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(2)), and its aprotic analogue, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmimNTf(2)), 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 Ehirn and Emim(+) cations. The experiments and simulations show a significant dynamical and structural change in EhimNTf(2) between the 2:1 ion pair:water and the 1:1 ion pair:water concentrations. The OHD-OKE data show that EmimNTf(2)/ water mixtures exhibit hydrodynamic behavior at all water concentrations up to saturation. In contrast, EhimNTf(2)/water mixtures deviate from hydrodynamic behavior at water concentrations above 2:1. At the 1:1 concentration, the orientational randomization of the Ehirn(+) 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(4) over other sites on the Ehim(+) and Emim(+) cations. Atomistic simulation results demonstrate that in EhimNTf(2) 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 Ehimilwater 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 center dot center dot center dot OH2 hydrogen bonds slow the orientational relaxation of Ehim cations as observed by the OHD-OKE experiments.