(193ai) Critical, Interfacial and Derivative Properties of Ionic Liquids by a Molecular-Based Equation of State

In the recent years, the study of ionic liquids has raised a great interest in both the scientific and industrial communities. The particular physicochemical properties of those fluids (negligible vapor pressure, wide liquid range, etc.) make them an attractive option for many industrial applications including gas absorption and separation processes, among others.  Most of the devoted research has been focused on studying their density and the solubility of different liquids and gases on them. However, few works have been devoted to other important properties that could provide additional insight about those fluids, such as the critical, interfacial and derivative properties.

Precise measurements of ionic liquids (IL) physical properties near the critical point are an unexplored field. Experimental data at these extreme conditions are very difficult to achieve due to the low vapor pressures and the thermal degradation of ionic liquids far before the critical region is reached. However, in order to design thermally stable ILs information about the critical region is a requirement for industrial purposes. Hence, the need of a reliable prediction of the critical properties of ionic liquids has pushed the development of theoretical methods based on molecular-based approaches with physical meaning.  In this contribution within the framework of the soft-SAFT EoS [1] coupled with the Density Gradient Theory (DGT) [2], the surface tension as well as the critical temperature, pressure and density have been estimated, for three different ionic liquid families, and compared with those reported in the literature from experimental [3-5] or simulation data [6].

In addition, for the [C_n-mim][Tf₂N] ILs family a correlation for the influence parameter as a function of the molecular weight was obtained, empowering the predictive capabilities of the equation for interfacial tensions of compounds of the family for which experimental data is scarce or unavailable [3]. Then, surface thermodynamic properties were also derived from the dependence of the surface tension values, and compared with those obtained within the same framework [4-6]. Finally, several second-order thermodynamic properties such as the heat capacities, the isothermal compressibilities or the Joule-Thomson coefficient are evaluated with no additional parameters. This information is valuable in order to discriminate the best model to represent the ionic liquid as it represents a strong test for any equation of state.

The results presented here show the robustness of using an accurate and versatile equation of state for the evaluation of derivative, interfacial [7] and surface properties [8] with a very modest computational effort.

F. Llovell acknowledges a JAE-Doc grant. This work is partially financed by the Spanish Government under projects CTQ2008-05370, CTQ2011-23255 and CEN2008-01027, a CENIT project belonging to the Programa Ingenio 2010). Additional support from the Catalan Government is also acknowledged (2009SGR-666).

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[6]  V. C. Weiss, B. Heggen  and F. Müller-Plathe J. Phys. Chem. C 114 (2010) 3599–3608

[7]   O.Vilaseca and L.F. Vega, Fluid Phase Equilib. 2010. doi:10.1016/j.fluid.2010.09.018

[8]   O.Vilaseca and L.F. Vega, Submitted for publication