Impact of Hydrogen Bonding on the Dynamics and Structure of Protic Ionic Liquid/Water Binary Mixtures

The orientational dynamics and microscopic liquid structure of a protic ionic liquid, 1-ethylimidazolium bis(trifluoromethylsulfonyl)imide (EhimNTf₂), and its aprotic analogue, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmimNTf₂), were studied at various water concentrations using optical heterodyne-detected optical Kerr effect (OHD-OKE) spectroscopy, linear infrared spectroscopy, and atomistic simulations. The OHD-OKE experiments essentially measure the orientational relaxation of the Ehim⁺ and Emim⁺ cations. The experiments and simulations show a significant dynamical and structural change in EhimNTf₂ between the 2:1 ion pair:water and the 1:1 ion pair:water concentrations. The OHD-OKE data show that EmimNTf₂/water mixtures exhibit hydrodynamic behavior at all water concentrations up to saturation. In contrast, EhimNTf₂/water mixtures deviate from hydrodynamic behavior at water concentrations above 2:1. At the 1:1 concentration, the orientational randomization of the Ehim⁺ cation is slower than that predicted using viscosity data. Atomistic simulation results reveal the microscopic ionic structures of dry liquids and the preferential hydrogen bonding of water to the H atom of the N-H of Ehim⁺ over other sites on the Ehim⁺ and Emim⁺ cations. Atomistic simulation results demonstrate that in EhimNTf₂ RTIL/water mixtures there is a substantial jump in the formation of water-water hydrogen bonds in addition to N-H-water hydrogen bonds upon increasing the water concentration from 2:1 to 1:1. Water-water hydrogen bonding strengthens the spatial coordination of the H atom of the N-H moiety of Ehim⁺ to neighboring water molecules through preferential hydrogen bonding. The jump in the concentration of water-water hydrogen bonds occurs at the Ehim⁺/water concentration at which the orientational relaxation deviates from hydrodynamic behavior. This structural observation is confirmed with FT-IR spectra that show asymmetry in the peak for the O-D stretch that is indicative of water clusters. The formation of water clusters and the strengthening of the N-H···OH₂ hydrogen bonds slow the orientational relaxation of Ehim⁺ cations as observed by the OHD-OKE experiments.