PGT Measurements of Imidazolium-Based Ionic Liquids Ramesh L. Gardas, ‡ Mara G. Freire, ‡ Pedro J. Carvalho, ‡ Isabel M. Marrucho, ‡ Isabel M. A. Fonseca, § Abel G. M. Ferreira,* ,§ and Joa ˜ o A. P. Coutinho ‡ CICECO, Departamento de Quı ´mica, Universidade de Aveiro, 3810-193 Aveiro, Portugal, and Departamento de Engenharia Quı ´mica, Faculdade de Cie ˆncias a Tecnologia, Universidade de Coimbra, Polo II, Pinhal de Marrocos, 3030-290 Coimbra, Portugal Experimental density measurements are reported, and the derived thermodynamic properties, such as the isothermal compressibility, the isobaric expansivity, and the thermal pressure coefficient are presented as Supporting Information for several imidazolium-based ionic liquids (ILs), namely, 1-ethyl-3-methyl-imidazolium bis- (trifluoromethylsulfonyl)imide [C 2 mim][NTf 2 ], 1-heptyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide [C 7 mim][NTf 2 ], 1-octyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide [C 8 mim][NTf 2 ], 1-ethyl-3-methyl- imidazolium tetrafluoroborate [C 2 mim][BF 4 ], and 1-butyl-3-methyl-imidazolium tricyanomethane [C 4 mim][C(CN) 3 ] in the pressure (0.10 < p/MPa < 30.00) and temperature (293.15 < T/K < 393.15) domains. These ILs were chosen to provide an understanding of the influence of the cation alkyl chain length and the anion influence on the properties under study. Experimental densities are correlated with the Tait equation with an average absolute deviation (AAD) less than 0.04 %. Experimental densities are in good agreement with the densities obtained by some recent predictive methods proposed in the literature. Introduction Ionic liquids (ILs) are a special class of molten salts having an appreciable liquid range. A commonly accepted practical definition for an ionic liquid is a salt with a melting temperature below the boiling point of water. 1 Most salts identified in the literature as ionic liquids are liquid at room temperature and often at substantially lower temperatures. Apart from these unusually low melting temperatures, ILs are characterized by a negligible vapor pressure, 2,3 a broad liquid range, 4 and a very rich and complex behavior as solvents 5-8 that can be modified by changing the nature of the cation or anion. For the successful and large-scale use of ILs, an intensive and systematic investigation of their physicochemical properties is necessary. These properties are of interest from the point of view of both fundamental and applied research. On the other hand, a detailed knowledge of the thermophysical properties of ILs is important in relating microscopic and macroscopic behavior. 9-12 Studies on physical and chemical properties of ionic liquids, property measurement methodology, high-quality data on reference systems, standards for reporting thermody- namic data, and the creation of a comprehensive database have been promoted by NIST, IUPAC, and DDB (the Dortmund Data Bank). 13-15 During the past few years, investigations of ther- mophysical and thermodynamic properties have increased remarkably, but they are by no means exhaustive. 16-31 Ionic liquids usually consist of a large, asymmetric organic cation coupled with a generally smaller, weakly coordinating anion. Because a large number of cationic and anionic structure combinations are possible, their physicochemical properties can be easily tuned by changing the structure of the component ions. Thus, a goal of the present study is to present reliable data for the density of five ILs and their temperature and pressure dependence. As it is impossible to measure all the possible combinations of anions and cations, it is necessary to make accurate measurements on selected systems to provide results that may be used to develop correlations and to test predictive methods. The most commonly used cations are those of the 1-alkyl- 3-methyl-imidazolium family, [C n mim]. On the other hand, the bis(trifluoromethylsulfonyl)imide anion, [NTf 2 ], has gained in recent years some importance among the commonly used anions due to its stability to moisture, air, and high-temperature conditions. The ILs based on the [BF 4 ] anion are historically important and commonly investigated, despite the fact that tetrafluoroborate can undergo hydrolysis producing HF in contact with water, 32,33 mainly at high temperatures. 34 This paper is a continuation of previous work 35 on high- pressure densities and derived thermodynamic properties of imidazolium-based ILs. In this work, the experimental measure- ments of the pressure (0.10 < p/MPa < 30.00) and temperature (293.15 < T/K < 393.15) dependence of the density and derived thermodynamic properties, such as the isothermal compress- ibility, the isobaric expansivity, and the thermal pressure coefficient of several imidazolium-based ILs, are presented as Supporting Information. The objective of this work is to contribute for the databank of thermodynamic properties of pure ILs and to investigate the relationship between ionic structures and their density, to establish principles for the molecular design of ILs. For that purpose, the [C 2 mim] cation was studied in combination with two anions, [NTf 2 ] and [BF 4 ], to conclude about the anion effect. On other hand, the [NTf 2 ] anion was combined with three different cations, [C 2 mim], [C 7 mim], and [C 8 mim], to study the effect of alkyl chain length on the imidazolium ring on the density and derived properties. This completes the characteriza- tion of the [C n mim][NTf 2 ] series, with n ranging from 2 to 8, being carried by us 35,36 and Rebelo and co-workers. 24,26 To the best of our knowledge, this is also the first description of the * To whom correspondence should be addressed. E-mail: abel@eq.uc.pt. ‡ Universidade de Aveiro. § Universidade de Coimbra. 1881 J. Chem. Eng. Data 2007, 52, 1881-1888 10.1021/je700205n CCC: $37.00 © 2007 American Chemical Society Published on Web 07/28/2007