Triborheology and molecular structure of novel ionic liquids influenced by flow and electric fields (IL-Triborheology)
Zusammenfassung der Projektergebnisse
We performed a fundamental study of physical properties of a variety of novel, mostly imidazolium based, ionic liquids (ILs) with different molecular structures in a broad temperature range utilizing broadband dielectric spectroscopy (DS), positron annihilation lifetime spectroscopy (PALS),rheology,tribology, X-ray-scattering and X-ray photoelectron spectroscopy (XPS) measurements, UV/Vis absorption and ab-initio calculations. The main goal was to establish relationships between physical (with emphasis on triborheological) properties of ILs and to understand how these properties are linked to the molecular structure of ILs. Studied ILs can be classified in three groups by their structure: (1)-energetic azoliumazolate ILs (synthesized by R. Rogers, USA), (2) - ILs with imidazolium-based cations with different chain length and varying anion structure (synthesized by Iolitec, Germany) and (3) – ILs with borate anion and different cation structure (synthesized by I.Krossing, Germany). For the ILs with low melting points, which show glassy dynamics in rheology and dielectric spectroscopy, the important physical characteristics such as melting, glass-transition and Vogel temperatures, fragility, volume expansion coefficient, and fractional free volume have been estimated.All studied room temperature ILs can be classified as fragile glassformers with intermediate fragility, ranging from 50 to 110. Studies of the energetic ILs revealed that ILs with organic azolate anions are complex fluids formed by heterogeneous nanoscale polar and apolar domains, which show structure formation in their liquid phase. The degree of structuring depends on chemical structureand type and strength of interactions (ionic, H-bonding, pi-interactions). Among studied ILs the Butylmethylimidazolium5-amino-tetrazolate (BMIm5AT) exhibited most peculiar physical behavior: high elasticity in low frequency zone, break of time-temperature superposition and at higher levels of structuring even temporal network formation. X-ray scattering measurements reveal a „nanometer“ ordered organization, while ab-initio calculations confirmed unique role of hydrogen bonding for this IL. Different types of interactions are the dominant factors which determine the structuring process and physical properties of ILs. How to differentiate between different types of interactions and how to control those is the challenge of the upcoming research. For the second group of ILs the influence of the anions molecular structure and the tail length of the cation on triborheological properties of 14 imidazolium-based ILs was explored. These investigations showed that the ILs with weakly coordinating fluorine-containing anions [BF4] and [PF6] and short alkyl chain lengths yielded low friction coefficients, whereas the wear rates were lower at longer chain lengths. The fluorine-based ILs with the lowest friction showed also the strongest shear-thinning effect in rheology (structural viscosity) and tribochemical interactions and reactions with the steel surface. We further explored the ILs with novel weakly-coordinating borate anion [B(hfip)4] where (hfip)=OC(H)(CF3)2, varying the structure of imidazolium cations. The molecular volume of the borate anion exceeds 3-4 times the molecular volume of the cations used. All studied 5 ILs showed complex physical behavior dominated by borate anion. Rheology revealed power-law dependence of viscosity on shear rate with the exponent varying from 0.7 to 0.9 in the whole 5 decade range of probed shear rates. Such strong shear-thinning behavior implies formation of “oriented domains” in the flow field. Polarizing microscopy revealed birefringent domains, which persist up to high melting temperatures. More work should be done to clarify complex triborheological behavior in Borate-based ILs. The results indicate that the relations of structure and the tribological properties of ILs are complex. Key factors that should be considred in the design of IL-based lubricants are 1) the strength of coordinating ability of the anion, 2) shear-rate dependent viscosity and 3) tribochemical surface reactions.
Projektbezogene Publikationen (Auswahl)
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“Peculiar Behavior of AzoliumAzolate Energetic Ionic Liquids", JPC Letters, v. 2, p. 2571-2576, 2011
N.V. Pogodina, E. Metwalli, P. Müller-Buschbaum, K. Wendler, R. Lungwitz, S. Spange, J.L. Shamshina, R.D. Rogers and Ch. Friedrich
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„Influence of structural variations on Imidazolium-based Ionic Liquids,“ Lubrication Science
C. Dold, T. Amann und A. Kailer
