(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 10oC to 50oC at low pressures (1-2 atm).
Values for the Henry's Law constant for CO2 ranged from ~ 25 - 50 bars for measurements between 10oC and 50oC. 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.