Structural aspect on “Salting-in” mechanism of PEG chains into a phosphonium-based ionic liquid using lithium salt

•Salting-in dissolution mechanism of PEG into a phosphonium-based ionic liquid with Li salt was investigated.•Raman spectroscopy, X-ray scattering, and MD simulations were conducted to elucidate Li-ion coordination structures.•Li ions are preferentially coordinated with PEG, rather than TFSA anions, to form Li+–PEG complexes.•Li ions bound to the PEGs act as a solvation site, resulting in the salting-in dissolution of PEG. A study on the “salting-in” mechanism of poly(ethylene glycol) (PEG) into triethylpentylphosphonium bis(trifluoromethanesulfonyl)amide ([P2225][TFSA]) ionic liquid with LiTFSA salt is presented. There was no dissolution of PEG chains into [P2225][TFSA] in the absence of LiTFSA salt. However, salting-in dissolution occurred when LiTFSA salt was added irrespective of the molecular weight of PEG. Raman spectral analysis revealed that the TFSA anions trapped within the Li-ion coordination complex (i.e., [Li(TFSA)2]–) in [P2225][TFSA] solution are easily decoordinated when PEG-chains are added to form stable Li+⋯PEG complexes; in other words, Li ions preferentially interact with O atoms of PEG over those of TFSA anion. High-energy X-ray total scattering (HEXTS) and all-atom molecular dynamics (MD) simulations were combined to elucidate the detailed structures of the salting-in dissolved PEG chains at the molecular level. Some of the notable characteristics are as follows: (1) In PEG/[P2225][TFSA] solution (without LiTFSA salt), the PEG chains struggle to interact with the P2225 cations because of weaker PEG–P2225+ interactions, resulting in the incompatibility of PEG/[P2225][TFSA] with shrink polymer chains. (2) When LiTFSA is added to the solution, the Li ions bound to the PEG chains act as a solvation site for solvent IL (in this case, TFSA anions). (3) Interactions between PEG-coordinated Li+ and TFSA– trigger the polymer chains to expand, resulting in “salting-in” dissolution of PEG.