Translational and Rotational Diffusion in Ionic Liquids

Open Access
- Author:
- Kaintz, Anne Elizabeth
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 19, 2012
- Committee Members:
- Mark Maroncelli, Dissertation Advisor/Co-Advisor
Mark Maroncelli, Committee Chair/Co-Chair
Alan James Benesi, Committee Member
David Lawrence Allara, Committee Member
John B Asbury, Committee Member
Ralph H Colby, Special Member - Keywords:
- diffusion
hydrodynamic
Inversion Recovery
ionic liquid
PFG-NMR
rotation
Stokes Einstein Debye
translation
viscosity - Abstract:
- In this work, we have investigated both translational and rotational diffusion in neat ionic liquids (ILs) and in IL solutions using nuclear magnetic resonance (NMR) methods. Translational diffusion studies focus on ionic liquid solvents N-alkyl-N-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imides [Prn1][Tf2N], with n = 3, 4, 5, 6, 8, 10, and trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide [P14,6,6,6][Tf2N]. Solutes include fused or bridged aromatics, fluorinated and nitrile-substituted benzenes, tetraphenylphosphonium benzoate, and other ionic liquids. Translational diffusion coefficients were measured using the longitudinal-eddy-current delay (LED) stimulated echo NMR pulse sequence with bipolar gradient pulse pairs. Applied field strengths were 400 and 850 MHz for 1H frequency. Additional data were collected, from various sources in the literature, for both IL solvent and conventional solvent systems. These data were used both as a point of comparison for our own measurements and as a broader sampling of solutes and solvents, allowing for an assessment of the effect of solute-solvent properties on the friction coefficient. Although the diffusion of solutes has been widely studied in conventional solvents and, to a lesser degree, in ionic liquids, many deviations from hydrodynamic predictions continue to be reported, often accompanied by their own competing models and hypotheses. One common deviation is that of sub-slip diffusion of small solutes in dilute solution. Study of such cases has been difficult because many of the more commonly-used analysis techniques are unable to measure small solutes or are prone to error. By contrast, NMR spectroscopy is ideal for such studies in that it is applicable for nearly all solutes, and provides more reproducible data than do several competing techniques. Despite this fact, NMR has been little used in studying dilute small molecule diffusion in ionic liquids. As a result, our work in this area provides a significant amount of new data and insight. We find that deviations of translational diffusion coefficients from the Stokes-Einstein (SE) equation in ILs are analogous but more pronounced than those in conventional solvents, in part due to the typically larger size of IL solvents. The ratio of solute-to-solvent size in IL solutions has a significant effect on the friction coefficient for translational diffusion, as it does for conventional solutions. The friction coefficient is also affected, in both conventional solvents and ILs, by the difference in the intermolecular forces of the solute and of the solvent. We find that the effect of solute shape on translational friction coefficient is minimal with respect to other sources of deviation from SE behavior. We also consider several SE corrections which were proposed by other researchers for conventional solutions, and assess their accuracy for IL solutions. Rotational diffusion studies focused on a different set of (deuterated) samples; neat 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide [Im21][Tf2N], and a dilute solution of benzene in1-butyl-3-methyl-imidazolium tetrafluoroborate [Im41][BF4]. A series of solutions spanning 0 to 1 mole fraction of [Im21][Tf2N] in tetrahydrofuran (THF) was also analyzed. Rotational diffusion coefficient measurements utilized a deuterium inversion recovery NMR pulse sequence, with spectrometers ranging in magnetic field strength from 300 to 850 MHz proton frequencies. The calculation of rotational correlation times, c, from NMR longitudinal decay times, T1, is so complex that it has often been simplified to the point of inaccuracy. Any given molecule will likely have multiple rotational correlation times, depending on structure, symmetry, and internal rotational dynamics. All correlation times, which may be similar in value, are comprised by a single T1 for each observed nucleus in a molecule. The fractional contribution of each c to an observed value of T1 depends upon the placement of the observable nucleus within the molecule. These multiple correlation times, combined with such factors as the structure and symmetry of the molecule, result in strikingly disparate correlations between c and T1 for different molecules. Currently, it is common practice to calculate c using a single exponential dependence, the simplest of all relations to T1, despite the fact that this is only expected to be strictly correct for spherical molecules. In this work, we present a method of interpreting NMR data in conjunction with molecular dynamics (MD) simulations in order to allow for a more accurate calculation of c. The temperature dependant rotational diffusion data we acquire by this method also contributes to the understanding of sub-slip rotations of small solute molecules in ionic liquids. Although many authors have proposed many different hypotheses in an attempt to explain this behavior, there is still no consensus. A systematic study by NMR has the advantage of accommodating smaller solute molecules than is possible using several of the more commonly-used techniques, while simultaneously providing greater reproducibility than can many other methods. In comparing rotational diffusion measurements made over a range of temperatures with three different magnetic field strengths to molecular dynamics simulations, we are able to fit various possible models for rotational correlation functions. Specifically, we generate time correlation functions via variable-temperature MD simulations, fit them to a parameterized functional form, in order to represent the simulated time and temperature dependence. Varying the parameters of such representations enables us to extract more meaningful rotational correlation functions and their temperature dependence from the measured NMR T1 data. For both benzene and 1-ethyl-3-methyl-imidazolium, we find very fast dynamics that fall within the extreme narrowing regime at laboratory-accessible conditions, as well as slower dynamics. The faster dynamics likely correspond to in-plane rotations, while the slower correspond to tumbling. The slower component may be described with a single exponential or stretched exponential decay, but the faster component requires a bi-exponential. Solutions of [Im21][Tf2N] and THF display a single-exponential decay with increasing viscosity.