R. M. Lynden-Bell

Intermolecular potentials for simulations of liquid imidazolium salts

Intermolecular potentials suitable for molecular dynamics or Monte Carlo simulations have been developed for dimethyl imidazolium and methyl ethyl imidazolium ions. The predicted crystal structures were compared with experimental crystal structures for chloride and PF− 6 salts and found to be satisfactory when the dominant electrostatic interactions were modelled by either an accurate distributed multipole description or a simplified atomic point charge model. A further simplification of using united atoms in place of methyl or methylene groups on the side chains gave a much less satisfactory reproduction of the crystal structures. Liquid dimethyl imidazolium chloride and dimethyl imidazolium PF− 6 were simulated using the explicit atom and united atom potentials. The local structure showed a strong preference for the chloride ions to be located in certain regions around the cation, and a similar, but less strong localization of the larger PF− 6. Significant differences in density and diffusion rates were found when the explicit atom model was replaced by the cheaper united atom model, showing that the latter potential is significantly poorer for modelling both the static solid and dynamic liquid simulations.

From hydrophobic to hydrophilic behaviour: A simulation study of solvation entropy and free energy of simple solutes

We describe atomistic simulations of the free energy and entropy of hydration of ions in aqueous solution at 25 °C using a simple point charge model (SPC/E) for water and charged spherical Lennard-Jones solutes. We use a novel method with an extended Lagrangian or Hamiltonian in which the charge and the size of the ions are considered as dynamical variables. This enables us to determine thermodynamic properties as continuous functions of solute size and charge and to move smoothly from hydrophilic to hydrophobic solvation conditions. On passing between these extremes, the entropy of solvation goes through maxima. For example it shows a double maximum as a function of charge at constant size and a single maximum as a function of size at constant (non-zero) charge. These maxima correspond to extremes of structure-breaking and are associated with the disappearance of the second solvation shell in the radial distribution function; no anomalies are seen in the first shell. We also present direct evidence of th...

Mobility and solvation of ions in channels

In this paper we present some results from a simulation study of the mobility and solvation of ions and uncharged molecules in aqueous solution in smooth cylindrical channels at room temperature. This ideal system provides a reference system with which to compare the behavior of water and ions in real porous materials such as zeolites, bucky tubes, and biological channels. We find that in channels of radii between 2.5 and 5.5 A the water molecules form a cylindrical solvation shell inside the channel walls with some evidence of a second shell in the center of the largest channel. Not all protons are involved in hydrogen bonding and a number point toward the walls. We attribute this to the concavity of the surface. When a sodium ion is added it tends to lie in the center of the channel where it can form the most complete solvation shell. Its diffusion rate decreases in smaller channels until it moves too slowly in a channel of 2 A radius to be detected in our simulations. This decrease is only partly due to an increase in the mean square force on the ion. A range of ions of different sizes were studied in a channel with radius 3 A. While the smaller of these ions (F−, Na+, and Ca++) lie preferentially in the center of the channel, larger ions (Cl−, I−, and Cs+) penetrate some way into the layer of water inside the wall and methane and ions with the charges turned off move next to the wall. Landau free energy analysis shows that this change is due to the balance between entropy and energy. The behavior in smooth channels is quite the opposite of what has been observed in experimental studies and simulations of Gramicidin (pore radius of 2 A), where Cs+ lies closer to the center of the channel and Na+ lies off the axis. This difference can be attributed to the specific molecular structure of the gramicidin pores (e.g., the presence of carbonyl groups). As in bulk solutions, the mobilities of the ions in smooth channels increase to a maximum with ion size and decrease with increasing magnitude of the charge on the ion, while uncharged species diffuse much more rapidly and show a monotonic decrease with size. This behavior is related to the characteristics of the fluctuations of the forces on the solute molecules.

Why are aromatic compounds more soluble than aliphatic compounds in dimethylimidazolium ionic liquids? A simulation study

Abstract Molecular dynamics simulations of solutions of benzene in dimethylimidazolium chloride and dimethylimidazolium hexafluorophosphate have been performed with a view to answering the question posed in the title. The difference between the chemical potential of a normal model of benzene and one with no charges was found to depend on the solvent but is at least 4 kBT. This difference is sufficient to account for the observed solubility differences. There are substantial changes in the local structure around benzene with and without charges.

Solvation of small molecules in imidazolium ionic liquids: a simulation study

Molecular dynamics simulations of a number of small molecules dissolved in dimethylimidazolium chloride at 400 K have been performed. The molecules chosen were water, methanol, dimethyl ether and propane, which have a range of properties from polar and hydrogen-bonding to non-polar. The local structure was analysed through ranked radial distribution functions and three dimensional probability functions, and the interaction energy with the anions and cations determined. The energetics and local structure show that the strongest interactions are hydrogen bonding to the chloride ion for water and methanol, while dimethyl ether and propane interact more strongly with the cation.

Gas—liquid interfaces of room temperature ionic liquids

The structure of the gas-liquid surface of dimethylimidazolium chloride has been studied using atomistic simulation. We find that there is a region of enhanced density immediately below the interface in which the cations are oriented with their planes perpendicular to the surface and their dipoles in the surface plane. There is negligible segregation of cations and anions. The temperature dependence of the surface tension is predicted to be anomalously low or be reversed in sign. The vapour-liquid interfaces between mixtures of water and dimethylimidazolium chloride show similar regions of enhanced density and preferential orientation of the cations. Water molecules also show preferential orientation in the interface region and are preferentially adsorbed on the vapour side of the interface. The surface tension decreases with increase in the mole fraction of water.

Electrode screening by ionic liquids

In this work we are concerned with the short-range screening provided by the ionic liquid dimethylimidazolium chloride near a charged wall. We study the free energy profiles (or potentials of mean force) for charged and neutral solutes as a function of distance from a charged wall. Four different wall charge densities are used in addition to a wall with zero charge. The highest magnitude of the charge densities is ±1 e nm(-2) which is close to the maximum limit of charge densities accessible in experiments, while the intermediate charges ±0.5 e nm(-2) are in the range of densities typically used in most of the experimental studies. Positively and negatively charged solutes of approximately the size of a BF ion and a Cl(-) ion are used as probes. We find that the ionic liquid provides excellent electrostatic screening at a distance of 1-2 nm. The free energy profiles show minima which are due to layering in the ionic liquid near the electrodes. This indicates that the solute ions tend to displace ionic liquid ions in the layers when approaching the electrode. The important role of non-electrostatic forces is demonstrated by the oscillations in the free energy profiles of uncharged solutes as a function of distance from the wall.

Simulation of the surface structure of butylmethylimidazolium ionic liquids

Molecular dynamics simulations of the liquid/vacuum surfaces of the room temperature ionic liquids [bmim][PF(6)], [bmim][BF(4)] and [bmim][Cl] have been carried out at various temperatures. The surfaces are structured with a top monolayer containing oriented cations and anions. The butyl side chains tend to face the vacuum and the methyl side chains the liquid. However, as the butyl chains are not densely packed, both anions and rings are visible from the vacuum phase. The effects of temperature and the anion on the degree of cation orientation is small, but the potential drop from the vacuum to the interior of the liquid is greater for liquids with smaller anions. We compare the simulation results with a range of experimental observations and suggest that neutron reflection from samples with protiated butyl groups would be a sensitive probe of the structure.

Ab initio simulation of charged slabs at constant chemical potential

We present a practical scheme for performing ab initio supercell calculations of charged slabs at constant electron chemical potential μ, rather than at constant number of electrons Ne. To this end, we define the chemical potential relative to a plane (or “reference electrode”) at a finite distance from the slab (the distance should reflect the particular geometry of the situation being modeled). To avoid a net charge in the supercell, and thus make possible a standard supercell calculation, we restore the electroneutrality of the periodically repeated unit by means of a compensating charge, whose contribution to the total energy and potential is subtracted afterwards. The “constant μ” mode enables one to perform supercell calculation on slabs, where the slab is kept at a fixed potential relative to the reference electrode. We expect this to be useful in modeling many experimental situations, especially in electro-chemistry.

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.