Abstract
An increasing lack of single ion cation–anion associations (ion pairing) in ionic liquids suggests a structural motif that stands in contradiction to the single ion pair structure of their vapor phase, which was evidenced by different experimental and theoretical studies. Therefore, a structural rearrangement has to occur en route from the liquid to the vapor. In this study, we propose a detailed four-step evaporation mechanism for ionic liquids, providing a refined perspective on the theory of this process based on the connection between ion pairing and volatility. The process involves diffusion of ions from the bulk to the surface, where they float around until a well-defined ion pair is formed with a counterion, leading to the departure from the surface into the vacuum. To assess the validity of this scheme, we performed a series of classical and ab initio molecular dynamics simulations based on the most sophisticated methods and force fields available for ionic liquids.
Highlights
Ionic liquids (ILs) had been often described as ‘‘nonvolatile’’ replacement for organic solvents [1,2,3,4]; in processes where other possible advantageous properties of ILs are utilized, the low vaporizability became a limiting obstacle in the effective purification via distillation
We propose a detailed four-step evaporation mechanism for ionic liquids, providing a refined perspective on the theory of this process based on the connection between ion pairing and volatility
The cornerstone of the above-described mechanistic picture is the gradual change in the ion pairing from the liquid to the vapor, which should be at a maximum either at the surface, or very close to it
Summary
Ionic liquids (ILs) had been often described as ‘‘nonvolatile’’ replacement for organic solvents [1,2,3,4]; in processes where other possible advantageous properties of ILs are utilized, the low vaporizability became a limiting obstacle in the effective purification via distillation. At the interface region of the IL there should be more ion pair-like structures (IPs), than in the bulk phase.
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