Membrane Drying of Ionic Liquid

Room temperature ionic liquids (RTILs) exist as liquid salts at ambient temperatures. RTILs typically consist of a bulky, asymmetric organic cation with either an organic or inorganic anion. Consequently, RTILs possess negligible vapor pressure and are touted as environmental benign, low-emission solvents that can be readily recycled [1].

Molecular interactions that lead to the formation of aggregate structures in aqueous IL solutions have been the subject of recent studies [2]. Water molecules can form hydrogen bonds with IL cations and anions. At low water concentrations, water molecules complex primarily with anions rather than with other water molecules and form clusters at high water concentrations.

Complex formation can affect fluid properties, notably diffusivity. The diffusivities of the [C₂mim][OAc] cation and anion as well as that for water are measured experimentally using NMR. The results indicate that although the anion is smaller than the cation, the anion diffusivity can be lower. This surprising result is attributed to water complexation. Diffusivities are reported for a range of temperatures.

The removal of water from aqueous RTIL solutions using pervaporation also is reported. The driving force for separation is the difference in water partial pressure across the membrane. A number of membranes of varying composition and molecular weight cut-off are evaluated for pervaporation with a dry gas sweep. The dependence of overall water transfer rates on temperature, liquid flow and gas flow is evaluated. The mass transfer resistance of each flow channel and the membrane are determined from the results. The effects of membrane properties and the temperature dependence of water vapor pressure on performance are reported. Additionally, the impact of the concentration dependent diffusivity and viscosity on the liquid resistance is quantified.

Reference:

1. K. R. Seddon, Ionic Liquids for clean Technology, J. Chem. Tech. Biotechnol. 68 (1997) 351-356.

2. W. Jiang, Y. Wang, G. A. Voth, Molecular Dynamics Simulation of Nanostructural Organization in Ionic Liquid/Water Mixtures, J. Phys. Chem. B., 111 (2007) 4812-4818.