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 (EhimNTf), and its aprotic analogue, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmimNTf), 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 EhimNTf between the 2:1 ion pair:water and the 1:1 ion pair:water concentrations. The OHD-OKE data show that EmimNTf/water mixtures exhibit hydrodynamic behavior at all water concentrations up to saturation. In contrast, EhimNTf/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 EhimNTf 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···OH hydrogen bonds slow the orientational relaxation of Ehim cations as observed by the OHD-OKE experiments.