Microviscosity Offered by Ionic Liquids and Ionic Liquid–Glycol Mixtures Is Probe Dependent

Interactions present within the solubilizing media constituted of ionic liquids (ILs) govern the outcomes of chemical processes carried out within such media by controlling the behavior of solutes dissolved therein. Fluidity afforded by IL-based media, in this context, not only reveals interactions present within the system, but it also helps decide whether the system is suitable for an application. The response of spectroscopic microviscosity probes dissolved in IL and IL-based solvents, in this regard, reveals information on both solute–solvent and solvent–solvent interactions present within the system. Interactions present within the cybotactic region of a microviscosity probe strongly depend on the functionalities present on the probe. Five different fluorescence probes representing three different classes of microviscosity reporting systems based on intramolecular excimer formation, steady-state fluorescence anisotropy, and fluorescence intensity are used to explore fluidity afforded by 11 ILs and their equimolar mixtures with tetraethylene glycol (TEG), a solvent having the ability to readily hydrogen bond, under ambient conditions. ILs are categorized into four sets: first having the same cation and different anions to assess the role of the anion, second having the same anion and different cations to assess the role of the cation, and the last two having C2–H substituted imidazolium cations with different anions to reveal the role of C2–H, if any, in H-bonding interactions involving ILs. The responses of all of the microviscosity probes within ILs and (IL + TEG) equimolar mixtures are compared with the bulk dynamic viscosities of the corresponding systems. The overall investigation reveals the lack of a trend between the probe responses representing microviscosity and the dynamic viscosities of IL and (IL + TEG) mixtures. The cybotactic region viscosity of the (IL + TEG) equimolar mixture depends on the identity of the probe; it may be higher than, lower than, or in between the microviscosities reported in both neat IL and neat TEG. Evidence of the role of C2–H on the imidazolium cation in intraspecies (within IL) or interspecies (between IL and TEG) H-bonding was not found.