Synthesis and Thermophysical Properties of Imidazolium-Based Bronsted Acidic Ionic Liquids Nawshad Muhammad,* ,† Zakaria Man, ‡ Yasir A. Elsheikh, § M. Azmi Bustam, ‡ and M.I. Abdul Mutalib ‡ † Interdisciplinary Research Center in Biomedical Materials, COMSATS Institute of Information Technology, Lahore, Pakistan ‡ PETRONAS Ionic Liquid Center, Department of Chemical Engineering, Universiti Teknologi PETRONAS (UTP), 31750 Tronoh, Perak, Malaysia § Department of Chemical and Materials Engineering Faculty of Engineering, King Abdulaziz University, P.O. Box 344, Rabigh, 21911 Saudi Arabia ABSTRACT: A range of imidazolium-based Bronsted acidic ionic liquids (ILs) have been synthesized by varying the side chain length of the alkyl and alkyl sulfonic group incorporated to the C1 and C3 positions in the imidazolium ring respectively with a fixed HSO 4 - anion. The synthesized ILs were characterized using NMR for structure confirmation and CHNS for elemental analysis. The thermal properties such as thermal decomposition temperature were determined using thermogravimetric analysis, and several key physical properties such as viscosity and density were measured within a temperature range of 20 °C to 80 °C using an Anton Paar viscometer and densitometer. The density measurement results and established equations were used to calculate the molecular volumes, standard entropies, crystal lattice energies, and thermal expansion coefficients of the synthesized ILs. ■ INTRODUCTION Room temperature ionic liquids (ILs) are typically salts that contain at least one organic cation and organic or inorganic anion, and have melting points below 100 °C. Such materials are in demand as alternatives to traditional molecular solvents owing to their desirable properties such as high chemical and thermal stabilities and extremely low flammabilities and vapor pressures. 1,2 The wide range of possible cation-anion combinations has enabled ILs to be developed with a specific set of desired physicochemical properties or for specific applications. The present applications of ILs in the field of catalysis are found to be numerous and cover many areas. One of the main areas of interest is to replace the use of traditional liquid acids in catalytic reactions. 3 Physiochemical properties of ILs such as melting point, viscosity, density, and miscibility with water was found to vary with the length of the alkyl chain on the cation or the type of anion incorporated to the ILs. It has been reported that 1-alkyl- 3-methylimidazolium with tetrafluoroborates 4 and hexafluor- ophosphates anions 5 formed liquid crystalline when the alkyl chain length exceeded more than 12 carbon atoms. Below this chain length, the ILs formed liquid at room temperature. In another account, the hydrophobic property of an IL was found to favor solvent extraction for product separation that is based on relative solubility between phases. Generally, the cation part of ILs appears to have bulky and uneven structures which result in a low degree of packing symmetry. This results in reduced lattice energy of the salt crystalline formed, thereby decreasing its melting point. 6 In this study a number of Bronsted acidic ILs namely 1- methyl-3-(3-sulfopropyl) imidazolium hydrogensulfate (MSPIMHSO4), 1-butyl-3-(3-sulfopropyl) imidazolium hydro- gensulfate (BSPIMHSO4), 1-methyl-3-(3-sulfobutyl) imidazo- lium hydrogensulfate (MSBIMHSO4) and 1-butyl-3-(3-sulfo- butyl) imidazolium hydrogensulfate (BSBIMHSO4) were prepared by incorporating a methyl or butyl group at the position of C1 and a propyl or butyl attached with a sulfonate group at the position of C3 within the imidazolium ring with fixed HSO 4 - anion. The structures of the synthesized ILs were confirmed using NMR, and the elemental analysis was conducted using CHNS. A number of thermophysical proper- ties of the synthesized ILs such as thermal stability, viscosity, and density were measured with respect to temperature for a range between 20 and 80 °C. The effects of the alkyl chain length attached to the imidazolium cation were investigated for thermal stability, viscosity, and density. The values such as molecular volumes, standard entropies, crystal lattice energies, and thermal expansion coefficients were then calculated for all the synthesized ILs. ■ METHODS Materials. The following chemicals were obtained from Sigma-Aldrich (Malaysia): 1-methylimidazole (99.5 %), 1- butylimidazole (99.0 %), 3-propanesultone (99.0 %), toluene Received: March 12, 2013 Accepted: February 6, 2014 Published: February 14, 2014 Article pubs.acs.org/jced © 2014 American Chemical Society 579 dx.doi.org/10.1021/je400243j | J. Chem. Eng. Data 2014, 59, 579-584