Structure and dynamics of ionic liquids: Trimethylsilylpropyl-substituted cations and bis(sulfonyl)amide anions

Ionic liquids with cationic organosilicon groups have been shown to have a number of useful properties, including reduced viscosities relative to the homologous cations with hydrocarbon substituents on the cations. We report structural and dynamical properties of four ionic liquids having a trimethylsilylpropyl functional group, including 1-methyl-3-trimethylsilylpropylimidazolium (Si-C₃-mim⁺) cation paired with three anions: bis(fluorosulfonyl)imide (FSI⁻), bis(trifluoromethanesulfonyl)imide (NTf2−), and bis(pentafluoroethanesulfonyl)imide (BETI⁻), as well as the analogous N-methyl-N-trimethylsilylpropylpyrrolidinium (Si-C₃-pyrr⁺) cation paired with NTf2−. This choice of ionic liquids permits us to systematically study how increasing the size and hydrophobicity of the anions affects the structural and transport properties of the liquid. Structure factors for the ionic liquids were measured using high energy X-ray diffraction and calculated from molecular dynamics simulations. The liquid structure factors reveal first sharp diffraction peaks (FSDPs) for each of the four ionic liquids studied. Interestingly, the domain size for Si-C₃-mim⁺/NTf2− indicated by the maxima for these peaks is larger than for the more polar ionic liquid with a similar chain length, 1-pentamethyldisiloxymethyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide (SiOSi-mim⁺/NTf2−). For the series of Si-C₃-mim⁺ ionic liquids, as the size of the anion increases, the position of FSDP indicates that the intermediate range order domains decrease in size, contrary to expectation. Diffusivities for the anions and cations are compared for a series of both hydrocarbon-substituted and silicon-substituted cations. All of the anions show the same scaling with temperature, size, and viscosity, while the cations show two distinct trends - one for hydrocarbon-substituted cations and another for organosilicon-substituted cations, with the latter displaying increased friction.