Published: May 02, 2011 r2011 American Chemical Society 6572 dx.doi.org/10.1021/jp1115614 | J. Phys. Chem. B 2011, 115, 6572–6584 ARTICLE pubs.acs.org/JPCB Effect of Cation Symmetry on the Morphology and Physicochemical Properties of Imidazolium Ionic Liquids Wei Zheng, † Ali Mohammed, ‡ Larry G. Hines, Jr., § Dong Xiao, § Omar J. Martinez, § Richard A. Bartsch, § Sindee L. Simon, † Olga Russina, || Alessandro Triolo,* ,^ and Edward L. Quitevis* ,‡,§ † Department of Chemical Engineering, ‡ Department of Physics, and § Department of Chemistry & Biochemistry, Texas Tech University, Lubbock, Texas 79409, United States ) Department of Chemistry, University of Rome “Sapienza”, Rome, Italy ^ Istituto Struttura della Materia, National Research Council of Italy, Rome, Italy b S Supporting Information 1. INTRODUCTION Room-temperature ionic liquids (ILs) are defined as salts with melting points at or below 373 K. A typical IL is composed of a bulky organic cation and an inorganic or organic anion. The ability to tune the physicochemical properties of ILs by changing the structure of the ions has led to their being called “designer” solvents. 1,2 Because of their unique properties and potential use in a variety of areas such as synthesis, chemical separations, biocatalysis, electrochemistry, solar cells, tribology, and iongel technology, ILs have recently been the subject of a large amount of research. 313 Given that the number of ILs is estimated to be a million, 14 a molecular-level understanding of ILs is clearly necessary for the rational design of ILs for current and future applications. There have been numerous attempts to gain a fundamental understanding of the physicochemical properties of ILs. 1523 In particular, Watanabe and co-workers 1517 used the method of pulsed-field gradient spinecho (PGSE) NMR to obtain cationic and anionic self-diffusion coefficients of imidazolium-based ILs. By correlating microscopic self-diffusion coefficients and macroscopic transport properties, such as viscosity and ionic conductivity, of a series of ILs with varying anion and alkyl chain length, Watanabe and co-workers were able to provide insights into the nature of the interionic forces that underlie the physicochemical properties of ILs. Another unique property of ILs is the nanoscale heterogeneity in their liquid state. Small-wide-angle X-ray scattering (SWAXS) measurements have provided the most direct evidence for this structural heterogeneity. SWAXS measurements 2426 on ILs based on the 1-alkyl-3-methylimidazolium cation [C n C 1 im] þ with the anions [Cl]  , [BF 4 ]  , and [PF 6 ]  for even n and bis{(trifluoromethane)sulfonyl}amide ([NTf 2 ]  ) for both even and odd n have shown that a low Q (momentum transfer) peak can be observed in the diffraction pattern from ILs bearing a long enough alkyl chain, in agreement with structural findings from molecular dynamics (MD) simulations. 2732 These MD simula- tions indicate that the polar head groups and the alkyl chains tend to segregate, thus leading to structural domains whose average Received: December 5, 2010 Revised: March 31, 2011 ABSTRACT: In this paper, the morphology and bulk physical properties of 1,3-dialkylimidazolium bis{(trifluoromethane) sulfonyl }amide ([(C N/2 ) 2 im][NTf 2 ]) are compared to that of 1-alkyl-3-methylimidazolium bis{(trifluoromethane)sulfonyl} amide ([C N1 C 1 im][NTf 2 ]) for N = 4, 6, 8, and 10. For a given pair of ionic liquids (ILs) with the same N, the ILs differ only in the symmetry of the alkyl substitution on the imidazolium ring of the cation. Small-wide-angle X-ray scattering measurements indicate that, for a given symmetric/asymmetric IL pair, the structural heterogeneities are larger in the asymmetric IL than in the symmetric IL. The correlation length of structural heterogeneities for the symmetric and asymmetric salts, however, is described by the same linear equation when plotted versus the single alkyl chain length. Symmetric ILs with N = 4 and 6 easily crystallize, whereas longer alkyl chains and asymmetry hinder crystallization. Interestingly, the glass transition temperature is found to vary inversely with the correlation length of structural heterogeneities and with the length of the longest alkyl chain. Whereas the densities for a symmetric/asymmetric IL pair with a given N are nearly the same, the viscosity of the asymmetric IL is greater than that of the symmetric IL. Also, an evenodd effect previously observed in molecular dynamics simulations is confirmed by viscosity measurements. We discuss in this paper how the structural heterogeneities and physical properties of these ILs are consistent with alkyl tail segregation.