Ionic Structures of Nanobased FeCl 3 /[C 4 mim]Cl Ionic Liquids Ji-Guang Li, † Yu-Feng Hu,* ,† Shu-Feng Sun, ‡ Shan Ling, † and Jin-Zhu Zhang † † State Key Laboratory of Heavy Oil Processing and High Pressure Fluid Phase Behavior & Property Research Laboratory, China University of Petroleum, Beijing 102249, China ‡ The Centre for Biological Imaging, Institute of Biophysics, Chinese Academy of Sciences, Beijing 100101, China ABSTRACT: We synthesized a series of FeCl 3 /[C 4 mim]Cl (iron(III) chloride with 1-butyl-3-methylimidazolium chloride) ionic liquids. The temperature dependence of the Raman spectra of the FeCl 3 /[C 4 mim]Cl ionic liquids was measured for the first time to analyze their ionic species. In addition, the infrared spectra, thermodynamic properties, and freeze-fracture trans- mission electron microscopy were combined together with the Raman spectra to reveal the microscopic information of the FeCl 3 /[C 4 mim]Cl ionic liquids. When the mole ratio of FeCl 3 / [C 4 mim]Cl is less than 1, the Raman scattering identifies the presence of [FeCl 4 ] - in the ionic liquid. When FeCl 3 is in excess, [Fe 2 Cl 7 ] - begins to appear. Besides, we found that the relative intensity of the two symmetry vibrations of [Fe 2 Cl 7 ] - is changing as the temperature is varied. The strength of the interionic interactions in FeCl 3 /[C 4 mim]Cl ionic liquids follows the order 1/1.5 > 1.5/1 > 1/1 due to the formation of [FeCl 4 ] - and [Fe 2 Cl 7 ] - . The nanostructures in FeCl 3 /[C 4 mim]Cl ionic liquids were observed for the first time by using biological imaging. The sizes of the local domains are found to be tens of nanometers. 1. INTRODUCTION Ionic liquids are composed solely of ions and are liquid at ambient temperature. 1-3 For their specific properties, such as nonflammability, high thermal stability, wide temperature range in the liquid state and extremely low volatility, they are attracting more and more interest as alternatives to conven- tional molecular liquids. 1-3 FeCl 3 /[C 4 mim]Cl ionic liquids have all characteristics of conventional ionic liquids including high temperature stability, high solvation ability, general nonflammability, wide liquidus range, and negligible vapor pressure. 4-9 Compared to conven- tional ionic liquids, magnetic FeCl 3 /[C 4 mim]Cl ionic liquids have some unique characteristics: strong magnetic field response, 4-9 paramagnetic, 4,10 possible local ordering of the magnetic anions, 8,10 single-component materials that are free from phase separation, 4 and moisture-stable and intermediate Lewis acidity. 11,12 It is even expected that ferromagnetic ionic liquids will be synthesized in the near future. 8 Magnetic ionic liquids FeCl 3 /[C 4 mim]Cl were discovered as a key material for green chemistry on solvents and catalysts in these years. 5,13,14 Recently, nanostructured conducting polymers were synthe- sized by simply adding monomers into a 1/1 FeCl 3 /[C 4 mim]Cl ionic liquid, and magnetic field seems to affect these local structures and hence the resultant polymer nanostructures. 6,9,15 These facts suggest that magnetic ionic liquids could be widely used as magnetic liquids consisting of nanoparticles. 4 When FeCl 3 /[C 4 mim]Cl ionic liquids are involved in green process, we can find that ionic species, interionic interactions, and temperature dependence of nanostructures are key fac- tors. 5,6,9,13-15 However, the microscopic information about the ionic state remains incompletely understood and there are no available reports regarding the nanostructure of FeCl 3 / [C 4 mim]Cl ionic liquids. Therefore, it is of great importance to understand the temperature-dependence of the structures of the FeCl 3 /[C 4 mim]Cl ionic liquids. In this work, the temperature-dependence of the Raman spectra of FeCl 3 /[C 4 mim]Cl ionic liquids was measured for the first time. In addition, the infrared (IR) spectra, conductivity, and glass transition results were combined to compare to the Raman spectra results and to investigate the interionic interactions between ions. The temperature dependence of liquid nanostructures of FeCl 3 /[C 4 mim]Cl ionic liquids was measured for the first time using freeze-fracture transmission electron microscopy (FF-TEM). 2. EXPERIMENTAL DETAILS 2.1. Materials. Anhydrous FeCl 3 with the purity of more than 98% was purchased from Acros and used without further purification. N-methylimidazole and n-C 4 H 9 Cl with the claimed purities of >99% were supplied by Shanghai Jiachen Chemical Co., Ltd., and were refined by the traditional methods before use. FeCl 3 /[C 4 mim]Cl ionic liquids were prepared using the well-established procedures of our and other previous works. 5,8,16 Both the literature results 5 and our preliminary experiments showed that, after having been dried in a vacuum box at 333 K for 24 h, the prepared ionic liquids could be dried to constant weights. Therefore, the ionic liquids were dried in the vacuum box at 333 K for 24 h and then were dried in the vacuum box at 333 K for another one or two 24 h to make sure the ionic liquids were of constant weight. Finally, the ionic Received: July 17, 2011 Revised: December 22, 2011 Published: December 28, 2011 Article pubs.acs.org/JPCB © 2011 American Chemical Society 6461 dx.doi.org/10.1021/jp206819h | J. Phys. Chem. B 2012, 116, 6461-6464