Physicochemical and Structural Properties of a Hydrophobicity/Hydrophilicity Switchable Ionic Liquid.

Herein, we report the physicochemical and structural properties of a new solubility-switchable ionic liquid (IL) comprising the glycerammonium (GA) cation with a hydrophilic group, the GA cation attached to an acetal-based protective group [protected GA (PGA)], and bis(trifluoromethanesulfonyl)amide (TFSA). The interionic volumes (Vinter) of the hydrophobic [PGA][TFSA] and hydrophilic [GA][TFSA] ILs were evaluated based on solution density, revealing weaker ion-ion interactions in these relative to conventional ILs. The [PGA][TFSA] and [GA][TFSA] also exhibit poor ion-conducting properties, with up to an order of magnitude lower ionic conductivity (σ) and self-diffusion coefficient (D), as compared with conventional ILs. Radial distribution functions derived from high-energy X-ray total scattering experiments [Gexp(r)] and molecular dynamics (MD) simulations [GMD(r)] indicate that nearest-neighbor ion-ion interactions in the [PGA][TFSA] and [GA][TFSA] are comparable to those in imidazolium-based IL. Conversely, these are appreciably weakened at the second- and third-neighbors and thus less structured in the long range (r > 12 Å) and very different from the highly ordered imidazolium IL. The atom-atom pair correlation function derived from the MD simulations disclose that at a local scale, specific interactions are absent, with only an electrostatic interaction in the [PGA][TFSA], whereas the GA cations interact with TFSA anions via hydrogen bonding of diol groups in the GA and O atoms in the TFSA. No hydrogen bonding group within the PGA cation leads to weak ion-hydration resulting in a phase separation of [PGA][TFSA] and water; in contrast, the GA cations are easily hydrogen-bonded with water molecules to be miscible in aqueous solutions.