Study on cloud point pressure of [Emim][Tf2N] in supercritical carbon dioxide microemulsions based on non-ionic surfactant and role of solubilized water

Abstract

We studied the solubilization effect of the supercritical carbon dioxide (scCO2) microemulsion based on non-ionic surfactants (Ls-45, Ls-54, Ls-36, Dynol-604, and TMN-6) for 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim][Tf2N]) ionic liquid (IL) as well as the influencing factors by analyzing the cloud point pressure (CPP) curves, which served as a function of dissolved ILs concentration. The [Emim][Tf2N]/surfactant/scCO2 microemulsion system was characterized by small-angle X-ray scattering (SAXS), which was proved to be a useful method to provide structure and size information of microemulsion. The effect of water on [Emim][Tf2N]-in-scCO2 (ILs/CO2) microemulsions was studied. The results showed that the ILs/CO2 microemulsions could be constructed by five surfactants selected in this paper, and in the Ls-mn system, the Ls-45/[Emim][Tf2N]/scCO2 system had the lowest CPP. However, the microemulsion constructed by Dynol-604 and TMN-6 surfactants had a poor solubilization effect, and the TMN-6 system was more sensitive to temperature. Besides, small amounts of water could reduce the CPP of the system to a certain extent, making the [Emim][Tf2N]/surfactant/scCO2 microemulsion system more stable, which had a positive effect on the solubilization of [Emim][Tf2N], especially the Ls-36 system. Moreover, the size of microemulsion droplets in [Emim][Tf2N]/surfactant/scCO2 systems was obtained by using the method of SAXS experiment, which also confirmed the solubilization of [Emim][Tf2N] in scCO2 microemulsion system. The structure of ILs/CO2 microemulsions constructed with a non-ionic surfactant was speculated. This study can provide a reference for the construction of ILs/CO2 microemulsions, which was helpful in understanding the microstructure and solubilities of ionic liquids in scCO2 microemulsions.