Solvation Structure of Poly(ethylene glycol) in Ionic Liquids Studied by High-energy X-ray Diffraction and Molecular Dynamics Simulations

Solvation structures of poly(ethylene glycol) (PEG) in 1-ethyl-3-methylimidazolium-based ionic liquids solution were studied by using high-energy X-ray diffraction (HEXRD) and molecular dynamics (MD) simulation in terms of dependences on the anion-type for ILs, molecular weight (M-w), concentration and shape (linear or tetra-armed) for PEG. It was found from HEXRD experiment that there is no M-w and shape dependences for PEG in 1-ethyl-3-methylimidazolium bis-(trifluoromethansulfonyl)amide, [C(2)mIm(+)][TFSA(-)] solution, implying that the local structure (r < 15 angstrom) is not influenced by M-w and shape of PEG. From the HEXRD results with the aid of MD simulations, it was found that C(2)mIm cation preferentially solvates to PEG in all the ILs, and TFSA is distributed randomly with van der Waals and weak electrostatic interactions. The atom-atom pair correlation and space distribution functions evaluated from the MD simulations revealed that hydrogen bonding interactions are formed between C2 carbon within the imidazolium ring and O (PEG), which is essentially different from C4 carbon-O and C5 carbon-O interactions. Finally, we discussed the relationship between the obtained microscopic solvation structures and the macroscopic properties of PEG-based gel (swelling ratio and chi parameters).