Systematic Study of the Thermophysical Properties of Imidazolium- Based Ionic Liquids with Cyano-Functionalized Anions Catarina M. S. S. Neves, † Kiki Adi Kurnia, † Joa ̃ o A. P. Coutinho, † Isabel M. Marrucho, †,‡ Jose ́ N. Canongia Lopes, § Mara G. Freire,* ,†,‡ and Luís Paulo N. Rebelo ‡ † Departamento de Química, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal ‡ Instituto de Tecnologia Química e Bioló gica, UNL, Avenida Repú blica, Apartado 127, 2780-901 Oeiras, Portugal § Centro de Química Estrutural, Instituto Superior Te ́ cnico, 1049-001 Lisboa, Portugal * S Supporting Information ABSTRACT: In the past few years, ionic liquids (ILs) with cyano- functionalized anions have shown to be improved candidates for electrochemical and separation applications. Nevertheless, only scattered data exist hitherto and a broad analysis of their structure- property relationship has yet to be attempted. Therefore, in this work, a systematic study of the densities, viscosities and refractive indices of imidazolium-based ILs with cyano-functionalized anions was carried out at 0.1 MPa within a broad temperature range (from 278 to 363 K). The ILs under study are based on 1-alkyl-3-methylimidazolium cations (alkyl = ethyl, butyl and hexyl) combined with the [SCN] - , [N(CN) 2 ] - , [C(CN) 3 ] - and [B(CN) 4 ] - anions. The selected matrix of cation/anion combinations allows us to provide a detailed and comprehensive investigation of the influence of the -CN group through an analysis of the thermophysical properties of the related ILs. The results show that, regardless of the cation, the densities decrease with an increase in the number of cyano groups or anion molecular weight. Moreover, for a fixed cation and temperature, the refractive index of the ILs decreases according to the rank: [SCN] - > [N(CN) 2 ] - ≈ [C(CN) 3 ] - > [B(CN) 4 ] - . On the other hand, no clear trend was observed for the viscosity of ILs and the respective number of -CN groups. The viscosity dependence on the cyano-functionalized anions decreases in the order: [SCN] - > [B(CN) 4 ] - > [N(CN) 2 ] - > [C(CN) 3 ] - . The isobaric thermal expansion coefficient, the derived molar refraction, the free volume, and the viscosity energy barrier of all compounds were estimated from the experimental data and are presented and discussed. Finally, group contribution models were applied, and new group contribution parameters are presented, extending these methods to the prediction of the ILs properties. ■ INTRODUCTION Ionic liquids (ILs) have received enormous attention during the past decade as replacement solvents for currently used volatile organic solvents. 1 Chemically, ILs are composed of bulky organic cations coupled either with organic or inorganic anions. Different combinations of their ions and the introduction of specific and functionalized groups lead to significant changes in their thermophysical properties and phase behavior and, therefore, ILs have been labeled as “designer solvents” and “task-specific fluids”. ILs thus provide the scientific community a plethora of research opportunities for exploring their unique and interesting properties. Most of the ILs studied to date present negligible volatility and nonflammability; this combi- nation has made a crucial contribution toward their selection for a large range of applications. One of the severest barriers to the industrial application of ILs is the relatively high viscosities of some fluids compared to those of molecular organic solvents, a fact that leads to lower mass transfer coefficients and higher energy requirements. Hence, the search for new and more versatile ILs is also being driven by the need for low viscosity materials. A considerable amount of work focusing on ILs containing fluoride-based anions has been published, particularly due to their low viscosity coupled with high thermal stability. 2-4 However, ILs with cyano-functionalized anions are a set of fluids that have been somewhat underexplored despite their remarkably low viscosities. In 2001, MacFarlane et al. 5 reported the synthesis of novel ILs containing the dicyanamide anion - exceptional fluids with low melting temperatures and significantly low viscosities. Although this finding was shown more than a decade ago, the subsequent studies of the application of ILs with -CN groups have mostly centered on their use as electrolytes and in dye- sensitized solar cells prompted by the observation that these anions can produce some of the most fluid and conductive ILs. 6-9 More recently, this type of ILs has also been tested in Received: June 15, 2013 Revised: August 13, 2013 Published: August 13, 2013 Article pubs.acs.org/JPCB © 2013 American Chemical Society 10271 dx.doi.org/10.1021/jp405913b | J. Phys. Chem. B 2013, 117, 10271-10283