Thermophysical and Thermodynamic Properties of 1-Butyl-3-methylimidazolium Tetrafluoroborate and 1-Butyl-3-methylimidazolium Hexafluorophosphate over an Extended Pressure Range Ricardo Gomes de Azevedo, † Jose ´ M. S. S. Esperanc ¸ a, † Vesna Najdanovic-Visak, † Zoran P. Visak, † Henrique J. R. Guedes, ‡ Manuel Nunes da Ponte, † and Luı ´s P. N. Rebelo* ,† Instituto de Tecnologia Quı ´mica e Biolo ´gica, ITQB 2, Universidade Nova de Lisboa, Av. Repu ´ blica, Apartado 127, 2780-901 Oeiras, Portugal, and REQUIMTE, Departamento de Quı ´mica, Faculdade de Cie ˆncias e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal The current study focuses on 1-butyl-3-methylimidazolium tetrafluoroborate, [bmim][BF 4 ], and 1-butyl- 3-methylimidazolium hexafluorophosphate, [bmim][PF 6 ]. The objective is to study the influence of pressure as well as that of the anion on several properties of this type of ionic liquid. The speed of sound and densities in pure ionic liquids (ILs) as a function of temperature and pressure have been determined. Several other thermodynamic properties such as compressibilities, expansivities, and heat capacities have been obtained. To the best of our knowledge, this research comprises both the first speed of sound data and the first evaluation of heat capacities at high pressures for ILs. Speed of sound measurements have been carried out in broad ranges of temperature (283 < T/K < 323) and pressure (0.1 < p/MPa < 150), sometimes inside the metastable liquid region using a nonintrusive microcell. The T-P melting line of [bmim][PF 6 ] has also been determined by an acoustic method. Density measurements have been performed for broad ranges of temperature (298 < T/K < 333) and pressure (0.1 < p/MPa < 60) using a vibrating tube densimeter. The pressure dependence of the heat capacities, which is generally mild, is found to be highly dependent on the curvature of the temperature dependence of the density. Introduction Despite the fact that hexafluorophosphate, [PF 6 ] - , and tetrafluoroborate, [BF 4 ] - , based ILs can undergo hydrolysis producing HF in contact with water 1 (mainly at high temperatures 2 ), they are historically the most important and most commonly investigated. We have thus chosen these anions coupled with the much studied cation 1-butyl 3-methylimidazolium for our initial speed of sound studies. Despite the increasing attention that room-temperature ionic liquids (RTILs) have recently received with respect to their thermophysical and thermodynamic properties, overall information is still scarce. Examples of very recent reviews and systematic studies can be found, for instance, in references 3-6 and references therein. The speed of sound, u, is a property that can be experimentally determined with great precision over a broad range of temperature and pressure. Because u can be related to the first pressure partial derivative of density, accurate speed of sound data can be used to enhance the development of equations of state. Furthermore, it is a very useful source of information for computing values of other thermodynamic properties that are difficult to obtain at extreme experimental conditions, such as calorimetric data at high pressures. One possible method for deriving a wealth of thermodynamic properties consists of the mea- surement of the whole (p, F, T) and (p, u, T) surfaces to obtain by differentiation the derived properties of the investigated compound. This method has proven useful, for instance, in the calculation of derived properties of proto- nated and deuterated acetone. 7,8 However, commonly ac- cepted as a more reliable method for deriving thermody- namic properties from speed of sound data is that which allows, by integration, the calculation of (p, F, T) and (p, C p , T) surfaces from merely one isobar of both density and isobaric heat capacity. 9-12 This method allows one to calculate other thermophysical properties such as isen- tropic (κ S ) and isothermal (κ T ) compressibilities, isobaric thermal expansivities (R p ), isobaric (C p ) and isochoric (C v ) heat capacities, and thermal pressure coefficients (γ v ) over the entire range of pressure of the speed of sound measure- ments. Whenever available, these values are compared with literature data obtained via other methods. Experimental Section Acoustic Cell and Densimeter. Measurements of the speed of sound were performed in the temperature range of 283 < T/K < 323 and pressure range of 0.1 < P/MPa < 150. The speed of propagation of sound waves in liquids was determined using a nonintrusive method at an opera- tional frequency of 0.5 MHz. We used the same experi- mental setup as previously described in great detail elsewhere. 7 Pressure was measured using an Omega pres- sure transducer, which was calibrated against a high- accuracy Heise gauge. The pressure precision and accuracy are better than (0.05%. The acoustic cell was placed in a Hart Scientific calibration bath (stability (0.001 K), and temperature was measured by a four-wire platinum resis- tance thermometer (PRT) coupled to a Keithley digital multimeter (model DMM 199). The PRT was previously * To whom correspondence should be addressed. E-mail: luis.rebelo@ itqb.unl.pt. Phone: +351-21 4469 441. Fax: +351-21 4411 277. † Instituto de Tecnologia Quı ´mica e Biolo ´gica. ‡ REQUIMTE. 997 J. Chem. Eng. Data 2005, 50, 997-1008 10.1021/je049534w CCC: $30.25 © 2005 American Chemical Society Published on Web 04/21/2005