J. Phys. Chem. B 2008, 112, 81-92
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Physical Properties of Ionic Liquids Consisting of the 1-Butyl-3-Methylimidazolium Cation with Various Anions and the Bis(trifluoromethylsulfonyl)imide Anion with Various Cations Hui Jin,† Bernie O’Hare,† Jing Dong,† Sergei Arzhantsev,† Gary A. Baker,‡ James F. Wishart,£ Alan J. Benesi,† and Mark Maroncelli*,† Department of Chemistry, 104 Chemistry Building, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802, Chemical Sciences DiVision, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6110, and Chemistry Department, BrookhaVen National Laboratory, Building 555A, Upton, New York 11973-5000 ReceiVed: August 11, 2007; In Final Form: September 21, 2007
Physical properties of 4 room-temperature ionic liquids consisting of the 1-butyl-3-methylimidazolium cation with various perfluorinated anions and the bis(trifluoromethylsulfonyl)imide (Tf2N-) anion with 12 pyrrolidinium-, ammonium-, and hydroxyl-containing cations are reported. Electronic structure methods are used to calculate properties related to the size, shape, and dipole moment of individual ions. Experimental measurements of phase-transition temperatures, densities, refractive indices, surface tensions, solvatochromic polarities based on absorption of Nile Red, 19F chemical shifts of the Tf2N- anion, temperature-dependent viscosities, conductivities, and cation diffusion coefficients are reported. Correlations among the measured quantities as well as the use of surface tension and molar volume for estimating Hildebrand solubility parameters of ionic liquids are also discussed.
1. Introduction Since the introduction of air- and water-stable room-temperature ionic liquids (ILs) in 1992,1 research into potential applications of these materials has grown at an ever accelerating rate. Room-temperature ionic liquids are now being explored in virtually all areas of chemistry, as solvents for organic and inorganic synthesis,2 as electrolytes in batteries3 and solar cells,4 as new types of energetic materials,5 as stationary phases in chromatography6 and in a variety of other analytical applications,7-9 as well as being the subject of fundamental study in physical chemistry.10 To support all of these endeavors, it is important to know various physical properties of the ILs involved. For this reason, a number of groups, most notably those of MacFarlane and Forsyth,11-14 Rogers,15 Seddon,16,17 Brennecke,18,19 and Watanabe20-24 have been actively gathering physical property data on ILs. A number of compilations25,26 as well as an on-line database28 providing easy access to such data are beginning to appear. However, the number of ionic liquids for which a range of physical properties have been accurately characterized is still limited, especially in comparison to the huge number of ILs currently available. Over the past several years, our group has reported spectroscopic studies of solvation in a number of related ionic liquids,29-35 few of which have been thoroughly characterized. The purpose of the present paper is to provide basic physical property data on these ILs, which we hope will be of value to other workers. The 16 ionic liquids studied in the present work (Figure 1) comprise three groups. The first group is a series of liquids based on the 1-butyl-3-methylimidazolium cation (Im41+) with the anions BF4-, PF6-, bis(trifluoromethylsulfonyl)imide (Tf2N-), * To whom correspondence should be addressed. E-mail: maroncelli@ psu.edu. † The Pennsylvania State University. ‡ Oak Ridge National Laboratory. £ Brookhaven National Laboratory.
Figure 1. Structures of the ions studied in this work and their designations.
and tris(trifluoromethylsulfonyl)methide (Tf3C-). Except for [Im41+][Tf3C-], various properties of these liquids have been reported by several groups, and they therefore provide useful comparisons for the present measurements. The remaining liquids all contain the Tf2N- anion. The second group involves two homologous series of cations, an N-alkyl-N-methylpyrrolidinium (Prn1+) series and a dimethyl(isopropyl)alkylammonium (Nipn11+) series having normal alkyl groups of n ) 3, 4, 6, and 10 carbon atoms in length. The cations of the final group are related to the previous imidazolium, pyrrolidinium, and ammonium series via substitution (or addition) of a hydroxyl functionality. With the exception of [Pr41+][Tf2N-] and some fragmentary data on [Pr31+][Tf2N-], none of the ILs in the latter two groups have been previously characterized. The experimental properties reported here include phasetransition temperatures, densities, refractive indices, surface
10.1021/jp076462h CCC: $40.75 © 2008 American Chemical Society Published on Web 12/11/2007
82 J. Phys. Chem. B, Vol. 112, No. 1, 2008 tensions, several solvatochromic measures of polarity, viscosities, conductivities, and cation diffusion coefficients. The “static” properties, which are relatively insensitive to temperature, were measured at a single temperature (20 or 25 °C), whereas viscosities are reported as functions of temperature to enable estimations of dynamical quantities at other temperatures. In addition, we report a number of characteristics of the individual ions derived from electronic structure calculations and also consider estimation of Hildebrand solubility parameters from measured values of surface tension and molar volumes. 2. Materials and Methods Computations: Density functional (DFT) calculations were performed using the Gaussian03 program36 at the B3LYP/631G(d,p) level of theory. Molecular volumes were estimated from the 0.001e/a03 contours and electrostatic properties calculated using atomic charges derived from fitting the electrostatic potential generated by these wave functions.37 To sample conformations of cations having flexible aliphatic chains, molecular mechanics calculations were performed using the MacroModel program38 with the MMFF94s force field.39 The conformational search algorithm employed was based on a multiple minimum Monte Carlo algorithm.40 The resulting structures were then used as input to the DFT calculations. Materials: 1-Butyl-3-methylimidazolium tetrafluoroborate [Im41+][BF4-] was obtained from Sigma-Aldrich (97%; 99.5%,
