Article pubs.acs.org/JPCB
How Does the Alkyl Chain Length of an Ionic Liquid Influence Solute Rotation in the Presence of an Electrolyte? Sugosh R. Prabhu and G. B. Dutt* Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India S Supporting Information *
ABSTRACT: Fluorescence anisotropies of a nonpolar solute, 9-phenylanthracene (9-PA), have been measured in 1-alkyl-3methylimidazolium (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.
1. INTRODUCTION The nanostructural organization of ionic liquids has been extensively investigated using numerous experimental techniques and theoretical methods.1−15 The consensus that emerged from these studies suggests that unlike conventional solvents, ionic liquids are not homogeneous at the microscopic level but segregate into polar and nonpolar domains due to the presence of a cooperative network of hydrogen bonds between the cations and anions.15 Besides comprehending the nanostructural organization of ionic liquids, considerable progress has been made toward understanding the dynamics of the solutes dissolved in these systems.16−40 Results obtained from some of these studies have established that the organized structure is responsible for the dynamic heterogeneity displayed by the chemical reactions in ionic liquids. Essentially, dynamic heterogeneity has been defined as the presence of a distribution of relaxation rates for molecules residing in different local environments of the ionic liquids.19 Although many studies have focused on the fundamental aspects of structure and dynamics in neat ionic liquids, little information is available concerning these features in the presence of additives such as electrolytes.41−44 In other words, how the addition of an electrolyte influences the nanostructural organization of the ionic liquid and the subsequent effect on solute dynamics have not been examined thoroughly. Recently, studies conducted by Fayer’s group and our group have addressed the issue of solute rotation in ionic liquid−electrolyte systems.45−47 Fluorescence anisotropy stud© XXXX American Chemical Society
ies involving nonpolar solutes in imidazolium-based ionic liquids in the presence of salts such as lithium bis(trifluoromethylsulfonyl)imide (LiTf2N) and lithium tetrafluoroborate (LiBF4) reveal that solute rotation becomes progressively faster with an increase in the concentration of the electrolyte.45,46 In a study carried out by Lawler and Fayer,45 it has been observed that the in-plane and out-of-plane friction coefficients of the nonpolar solute, perylene, decrease significantly in 1-buty l-3-meth ylimidazolium bis(trifluoromethylsulfonyl)imide with an increase in the concentration of LiTf2N. In a similar vein, we have noticed that the rotational diffusion of another nonpolar solute, 9-phenylanthracene (9-PA), in 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4] gradually becomes faster with an increase in [LiBF4].46 Even though the electrolyte ions are solubilized in the polar domains of the ionic liquids, they alter the packing of the alkyl chains in such a way that the rotation of the solutes located in the nonpolar domains is affected. Thus, these studies conclusively demonstrate that addition of electrolytes perturbs the organized structure of the imidazolium ionic liquids. However, it may be noted that similar studies carried out with a pair of structurally similar nondipolar solutes in a protic ionic liquid, ethylammonium nitrate (EAN), in the presence of Received: September 7, 2016 Revised: October 28, 2016 Published: December 1, 2016 A
DOI: 10.1021/acs.jpcb.6b09032 J. Phys. Chem. B XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry B
Chemie Pvt. Ltd. and Aldrich, respectively. The stated purity of the ionic liquids is >99%, with
