Results of molecular dynamics simulations are reported in which the structure and dynamics of the reacted and unreacted forms of the task specific ionic liquid (TSIL) tetrabutylphosphonium 2-cyanopyrrolide are computed. This particular ionic liquid is one of several newly discovered TSILs containing aprotic heterocyclic anions designed specifically for CO2 capture. The physical properties, liquid structure and dynamics of the ionic liquid are computed as a function of extent of reaction with CO2. Translational and rotational dynamics show little change upon reaction with CO2, in sharp contrast to traditional TSILs and consistent with experimental viscosity measurements. It is shown that this is due to the failure of a hydrogen bond network to form upon reaction with CO2.
Molecular dynamics simulations were also carried out to investigate the solubility and dynamics of water in five different ionic liquids capable of chemically reacting with CO2. The ionic liquids all have a common tetrabutylphosphonium cation paired with five different aprotic heterocyclic anions. These ionic liquids have properties that make them attractive candidates for use in CO2 capture applications, which undergo a solid to liquid phase transition when it reacts with CO2. The heat of fusion can be utilized to reduce the energy requirements for CO2 capture from the flue gas. However, the impact of water on their properties is unknown. The simulations show that the ionic liquid having a 2-cyanopyrrolide anion is the most hydrophobic of all the liquids studied, but that upon reaction with CO2 it becomes much more hydrophilic. The other ionic liquids investigated show little change in water solubility upon reaction with CO2. Henry?s Law constants, enthalpies and entropies of water absorption, liquid structure, hydrogen bonding and self-diffusivities were computed and found to correlate well with the strength of interaction between water and the anions and the relative hydrophobicity of the liquids.