Liquid Structure of CO₂-Reactive Aprotic Heterocyclic Anion Ionic Liquids from X-ray Scattering and Molecular Dynamics

A combination of X-ray scattering experiments and molecular dynamics simulations were conducted to investigate the structure of ionic liquids (ILs) which chemically bind CO₂. The structure functions were measured and computed for four different ILs consisting of two different phosphonium cations, triethyloctylphosphonium ([P₂₂₂₈]⁺) and trihexyltetradecylphosphonium ([P₆₆₆₁₄]⁺), paired with two different aprotic heterocyclic anions which chemically react with CO₂, 2-cyanopyrrolide, and 1,2,4-triazolide. Simulations were able to reproduce the experimental structure functions, and by deconstructing the simulated structure functions, further information on the liquid structure was obtained. All structure functions of the ILs studied had three primary features which have been seen before in other ILs: a prepeak near 0.3-0.4 Å^-1 corresponding to polar/nonpolar domain alternation, a charge alternation peak near 0.8 Å^-1, and a peak near 1.5 Å^-1 due to interactions of adjacent molecules. The liquid structure functions were only mildly sensitive to the specific anion and whether or not they were reacted with CO₂. Upon reacting with CO₂, small changes were observed in the structure functions of the [P₂₂₂₈]⁺ ILs, whereas virtually no change was observed upon reacting with CO₂ in the corresponding [P₆₆₆₁₄]⁺ ILs. When the [P₂₂₂₈]⁺ cation was replaced with the [P₆₆₆₁₄]⁺ cation, there was a significant increase in the intensities of the prepeak and adjacency interaction peak. While many of the liquid structure functions are similar, the actual liquid structures differ as demonstrated by computed spatial distribution functions. (Graph Presented).