Improved United-Atom Force Field for 1-Alkyl-3-methylimidazolium Chloride

Abstract

We have developed a united atom (UA) nonpolarizable force field for 1-alkyl-3-methyl-imidazolium chloride ([C n mim][Cl], n ) 1, 2, 4, 6, 8), a potential solvent for the pretreatment of lignocellulosic biomass. The charges were assigned by fitting the electrostatic potential surface (ESP) of the ion pair dimers. The LennardJones parameters of the hydrogen atoms on the imidazolium ring were adjusted to agree with the ab initio optimized geometries of isolated ion pairs. Molecular dynamics (MD) simulations were performed for a wide range of temperatures to validate the force field. Substantial improvements were found in both the dynamical properties and the fluid structures, as compared to those predicted using our previously developed UA force field (UA2006) (Phys. Chem. Chem. Phys. 2006, 8, 1096. Liquid densities were found to lie within 2% experimental data. The simulated heats of vaporization decreased about 30% relative to that predicted using the UA2006 force field. The site-site radial distribution functions between the hydrogen atoms on the imidazolium ring and the chloride anions were in good agreement with those determined by ab initio molecular dynamics. The newly developed force field gives a much better description of the self-diffusion coefficients and shear viscosities, which usually deviate by 1 order of magnitude when determined using other force fields.

Multiscale modeling of the trihexyltetradecylphosphonium chloride ionic liquid

“…61,62,69 Additionally, the calculated cation-cation, cation-anion, and anion-anion pair RDFs from UA models at other temperatures show qualitatively similar features as those obtained from atomistic simulations, and are hence not shown here. The sharp peak registered at approximately 0.40 nm in the P-Cl RDFs indicates a wellorganized ionic structure with a distinct first coordination shell of Cl anions around [P 6,6,6,14 ] cations due to strong electrostatic interactions.…”

“…The force field parameters for the retained atoms in the UA [P 6,6,6,14 ] model are derived from the corresponding atomistic force field without adjustment, and thus have the same potential functional forms as shown in eqn (1), while the vdW parameters for the united methylene CH 2 and methyl CH 3 sites are adopted from the transferable potentials for the phase equilibria (TraPPE) force field developed by the Siepmann group. 62,63 Therefore, in the current work, the partial charges for UA sites are determined by simply summing up the atomic charges of their respective atomic constituents, and then scaling the total ion charge as 0.8 e for the UA [P 6,6,6,14 ] cationic model. However, the center of mass (COM) of the UA group does not essentially locate at the carbon center.…”

“…[39][40][41]43,59,60 The united-atom (UA) model is traditionally one of the most popular coarse-graining schemes used to model IL systems [61][62][63][64] as well as hydrocarbon systems in general. One critical problem in computational studies is that the bulky and asymmetric tetraalkylphosphonium cations generally consisting of a large number of atoms, and the strong correlations between ionic groups due to long-range electrostatic interactions, lead to their viscous behavior, and thus molecular simulations at large length and long time scales are needed to obtain reliable results.…”

“…One critical problem in computational studies is that the bulky and asymmetric tetraalkylphosphonium cations generally consisting of a large number of atoms, and the strong correlations between ionic groups due to long-range electrostatic interactions, lead to their viscous behavior, and thus molecular simulations at large length and long time scales are needed to obtain reliable results. [61][62][63][64][67][68][69] Currently, several coarse-graining approaches are available to determine effective interaction potentials between UA sites from atomistic simulation results. In addition to the benefit of computational efficiency, it is also expected that coarse-grained models can reveal essential structural properties of ILs at a mesoscopic level by integrating over less important degrees of freedom at the atomic scale.…”

Viscosities of the Mixtures of 1-Ethyl-3-Methylimidazolium Chloride with Water, Acetonitrile and Glucose: A Molecular Dynamics Simulation and Experimental Study

“…The united-atom force field developed by Liu et al 34 was used to describe [emim][Cl]. The SPC/E model 35 was used to describe water.…”