Published: August 04, 2011 r2011 American Chemical Society 10919 dx.doi.org/10.1021/jp2049316 | J. Phys. Chem. B 2011, 115, 10919–10926 ARTICLE pubs.acs.org/JPCB High-Accuracy Vapor Pressure Data of the Extended [C n C 1 im][Ntf 2 ] Ionic Liquid Series: Trend Changes and Structural Shifts Marisa A. A. Rocha, † Carlos F. R. A. C. Lima, † Lígia R. Gomes, § Bernd Schr€ oder, ‡ Jo ~ ao A. P. Coutinho, ‡ Isabel M. Marrucho, ‡,|| Jos  e M. S. S. Esperanc -a, || Luís P. N. Rebelo, || Karina Shimizu, # Jos  e N. Canongia Lopes, ||,# and Luís M. N. B. F. Santos* ,† † Centro de Investigac - ~ ao em Química, Departamento de Química e Bioquímica, Faculdade de Ci ^ encias da Universidade do Porto, R. Campo Alegre 687, P-4169-007 Porto, Portugal ‡ CICECO, Departamento de Química, Universidade de Aveiro, P-3810-193 Aveiro, Portugal § CIAGEB, Faculdade de Ci ^ encias da Sa ude da UFP, Universidade Fernando Pessoa, R. Carlos da Maia 296, P-4200-150 Porto, Portugal ) Instituto de Tecnologia Química e Biol ogica, ITQB2, Universidade Nova de Lisboa, Av. Rep ublica, Apartado 127, P-2780-901 Oeiras, Portugal # Centro de Química Estrutural/IST, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal b S Supporting Information 1. INTRODUCTION Many properties of ionic liquids, for example, melting tem- perature, acidity and basicity, surface tension, or solubility in water and organic solvents can be adjusted by modifying the cations, anions, or both. Many authors have claimed that ionic liquids share common characteristics such as moderate-to-low electrical conductivity, moderate-to-high viscosity, good thermal stability, and extremely low vapor pressure at room temperature. However, extensive studies on the properties of ILs in the past decade 1 have shown that almost none of these properties are shared across the IL range; therefore, none of them can be considered in and of themselves as a defining characteristic of an ionic liquid. Over recent years, a growing number of applications for these fluids have been developed. Nowadays, they can be used in multiple industrial processes, 2 including, cellulose processing, 3,4 gas hand- ling, solar energy conversion, 5 and waste recycling. Besides their advantages for specific applications (catalytic activity, solvent effects, separation phenomena), ionic liquids have been designated as “green solvents” because of their low volatility, which prevents their spread into the atmosphere. Nonetheless, it has been also found that they are not free of toxicity, with aquatic toxicities at the same level of some organic solvents, 6 causing some controversy concerning the application of the “green solvent” label. 7 In fact, their extremely low volatilities limit the range of techniques available for their separation and purification, reducing the options for multiple recycling cycles during or after the manufacturing process as well as their recovery, storage, and so on. It has been discovered that, at moderate temperatures, some ionic liquids may present a range of non-negligible vapor Received: May 26, 2011 Revised: July 31, 2011 ABSTRACT: For the first time, two distinct trends are clearly evidenced for the enthalpies and entropies of vaporization along the [C n mim][Ntf 2 ] ILs series. The trend shifts observed for Δ l g H m o and Δ l g S m o , which occur at [C 6 mim][Ntf 2 ], are related to structural modifications. The thermodynamic results re- ported in the present article constitute the first quantitative experimental evidence of the structural percolation phenomen- on and make a significant contribution to better understanding of the relationship among cohesive energies, volatilities, and liquid structures of ionic liquids. A new Knudsen effusion apparatus, combined with a quartz crystal microbalance, was used for the high-accuracy volatility study of the 1-alkyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide series ([C n mim][Ntf 2 ], where n = 2, 3, 4, 5, 6, 7, 8, 10, 12). Vapor pressures in the (450500) K temperature range were measured, and the molar standard enthalpies, entropies, and Gibbs energies of vaporization were derived. The thermodynamic parameters of vaporization were reported, along with molecular dynamic simulations of the liquid phase structure, allowing the establishment of a link between the thermodynamic properties and the percolation phenomenon in ILs.