Translational and Rotational Diffusion in Ionic Liquids Through NMR Spectroscopy
Open Access
- Author:
- Shadeck, Michael Thomas
- Graduate Program:
- Chemistry
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- June 11, 2015
- Committee Members:
- Mark Maroncelli, Thesis Advisor/Co-Advisor
Karl Todd Mueller, Thesis Advisor/Co-Advisor
William George Noid, Thesis Advisor/Co-Advisor
Michael Anthony Hickner, Thesis Advisor/Co-Advisor - Keywords:
- Diffusion
Ionic Liquids
Nuclear Magnetic Resonance
Translational Diffusion
Rotational Diffusion - Abstract:
- In this work, translational and rotational diffusion in neat ionic liquids and dilute ionic liquid solutions were investigated using nuclear magnetic resonance (NMR) spectroscopy. Two distinct studies of translational diffusion were performed. The first focuses on solutions prepared in 1-alkyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imde ionic liquids ([Prn1][Tf2N], n=3,4, and10) with quasi-spherical solutes, such as methane, ammonium, and tetramethylsilane. The second study entails measurement of self-diffusion coefficients in homologous series of neat ionic liquids: 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [CnC1im][Tf2N] and [(n-2)mCn-1C1im][Tf2N] n = 3, 4, 5, 6, 7. The majority of the measurements of translational diffusion were carried out using a Bruker AV-III-850 NMR spectrometer with a Diff-30 probe with triple axis gradients using a longitudinal-eddy-current delay stimulated echo NMR pulse sequence. The Stokes-Einstein (SE) model is a starting point for understanding the solute diffusion coefficients measured. While the SE model can often predict diffusion coefficients that are close to experimental values, there are several factors that can lead to deviations. The primary simplification of this model is to treat the solvent as a structureless continuum, thereby implicitly assuming that the solute is much larger than the size of the solvent molecules. Since ionic liquids are typically larger than conventional solvents, deviations from SE predictions are commonly observed for smaller solutes translating in ionic liquids. In addition to size, intermolecular interactions between the solute and solvent hinder the diffusion, leading to deviations. In the first translational diffusion study, charged and uncharged quasi-spherical solutes with various sizes were chosen to better understand deviations from SE predictions. By selecting these types of probes, shape effects are minimized and any deviation would be attributed to the fiction coefficient of the solute. The data collected here were important in verifying the accuracy of molecular dynamics simulations of these same systems. These simulations were able to provide new insight into the mechanisms of diffusion of small neutral and charged solutes in ionic liquids. Translational diffusion experiments were also performed on two series of neat ionic liquids which differed in either having a linear or branched alkyl chain on the cation. These liquids were provided by the Quitevis group from Texas Tech University. From their findings, it was observed that ionic liquids with the same composition but different alkyl chain connectivity exhibited large deviations in viscosity at certain chain lengths, while other physical properties, such as density, remained consistent. The self-diffusion coefficients of the cations and anions of two homologous series of neat ionic liquids were investigated to determine if analogous behavior was observed when compared to the viscosity. Rotational diffusion studies analyzed dilute solutions of p-xylene-d10 (pXy0), 1,4-dimethylpyridinium-d7 hexafluorophosphate (DMPy+), and p-tolunitrile-d7 (CMBzµ) in 1-butyl-3-methylimidazolium tetrafluoroborate ([Im41][BF4]). For rotational diffusion experiments, data were collected using the DPX-300 spectrometer with a broadband multinuclear probe in addition to the AV-III-850. The pulse sequence used in these experiments was the inversion recovery for deuterium. Determining rotational correlation times,τ_c, from deuterium longitudinal relaxation, ¬T1, is often oversimplified to the point of inaccuracy as a result of assuming exponential relaxation of rotational correlations. Factors such as symmetry and structure lead to molecules possessing several unique rotations and thus rotational correlation functions will have much more complex functional forms. In a previous study, correlation times for the rotational dynamics of benzene in ionic liquids were studied by fitting NMR data with the aid of MD simulations, eliminating the need for simplification. In the present study, the same type of analysis was used to determine rotational correlation times for probes having similar size and shape, but very different solvent interactions. The goal was to investigate the effect intermolecular interactions have on rotational diffusion.