Dynamic Acid/Base Equilibrium in Single Component Switchable Ionic Liquids and Consequences on Viscosity

The deployment of transformational nonaqueous CO₂-capture solvent systems is encumbered by high viscosities even at intermediate uptakes. Using single-molecule CO₂ binding organic liquids as a prototypical example, we present key molecular features that control bulk viscosity. Fast CO₂-uptake kinetics arise from close proximity of the alcohol and amine sites involved in CO₂ binding in a concerted fashion, resulting in a Zwitterion containing both an alkyl-carbonate and a protonated amine. The population of internal hydrogen bonds between the two functional groups determines the solution viscosity. Unlike the ion pair interactions in ionic liquids, these observations are novel and specific to a hydrogen-bonding network that can be controlled by chemically tuning single molecule CO₂ capture solvents. We present a molecular design strategy to reduce viscosity by shifting the proton transfer equilibrium toward a neutral acid/amine species, as opposed to the ubiquitously accepted zwitterionic state. The molecular design concepts proposed here are readily extensible to other CO₂ capture technologies.