Melting Behavior and Ionic Conductivity in Hydrophobic Ionic Liquids Miriam Kunze, † Maria Montanino, ‡ Giovanni B. Appetecchi, ‡ Sangsik Jeong, † Monika Scho ¨nhoff, † Martin Winter, † and Stefano Passerini* ,† Department of Physical Chemistry, Westfälische Wilhelms-UniVersita ¨t Mu ¨nster, Corrensstrae 28/30, 48149 Mu ¨nster, Germany, and Agency for the New Technologies, Energy and the EnVironment (ENEA), Via Anguillarese 301, 00123 Rome, Italy ReceiVed: October 16, 2009; ReVised Manuscript ReceiVed: December 9, 2009 Four room-temperature ionic liquids (RTILs) based on the N-butyl-N-methyl pyrrolidinium (Pyr 14 + ) and N-methyl-N-propyl pyrrolidinium cations (Pyr 13 + ) and bis(trifluoromethanesulfonyl)imide (TFSI - ) and bis(fluorosulfonyl)imide (FSI - ) anions were intensively investigated during their melting. The diffusion coefficients of 1 H and 19 F were determined using pulsed field gradient (PFG) NMR to study the dynamics of the cations, anions, and ion pairs. The AC conductivities were measured to detect only the motion of the charged particles. The melting points of these ionic liquids were measured by DSC and verified by the temperature-dependent full width at half-maximum (FWHM) of the 1 H and 19 F NMR peaks. The diffusion and conductivity data at low temperatures gave information about the dynamics at the melting point and allowed specifying the way of melting. In addition, the diffusion coefficients of 1 H(D H ) and 19 F(D F ) and conductivity were correlated using the Nernst-Einstein equation with respect to the existence of ion pairs. Our results show that in dependence on the cation different melting behaviors were identified. In the Pyr 14 - based ILs, ion pairs exist, which collapse above the melting point of the sample. This is in contrast to the Pyr 13 -based ILs where the present ion pairs in the crystal dissociate during the melting. Furthermore, the anions do not influence the melting behavior of the investigated Pyr 14 systems but affect the Pyr 13 ILs. This becomes apparent in species with a higher mobility during the breakup of the crystalline IL. Introduction Ionic liquids (ILs) are molten salts with a melting point below 100 °C. Not only this fact, but also that these liquids have a low vapor pressure and a high chemical and electrochemical stability make them labeled as green solvents. 1-5 Therefore, ILs are being investigated for a wide range of applications. One field is the utilization as electrolyte component in electrochemi- cal devices. This includes lithium batteries, 6-14 fuel cells, 15 electrochemical (super or ultra) capacitors, 16 electrochemical actuators, 17 light-emitting electrochemical cells, 18 etc. For these electrochemical applications, one has to have exact requirements including a high chemical, thermal, and electrochemical stability, a wide liquid range for operation at low and high temperatures, and a high ionic conductivity. Especially for lithium batteries the ionic conductivity plays an important role. In case a lithium salt is added to the IL, it is generally found that the Li + conductivity scales with the conductivity of the IL. Thus, batteries can be discharged at faster rates with high conductivity ILs. Furthermore, the use (charge and discharge) of the lithium batteries shall be possible at very low temperatures. Because of that fact, it is important to gain knowledge of the molecular motion at low temperatures and throughout the melting transition of the material. Until now, in the literature it is often reported that above the melting temperature T M ion pair formation is taking place in the IL. 19-22 Here, we report the physical and electrochemical properties of four different ILs based on the N-butyl-N-methyl pyrroli- dinium cation (Pyr 14 + ), N-methyl-N-propyl pyrrolidinium cation (Pyr 13 + ), bis(trifluoromethanesulfonyl)imide (TFSI - ), and bis- (fluorosulfonyl)imide (FSI - ) anion (Scheme 1) over a wide temperature range including the dynamics below T M . Mixtures of these ionic liquids with lithium TFSI have been proved successfully for lithium insertion in graphite. 23-25 Hence, the melting temperatures T M were determined by DSC and NMR, and for the dynamic processes the measured diffusion coef- * Corresponding author. E-mail: stefano.passerini@uni-muenster.de. † Westfaelische Wilhelms-Universita ¨t Mu ¨nster. ‡ ENEA. SCHEME 1: Cations and Anions Used for the Ionic Liquids J. Phys. Chem. A 2010, 114, 1776–1782 1776 10.1021/jp9099418  2010 American Chemical Society Published on Web 01/08/2010