B. L. Bhargava

Refined potential model for atomistic simulations of ionic liquid [bmim][PF6].

Refined parameters of an atomistic interaction potential model for the room temperature ionic liquid 1-n-butyl,3-methylimidazolium hexafluorophosphate are presented. Classical molecular dynamics simulations have been carried out to validate this fully flexible all-atom model. It predicts the density of the liquid at different temperatures between 300 and 500 K within 1.4% of the experimental value. Intermolecular radial distribution functions and the spatial distribution functions obtained from the new model are in close agreement with ab initio simulations. The calculated diffusion coefficients of ions and the surface tension of the liquid agree well with experiment.

Dynamics in a room-temperature ionic liquid: a computer simulation study of 1,3-dimethylimidazolium chloride.

The transport properties and solvation dynamics of model 1,3-dialkylimidazolium chloride melt at 425 K is studied using molecular-dynamics simulations. Long trajectories of a large system have been generated and quantities such as the self-diffusion coefficient of ions, shear viscosity, and ionic conductivity have been calculated. Interestingly, the diffusion of the heavier cation is found to be faster than the anion, in agreement with experiment. The interaction model is found to predict a higher viscosity and lower electrical conductivity compared to experimental estimates. Analysis of the latter calculations points to correlated ion motions in this melt. The solvation time correlation function for dipolar and ionic probes studied using equilibrium simulations exhibits three time components, which include an ultrafast (subpicosecond) part as well as one with a time constant of around 150 ps. The ultrafast solvent relaxation is ascribed to the rattling of anions in their cage, while the slow component could be related to the reorientation of the cations as well as to ion diffusion.

Molecular dynamics studies of cation aggregation in the room temperature ionic liquid [C10mim][Br] in aqueous solution.

The structure of an aqueous 1-n-decyl-3-methylimidazolium bromide solution and its vapor-liquid interface has been studied using molecular dynamics (MD) simulations. Starting from an isotropic solution, spontaneous self-assembly of cations into small micellar aggregates has been observed. The decyl chains are buried inside the micelle to avoid unfavorable interactions with water, leaving the polar headgroups exposed to water. The cation aggregation numbers, ranging from 15 to 24 compare favorably with experimental estimates. Results are presented for the organization of solvent around the cations. The structure of the aggregates as determined from the present MD simulations does not support the staircase model proposed on the basis of nuclear magnetic resonance studies on similar aqueous ionic-liquid solutions. The distribution of ions in bulk solutions and at an air/water interface is also discussed.

Initial stages of aggregation in aqueous solutions of ionic liquids: molecular dynamics studies.

Structures formed by 1-alkyl-3-methylimidazolium bromide aqueous solutions with decyl, dodecyl, tetradecyl, and hexadecyl chains have been studied using molecular dynamics (MD) simulations. Spontaneous self-assembly of the amphiphilic cations to form quasi-spherical polydisperse aggregates has been observed in all of the systems, with the size and nature of the aggregates varying with chain length. In all systems, the cation alkyl tails are buried deep inside the aggregates with the polar imidazolium group exposed to exploit the favorable interactions with water. Aggregation numbers steadily increase with the chain length. The hexadecyl aggregates have the most ordered internal structure of the systems studied, and the alkyl chains in these cations show the least number of gauche defects.

Formation of micelles in aqueous solutions of a room temperature ionic liquid: a study using coarse grained molecular dynamics

A coarse grained force field model has been developed to study aqueous solutions of the room temperature ionic liquid 1-n-decyl-3-methylimidazolium bromide using molecular dynamics (MD) simulations. Spontaneous self-aggregation of amphiphilic cations to form quasi-spherical micelles has been observed in dilute solutions. Two types of growth mechanism have been observed during the MD simulations, including the fusion of small micelles to form larger ones. Aggregates are found to be poly-disperse and the aggregation number, as determined from the MD simulations, compares well with results from small angle neutron scattering. The dynamics of micelles is discussed. Formation of the hexagonal columnar phase at a concentration of 37% (w/w) of water has been observed with a spacing of the columns in good agreement with the experimentally determined value. The vapor–liquid interface of an aqueous ionic liquid solution has been investigated and the structure at the interface has been characterized. The surface area per cation agrees with the experimentally determined value for a similar ionic liquid containing the chloride anion.

Aqueous solutions of imidazolium ionic liquids: molecular dynamics studies

Molecular dynamics (MD) simulations have been carried out at room temperature on a series of aqueous 1-n-alkyl-3-methylimidazolium bromide ([Cnmim][Br]) solutions with alkyl chain-lengths ranging from ethyl to octyl. The computed cation distributions have been found to be inhomogeneous, with the degree of organization of their tails increasing with the length of the alkyl chain. Cation diffusion decreases with increasing chain length partly due to the formation of aggregates. Anions, which are partially associated with cations, also show a similar behavior despite being the same in all of the solutions. Aqueous [C2mim][Br] solution has been found to remain isotropic even at high concentration of ionic liquid (IL). While cations in [C4mim][Br] solution weakly associate to form small clusters, those in [C6mim][Br] solution form small aggregates. Definite aggregate formation has been observed in [C8mim][Br] solution. Aggregates are poly-disperse and their size varies from 10 to 25 cations. The most probable value of the aggregation number agrees with experimental results. Aggregates are quasi-spherical objects with the alkyl tails forming the core and the imidazolium head-groups exposed to water. The structure observed in the MD simulations does not support the stair-like model proposed based on a nuclear magnetic resonance study on [C8mim][Br] solution. The van der Waals interactions between the alkyl tails are responsible for the aggregation of cations.

Formation of Interconnected Aggregates in Aqueous Dicationic Ionic Liquid Solutions.

The structure and organization in an aqueous solution of a gemini surfactant, the dicationic ionic liquid 1,3-bis(3-decylimidazolium-1-yl) propane bromide, and its vapor-liquid interface have been studied using molecular dynamics simulations at room temperature. Starting from a uniform distribution of cations, the system is found to spontaneously evolve forming cross-linked cationic micellar aggregates. Alkyl tails are typically found buried inside the aggregates to minimize their unfavorable interactions with water, whereas the polar head groups are present at the micellar surfaces, exposed to water. Anions are found throughout the solution and are not strongly bound to the cations. Cationic micellar aggregates exhibit an interesting behavior: interconnection mediated by head groups, a phenomenon which is not observed in monocationic ionic liquid solutions. The structure of the vapor-liquid interface of the solution, the structure of the micellar aggregates, and the distribution of counterions are also discussed.

Nanoscale organization in aqueous dicationic ionic liquid solutions.

Coarse-grained molecular dynamics simulations have been performed on aqueous solutions of the gemini dicationic ionic liquid 1,5-bis(3-decylimidazolium-1-yl) pentane bromide at room temperature to study the structure and organization in the solution. Several trajectories corresponding to different concentrations of the ionic liquid (IL) in the solution were generated for about a microsecond. The 40% (w/w) aqueous mixture evolves to a hexagonal structure starting from a random distribution of ions. Spontaneous aggregation of cations was observed in solutions at lower concentrations. Unlike the monocationic ILs, which form spherical aggregates, the aggregates observed in the dicationic IL solution displayed a near-hexagonal arrangement of the hydrophobic cores that were connected to each other by hydrophilic head groups. Anions were found to be present close to the polar head groups. The formation of the interlinked aggregates from a random distribution and the organization at the vapor-liquid interface are also discussed.

Effect of cation asymmetry on the aggregation in aqueous 1-alkyl,3-decylimidazolium bromide solutions: molecular dynamics studies.

Self-assembly of cations in aqueous solutions of 1-alkyl,3-decylimidazolium bromide (with four different alkyl chains, methyl, butyl, heptyl, and decyl chain,) have been studied using atomistic molecular dynamics simulations. Polydisperse aggregates of cations are formed in the solution with alkyl tails in the core and the polar head groups present at the surface of the aggregates. The shape of the aggregates is dictated by the length of the alkyl chain. Aggregation numbers increase steadily with the increasing alkyl chain length. The greater asymmetry in the two-substituent chain length leads to a different surface structure compared to that of the cations with alkyl chains of similar length.

Ionic Liquids at Nonane–Water Interfaces: Molecular Dynamics Studies

The structures of ternary systems with water, nonane, and an ionic liquid, with the ionic liquid placed between water and nonane, have been studied using atomistic molecular dynamics simulations. Three different ionic liquids with 1-n-butyl-3-methylimidazolium cation and bromide, tetrafluoroborate, and trifluoromethanesulfonate anions have been studied. The ionic liquids disperse into the aqueous phase quickly and are solubilized in water within 15 ns to form two equivalent nonane-aqueous ionic liquid interfaces. The interfacial region is enriched with ionic liquids due to the amphiphilicity of the cations. The presence of ionic liquids at the interface reduces the interfacial tension between the nonane and water, thus facilitating the mixing of aqueous and nonane phases. The reduction in the interfacial tension is found to be inversely related to the solubility of the corresponding ionic liquid in water. The butyl chains of the cations and the trifluoromethanesulfonate anions present in the interfacial region are found to be preferentially oriented parallel to the interface normal.