We develop ab initio force fields for alkylimidazolium-based ionic liquids (ILs) that predict the density, heats of vaporization, diffusion, and conductivity that are in semiquantitative agreement with experimental data. These predictions are useful in light of the scarcity of and sometimes inconsistency in experimental heats of vaporization and diffusion coefficients. We illuminate physical trends in the liquid cohesive energy with cation chain length and anion. These trends are different than those based on the experimental heats of vaporization. Molecular dynamics prediction of the room temperature dynamics of such ILs is more difficult than is generally realized in the literature due to large statistical uncertainties and sensitivity to subtle force field details. We believe that our developed force fields will be useful for correctly determining the physics responsible for the structure/property relationships in neat ILs.

Molecular simulations play an important role in establishing structure-property relations in complex fluids such as room-temperature ionic liquids. Classical force fields are the starting point when large systems or long times are of interest. These force fields must be not only accurate but also transferable. In this work, we report a physically motivated force field for the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) based on symmetry-adapted perturbation theory. The predictions (from molecular dynamics simulations) of the liquid density, enthalpy of vaporization, diffusion coefficients, viscosity, and conductivity are in excellent agreement with experiment, with no adjustable parameters. The explicit energy decomposition inherent in the force field enables a quantitative analysis of the important physical interactions in these systems. We find that polarization is crucial and there is little evidence of charge transfer. We also argue that the often used procedure of scaling down charges in molecular simulations of ionic liquids is unphysical for [BMIM][BF4]. Because all intermolecular interactions in the force field are parametrized from first-principles, we anticipate good transferability to other ionic liquid systems and physical conditions.

Polymers exhibit interesting phase behavior in room temperature ionic liquids. For example poly(ethylene oxide) (PEO) displays a lower critical solution temperature (LCST) in [BMIM]- [BF 4 ] with a critical temperature and concentration that are only weakly dependent on molecular weight, contrary to the behavior of polymers in other solvents. To shed light on the mechanism of the LCST, we study the phase behavior of PEO in [BMIM][BF 4 ] using molecular dynamics (MD) simulations. The simulations show the signature of a phase transition as the temperature is increased. At low temperatures, interactions similar to a hydrogen bond are found between the imidazolium hydrogen and the PEO oxygen (HI−O Hbond) and the imidazolium hydrogen and the anion fluorines (HI−F H-bond). These interactions stabilize the mixed phase. A potential of mean force (PMF) analysis shows an entropic cost associated with the HI−O H-bond, which makes the bond formation unfavorable at higher temperatures, while the HI−F Hbond does not show a significant temperature dependence: This suggests that LCST phase separation is driven by the entropic penalty of the polymer for a PEO-cation hydrogen bond. We test the effect of scaling the charges on the [BMIM] [BF 4 ]. Interestingly, the scaled charge force-field does not predict a phase separation at any temperature, thus, emphasizing the pitfalls of charge scaling for mixtures. I onic liquids have attracted great attention in the past decade due to their interesting unique properties, such as negligible vapor pressure, high thermal and chemical stability, relatively high ionic conductivity, and nonflammability. These properties have made them an excellent candidate for the next generation solvents that could potentially replace traditional organic solvents in many areas. When mixed with certain polymers, ionic liquids have potential in materials design because the polymers can provide mechanical integrity and structural persistence that ionic liquids lack. 1 These include applications as membranes for fuel cells, polymerized ion gels for gas separation, basis of electromechanical actuators, and electrolytes in lithium batteries. Understanding the phase behavior and miscibility of polymers in ionic liquids is therefore crucial for materials design. There is also fundamental interest because the phase behavior is unusual. Poly(ethylene oxide) (PEO) displays a lower critical solution temperature (LCST) in 1-ethyl-3-methylimidazolium tetrafluoroborate ( [EMIM][BF 4 ]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF 4 ]).Interestingly, unlike typical polymer solutions, the critical composition occurs at high polymer concentrations, and the critical temperature is insensitive to polymer molecular weight. 2,3 In this work, we use atomistic computer simulations to study the phase behavior of PEO in [BMIM] [BF 4 ] .An LCST usually occurs when both enthalpy of mixing and entropy of mixing are negative. The negative entropy of mixing is often explained either by specific interactions 4−6 or compre...

The behavior of polymers in ionic liquids is of technological and scientific interest. In this work we present atomistic simulations for the properties of isolated poly(ethylene oxide) (PEO) in the ionic liquid 1-butyl 3-methylimidazolium tetrafluoroborate ([BMIM][BF 4 ]) and compare to the properties of the same polymer in water, at room temperature, for degrees of polymerization N ranging from 9 to 40. PEO chains are much more expanded in [BMIM][BF 4 ] than in water. The root-mean-square radius of gyration, R g , scales as R g ∼ N ν with ν ≈ 0.9, and the distribution of end-to-end distance is bimodal, with coexisting extended and hairpin-like conformations. The simulations are consistent with polyelectrolyte behavior, i.e., R g ∼ N, but the chains might be too short to be in the true scaling regime. (For comparison, R g ∼ N 0.5 in water.) [BMIM][BF 4 ] is a much better solvent than water: In [BMIM][BF 4 ] the solvation free energy of the monomer is 50% more negative, and the potential of mean force between two PEO 9-mers is significantly more repulsive than in water; the repulsion comes from energetic polymer−solvent interactions. The simulations suggest that the conformational behavior of PEO in ionic liquids is different from that in other common solvents, and computational studies of long chains will be interesting.

A new coarse-grained force field is developed for polyethylene glycol (PEG) in water. The force field is based on the MARTINI model but with the big multipole water (BMW) model for the solvent. The polymer force field is reparameterized using the MARTINI protocol. The new force field removes the ring-like conformations seen in simulations of short chains with the MARTINI force field; these conformations are not observed in atomistic simulations. We also investigate the effect of using parameters for the end-group that are different from those for the repeat units, with the MARTINI and BMW/MARTINI models. We find that the new BMW/MARTINI force field removes the ring-like conformations seen in the MARTINI models and has more accurate predictions for the density of neat PEG. However, solvent-separated-pairs between chain ends and slow dynamics of the PEG reflect its own artifacts. We also carry out fine-grained simulations of PEG with bundled water clusters and show that the water bundling can lead to ring-like conformations of the polymer molecules. The simulations emphasize the pitfalls of coarse-graining several molecules into one site and suggest that polymer-solvent systems might be a stringent test for coarse-grained force fields.