How Does the Alkyl Chain Length of an Ionic Liquid Influence Solute Rotation in the Presence of an Electrolyte?

Fluorescence anisotropies of a nonpolar solute, 9-phenylanthracene (9-PA), have been measured in 1-alkyl-3-methylimidazolium (alkyl = methyl, butyl, octyl, and dodecyl) bis(trifluoromethylsulfonyl)imides ([RMIM][Tf₂N]) with varying amounts (0-0.3 mole fraction) of lithium bis(trifluoromethylsulfonyl)imide (LiTf₂N). This study has been carried out to understand how the length of the alkyl chain and the concentration of the electrolyte influence the rotational diffusion of a nonpolar solute. It has been observed that the addition of an electrolyte to the ionic liquid increases the bulk viscosity of the system significantly, as the Li⁺ cations strongly coordinate with the [Tf₂N] anions in the polar domains. The reorientation times of 9-PA have been analyzed with the aid of Stokes-Einstein-Debye hydrodynamic (SED) theory, and they fall within the broad limits set by the hydrodynamic slip and stick boundary conditions. However, deviations from the SED theory have been noticed upon addition of LiTf₂N, and the influence of the electrolyte is more pronounced in the case of ionic liquids with shorter alkyl chains. The observed trends have been rationalized in terms of electrolyte-induced structural changes in these ionic liquids.