Orientational Dynamics of Room Temperature Ionic Liquid/Water Mixtures: Water-Induced Structure

• Published in: The journal of physical chemistry. B Vol. 116; no. 6; pp. 1777 - 1787
• Main Authors: Sturlaugson, Adam L, Fruchey, Kendall S, Fayer, Michael D
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 16.02.2012
• Subjects: Cations; Chains; Decomposition; Diffusion rate; Dynamics; Ionic liquids; Liquids; Mathematical models
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp209942r

Optical heterodyne detected optical Kerr effect (OHD-OKE) measurements on a series of 1-alkyl-3-methylimidazolium tetrafluoroborate room-temperature ionic liquids (RTILs) as a function of chain length and water concentration are presented. The pure RTIL reorientational dynamics are identical in form to those of other molecular liquids studied previously by OHD-OKE (two power laws followed by a single exponential decay at long times), but are much slower at room temperature. In contrast, the addition of water to the longer alkyl chain RTILs causes the emergence of a long time biexponential orientational anisotropy decay. Such distinctly biexponential decays have not been seen previously in OHD-OKE experiments on any type of liquid and are analyzed here using a wobbling-in-a-cone model. The slow component for the longer chain RTILs does not obey the Debye–Stokes–Einstein (DSE) equation across the range of solutions, and thus we attribute it to slow cation reorientational diffusion caused by a stiffening of cation alkyl tail–tail associations. The fast component of the decay is assigned to the motions (wobbling) of the tethered imidazolium head groups. The wobbling-in-a-cone analysis provides estimates of the range of angles sampled by the imidazolium head group prior to the long time scale complete orientational randomization. The heterogeneous dynamics and non-DSE behavior observed here should have a significant effect on reaction rates in RTIL/water cosolvent mixtures.