Rotational Diffusion of Organic Solutes in 1-Methyl-3-octylimidazolium Tetrafluoroborate–Diethylene Glycol Mixtures: Influence of Organic Solvent on the Organized Structure of the Ionic Liquid

Rotational diffusion of two structurally similar organic solutes, 9-phenylanthracene (9-PA) and rhodamine 110 (R110), has been investigated in 1-methyl-3-octylimidazolium tetrafluoroborate–diethylene glycol ([MOIM]­[BF₄]–DEG) mixtures to understand the influence of organic solvent on the organized structure of the ionic liquid. The reorientation times (τ_r) of nonpolar and charged solutes have been measured as a function of viscosity (η) by changing the temperature (<i>T</i>) as well as the composition of the ionic liquid–organic solvent mixture. These results when analyzed using the Stokes–Einstein–Debye (SED) hydrodynamic theory follow the relationship τ_r = <i>A</i>(η/<i>T</i>)^<i>n</i>, where <i>A</i> is the ratio of hydrodynamic volume of the solute to Boltzmann constant. However, in neat [MOIM]­[BF₄] and up to 0.4 mole fraction of DEG (<i>x</i>_DEG), significant deviations from the SED hydrodynamic theory have been noticed with <i>n</i> being much less than unity. As <i>x</i>_DEG is increased further, the parameters <i>A</i> and <i>n</i> increase considerably for both solutes, and their rotational diffusion follows the predictions of the SED hydrodynamic theory. It has also been observed that the trends in the variation of τ_r with η/<i>T</i> for 9-PA and R110 are not similar. These observations have been rationalized by taking into consideration the organized structure of the ionic liquid, which gradually becomes homogeneous at the microscopic level with the addition of the organic solvent