Evaluation of Thermophysical Properties of Imidazolium-Based Phenolate Ionic Liquids Syed Nasir Shah, † Kallidanthiyil Chellappan Lethesh,* ,† M. I. Abdul Mutalib, ‡ and Rashidah Binti Mohd Pilus § † PETRONAS Ionic Liquid Centre, ‡ Department of Chemical Engineering, and § Department of Petroleum Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia * S Supporting Information ABSTRACT: Thermophysical properties of imidazolium-based phenolate ionic liquids (ILs) were investigated over a wide temperature range. Different alkyl groups such as ethyl, butyl, hexyl, octyl, and decyl were tethered to the 1-methylimidazolium cation to study the effect of alkyl chain length on thermophysical properties such as density, viscosity, refractive index, heat capacity, and surface tension. The thermal stability of the phenolate ILs was investigated using thermogravimetric analysis. From the experimental values of density and surface tension, the molecular volume, standard molar entropy, lattice energy, surface entropy, and surface enthalpy of the ILs were calculated at 303.15 K. ■ INTRODUCTION Ionic liquids (ILs) have been known for quite a long time, but in the last two decades there has been a significant increase in the application of ILs in the chemical industry as well as in academia. The commercialization of BASIL and DIFASOL processes has raised a global interest for industries around the world to work in the field of ILs. 1 ILs are molten salts composed of cations and anions. 2 The properties of ILs can be tuned by the careful choice of cations and anions. This tuning of properties makes ILs appropriate for a large number of applications. The cations of ILs are organic, and the anions can be either organic or inorganic. The structural modifications of cations and anions enables the production of almost 10 6 ILs. This value is significantly higher than the small number of conventional solvents used in industrial applications. 1 These applications include separation processes, purification, catalysis, electrochemistry, solar cells, biomass dissolution, etc. 3-10 Biomass dissolution can be done with ILs having anions with H-bond accepting nature. 7,11-13 Binnemans and co-workers utilized the Lewis basicity of para substituted tert-butyl phenolate anion-based ILs (phenolate platform) for the development of a new route for the synthesis of hydrophilic ILs with very low halide content. 14 They also utilized the phenolate platform for the synthesis of highly base stable quarternary ILs. 15 It has been reported that ILs with phenolate anion are an effective medium for CO 2 captutre applications. 16 However, comprehensive study of the thermophysical proper- ties of phenolate anion-based ILs is limited. In this work a detailed study of the thermophysical properties of phenolate- based ILs with the most commonly used imidazolium cation is presented. An overview of ILs used in this study is given in Figure 1. ■ EXPERIMENTAL SECTION Materials and Methods. The chemicals used for the synthesis of ILs were purchased from Acros Organics (Geel Belgium) and Sigma-Aldrich (Bornem, Belgium).The synthesis of the ILs was carried out according to a reported procedure. 17 However, rigorous purification and drying methods were applied to reduce the halide content and water content in the ILs compared to that of ILs in the previous report. The synthesized ILs were washed ten times with ethyl acetate (10 × 10 mL), and the ILs were dried in a vacuum line at 60 °C for 48 h. The water content and halide content of the ILs were reduced remarkably after the drying and purification. A Bruker Avance 500 spectrometer was used to record the 1 H and 13 C NMR spectra. Water content of ILs was measured using a coulometric Karl Fischer titrator (Mettler Toledo, model DL39). The density and viscosity of the ILs was measured using the Anton Paar viscometer (model SVM3000) having a temperature uncertainty of u(T) = ± 0.01 °C, viscosity uncertainty of u(μ)= ± 0.32%, and uncertainty in the density calculations u(ρ)= ± 5 × 10 -6 g·cm -3 . Surface tension was measured using the pendant drop method. The temperature range was from 293.15 to 353.15 K with uncertainty of u(σ)= ± 0.04 mN·m -1 and u(T) = ± 0.01 K. An ATAGO programmable digital refractometer (RX-5000α) was used to measure the refractive indices of all the ILs. The temperature window was from 293.15 to 333.15 K. The uncertainty in measurement for this equipment is of the order of ±4 × 10 -5 , and the temperature accuracy is ±0.05 K. Thermal decom- Received: December 30, 2014 Revised: March 18, 2015 Accepted: March 27, 2015 Published: March 27, 2015 Figure 1. Overview of the chemical structures and abbreviation of ILs used in this study. Article pubs.acs.org/IECR © 2015 American Chemical Society 3697 DOI: 10.1021/ie505059g Ind. Eng. Chem. Res. 2015, 54, 3697-3705