Ion pairing and anion-driven aggregation of an ionic liquid in aqueous salt solutions

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.