Study on the Phase Behavior and Molecular Dynamics Simulation of a Supercritical Carbon Dioxide Microemulsion Containing Ionic Liquid

We investigated the solubilization effect of a supercritical carbon dioxide microemulsion based on LS-<i>mn</i> (LS-36, LS-45, and LS-54) surfactants for 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]­[BF₄]) as well as the influencing factors by analyzing the cloud point pressure (CPP) curves, which served as a function of dissolved IL concentration. Results show that increased water content (<i>W</i>₀) could enhance [Bmim]­[BF₄] dissolution. Under the same conditions, microemulsion systems consisting of LS-54, LS-45, and LS-36 have the gradually decreased the dissolving capacity for [Bmim]­[BF₄]. However, variation of the surfactant concentration has little influence on dissolving [Bmim]­[BF₄]. In addition, introducing appropriate amounts of ethanol could decrease the CPP of the system, which did not bring significant enhancement on [Bmim]­[BF₄] dissolution. Molecular dynamics (MD) simulation was implemented to give an insight about the microstructure and prove the formation of the microemulsion

Molecular dynamics simulations of CO₂ permeation through ionic liquids confined in γ-alumina nanopores

<p>CO₂ permeation through imidazolium-based ionic liquids (ILs, [BMIM][Ac], [EMIM][Ac], [OMIM][Ac], [BMIM][BF₄], and [BMIM][PF₆]) confined in 1.0, 2.0, and 3.5 nm γ-alumina pores was investigated using molecular dynamics simulation. It was found that the nanopore confinement effect influenced the structure of confined ILs greatly, resulting in a layered structure and anisotropic orientation of ILs. In the center of 2.0-nm pore, the long alkyl chain of [BMIM]⁺ tended to be parallel to the wall, providing a straight diffusion path benefiting the CO₂ permeation. The CO₂ diffusion coefficients in confined [EMIM][Ac], [BMIM][Ac], and [OMIM][Ac] were 2.3–4.1, 2.4–6.4, and 14.4–21.7 × 10⁻¹⁰ m² s⁻¹, respectively. This order was opposite to that in the bulk ILs, because the longer alkyl chain led to a more ordered structure, facilitating CO₂ diffusion. In addition, the CO₂ solubilities were 445–722 mol m⁻³ MPa⁻¹ for the five ILs confined in 1.0 nm pore, which were larger than those in 2.0 and 3.5 nm pores (196–335 mol·m⁻³ MPa⁻¹), due to the larger free volume. Both parallel orientation of alkyl chain and large free volume could increase the CO₂ permeability in confined ILs.</p

Solubility of Ionic Liquid [Bmim]Ac in Supercritical CO₂ Containing Different Cosolvents

The solubility of the ionic liquid (IL) 1-butyl-3-methylimidazolium acetate ([Bmim]­Ac) in supercritical carbon dioxide (scCO₂) with cosolvents, including ethanol, acetone, dimethyl sulfoxide (DMSO), and acetonitrile, was determined at 40, 50, and 60 °C and with a pressure up to 15.0 MPa. The results showed that the addition of cosolvents has a significant effect on the solubility of [Bmim]­Ac in scCO₂. The ability of different cosolvents to enhance the solubility of [Bmim]Ac in scCO₂ is in the following order: ethanol > DMSO > acetone > acetonitrile. The solubility of [Bmim]Ac in the scCO₂/cosolvent mixture increased dramatically as the cosolvent concentration exceeded 20.0 mol %. The effect of the temperature on the solubility is more complicated. The solubility of [Bmim]Ac in scCO₂/cosolvent increased slowly when using ethanol as the cosolvent, decreased slowly when using acetone or acetonitrile as the cosolvent, and increased first and then decreased when using DMSO as the cosolvent as the temperature increased from 40 to 60 °C. Moreover, the values of solubility increased as the pressure increased from 8 to 15 MPa. The increased tendency in the high pressure area is more obvious. The maximum solubility is 3.66 × 10^–2 mol %, which can be obtained when using 26.0 mol % ethanol at 60 °C, 14.55 MPa, and the minimum solubility is 1.89 × 10^–4 mol %, which can be obtained when using 10.5 mol % DMSO at 40 °C, 9.93 MPa. The modified Christal equation was used to correlate the solubility data, and the average absolute relative deviations are in the range of 4.36–14.35%. The maximum correlation accuracy is obtained when using ethanol as the cosolvent, and the minimum value for the system is obtained when using DMSO as the cosolvent

Critical Microemulsion Concentration and Molar Ratio of Water-to-Surfactant of Supercritical CO₂ Microemulsions with Commercial Nonionic Surfactants: Experiment and Molecular Dynamics Simulation

The critical microemulsion concentration (cμc) and the molar ratio of water-to-surfactant (<i>W</i>₀) of supercritical CO₂ (scCO₂) microemulsion that uses different nonionic hydrocarbon surfactants (LS-36, LS-45, LS-54, DYNOL-604, TMN-6) were examined at temperatures from 35 to 45 °C and pressures up to 19 MPa. The results show that the cμc mainly depends on the structure of the surfactant. The surfactant with more hydrophilic structure, such as the ethylene oxide (EO) group and hydroxyl, tends to produce a higher cμc. In addition, the cμc increases with the increase of the ratio of ethylene oxide (EO) group number to the propylene oxide (PO) group number of the surfactant. The capacity of the microemulsion system to dissolve water, which is characterized by <i>W</i>₀, is related to the concentration and structure of surfactant. It is found that a higher solubility of surfactant in CO₂ favors the system to dissolve water at lower pressure. At higher pressure, the stronger hydrophilicity of surfactant and the higher surfactant concentration are beneficial for microemulsions to contain more water. The molecular dynamics (MD) simulation, which was conducted in the NPT ensemble, shows the spontaneous evolution of a surfactant cluster and microstructure of microemulsion at different conditions. It demonstrates that the microemulsion system with more water molecules can form a larger water cluster and catch more surfactants although a few surfactants dissociate in the continuous phase. The experimental data and MD simulation results provide useful infomation for the structure regulation of the scCO₂ microemulsion and expand the study to the microscopic scale