title: Anion-Functionalized Task-Specific Ionic Liquids: Molecular Origin of Change in Viscosity upon CO2 Capture.
abstract: The structure and dynamics of a task-specific ionic liquid (TSIL), trihexyl(tetradecyl)phosphonium imidazolate, before and after absorbing CO(2) were studied with a molecular dynamics (MD) simulation. This particular ionic liquid is one of several newly discovered azole-based TSILs for equimolar CO(2) capture. Unlike other TSILs whose viscosity increases drastically upon reaction with CO(2), its viscosity decreases after CO(2) absorption. This unique behavior was confirmed in our MD simulation. We find that after CO(2) absorption the translational dynamics of the whole system is accelerated, accompanied by an accelerated rotational dynamics of the cations. Radial distribution function and spatial distribution function analyses show that the anions become asymmetric after reaction with CO(2), and this causes the imbalance of the interaction between the positive and negative regions of the ions. The interaction between the phosphorus atom of the cation and oxygen atoms of the carboxyl group on the anion is enhanced, while that between the phosphorus atom and the naked nitrogen atom of the anion is weakened. The ion-pair correlation functions further support that the weakened interaction leads to faster dissociation of cation-anion pairs, thereby causing an accelerated dynamics. Hence, the asymmetry of anions influences the dynamics of the system and affects the viscosity. This insight may help design better TSILs with decreased viscosity for CO(2) capture.
authors.affiliation: Institute of New Energy Material Chemistry, College of Chemistry, Tianjin Key Laboratory of Metal- and Molecular-Based Material Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University , Tianjin 300071, China.
authors.forename: Ailin
authors.initials: A
authors.lastname: Li
authors.affiliation: Department of Chemistry, University of California , Riverside, California 92521, United States.
authors.forename: Ziqi
authors.initials: Z
authors.lastname: Tian
authors.affiliation: Institute of New Energy Material Chemistry, College of Chemistry, Tianjin Key Laboratory of Metal- and Molecular-Based Material Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University , Tianjin 300071, China.
authors.forename: Tianying
authors.initials: T
authors.lastname: Yan
authors.affiliation: Department of Chemistry, University of California , Riverside, California 92521, United States.
authors.forename: De-en
authors.initials: DE
authors.lastname: Jiang
authors.affiliation: Chemical Sciences Division, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831, United States.|Department of Chemistry, University of Tennessee , Knoxville, Tennessee 37996-1600, United States.
authors.forename: Sheng
authors.initials: S
authors.lastname: Dai