Hydrogen-bond rich ionic liquids with hydroxyl cationic tails (original) (raw)
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Ab initio molecular dynamics simulation of ionic liquids
Chemical Physics, 2007
Ab initio Car-Parinnello molecular dynamics is used to simulate the structure and the dynamics of 1-butyl-3-methylimidazolium iodide ͓͑bmim͔I͒ 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-plane͒ 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 ͓bmim͔I are in quite agreement with the experiments.
The Journal of Physical Chemistry B, 2008
Molecular dynamics (MD) simulations have been performed to investigate the structure and dynamics of an energetic ionic liquid, 1-hydroxyethyl-4-amino-1,2,4-triazolium nitrate (HEATN). The generalized amber force field (GAFF) was used, and an electronically polarizable model was further developed in the spirit of our previous work (Yan, T.; Burnham, C. J.; Del Popolo, M. G.; Voth, G. A. J. Phys. Chem. B 2004, 108, 11877). In the process of simulated annealing from a liquid state at 475 K down to a glassy state at 175 K, the MD simulations identify a glass-transition temperature region at around 250-275 K, in agreement with experiment. The self-intermediate scattering functions show vanishing boson peaks in the supercooled region, indicating that HEATN may be a fragile glass former. The coupling/decoupling of translational and reorientational ion motion is also discussed, and various other physical properties of the liquid state are intensively studied at 400 K. A complex hydrogen bond network was revealed with the calculation of partial radial distribution functions. When compared to the similarly sized 1-ethyl-4-methyl-1,4-imidazolium nitrate ionic liquid, EMIM + / NO 3 -, a hydrogen bond network directly resulting in the poorer packing efficiency of ions is observed, which is responsible for the lower melting/glass-transition point. The structural properties of the liquid/vacuum interface shows that there is vanishing layering at the interface, in accordance with the poor ion packing. The effects of electronic polarization on the self-diffusion, viscosity, and surface tension of HEATN are found to be significant, in agreement with an earlier study on EMIM + /NO 3 -(Yan, T.; Burnham, C. J.; Del Popolo, M. G.; Voth, G. A.
ChemPhysChem, 2009
al, dynamical and thermodynamic properties of such kind of liquids. The crucial point is the derivation of a reliable force field that can describe the interactions between ions in an appropriate way. For H-bonded liquids such as water, alcohols or amides this is usually done by fitting charges and Lennard-Jones parameters to experimentally well-known properties. Force fields are parameterized to be in agreement with pair correlation functions from neutron or X-ray structure factors, self-diffusion coefficients from NMR and heats of vaporization from vapor pressure measurements. [6, 7] Until recently, most of these properties were not available for ionic liquids. Moreover, they are not easy to determine with sufficient accuracy, and they are effectively known only for a few members of some ionic liquid families. One of the first and most prominent force fields used for ILs was developed by two of us [J. N. Canongia Lopes, A. A. H. Pµdua, (CLaP)] using Lennard-Jones parameters taken from the OPLS (see below) force field describing similar organic (and neutral) molecules as a starting point. The missing parameters (mainly atomic point charges and dihedral angles) were parameterized using data obtained using ab initio and MD calculations. Due to the aforementioned scarcity of data, the force field could only be validated by comparison of the MD simulation results with the corresponding crystalline structures and liquid densities of selected ionic liquids. A series of four articles [8-11] represent the CLaP force field at this stage and establish a general protocol for the molecular simulation of common ionic liquids within the framework of statistical mechanics. It must be stressed that the force field has been developed in the spirit of the OPLS model and is thus primarily oriented towards the calculation of structural and equilibrium thermodynamic properties of liquid phases. Moreover, the force field was built in a stepwise manner that allows the construction of models for entire families of ionic liquids and allows the mutual interchange of their anions or cations without further reparameterization. However, the wide coverage of such a protocol comes at the expense of its accuracy for specific cases. For the ionic liquids of the 1-alkyl-3-methylimidazolium bistriflamide family, [C n mim]A C H T U N G T R E N N U N G [NTf 2 ], the measured density and the MD density differ by about 3 % when using the CLaP force field. It is more difficult to assess the ability of this force field to predict transport properties. Calculation of diffusion coefficients through equilibrium molecular dynamics methods appears to lead to values that are one order of magnitude lower than experimental ones. It has been shown that inclusion of polarizable charges leads to faster dynamics when using equilibrium methods, [12] resulting in a drop of about 1/3 in viscosity and a threefold increase in the ion diffusion coefficients. But other studies [13-15] have shown that the use of nonequilibrium methods can lead to viscosities and diffusion coefficients that agree very well with experiment even for fixed-charge models, and in some of these calculations ions were parameterized through the CLaP model. Regarding transport properties, the roles of the model or of the simulation method in attaining quantitative predictions are not yet completely resolved. Finally, the estimated enthalpies of vaporization are too large (by 20 and 50 %) when compared to the available experimental data.
Understanding Ionic Liquids through Atomistic and Coarse-Grained Molecular Dynamics Simulations
Accounts of Chemical Research, 2007
Understanding the physical properties of ionic liquids (ILs) via computer simulation is important for their potential technological applications. The goal of our IL research is to obtain a unified understanding of the properties of ILs with respect to their underlying molecular structure. From atomistic molecular dynamics simulations, the many-body electronic polarization effect was found to be important for modeling ILs, especially their dynamics. The multiscale coarse-graining methodology has also been employed to increase the simulation speed by a factor of 100 or more, thereby making it possible to study the mesoscopic behavior of ILs by computer simulations. With these simulation techniques, ILs with an amphiphilic cation were found to exhibit a spatial heterogeneity due to the aggregation of their nonpolar alkyl tails. This spatial heterogeneity is a key feature in interpreting many physical phenomena of ILs, such as their heterogeneous selfdiffusion and surface layering, as well as their surfactant-like micelles formed in IL/water mixtures.
Simulations of Ionic Liquids, Solutions, and Surfaces
Accounts of Chemical Research, 2007
We have been using atomistic simulation for the last 10 years to study properties of imidazolium-based ionic liquids. Studies of dissolved molecules show the importance of electrostatic interactions in both aromatic and hydrogen-bonding solutes. However, the local structure strongly depends upon ion-ion and solute-solvent interactions. We find interesting local alignments of cations at the gas-liquid and solid-liquid interfaces, which give a potential drop through the surface. If the solid interface is charged, this charge is strongly screened over distances of a few nanometres and this screening decays on a fast time scale. We have studied the sensitivity of the liquid structure to force-field parameters and show that results from ab initio simulations can be used in the development of force fields.
A systematic molecular simulation study of ionic liquid surfaces using intrinsic analysis methods
2012
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.
Physical Chemistry Chemical Physics, 2008
We performed a detailed molecular dynamics study of the interfacial structure of aqueous solutions of 1-butyl-3-methylimidazolium tetrafluoroborate in order to explain the anomalous dependence of the surface tension on concentration. At low concentrations the surface tension decreases with concentration. At higher concentrations the surface becomes saturated; a plateau is observed in simulations with a non-polarizable force field while a possible increase is detected in simulations with a polarizable force field. The structure is characterized by a surplus of cations at the surface (with hydrophobic butyl chains pointing toward vacuum) which at low concentrations is only partly compensated by the anions because of asymmetric solvation. A more hydrophobic 1-butyl-3-methylimidazolium hexafluorophosphate is also simulated for comparison.