group. The calculated surface tension of the liquids also decreased with increasing alkyl chain length, in agreement with experiment. The surface tension of an ionic liquid with symmetric cation was marginally higher than that of one with an asymmetric, isomeric cation.     

Structure of the liquid-vacuum interface of room-temperature ionic liquids: a molecular dynamics study

Molecular dynamics simulations for the liquid-vacuum interface of the ionic liquid 1-ethyl-3-methylimidazolium nitrate (EMIM+/NO3-) were performed for both electronically polarizable and nonpolarizable potential energy surfaces. The interfacial structural properties, such as the oscillation in the number density profile, the orientational ordering, and the local clustering in the interfacial region, were calculated. The simulations with both the polarizable and nonpolarizable model demonstrate the existence of an inhomogeneous interfacial structure normal to the surface layer. It was found for both models that the ethyl tail group on EMIM+ is likely to protrude outward from the surface. In the outmost surface layer, the cation is likely to lie on the surface with the imidazolium ring parallel to the interface, while there is a second region with enhanced density from that in the bulk where the cation preferably slants with the imidazolium ring tending to be perpendicular to the surface. The results also reveal that the electronic polarization effect is important for the ionic liquid interface. It is found that the cation is likely to be segregated at the ionic liquid surface for the polarizable model, while for the nonpolarizable model, the anion is found to be more likely to exhibit such behavior. The surface tension of the polarizable model (58.5 +/- 0.5 mN/m) is much smaller than that of the nonpolarizable model (82.7 +/- 0.6 mN/m), in better agreement with extrapolated experimental measurements on similar ionic liquid systems.     

Structure, dynamics, and the free energy of solute adsorption at liquid-vapor interfaces of simple dipolar systems: molecular dynamics results for pure and mixed Stockmayer fluids

We have performed molecular dynamics simulation studies of the structural, thermodynamic, and dynamical properties of liquid-vapor interfaces of pure and binary Stockmayer fluids of different polarity. The density profiles, the width of the liquid-vapor interface, and the orientational structure of the interfaces are calculated to characterize the structural aspects of the interfaces. Among the thermodynamic properties, we have computed the surface tension and also the free energy of transfer of a charged solute across the liquid-vapor interface for both pure and mixed fluids. Among the dynamical properties of the interfaces, we have calculated the time dependence of the velocity and angular velocity autocorrelation functions, continuous and intermittent survival probabilities, mean square displacements, diffusion coefficients, and also the dipole correlation functions and orientational relaxation times of interfacial solvent molecules. It is found that the width of the interfaces decreases with increase of concentration of the more polar component. The dipole vectors of the interfacial molecules tend to align parallel to the surfaces and this alignment is enhanced with increasing dipole moment of the fluid molecules. Also, the surface tension shows an increasing trend with increase of dipole moment of the molecules. The dynamical properties of the interfaces are found to be different from those of the corresponding bulk liquid phases. In general, the molecules at the interfaces are found to rotate and translate in the parallel direction at a somewhat faster rate than the bulk molecules. Also, on increase of concentration of the more polar component, the diffusion and orientational relaxation of interfacial molecules are found to show a weaker slowing down than those of the bulk molecules, which can be attributed to the preferential presence of the more polar component in the bulk liquid regions. The temporal behavior of the interfacial survival probabilities reveals a decrease of the survival times with increasing polarity, which can be attributed to a corresponding decrease in the interfacial thickness. Results are presented for both continuous and intermittent survival times and the origins of their differences are discussed. The free energy calculations reveal no minimum at the interfaces for adsorption of a charged solute, which shows that the ions would prefer to stay in the interior of the liquid phases, rather than at interfaces, for these model dipolar systems.     

Structure and dynamics of water confined in silica nanopores

We report the results of molecular simulation of water in silica nanopores at full hydration and room temperature. The model systems are approximately cylindrical pores in amorphous silica, with diameters ranging from 20 to 40 ?. The filled pores are prepared using grand canonical Monte Carlo simulation and molecular dynamics simulation is used to calculate the water structure and dynamics. We found that water forms two distinct molecular layers at the interface and exhibits uniform, but somewhat lower than bulk liquid, density in the core region. The hydrogen bond density profile follows similar trends, with lower than bulk density in the core and enhancements at the interface, due to hydrogen bonds between water and surface non-bridging oxygens and OH groups. Our studies of water dynamics included translational mean squared displacements, orientational time correlations, survival probabilities in interfacial shells, and hydrogen bond population relaxation. We found that the radial-axial anisotropy in translational motion largely follows the predictions of a model of free diffusion in a cylinder. However, both translational and rotational water mobilities are strongly dependent on the proximity to the interface, with pronounced slowdown in layers near the interface. Within these layers, the effects of interface curvature are relatively modest, with only a small increase in mobility in going from the 20 to 40 ? diameter pore. Hydrogen bond population relaxation is nearly bulk-like in the core, but considerably slower in the interfacial region.     

Intermolecular vibrations and diffusive orientational dynamics of Cs condensed ring aromatic molecular liquids

The ultrafast dynamics, including the intermolecular vibrations and the diffusive orientational dynamics, of the neat C(s) symmetry condensed ring aromatic molecular liquids benzofuran, 1-fluoronaphtalene, and quinoline were investigated for the first time by means of femtosecond Raman-induced Kerr effect spectroscopy. To understand the features of these C(s) condensed ring aromatic molecular liquids, reference singular aromatic molecular liquids, furan, fluorobenzene, pyridine, and benzene, were also studied. High quality low-frequency Kerr spectra of the aromatic molecular liquids were obtained by Fourier-transform deconvolution analysis of the measured Kerr transients. The Kerr spectra of the C(s) condensed ring aromatic molecular liquids are bimodal, as are those of the reference singular aromatic molecular liquids. The first moment of the intermolecular vibrational spectrum and the peak frequencies of the high- and low-frequency components in the broad spectrum band were compared with their molecular properties such as the rotational constants, molecular weight, and intermolecular (bimolecular) force. The comparisons show that the molecular volume (related to molecular weight and rotational constants) is a dominant property for the characteristic frequency of the entire intermolecular vibrational spectrum. The observed intramolecular vibrational modes in the Kerr spectra of the aromatic molecular liquids were also assigned on the basis of the ab initio quantum chemical calculation results. In their picosecond diffusive orientational dynamics, the slowest relaxation time constant for both the condensed ring and singular aromatic molecular liquids can be accounted for by the simple Stokes-Einstein-Debye hydrodynamic model.     

Structural characterization of interfacial n-octanol and 3-octanol using molecular dynamic simulations

Structurally isomeric octanol interfacial systems, water/vapor, 3-octanol/vapor, n-octanol/vapor, 3-octanol/water, and n-octanol/water are investigated at 298 K using molecular dynamics simulation techniques. The present study is intended to investigate strongly associated liquid/liquid interfaces and probe the atomistic structure of these interfaces. The octanol and water molecules were initially placed randomly into a box and were equilibrated using constant pressure techniques to minimize bias within the initial conditions as well as to fully sample the structural conformations of the interface. An interface formed via phase separation during equilibration and resulted in a slab geometry with a molecularly sharp interface. However, some water molecules remained within the octanol phase with a mole fraction of 0.12 after equilibration. The resulting "wet" octanol interfaces were analyzed using density profiles and orientational order parameters. Our results support the hypothesis of an ordered interface only 1 or 2 molecular layers deep before bulk properties are reached for both the 3-octanol and water systems. However, in contrast to most other interfacial systems studied by molecular dynamics simulations, the n-octanol interface extends for several molecular layers. The octanol hydroxyl groups form a hydrogen-bonding network with water which orders the surface molecules toward a preferred direction and produces a hydrophilic/hydrophobic layering. The ordered n-octanol produces an oscillating low-high density of oxygen atoms out of phase with a high-low density of carbon atoms, consistent with an oscillating dielectric. In contrast, the isomeric 3-octanol has only a single carbon-rich layer directly proximal to the interface, which is a result of the different molecular topology. Both 3-octanol and n-octanol roughen the water interface with respect to the water/vapor interface. The "wet" octanol phases, in the octanol/water systems reach bulk properties in a shorter distance than the "dry" octanol/vapor interfaces.     

Air-liquid interfaces of imidazolium-based [TF2N-] ionic liquids: insight from molecular dynamics simulations

We present molecular dynamics simulations of the air-liquid interface for three room temperature ionic liquids with a common anion: bis(trifluoromethylsulfonyl) imide ([Tf(2)N]), and imidazolium-based cations that differ in the alkyl tail length: 1-butyl-3-methylimidazolium ([C(4)mim]), 1-hexyl-3-methylimidazolium ([C(6)mim]), and 1-octyl-3-methylimidazolium ([C(8)mim]). The CHARMM type force field is used with the partial charges based on quantum calculations for isolated ion pairs. The total charge on cations and anions is around 0.9e and -0.9e, respectively, which somewhat mimics the anion to cation charge transfer and many-body effects. The surface tension at 300 K is computed using the mechanical route and its value slightly overpredicts experimental values. The air-liquid interface is analyzed using the intrinsic method of Identification of the Truly Interfacial Molecules. Structural and dynamic properties of the interfacial, sub-interfacial and central layers are determined. To describe the structure of the interface, we compute the surface roughness, number density and charge density profiles, and orientation ordering of the ions. We further determine the survival probability, normal and lateral self-diffusion coefficients, and re-orientation correlation functions to characterize the dynamics of the cations and anions in the layers. We found a significant enhancement of the cation density and preferential orientation ordering of both the cations and anions at the interface. Overall, the surface of the interfacial layer is smoother than the surface of the sub-interfacial layer and the roughness of both the interfacial and sub-interfacial layers increases with the increase of the length of the cation alkyl tail. Finally, the ions stay considerably longer in the interfacial layer than in the sub-interfacial layer and dynamics of exchange of the ions between the consecutive layers is related to the distinct diffusion and re-orientation dynamics behavior of the ions within the layers.     

Air-liquid interfaces of aqueous solutions containing ammonium and sulfate: spectroscopic and molecular dynamics studies

Investigations of the air-liquid interface of aqueous salt solutions containing ammonium (NH(4)(+)) and sulfate (SO(4)(2-)) ions were carried out using molecular dynamics simulations and vibrational sum frequency generation spectroscopy. The molecular dynamics simulations show that the predominant effect of SO(4)(2-) ions, which are strongly repelled from the surface, is to increase the thickness of the interfacial region. The vibrational spectra reported are in the O-H stretching region of liquid water. Isotropic Raman and ATR-FTIR (attenuated total reflection Fourier transform infrared) spectroscopies were used to study the effect of ammonium and sulfate ions on the bulk structure of water, whereas surface sum frequency generation spectroscopy was used to study the effect of these ions on the interfacial structure of water. Analysis of the interfacial and bulk vibrational spectra reveal that aqueous solutions containing SO(4)(2-) perturb the interfacial water structure differently than the bulk and, consistent with the molecular dynamics simulations, reveal an increase in the thickness of the interfacial region.     

Probing the neutral graphene-ionic liquid interface: insights from molecular dynamics simulations

We study basic mechanisms of the interfacial layer formation at the neutral graphite monolayer (graphene)-ionic liquid (1,3-dimethylimidazolium chloride, [dmim][Cl]) interface by fully atomistic molecular dynamics simulations. We probe the interface area by a spherical probe varying the charge (-1e, 0, +1e) as well as the size of the probe (diameter 0.50 nm and 0.38 nm). The molecular modelling results suggest that: there is a significant enrichment of ionic liquid cations at the surface. This cationic layer attracts Cl(-) anions that leads to the formation of several distinct ionic liquid layers at the surface. There is strong asymmetry in cationic/anionic probe interactions with the graphene wall due to the preferential adsorption of the ionic liquid cations at the graphene surface. The high density of ionic liquid cations at the interface adds an additional high energy barrier for the cationic probe to come to the wall compared to the anionic probe. Qualitatively the results from probes with diameter 0.50 nm and 0.38 nm are similar although the smaller probe can approach closer to the wall. We discuss the simulation results in light of available experimental data on the interfacial structure in ionic liquids.     

Contribution to understanding of the molecular dynamics in liquids

The dielectric relaxation spectroscopy is used for studying the orientational molecular dynamics in the isotropic (I) and nematic (N) phases of two mesogenic liquids composed of the molecules of similar structure and length, but of an essentially different polarity: n-heptylcyanobiphenyl, C(7)H(15)PhPhCN, 7CB (molecular dipole moment mu approximately 5D) and 4-(trans-4'-n-hexylcyclohexyl)isothiocyanatobenzene, C(6)H(13)CyHxPhNCS, 6CHBT (mu approximately 2.5D); advantageously, the temperatures of the I-N phase transition for the two compounds are very close to each other (T(NI) = 316.6 +/- 0.2 K). It is shown that regardless of the differences in polarity of 7CB and 6CHBT molecules and their abilities in dipolar aggregation, the values and temperature dependences of the relaxation time (corresponding to the rotational diffusion of the molecules around their short axis) are very close to each other, in both the isotropic and nematic phases of the liquids studied. Therefore, the data show that the dielectric relaxation processes occurring in dipolar liquids in the isotropic and nematic states lead through the rotational diffusion of individual molecules and the diffusion seems to be not influenced by the intermolecular interactions.     

离子液体表面活性剂1-丁基-3-甲基咪唑烷基硫酸酯的合成及其在水溶液中的聚集行为

采用离子交换法,由1-丁基-3甲基咪唑氯盐（C4mimCl）和烷基硫酸钠合成了一系列无卤素的阴离子表面活性离子液体—1-丁基-3-甲基咪唑烷基硫酸酯[C4mim][CnH2n 1SO4](n=8,12,16),利用表面张力仪、稳态荧光光谱等手段考察了表面活性离子液体在水溶液表面及体相中的聚集行为,结果表明,与传统无机反离子相比,有机咪唑阳离子[C4mim] 作为反离子的离子液体型表面活性剂具有较高的表面活性,[C4mim] 产生的氢键引起的抑制分子规则排列的作用小于其促进分子有序排列的疏水作用。长烷基链的阴离子是界面膜及胶束的主要组成成分,阴离子疏水烷基碳链的增长虽然可促进胶束的形成,但却在一定程度上抑制[C4mim] 离子参与界面或胶束的形成;阴离子所带烷基链越长,越不利于阳离子[C4mim]＋参与界面膜或胶束的形成,界面膜或胶束中表面活性剂排布越松散,即界面张力越大,体系中胶束聚集数较小。     

Effect of electric field on liquid infiltration into hydrophobic nanopores

Understanding the variation of nanofluidic behavior in the presence of an external electric field is critical for controlling and designing nanofluidic devices. By studying the critical infiltration pressure of liquids into hydrophobic nanopores using molecular dynamics (MD) simulations and experiments, important insights can be gained on the variation of the effective liquid-solid interfacial tension with the magnitude and sign of electric field, as well as its coupling with the pore size and the solid and liquid species. It is found that the effective hydrophobicity reduces with the increase of electric intensity and/or pore size, and the behavior is asymmetric with respect to the direction of the electric field. The underlying molecular mechanisms are revealed via the study of the density profile, contact angle, and surface tension of confined liquid molecules.     

Structure and Energetics of model amphiphilic molecules at the water liquid-vapor interface. A molecular dynamics study

A molecular dynamics study of adsorption of p-n-pentylphenol at infinite dilution at the water liquid-vapor interface is reported. The calculated free energy of adsorption is -8.8 +/- 0.7 kcal/mol, in good agreement with the experimental value of -7.3 kcal/mol. The transition between the interfacial region and the bulk solution is sharp and well-defined by energetic, conformational, and orientational criteria. At the water surface, the phenol head group is mostly immersed in aqueous solvent. The most frequent orientation of the hydrocarbon tail is parallel to the interface, due to dispersion interactions with the water surface. This arrangement of the phenol ring and the alkyl chain requires that the chain exhibits a kink. As the polar head group is being moved into the solvent, the chain length increases and the tail becomes increasingly aligned toward the surface normal, such that the nonpolar part of the molecule exposed to water is minimized. The same effect was achieved when phenol was replaced by a more polar head group, phenolate. This result underscores the difference between hydrophobic hydration at the surface and in the bulk solvent, when nonpolar molecular fragments adopt compact conformations.     

Ab initio molecular dynamics simulation of ionic liquids

Ab initio Car-Parinnello molecular dynamics is used to simulate the structure and the dynamics of 1-butyl-3-methylimidazolium iodide ([bmim]I) ionic liquid at 300 K. Site-site pair correlation functions reveal that the anion has a strong interaction with any three C-H's of the imidazolium ring. The ring bends over and wraps around the anion such that the two nitrogen atoms take a distance to the anion. Electron donating butyl group contributes the electronic polarization in addition to geometrical (out-of-plane) polarization of the ring due to the liquid environment. This facilitates bending of the ring along the axis passing through nitrogen atoms. The average bending angle depends largely on the alkyl chain length and slightly on the anion type. Redistribution of electron density over the ring caused by the electron donating alkyl group provides additional independent evidence to the instability of lattice structure, hence the low melting point of the ionic liquid. Simulated viscosity and diffusion coefficients of [bmim]I are in quite agreement with the experiments.     

Detailed intermolecular structure of molecular liquids containing slightly distorted tetrahedral molecules with C(3v) symmetry: chloroform, bromoform, and methyl-iodide

Analyses of the intermolecular structure of molecular liquids containing slightly distorted tetrahedral molecules of the CXY(3)-type are described. The process is composed of the determination of several different distance-dependent orientational correlation functions, including ones that are introduced here. As a result, a complete structure classification could be provided for CXY(3) molecular liquids, namely for liquid chloroform, bromoform, and methyl-iodide. In the present work, the calculations have been conducted on particle configurations resulting from reverse Monte Carlo computer modeling: these particle arrangements have the advantage that they are fully consistent with structure factors from neutron and x-ray diffraction measurements. It has been established that as the separation between neighboring molecules increases, the dominant mutual orientations change from face-to-face to edge-to-edge, via the edge-to-face arrangements. Depending on the actual liquid, these geometrical elements (edges and faces of the distorted tetrahedra) were found to contain different atoms. From the set of liquids studied here, the structure of methyl-iodide was found to be easiest to describe on the basis of pure steric effects (molecular shape, size, and density) and the structure of liquid chloroform seems to be the furthest away from the corresponding "flexible fused hard spheres" like reference system.     

Molecular level structure of the liquid/liquid interface. Molecular dynamics simulation and ITIM analysis of the water-CCl4 system

The molecular level properties of the liquid/liquid interface between water and CCl(4) are analysed in detail on the basis of molecular dynamics computer simulation. This analysis requires a full list of the molecules that are right at the interface in both phases. Such a list can be provided by the novel method for identifying truly interfacial molecules (ITIM). The full list of the truly interfacial molecules various properties (e.g., width, molecular level roughness) of the interface can be meaningfully analysed. The residence time of the molecules at the interface, the percolation of the water molecules at the interfacial layer as well as in the second layer beneath the surface, the preferred orientations of the interfacial water molecules and the dependence of these orientational preferences on the local curvature of the interface are also analysed and discussed in detail.