Why Is the Partial Molar Volume of CO2 So Small When Dissolved in a Room Temperature Ionic Liquid? Structure and Dynamics of CO2 Dissolved in [Bmim+] [PF6-]

Abstract

When supercritical CO₂ is dissolved in an ionic liquid, its partial molar volume is much smaller than that observed in most other solvents. In this article we explore in atomistic detail and explain in an intuitive way the peculiar volumetric behavior experimentally observed when supercritical CO₂ is dissolved in 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim⁺] [PF₆^-]). We also provide physical insight into the structure and dynamics occurring across the boundary of the CO₂ ionic liquid interface. We find that the liquid structure of [Bmim⁺] [PF₆^-] in the presence of CO₂ is nearly identical to that in the neat ionic liquid (IL) even at fairly large mole fractions of CO₂. Our simulations indicate, in agreement with experiments, that partial miscibilities of one fluid into the other are very unsymmetrical, CO₂ being highly soluble in the ionic liquid phase while the ionic liquid is highly insoluble in the CO₂ phase. We interpret our results in terms of the size and shape of spontaneously forming cavities in the ionic liquid phase, and we propose that CO₂ occupies extremely well-defined locations in the IL. Even though our accurate prediction of cavity sizes in the neat IL indicates that these cavities are small compared with the van der Waals radius of a single carbon or oxygen atom, CO₂ appears to occupy a space that was for the most part a priori “empty”.