Journal of Physical Chemistry B, Vol.124, No.18, 3784-3790, 2020
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 (V-inter) 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 (sigma) and self-diffusion coefficient (D), as compared with conventional ILs. Radial distribution functions derived from high-energy X-ray total scattering experiments [G(exp)(r)] and molecular dynamics (MD) simulations [G(MD)(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 angstrom) 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.