Rotational Diffusion of Charged and Nondipolar Solutes in Ionic Liquid–Organic Solvent Mixtures: Evidence for Stronger Specific Solute–Solvent Interactions in Presence of Organic Solvent

Abstract

Rotational diffusion of a charged solute, rhodamine 110 (R110), and a nondipolar solute, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP), has been investigated in ionic liquids, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][Tf2N]) and 1-butyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([BMIM][FAP]), with 0.8 mole fraction of dibenzyl ether (DBE). This study has been undertaken to find out how specific interactions between the solute and the ionic liquid are affected upon dilution with a nondipolar solvent. It has been observed that at a given viscosity (η) and temperature (T), the reorientation times of R110 increase by 40-60% in the ionic liquid-organic solvent mixtures compared to ones in the corresponding neat ionic liquids. In the case of DMDPP, the influence of DBE is less pronounced, and its reorientation times increase by 25-50% at a given η/T. The addition of DBE weakens the numerous interactions prevailing between the cations and the anions of the ionic liquids, which results in stronger specific interactions between the solutes and the constituent ions, consequently leading to slower rotation of the solutes.

Characterizing the Magnitude and Structure-Dependence of Free Charge Density Gradients in Room-Temperature Ionic Liquids

“…To characterize this gradient in RTILs, we measure the fluorescence anisotropy decay of selected chromophores in the RTILs as a function of the distance from the support (ITO) surface. The study of diffusion, solvation dynamics, and energy transfer in RTILs has proven to be an important means of understanding local organization and longer-range diffusional behavior in these systems. − In our work, the experimental signals acquired are the time-resolved emission transients polarized parallel ( I || ( t )) and perpendicular ( I ⊥ ( t )) to the incident excitation light pulse. The induced orientational anisotropy decay function, R ( t ), is the normalized difference between I || ( t ) and I ⊥ ( t ).…”

Rotational Dynamics in Ionic Liquids from NMR Relaxation Experiments and Simulations: Benzene and 1-Ethyl-3-Methylimidazolium

“…Nonpolar fluorophores sometimes exhibit times below slip predictions, 18,28,16 and molecules with charged functional groups may have rotation times greater than stick predictions. 19,21,26 Within a single solvent, rotation times typically conform to τ rot ∝ (η/T) p , and in most cases, p ≅ 1, as expected from hydrodynamics. However, much smaller powers are sometimes found.…”

“…In most cases, rotation times are found to fall between the limiting hydrodynamic predictions of stick and slip boundary conditions, but exceptions are also observed. Nonpolar fluorophores sometimes exhibit times below slip predictions, ,, and molecules with charged functional groups may have rotation times greater than stick predictions. ,, Within a single solvent, rotation times typically conform to τ rot ∝ (η/ T ) p , and in most cases, p ≅ 1, as expected from hydrodynamics. However, much smaller powers are sometimes found. ,− For example, in one study where pressure rather than temperature was varied, values of p as small as 0.55 and 0.39 were reported .…”

“…To date, the method most often applied to measure rotational dynamics in ionic liquids has been time-dependent fluorescence anisotropy of aromatic fluorophores. − Early studies ,, showed that rotational correlation functions in ionic liquids are often nonexponential but with correlation times τ rot that conform to the hydrodynamic expectation τ rot ∝ η/ T , where η is the solution viscosity and T the temperature. These early studies also reported rotation times to be consistent with extrapolations of the times observed in conventional solvents to the higher viscosities prevailing in ionic liquids.…”

“…These early studies also reported rotation times to be consistent with extrapolations of the times observed in conventional solvents to the higher viscosities prevailing in ionic liquids. Since then, a large number of solute + ionic liquid systems have been studied using fluorescence anisotropy, particularly by the groups of Dutt − and Sarkar, − and a variety of behaviors have been reported. In most cases, rotation times are found to fall between the limiting hydrodynamic predictions of stick and slip boundary conditions, but exceptions are also observed.…”