Surface Tension of 1‑Ethyl-3-methylimidazolium Ethyl Sulfate or 1‑Butyl-3-methylimidazolium Hexafluorophosphate with Argon and Carbon Dioxide Guillermo Reyes, † Marcela Cartes, † Carlos Rey-Castro, † Hugo Segura,* ,† and Andre ́ s Mejía* ,† † Departamento de Ingeniería Química, Universidad de Concepció n, POB 160-C, Correo 3, Concepció n, Chile † Departament de Química, Universitat de Lleida, Rovira Roure 191, 25198 Lleida, Catalonia, Spain * S Supporting Information ABSTRACT: Surface tensions of two ionic liquids (IL): 1-ethyl-3- methylimidazolium ethyl sulfate and 1-butyl-3-methylimidazolium hexafluor- ophosphate in pressurized atmospheres of argon and carbon dioxide have been measured over the temperature range (303 to 366) K and over the pressure range (0.1 to 15) MPa for the case of argon atmosphere and (0.1 to 5) MPa for the case of carbon dioxide atmosphere by using a pendant drop tensiometer. Based on the experimental measurements, the isothermal surface tension of all IL-gas systems studied decreases as the pressure increases, evidencing a gas adsorption at the IL interface. Isobaric surface tension of an IL-gas does not show a general pattern as the temperature increases. In order to verify the isothermal surface behavior, the relative Gibbs adsorption isotherms have been calculated from the surface tension data by using the theoretical Guggenheim model, corroborating the gas adsorption processes at the IL interface. Comparing the relative Gibbs adsorption isotherms, it is possible to conclude that the ILs studied have the capability to adsorb more carbon dioxide than argon. This fact provides relevant information to use the IL as a capturing agent for carbon dioxide and the use of argon to store pure ILs. ■ INTRODUCTION Ionic liquids (ILs) are part of the so-called neoteric solvents due to their tunable properties, relatively low environmental impact, 1-5 and reduced volatility, which have led to a wide range of applications in biology, 6,7 electrochemistry, 8,9 organic chemistry, separation, extraction, and catalysis 1,5-10 fields. As a rule of thumb, it is well-established that the IL cation is mostly responsible for the physical properties such as density and viscosity whereas the IL anion is predominantly responsible for the chemical properties such as reactivity and selectivity among others. 5,6,10 The large versatility resulting from the possible combinations of cations and anions gives ILs their tunable behavior and transforms them into design substances. Besides, ILs exhibit negligible vapor pressure and low toxicity, which make them environmentally friendly, and so their use has been implemented within so-called green chemistry. 3-5 Nowadays, mixtures of ILs with gases have gained considerable interest since ILs can be used as capturing agents for greenhouse gases such as methane (CH 4 ), water vapor (H 2 O), and carbon dioxide (CO 2 ). 11-13 For the case of the ILs studied here, namely, 1-butyl-3- methylimidazolium hexafluorophosphate or [bmim][PF 6 ] and 1-ethyl-3-methylimidazolium ethyl sulfate or [emim][C 2 SO 4 ], previous works have been focused on determining vapor-liquid equilibria, diffusion, and solubility of some gases. For the case of [bmim][PF 6 ], Morgan et al. 14 reported the vapor-liquid equilibria and diffusion coefficients of this IL with CO 2 , oxygen (O 2 ), nitrogen (N 2 ), and a series of hydrocarbons from methane (CH 4 ) to butane (C 4 H 10 ). Other authors have reported the solubility of gases such as CO 2 and argon (Ar). 15-18 For the case of [emim][C 2 SO 4 ], Blanchard et al. 13 presented the high-pressure phase behavior of CO 2 with this and other ionic liquids in the temperature range (313 to 333) K, while Jacquemin et al. 19 reported solubilities of hydrogen and CO 2 over the temperature range (283 to 343) K. For the mixture [emim][C 2 SO 4 ] + Ar, no previous work has been reported previously to the best of our knowledge. Despite the importance of diffusion and solubility of gases in [bmim][PF 6 ] and [emim][C 2 SO 4 ], the surface tensions of such systems have not been thoroughly studied so far. The surface tension of pure ILs and gas pressurized ILs plays key role in science and technology as it is directly involved in solubility, miscibility and mass transfer processes. 20,21 In spite of the importance of this thermophysical property, exper- imental surface tension data reported in the literature does usually refer to the pure IL systems only. For instance, surface tensions of pure [bmim][PF 6 ] 22-25 have been studied in the temperature range (288.14 to 393) K using techniques such as Received: December 18, 2012 Accepted: April 1, 2013 Published: April 12, 2013 Article pubs.acs.org/jced © 2013 American Chemical Society 1203 dx.doi.org/10.1021/je3013414 | J. Chem. Eng. Data 2013, 58, 1203-1211