Effect of the number, position and length of alkyl chains on the physical properties of polysubstituted pyridinium ionic liquids Pedro Verdía a , Marta Hernaiz b , Emilio J. González c , Eugénia A. Macedo c , Josefa Salgado d , Emilia Tojo a,⇑ a Organic Chemistry Department, Faculty of Chemistry, University of Vigo, 36310 Vigo, Spain b Tekniker, Otaola 20, P.K. 44, 20600 Eibar, Gipuzkoa, Spain c LSRE – Laboratory of Separation and Reaction Engineering, Associate Laboratory LSRE/LCM, Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, s/n, Porto 4200-465, Portugal d Applied Physics Department, Faculty of Physics, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain article info Article history: Received 5 April 2013 Received in revised form 17 September 2013 Accepted 19 September 2013 Available online 4 October 2013 Keywords: Ionic liquids Decomposition temperature Density Speed of sound Refractive index Viscosity abstract The knowledge of the physical properties of ionic liquids is of high importance in order to evaluate their potential applicability for a given purpose. In the last few years, ionic liquids have been proposed as promising solvents for extractive desulfurization of fuels. Among them, recent studies have shown that ionic liquids derived from pyridinium affords excellent S-compounds removal capacity. In this work, the thermal analysis of five ionic liquids derived from pyridinium cation polysubstituted with different alkyl chains was carried out by Differencial Scanning Calorimetry (DSC) and Thermal Gravimetric Analysis (TGA). Furthermore, the density, speed of sound, refractive index and dynamic viscosity for all the pure ionic liquids were also measured from T = (298.15 to 343.15) K. The effect of the number of cation alkyl chains, their length, and their position on the pyridinium ring, on the ionic liquid physical properties is also analyzed and discussed. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Nowadays, the emission of sulfur and nitrogen oxides to the atmosphere is one of the main environmental issues worldwide and the origin of acid rain. The main source of emissions of these compounds to the atmosphere is the combustion of fossil fuels containing sulfur compounds [1]. Moreover, the presence of sul- furic oxides in exhaust fumes compromises the efficiency of the catalytic converters employed in cars for reducing contaminating emissions. For all these reasons, current legislation has set very strict limits to the content of sulfur compounds in diesel and gasoline [2,3], and the obtaining of ultra-low sulfur-containing fuels has become one of the main challenges for refinery industries. The use of ionic liquids (ILs) as environmentally benign alterna- tive solvents has been extensively studied and reviewed in the last few years [4]. ILs present a series of characteristics, as thermal and chemical stability, wide liquid range, non-flammability, ability to dissolve a wide range of materials, compatibility with oxidant and reducing agents and negligible vapor pressure, that make their use advantageous over the use of volatile organic solvents. The absence of vapor pressure in ILs is of high importance under the environmental point of view, as ILs do not produce air pollution and can be easily recovered. One of the most promising industrial applications of ILs is their use in the extractive desulfurization of fuels, which has been recently reviewed [5]. Studies of Holbrey and co-workers [6] have shown that the extractive ability of ILs is mainly affected by the cation type, and vary following the sequence dimethylpyridinium > methylpyridini- um > pyridinium  imidazolium  pyrrolidinium. More recently, it has been shown that the tetraalkylsubstituted pyridinium ILs 1-ethyl-3,5-dimethyl-2-pentylpyridinium bis(trifluoromethanesul- fonyl)imide [ 1 E 3 M 5 M 2 PPy][NTf 2 ] and 1-butyl-3,5-dimethyl-2-pent- ylpyridinium bis(trifluoromethanesulfonyl)imide [ 1 B 3 M 5 M 2 PPy] [NTf 2 ] present high values of selectivity and solute distribution ratio when used as solvents for extraction of thiophene from heptane [7]. The knowledge of the physical properties of an IL is very impor- tant to evaluate its capacity for a particular application. Taking into account the interest of polysubstituted pyridinium based ILs as extractants for fuels, thermal decomposition temperature, density, speed of sound, refractive index, and viscosities of a series of five ILs derived from pyridinium cation substituted with different alkyl chains are reported in this work. The influence of the number of cation alkyl chains, their length, and their position on the pyridini- um ring, on the ionic liquid physical properties is also analysed and discussed. 0021-9614/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jct.2013.09.028 ⇑ Corresponding author. Tel.: +34 986 812 290; fax: +34 986 812 262. E-mail address: etojo@uvigo.es (E. Tojo). J. Chem. Thermodynamics 69 (2014) 19–26 Contents lists available at ScienceDirect J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct