Atomistic Simulations of Poly(ethylene oxide) in Water and an Ionic Liquid at Room Temperature

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][BF4]) 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] [BF4] than in water. The root-mean-square radius of gyration, R-g, scales as R-g similar to N-nu with nu approximate to 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 similar to N, but the chains might be too short to be in the true scaling regime. (For comparison, R-g similar to N-0.5 in water.) [BMIM]{BF4] is a much better solvent than water: In [BMIM] BF4] 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.