Monash University

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Properties of ionic liquids and ionic liquid mixtures.

posted on 2017-02-27, 22:30 authored by Annat, Gary
While much research into the field of ionic liquids has described applications for which these new and facile materials can be used, the origins of the desirable physical properties (i.e. high ionic conductivity, large electrochemical windows, high thermal stability, etc.), remains subject to empirical understanding and guess-work. The investigation of new salts from the> 1010 possibilities can be cumbersome as time is invested in either a wide range of promising materials that may yield limited success, or through systematic testing of whole families of ionic liquids to find the best performing material. Developing an understanding of the role different ions and functional groups play in the bulk physical properties of an ionic liquid is crucial in guiding future research to uncover modem materials for advanced practical applications. This work first analyses the physical properties of many different ionic liquids to gain insight into the liquid state of pure ionic liquids. Viscosity, ionic conductivity and density data are used to construct Walden Plots, to understand the freedom of movement of ions in the electrolyte, based on the Walden rule that states that the product of molar conductivity and viscosity is constant. It is proposed in this work that the observed deviation from this relationship is influenced by the size of the ions. Based on estimates of ion size using ab initio calculations, new deviations in molar conductivity in the Walden Plot (~W) are determined. Furthermore, using the Nernst-Einstein equation, ionicity values are determined from diffusion NMR analysis. The pure state IS also probed in detail for the ionic liquid trihexyltetradecylphosphonim chloride ([P6,6,6,14](Cl]) using wide angle X-ray scattering coupled with molecular dynamics simulations. Nanometer sized domains are observed in the liquid state, which is correlated by the computer simulations. These domains alternate between polar and non-polar, reflecting aggregation of the charged ions and aggregation of the uncharged alkyl chains on the phosphonium cation. While there are many new ionic liquids to explore, another avenue of research that is beginning to bloom is the study of mixtures of ionic liquids. The most obvious starting point is perhaps the study of ionic liquids combined with molecular solvents, as these latter materials have well documented and accurately measured properties. However, some of the properties that are so heavily sought after in ionic liquids are sacrificed in such mixtures. In contrast, ionic liquids mixed with other ionic liquids offer the possibility of improvement of undesirable properties without the loss of advantageous properties such as negligible volatility. As there is an overwhelmingly large range of ionic liquid in ionic liquid possible combinations, though, a guided and well constructed approach is required to make significant headway in the field. This work presents the study of a group of ionic liquids where the differences in constituent ions are chosen to yield significant information on how different ions interact, while the number of differences is kept to a minimum to avoid too many competing factors. The concept of "simple" mixing, in terms of the properties of ionic liquid mixtures, is clarified first in order to identify any unusual behaviour. Thus, equations for predicting viscosities in mixtures are confirmed, and analogous equations are used to describe molar conductivities. The greatest deviation from simple mixing is observed in mixtures of the N-methyl-N-propylpyrrolidinium ([C3mpyrt]+) cation and the large [P6,6,6,14]+ cation, used with the bis(trifluoromethylsulfonyl)amide ([NTf2]*) anion. These mixtures exhibit an immiscibility window, a lack of crystallisation in single phase mixtures, a large excess molar volume and significant departure from the expected viscosity. It is conjectured that the physical properties of the miscible composition in this mixture is the result of alkyl-rich domains in the liquid state, and that when the composition of [C3mpyr][NTf2] is in the majority these domains cannot stay in solution and force the ionic liquids to separate. In order to accurately perform NMR diffusion analyses of the ionic liquid binary mixtures, the exact procedure for the NMR diffusion experiments needed to be explored and clarified. It is observed that the standard pulse sequence traditionally used for diffusion experiments, the Hahn-Echo pulse sequence, yields anomalous results in high viscosity ionic liquids. As only the most fluid of ionic liquids give consistent results with this standard procedure a different pulse sequence is required. The stimulated echo sequence is shown to have no viscosity dependence and is therefore recommended for PFG-NMR studies on ionic liquids. Finally, mixtures of ionic liquids and molecular solvents will produce materials that are useful in some applications, and this work presents a study comparing analysis based on transport properties (Le. the Walden plot) against studies of the vapour pressure (Le. osmotic coefficient and activities). It is shown that both techniques give evidence of ion aggregation at low concentrations, but deviate from one another above -0.3 mole fraction ionic liquid. This is attributed to breakdown of the validity of osmotic coefficient measurements at high salt concentrations. An effect of solvent polarity on ion aggregation is also observed. This work gives significant advances in the probing of the state of ions within an ionic liquid, and gives insights into how ions interact with each other, other ionic liquids and molecular solvents. The findings here can serve as a basis for developing new ionic liquids, as well as direct investigations for new ionic liquid mixtures.


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Principal supervisor

Douglas R. MacFarlane

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Doctor of Philosophy

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Faculty of Science

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