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.     

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.     

The extent of molecular orientation at liquid/vapor interface of pyridine and its alkyl derivatives by molecular dynamics simulation

In this paper, molecular dynamics simulation was performed to investigate the liquid∕vapor interfacial structure of neat polar liquids. Large-scale ensembles of liquid pyridine and its alkyl derivatives, 4-methylpyridine and 4-ethylpyridine, were simulated by classical molecular dynamics at 298 K. For the liquid system of low polarity, the surface density profile of the atoms meet exactly at the middle of interfacial region, and atoms of hydrophobic nature can be hardly discriminated from hydrophilic ones in either vapor or liquid sides. For a liquid system of high polarity, the density profile of atoms with different nature is highly discriminated all over the interfacial region, and as the polarity increases, a dense region of atomic density is clearly developed in the subsurface region. The recognized bivariate method was also used to study the molecular orientational distribution quantitatively. Orientational analysis of the three liquid systems indicates that the pyridine ring plane in the outmost surface tends to be vertical. Its tendency in the innermost interfacial region is parallel. The orientational states available to 4-ethylpyridine and pyridine are discriminated by predicting the possibility of a bisector-wise tumbling for the ring plane in pyridine and a side-wise tumbling in 4-ethylpyridine. The orientational distribution maps explain the trend of experimental surface tension and surface entropy. As the dipole moment of these liquids increases with the alkyl chain length, the surface structural profile changes from a regular definite one to a surface of complex atomic structure involving a dense phase near the interface. The development of dense region in alkyl derivatives is the result of segregation of molecules due to the alkyl group, which is captured and discriminated by molecular dynamics simulation even when the length of a short alkyl chain is increased by one carbon atom.     

Liquid-phase structure of dialkylimidazolium ionic liquids from computer simulations

We present a detailed computational study of the structure of ionic liquids based on the imidazolium cation. Both imidazolium-ring stacking and hydrogen bonding behavior are investigated from radial and spatial orientational distribution functions, as well as orientational correlation functions. The alkyl chain size and anion effect on the liquid structure are provided and discussed. Our results support models for liquid organization comparable to those formulated on the basis of experimental observations.     

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.     

Quantum state resolved scattering from room-temperature ionic liquids: the role of cation versus anion structure at the interface

We present results on state-resolved scattering studies for seeded CO(2) supersonically cooled molecular beams (E(inc) = 61.9(40) kJ/mol) from a series of room-temperature ionic liquids (RTILs). These RTILs are composed of C(n)-methylimidazolium cations with BF(4)(-) or Tf(2)N(-) counteranions. The final rovibrational quantum state distributions from these nonequilibrium surface scattering collisions are monitored by high-resolution diode laser absorption spectroscopy as a function of (i) cation alkyl chain length and (ii) anion size, and analyzed to yield the propensity for thermal desorption (TD) versus impulsive scattering (IS) dynamics. For a fixed BF(4)(-) or Tf(2)N(-) counteranion, the distributions reveal an increase in the TD fraction (α) with the C atom number (n) in the alkyl side chain, which provides evidence for selective preference of nonpolar groups at the gas-liquid interface with increasing chain length. Conversely, for short carbon chains (n = 4), the thermal fraction decreases when the anion is changed from a compact and less polarizable BF(4)(-) to the bulkier and more polarizable Tf(2)N(-), whereas any sensitivity to anion identity essentially vanishes for longer alkyl chains (n = 8, 12). These combined data illustrate a number of interesting trends in anion versus cation competition for interfacial sites, specifically (i) the presence of interfacial anions at the surface layer for sufficiently short alkyl headgroups, (ii) inertial "stiffening" due to increasing average surface mass, as well as (iii) a propensity for larger anion sizes in the interfacial region. Finally, the TD probabilities follow the exact opposite trend in "bulk" Henry's Law solubility constants with respect to anion size, which further highlights the intrinsically nonequilibrium dynamics sampled by hyperthermal collisions at the gas-liquid interface.     

Structure and orientation of the imidazolium cation at the room-temperature ionic liquid/SiO2 interface measured by sum-frequency vibrational spectroscopy

In this study, we have examined both the effect of alkyl chain length and anion composition on the 1-alkyl-3-methylimidazolium (C(n)mim, n = 4, 6, 8, 10, and 12) structure and orientation at the room-temperature ionic liquid (RTIL)/SiO(2) interface by sum-frequency vibrational spectroscopy (SFVS). Four different anions were investigated in this study: tetrafluoroborate (BF(4)), hexafluorophosphate (PF(6)), bis(trifluoromethylsulfonyl)imide (BMSI), and bis(pentafluoroethylsulfonyl)imide (BETI). It was found that the alkyl chain in BMSI and BETI RTILs showed a decrease in gauche defects with an increase in chain length, whereas the alkyl chains of the BF(4) and PF(6) RTILs have virtually no gauche defects regardless of chain length. The tilt of the alkyl chain lies predominantly perpendicular to the surface for all the RTILs examined. A strong correlation between the HCCH vs tilt angle and alkyl chain length was observed; as the alkyl chain is lengthened the HCCH vs lies more perpendicular to the SiO(2) surface. The results of this study suggest that the length of the alkyl chain dictates to a large degree the orientation of the imidazolium cation at the surface, regardless of anion composition. To a lesser extent, the HCCH vs tilt of the imidazolium ring of the cation also appears to be correlated to the surface charge density of the SiO(2). As the SiO(2) surface charge density becomes more negative the HCCH vs tilt angle lies more parallel to the surface.     

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.     

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。     

表面活性离子液体参与构筑的胶束体系

离子液体(ILs)的可设计性,使得新型表面活性离子液体(SAILs)不断涌现,从而将具有不同功能基团的ILs引入到了传统的有序分子聚集体中,这也将有助于实现有序分子聚集体的可控性和功能化。本文综述了SAILs在水溶液及离子液体中的胶束化行为,重点总结了烷基碳链长度、阴离子类型和阳离子结构对SAILs聚集行为的影响,分类归纳了SAILs与传统表面活性剂复配体系的协同增效作用,并探讨了SAILs构建的胶束体系的发展方向。     

Tunable Lower Critical Solution Temperature of Poly(butyl acrylate) in Ionic Liquid Blends

We describe the lower critical solution temperature(LCST)-type phase behavior of poly(butyl acrylate)(PBA) dissolved in hydrophobic 1-alkyl-3-methylimidazolium bis{(trifluoromethyl) sulfonyl}amide ionic liquids(ILs). The temperature-composition phase diagrams of these PBA/ILs systems are strongly asymmetric with the critical composition shifted to low concentrations of PBA. As the molecular weight increases from 5.0×10~3 to 2.0×10~4, the critical temperature decreases by about 67 °C, and the critical composition shifts to a lower concentration.Furthermore, the LCST of PBA/ILs system increases as increasing the alkyl side chain length in the imidazolium cation. Using IL blends as solvents,the LCST of PBA can be tuned almost linearly over a wide range by varying the mixing ratio of two ionic liquids without modifying the chemical structure of the polymers.     

Novel ascorbic acid based ionic liquids for the in situ synthesis of quasi-spherical and anisotropic gold nanostructures in aqueous medium

A series of newly designed ascorbic acid based room temperature ionic liquids were successfully used to prepare quasi-spherical and anisotropic gold nanostructures in an aqueous medium at ambient temperature. The synthesis of these room temperature ionic liquids involves, first, the preparation of a 1-alkyl (such as methyl, ethyl, butyl, hexyl, octyl, and decyl) derivative of 3-methylimidazolium hydroxide followed by the neutralization of the derivatised product with ascorbic acid. These ionic liquids show significantly better thermal stability and their glass transition temperature (Tg) decreases with increasing alkyl chain length. The ascorbate counter anion of these ionic liquids acts as a reducing agent for HAuCl4 to produce metallic gold and the alkylated imidazolium counter cation acts as a capping/shape-directing agent. It has been found that the nature of the ionic liquids and the mole ratio of ionic liquid to HAuCl4 has a significant effect on the morphology of the formed gold nanostructures. If an equimolar mixture of ionic liquid and HAuCl4 is used, predominantly anisotropic gold nanostructures are formed and by varying the alkyl chain length attached to imidazolium cation of the ionic liquids, various particle morphologies can formed, such as quasispherical, raspberry-like, flakes or dendritic. A probable formation mechanism for such anisotropic gold nanostructures has been proposed, which is based on the results of some control experiments.     

Structure of ionic liquids of 1-alkyl-3-methylimidazolium cations: a systematic computer simulation study

Molecular dynamics simulations of room temperature molten salts (ionic liquids) containing imidazolium cations have been performed. Ten different systems were simulated at 323 K by using united atom force fields, in which the anion size (F-, Cl-, Br-, and PF6-) and the length of the alkyl chain of 1-alkyl-3-methylimidazolium cations (1-methyl-, 1-ethyl-, 1-butyl-, and 1-octyl-) were systematically varied. It is shown that the resulting equilibrium structures account for the observed features of experimental static structure factors when available. A detailed analysis of the simultaneous effect of changing the anion and the alkyl chain on the preferential location of nearest-neighbor anions around the cations is provided. It is shown that regions above and below the imidazolium ring are the preferential ones in case of large anions. By increasing the length of the alkyl chain, nearest-neighbor anions are pushed away from the volume occupied by the flexible alkyl chain. Partial structure factors of 1-butyl- and 1-octyl- derivatives display a peak at a wave vector smaller than the main peak, indicating the occurrence of an intermediate range order in these ionic liquids due to the presence of long alkyl chains.     

Temperature dependence of the electrical conductivity of imidazolium ionic liquids

The electrical conductivities of 1-alkyl-3-methylimidazolium tetrafluoroborate ionic liquids and of 1-hexyl-3-methylimidazolium ionic liquids with different anions were determined in the temperature range between 123 and 393 K on the basis of dielectric measurements in the frequency range from 1 to 10(7) Hz. Most of the ionic liquids form a glass and the conductivity values obey the Vogel-Fulcher-Tammann equation. The glass transition temperatures are increasing with increasing length of the alkyl chain. The fragility is weakly dependent on the alkyl chain length but is highly sensitive to the structure of the anion.     

Diffusion Measurements To Understand Dynamics and Structuring in Solutions Involving a Homologous Series of Ionic Liquids

The self-diffusion coefficients of each of the components in mixtures containing pyridine and each of the homologous series 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imides in acetonitrile were determined using NMR diffusometry (i. e., Pulsed Gradient Spin Echo). The nature of solvation was found to change significantly with the proportion of salt in the mixtures. Increased diffusion coefficients (when corrected for viscosity) for the molecular components were observed with increasing proportion of ionic liquid and with increasing alkyl chain length on the cation. Comparison of the molecular solvents suggests increased interactions in solution of the pyridine with other components of the mixture, consistent with the proposed interactions shown previously to drive changes in reaction kinetics. Discontinuities were seen in the diffusion data for each species in solution across different ionic liquids between the hexyl and octyl derivatives, suggesting a change in the structuring in solution as the alkyl chain on the cation changes and demonstrating the importance of such when considering homologous series.     

Temperature effect of hydrophobically modified polyethylene oxide at the air–water interface

Surface pressure (π)–area (A) isotherms of hydrophobically modified polyethylene oxide (HEUR) at the air–water interface was examined. Conformational transitions between pancake, mushroom, and brush states of the hydrophilic backbone influence the intermolecular interaction between the hydrophobic chains. We choose relatively long (18 carbons) hydrophobic ends, which have large hydrophobic interactions, and investigate the main chain effect by change in the length of the hydrophilic PEO chain. At high surface concentration region, the temperature coefficient of surface pressure, dπ/dT, was larger by increasing the portion of the hydrophobicity. This indicates an increase in surface energy and a decrease in surface enthalpy at high surface concentrations. As alkyl chains on both sides of HEURs are anchored at the air–water interface, restriction caused by the alkyl chain would be smaller for the long PEO chain, but the larger for the short PEO chain length.     

二茂铁在几种离子液体中的迁移行为

采用循环伏安法研究了二茂铁(Fc)在几种离子液体中的迁移行为. 计算了Fc在各离子液体中的扩散系数和粘度系数,探讨了离子液体粘度与离子液体结构之间的关系. 实验结果表明, 离子液体的粘度随阳离子取代基碳链长度的增加而增加, 随阴离子对称性的增加而增加.     

Spatial correlations of a flexible-chain oligomer in an ionic liquid

The effect of the length of the cationic tail of an ionic liquid on the dependence of the scale of structural heterogeneities of a flexible-chain oligomer dissolved in the liquid on the oligomer concentration is studied in terms of the integral equation theory. The structure of the ionic-liquid-oligomer mixture is analyzed with the use of the calculated partial structure factors. The effect of the alkyl chain length of the cation on the dependence of the characteristic scale of ordering of the oligomer chains on their concentration in the solution is the most significant for an ionic liquid with medium-length and long cationic tails. It is shown that the behavior of this dependence is affected by the type of ordering of the ionic-liquid ions. For comparison, a similar dependence for an oligomer in a molecular solvent is calculated and analyzed.     

Odd-even effects in the homologous series of alkyl-cyanobiphenyl liquid crystals: a molecular dynamic study

For the first five members of the homologous series of 4-n-alkyl-4'-cyanobiphenyls in the nematic and isotropic phases, molecular dynamics (MD) simulations were performed by using a united atom model. An odd-even effect was seen to be found in the molecular length, width, biaxiality, and inertia tensor anisotropy with the length of alkyl chain. The distributions of the molecular length, width, and inertia tensor anisotropy were presented. The orientational distributions of the last C--C bonds in the alkyl chain were also evaluated for nematic and isotropic phases.     

Structure of nonionic surfactant micelles in the ionic liquid ethylammonium nitrate

The structure of micelles formed by nonionic polyoxyethylene alkyl ether nonionic surfactants, C n E m , in the room-temperature ionic liquid, ethylammonium nitrate (EAN), has been determined by small-angle neutron scattering (SANS) as a function of alkyl and ethoxy chain length, concentration, and temperature. Micelles are found to form for all alkyl chains from dodecyl through to octadecyl. Dodecyl-chained surfactants have high critical micelle concentrations, around 1 wt%, and form weakly structured micelles. Surfactants with longer alkyl chains readily form micelles in EAN. The observed micelle structure changes systematically with alkyl and ethoxy chain length, in parallel with observations in aqueous solutions. Decreasing ethoxy chain length at constant alkyl chain length leads to a sphere to rod transition. These micelles also grow into rods with increasing temperature as their cloud point is approached in EAN.