Ion Pairing and Anion-Driven Aggregation of an Ionic Liquid in Aqueous Salt Solutions

Abstract

A relevant question regarding ionic liquids is whether they exist in aqueous solution as totally dissociated ions or as ionic pairs, molecularly dispersed or forming part of aggregates. Several methods were employed here to evaluate these points by comparing the results of an ionic liquid, 1-ethyl-3-methylimidazolium tosylate ([emim][TOS]) with its model compound p-toluenesulfonic acid (pTSA). Conductivity measurements of [emim][TOS] and pTSA dissolved in deionized water support the existence of small amounts of aggregates for both compounds, with a larger extent in the first case. However, apparent molar volume measurements provide no clear evidence for aggregation in aqueous solution or in NaCl aqueous solutions. In contrast, the excimer emission and the absorption and excitation spectra prove the existence of aggregates of [emim][TOS] and pTSA anions in aqueous solution, with and without salt, giving in addition structural information about them. It was thus found that only [emim][TOS] forms ion pairs in deionized water, which dissociate in the presence of NaCl. J-aggregates of pTSA and [emim][TOS] anions, with slip angles that decrease with increasing concentration, were observed through the excitation spectra, and the roles of the anion and cation as well as the effect of the presence of NaCl were analyzed. Thanks to aggregation-induced emission enhancement, fluorescence is much more sensitive than conductivity or density measurements to detect aggregation.

Engineering Quantum Dots with Ionic Liquid: A Multifunctional White Light Emitting Hydrogel for Enzyme Packaging

“…The bare QD shows an absorption edge at 315 nm (Figure S1, Supporting Information) and an emission peak at 587 nm (at λ ex = 330 nm; Figure A). Notably, the emission of QD at 587 nm is due to the ( 4 T 1 – 6 A 1 ) electronic transition of the Mn 2+ dopant ions, while the choline tosylate‐based IL exhibits its intrinsic fluorescent nature, as reported by earlier works 5,29,34. Whereas, only IL shows an emission peak at 425 nm (at λ ex = 330 nm) and absorption peak at 350 nm (Figure S2, Supporting Information).…”

“…This result suggests that the structural integrity of the QD in the presence of an IL is maintained. However, the presence of the characteristic peaks of the functional groups such as OH stretching (3380 cm −1 ), alkyl CH stretching (2930 and 2850 cm −1 ), aromatic CC stretching (1625 cm −1 ), CH 2 bending (1481 cm −1 ), the asymmetric stretching vibration of OSO (1192 and 1129 cm −1 ), the symmetric stretching vibration of OSO (1037 cm −1 ), CC stretching (1009 cm −1 ), the anti‐symmetric stretching of quaternary ammonia (952 cm −1 ), p ‐disubstituted benzene (810 cm −1 ), the stretching vibration of the CS (685 cm −1 ) in the IL in the FTIR spectrum of IL‐treated QDs supports the incorporation of IL on the surface of the QD (Figure 1E; Table S1, Supporting Information) 32–34. In addition, the changes in the surface charge from +16.0 to +11.6 mV for QD, upon addition of IL, supports the modification of the surface of the QD through electrostatic interaction between the anions of IL and the excess cations present on the surface of QD (Figure 1F).…”

Application of a Novel “Turn-on” Fluorescent Material to the Detection of Aluminum Ion in Blood Serum

“…PPPNa was precipitated in the PBS/THF mixture as the THF fraction increased to more than 70%. It can be explained that unlike that in pure water, the electrostatic interaction between the ions in PBS and PPPNa can reduce the dissociation and solubility of PPPNa, induce PPPNa to more readily and loosely aggregate, and thus decrease the degree of RIR of PPPNa and block the nonradiative transition of the excited states, ,,, causing different enhanced PL behaviors. ,,− The decrement of fluorescence at the THF contents over 70% is possibly due to the quickly formed agglomerates and precipitates. The light-scattering tails in the long wavelength regions of its transmittance spectra confirmed the formation of aggregates in high contents of poor solvents (Figure S5).…”

Solvent-mediated molar conductivity of protic ionic liquids

“…15 The effect of molecular solvents on the molar conductivity of PILs was investigated. The experimental molar conductance data for these PILs in different solvents collected at 298.15 K are given in the ESI.…”

“…These properties are influenced by anion-cation and ion-solvent interactions. Many experimental methods like spectroscopic techniques [13][14][15][16][17][18] and isothermal titration calorimetry have been developed to explore the anion-cation and ion-solvent interactions of ILs. 19,20 The theoretical studies have been receiving attention because of their advantages of producing electronic structures, anion-cation binding energy, etc.…”