(384c) Thermophysical and Electrochemical Properties of Self-Assembling Amphiphilic Ionic Liquids

Because of their low volatility, high thermal stability, and ability to conduct ions, ionic liquids are attractive as electrolytes for energy conversion and storage devices. Many of these ionic liquids are "fragile" liquids [1], that is, they undergo significant changes in molecular aggregation and fluid microstructure with relative small changes in temperature. We studied the influence of molecular self-assembly and temperature on the thermophysical and transport properties of ionic liquids with ammonium or imidazolium cations, and iodide, bistriflamide, mesylate, or triflate anions. Nonpolar alkyl groups, polar poly(ethylene glycol) (PEG) groups, and amphiphilic PEGylated fluoroalkyl groups were incorporated in the chemical structures of the cations. The selected cation and anion structures allowed us to systematically investigate the roles of charge delocalization and microstructure formation on ionic liquid properties.

The ionic liquids were characterized using techniques such as differential scanning calorimetry, thermogravimetry, cone-and-plate viscometry, pulsed-field gradient NMR spectroscopy, electrochemical impedance spectroscopy, and cyclic voltammetry. Molecular self-assembly was studied using self-consistent mean field simulations. The degree of dissociation of the cations and anions in the ionic liquid, that is, the ionicity of the ionic liquid, was characterized using measurements of ionic conductivities and ion diffusion coefficients.

PEGylation of the cations resulted in better solvation and cation–anion separation because of hydrogen bonding interactions of the oxygen atoms in PEG with the acidic protons of the cation [2]. The melting and crystallization phase transitions depended on the molecular weight of the PEG tails. Nonpolar alkyl groups, attached to cation, strongly influenced physical properties such as density, viscosity, and conductivity of the PEGylated ionic liquid. In ionic liquids with the fluorinated amphiphilic groups, the nonpolar fluoroalkyl groups exhibited microphase segregation from the polar PEG and ionic species in the liquid [3]. The aprotic ionic liquids with imidazolium cations and iodide anions exhibited greater ionicity than the protic ionic liquids with trialkylammonium or 3-alkylimidazol-1-ium cations and mesylate or triflate anions. In the case of protic ionic liquids, the mechanism of proton transport (vehicular vs. Grotthuss diffusion) was influenced by the fluid microstructure.

In this talk, the structure-property relationships developed in this study will discussed with the aim of optimal design electrolytes for devices such as dye-sensitized solar cells, lithium ion batteries, and high-temperature proton exchange membrane fuel cells.

REFERENCES

1. Angell, C. A. Formation of glasses from liquids and biopolymers. Science 1995, 267, 1924-1935.
2. Ganapatibhotla, L. V. N. R.; Zheng, J.; Roy, D.; Krishnan, S. PEGylated imidazolium iodide ionic liquid electrolytes: Thermophysical and electrochemical properties. Chem. Mater. 2010, 22, 6347-6360.
3. Ganapatibhotla, L. V. N. R.; Wu, L.; Zheng, J.; Jia, X.; Roy, D.; McLaughlin, J. B,; Krishnan, S. Ionic liquids with fluorinated block-oligomer tails: Impact of self-assembly on transport properties. J. Mater. Chem. 2011, 21, 19275-19285.