Effect of Alkyl Chain Length on the Rotational Diffusion of Nonpolar and Ionic Solutes in 1‑Alkyl-3-Methylimidazolium-bis(trifluoromethylsulfonyl)imides

Rotational diffusion of a nonpolar solute 9-phenylanthracene (9-PA) and a cationic solute rhodamine 110 (R110) has been examined in a series of 1-alkyl-3-methylimidazolium (alkyl = octyl, decyl, dodecyl, tetradecyl, hexadecyl, and octadecyl) bis­(trifluoromethylsulfonyl)­imides to understand the influence of alkyl chain length on solute rotation. In this study, reorientation times (τ_r) have been measured as a function of viscosity (η) by varying the temperature (<i>T</i>) of the solvents. These results have been analyzed using the Stokes–Einstein–Debye (SED) hydrodynamic theory along with the ones obtained for the same solutes in 1-alkyl-3-methylimidazolium (alkyl = methyl, ethyl, propyl, butyl, and hexyl) bis­(trifluoromethylsulfonyl)­imides (Gangamallaiah and Dutt, <i>J. Phys. Chem. B</i> <b>2012,</b> <i>116,</i> 12819–12825). It has been noticed that the data for 9-PA and R110 follows the relation τ_r = <i>A</i>(η/<i>T</i>)^<i>n</i> with <i>A</i> being the ratio of hydrodynamic volume of the solute to the Boltzmann constant and <i>n</i> = 1 as envisaged by the SED theory. However, upon increasing the alkyl chain length from methyl to octadecyl significant deviations from the SED theory have been observed especially from the octyl derivative onward. From methyl to octadecyl derivatives, the value of <i>A</i> decreases by a factor of 3 for both the solutes and <i>n</i> by a factor of 1.4 and 1.6 for 9-PA and R110, respectively. These observations have been rationalized by taking into consideration the organized structure of the ionic liquids, whose influence appears to be pronounced when the number of carbon atoms in the alkyl chain attached to the imidazolium cation exceeds eight