Thermochimica Acta 519 (2011) 44–49 Contents lists available at ScienceDirect Thermochimica Acta journal homepage: www.elsevier.com/locate/tca Molar heat capacity of four aqueous ionic liquid mixtures Hui-Chun Hu a , Allan N. Soriano a,b , Rhoda B. Leron a , Meng-Hui Li a,∗ a R&D Center for Membrane Technology and Department of Chemical Engineering, Chung Yuan Christian University, Chung Li 32023, Taiwan, ROC b School of Chemical Engineering and Chemistry, Mapúa Institute of Technology, Manila 1002, Philippines article info Article history: Received 20 August 2010 Received in revised form 13 February 2011 Accepted 17 February 2011 Available online 2 March 2011 Keywords: Aqueous ionic liquid mixture Excess molar heat capacity Molar heat capacity Redlich–Kister equation abstract As a continuation of our systematic study on physicochemical characterization of aqueous solu- tion of ionic liquids, new measurements of molar heat capacity for aqueous solutions of four ionic liquids were reported. The investigated ionic liquids were 1-butyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium bromide, 1-butyl-2,3-dimethylimidazolium tetrafluoroborate, and 1- butyl-2,3-dimethylimidazolium hexafluorophosphate. The molar heat capacities were measured at standard pressure and over the temperature range (303.2–353.2) K using a differential scanning calorime- ter (DSC) having an estimated experimental uncertainty of about ±2% with liquid water as reference. The measured molar heat capacities were reported as function of temperature and composition and the excess molar heat capacities were calculated using a Redlich–Kister type equation. For the studied ionic liquids, results showed variations in the dependence of the excess molar heat capacities on temperature and composition. The applied correlation satisfactorily represented the molar heat capacity measurements as shown by an acceptable overall average deviation of 0.09%. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Room-temperature ionic liquids (RTILs) are regarded to as promising green alternatives to volatile organic solvents. They are organic salts that consist entirely of ions and liquid at or below room temperature. They posses unique properties, includ- ing wide liquid range, extremely low vapor pressure, high thermal stability, high solvation capacity, and non-flammability, which sug- gest their possible applications in a wide variety of industrial and chemical processes [1]. Several studies reported that RTILs have the potential as absorption media in CO 2 capture, as work- ing fluids in electrochemical processes (i.e., batteries, capacitors, etc.), and as heat-transfer or thermal fluids [2–6] and the use of their aqueous mixtures in such applications maybe inevitable [7]. The suitability of RTILs for use as working fluids for a specific process depends greatly on their physical and thermodynamic properties. These parameters will be vital in the process design. For heat-transfer applications, for instance, one of the relevant properties is the heat capacity. This property indicates the ability of the substance to store heat; thus, it can be used to assess the efficacy of the substance as a thermal fluid. Several papers reported heat capacity data for a number of pure RTILs for a ∗ Corresponding author. Tel.: +886 3 265 4109; fax: +886 3 265 4199. E-mail address: mhli@cycu.edu.tw (M.-H. Li). variety of temperature ranges [1,2,4,8–13] while only few reported those of aqueous RTIL solutions. On the latter, available data are for water + RTIL solutions of 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylpyridinium tetrafluoroborate, 1-n-butyl-3-methylimidazolium bromide, 1-butyl-3-methylimi- dazoliumtosylate, 1-butyl-3-methylimidazolium hexafluoropho sphate, 1-ethyl-3-methylimidazolium ethylsulfate, 1-ethyl-3- methylimidazoliummethylsufate, 1-butyl-3-methylimidazolium trifluoromethanesulfonate, 1-ethyl-3-methylimidazolium triflu- oromethanesulfonate (or triflate), 1-ethyl-3-methylimidazolium trifluoroacetate, 1-ethyl-3-methylimidazolium dicyanamide, and 1-ethyl-3-methylimidazolium 2-(2-methoxyethoxy) ethylsulfate [7,8,10,14–18]. In fact of the four studied systems in this work, only that of 1-butyl-3-methylimidazolium bromide + water systems are available [10]. Thus, in this work, the molar heat capacities, C P , of aqueous mixtures of the RTILs 1-butyl-3-methylimidazolium chloride [Bmim][Cl], 1-butyl-3-methylimidazolium bromide [Bmim][Br], 1-butyl-2,3-dimethylimidazolium tetrafluoroborate [Bdmim] [BF 4 ], and 1-butyl-2,3-dimethylimidazolium hexafluorophosphate [Bdmim][PF 6 ] were determined at standard pressure, within the temperature range (303.15–353.15) K and over the entire range of mole fraction depending on the solubility of the RTIL in water. Also the excess molar heat capacity, C E P , was calculated using a Redlich–Kister type equation and the dependences of the measured C P and calculated C E P on system’s temperature and composition were presented. 0040-6031/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2011.02.027