(28c) Thermodynamic and Transport Characteristics of Co2 in Ionic Liquids

We have begun a fundamental investigation of the transport characteristics of gases and organic solutes in room temperature ionic liquids. Our goal is to combine information collected from a number of different experimental measurements in order to develop a fundamental understanding of the interactions governing transport in ionic liquids.

The solubility and diffusivity of target solutes in a collection of ionic liquids have been determined by measuring the decrease in pressure that results following the introduction of solute gas into a closed vessel containing a thin ionic liquid film. The viscosity of selected ionic liquids has also been measured. All measurements have been performed at temperatures ranging from 10^oC to 50^oC at low pressures (1-2 atm).

Values for the Henry's Law constant for CO₂ ranged from ~ 25 - 50 bars for measurements between 10^oC and 50^oC. Trends show H in 1-(3,4,5,6-perfluorohexyl)-3-methyl imidazolium Tf₂N < 1-butyl-3-methyl pyridinium Tf₂N < 1-butyl-3-methyl imidazolium Tf₂N ~ 1,2-dimethyl-3-propyl imidazolium Tf₂N, with values consistent with literature reports for similar ionic liquids.

Values for the diffusion coefficient for CO₂ in the same ionic liquids ranged from ~ 3 x 10^-6 cm²/s to 15 x 10^-6 cm²/s with trends between ionic liquids similar to those observed for the Henry's Law constant. These diffusion coefficients are an order of magnitude smaller than values reported for CO₂ diffusion in methanol and isooctane and are 5-10 times larger than values reported for CO₂ diffusion in ionic liquids with BF₄^- and PF₆^- anions (Chen and Chen, 1985; Shiflett and Yokoseki, 2005). Activation energies for diffusion ranged from 10 to 15 kJ/mol. These values are slightly greater than values reported for CO₂ in methanol and isooctane (9-10 kJ/mol) and considerably smaller than values reported for CO₂ in ionic liquids with BF₄^- and PF₆^- anions (24-27 kJ/mol). Literature values for self diffusion coefficients for ionic liquid anions and cations yield activation energies of ~ 30 kJ/mol for 1-butyl-3-methyl imidazolium Tf₂N (Todkuda et al, 2006) and ~ 40 kJ/mol for 1-butyl-3-methyl imidazolium PF₆ (Umecky et al, 2004). These trends for Tf₂N^- vs PF₆^- ionic liquid self diffusion are similar to the trends observed for CO₂ diffusion in these same ionic liquids.

To date, viscosity measurements have been performed with two ionic liquids. Results show that the viscosity of 1-butyl-3-methyl imidazolium Tf₂N is an order of magnitude larger than the viscosity of 1,2-dimethyl-3-propyl imidazolium Tf₂N, a surprising result given the similarity in chemical structure of these two liquids. The viscosity of 1-butyl-3-methyl imidazolium Tf₂N also shows a much stronger temperature dependence when compared to 1,2-dimethyl-3-propyl imidazolium Tf₂N, a trend which is qualitatively similar to that observed for CO₂ diffusion in these liquids.

Results will be presented and discussed in terms of our developing understanding of the relationship between transport characteristics and solute and ionic liquid structure.