Ind. Eng. Chem. Res. 2005, 44, 4815-4823
4815
Diffusivities of Gases in Room-Temperature Ionic Liquids: Data and Correlations Obtained Using a Lag-Time Technique David Morgan, Lee Ferguson, and Paul Scovazzo* Department of Chemical Engineering, University of Mississippi, 134 Anderson Hall, University, Mississippi 38677
Diffusivity and solubility data are presented for carbon dioxide, ethylene, propylene, 1-butene, and 1,3-butadiene in five imidazolium-based ionic liquids and one phosphonium-based ionic liquid covering a liquid viscosity range of 10-1000 cP. The data were obtained using a lag-time technique that involves analysis of both the transient and steady-state permeation regimes through a supported liquid film. In general, gas diffusion in ionic liquids (∼10-6 cm2/sec) is slower than that in traditional hydrocarbon solvents and water, but the dependence on viscosity is lower. Conversely, the dependence of diffusivity on temperature and the size of the solute gas is higher than that for nonpolar solvents. A correlation for gas diffusivity in ionic liquids at 30 °C is proposed in terms of the gas molar volume, the ionic liquid viscosity, and density, based on 30 data points with a coefficient of multiple determination r2 ) 0.975. 1. Introduction The defining characteristic of (room-temperature) ionic liquids (ILs) is that they are salts that remain in the liquid phase at or below 150 °C.1 Hence, the chosen terminology distinguishes these salts from traditional, high-temperature molten salts such as alkali or alkaline earth metal salts. As with any salt, ILs are a combination of a cation and an anion; however, the ionic moieties are often voluminous, asymmetric organic structures that allow for delocalization and screening of charge,2 which favors low-temperature melting and/or glass transitions. One major impetus for the continued investigation of ionic liquids is their unique and mutable solvent properties. Unlike most organic solvents, ionic liquids exhibit negligible vapor pressure and are nonflammable with high thermal stability, allowing for liquid ranges of >300 °C.3 In addition, the structure and functionality of each ionic moiety may be varied to increase or decrease the solubility of a myriad of compounds.4,5 Densities at ambient temperature are typically in the range of 1.1-1.6 g/mL6 but can be 0.1 mol/ (L atm) for carbon dioxide, water, and alkenes. However, excluding electrochemical studies involving the production of superoxides,14,15 there is little data reported for gas diffusivities in ionic liquids. The electrochemical studies16-19 do indicate diffusivities in the range of 10-6-10-9 cm2/s for several common electroactive metal * Corresponding author. E-mail:
[email protected].
complexes with a strong dependence on the ionic liquid viscosity, but no attempt has been made to extend the data or generalizations to small solutes. Therefore, it is not possible to have an introductory discussion of how gas diffusivities in ILs differ from those in conventional liquids. It is possible, however, to note that ILs have large molar volumes (Table 1) compared to those in conventional solvents (∼0.2-0.6 L/mol compared to water’s 0.018 L/mol), and this difference in molar volumes appears to influence diffusivity, as illustrated later in our paper. The main objective of our paper is to report the diffusivities of gases in ILs covering a broad range of viscosities (10-1000 cP). The second objective is to provide an insight into IL transport phenomena via the development of an IL-specific diffusivity correlation. We determined the gas diffusivities in ionic liquids using a lag-time technique. Daynes20 (1920) first proposed this method for polymeric films. By analyzing both the transient and steady-state regimes of permeation, permeability can be separated into its components, solubility and diffusivity. For many liquids, this approach is impractical for determining the diffusion coefficients, since liquid vaporization would result in a significant loss of the material being tested. However, ionic liquids are an exception because of their nonvolatility. While more accurate methods are available for the determination of gas diffusivities in liquids,21 the lag-time technique was chosen for this study because it can be performed with liquid sample volumes of
