Surface Tension of Binary Mixtures of 1‑Alkyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquids: Experimental Measurements and Soft-SAFT Modeling M. B. Oliveira, † M. Domínguez-Pe ́ rez, ‡ M. G. Freire, † F. Llovell, § O. Cabeza, ‡ J. A. Lopes-da-Silva, † L. F. Vega, §,∥ and J. A. P. Coutinho* ,† † Chemistry Department, CICECO& QOPNA, University of Aveiro, Campus Universita ́ rio de Santiago, 3810-193 Aveiro, Portugal ‡ Mesturas Group, Departamento de Física, Facultade de Ciencias, Universidade da Coruñ a, 15008 A Coruñ a, Spain § MATGAS Research Center (Carburos Meta ́ licos/Air Products, CSIC, UAB), Campus UAB, 08193 Bellaterra, Barcelona, Spain ∥ Carburos Meta ́ licos/Air Products Group, C/Aragó n, 300, 08069 Barcelona, Spain * S Supporting Information ABSTRACT: Ionic liquids have attracted a large amount of interest in the past few years. One approach to better understand their peculiar nature and characteristics is through the analysis of their surface properties. Some research has provided novel information on the organization of pure ionic liquids at the vapor−liquid interface; yet, a systematic study on the surface properties of mixtures of ionic liquids and their organization at the surface has not previously been carried out in the literature. This work reports, for the first time, a comprehensive analysis of the surface organization of mixtures of ionic liquids constituted by 1-alkyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ionic liquids, [C n mim]- [NTf 2 ]. The surface tension of mixtures composed of [C 4 mim][NTf 2 ] + [C n mim][NTf 2 ](n = 1, 2, 5, 6, 8, and 10) was experimentally determined, at 298.2 K and atmospheric pressure, in the whole composition range. From the experimental data, the surface tension deviations and the relative Gibbs adsorption isotherms were estimated showing how the surface composition of an ionic liquid mixture differs from that of the liquid bulk and that the surface is enriched by the ionic liquid with the longest alkyl chain length. Finally, the soft-SAFT equation of state coupled with the density gradient theory (DGT) was used, for the first time, to successfully reproduce the surface tension experimental data of binary mixtures of ionic liquids using a molecular-based approach. In addition, the DGT was used to compute the density profiles of the two components across the interface, confirming the experimental results for the components distribution at the bulk and at the vapor−liquid interface. 1. INTRODUCTION In the past few years, ionic liquids (ILs) have been the focus of extensive research by academia, and they are finding their place in the chemical industry. These molten salts, constituted by large organic cations and organic or inorganic anions, are unable to form an ordered crystal, and thus keep their liquid state at or near room temperature. 1 Their exceptional thermophysical properties, 2−4 such as negligible vapor pressure, high thermal and chemical stability, nonflammability, and high solvation capacity for a wide range of organic compounds, have largely contributed to their recognition as potential alternatives to organic molecular solvents actually employed in a wide range of processes. Nowadays, the applications of ionic liquids range, among others, from their use as solvents in organic synthesis 5 to their use as extractive media in separation methodologies. 6 In general, most applications concerning ionic liquids involve the presence of a second liquid/gas phase where the interface among the fluids plays a vital role. Because vapor absorption transfer rates are affected through that interface, 7 the knowledge of the surface tension of ionic liquids and their mixtures is essential for a proper design and operation of new industrial mass-transfer-based operations involving these fluids, such as distillation, extraction, and absorption. 8 Considering the wide variety of ionic liquids that can be designed by combining different cations and anions, the knowledge of their interfacial properties, namely surface and interfacial tensions, and the connection of these properties with the chemical structure of the compounds are crucial require- ments for designing or selecting an ionic liquid for a specific purpose. 9 Because surface tension is a measure of the cohesive Received: June 18, 2012 Revised: September 7, 2012 Published: September 10, 2012 Article pubs.acs.org/JPCB © 2012 American Chemical Society 12133 dx.doi.org/10.1021/jp3059905 | J. Phys. Chem. B 2012, 116, 12133−12141