Phase Behavior, Micro-structure And Applications Of Supercritical CO₂Microemulsion

Green solvents are the key of green chemical production. As one of the new green solvents, supercritical CO2(scCO2) microemulsion is becoming a hot topic of green chemistry and has potential application prospect in many fields, such as separation and extraction, chemical reactions, and preparation of nanomaterials. An understanding of the phase behavior of these complex microemulsion systems is the foundation for actual applications. Researches on selectively extracting target products from dilute aqueous solutions using scCO2microemulsions could overcome some disadvantages of the existing separation methods, such as high energy consumption, easy plug, complexity of processes and requirement for membrane with high selectivity, which have important application value. In this work, the phase behavior and micro-structure of the scCO2microemulsions were studied, with1,3-PDO dilute aqueous solutions to simulate the real fermentation broth, scCO2as the continuous phase and non-fluorous (dodecyl(poly(ethylene-methylethylene glycol)) ether) Ls-mn as surfactants. The main works are as follows:(1) The formation of scCO2microemulsions with non-fluorous Ls-mn as surfactants was proved by the phase behavior of the surfactant/CO2/water systems and UV-vis spectrum of methyl orange. The results showed that the polarity and the solubilization properties of the polar core increased with the increase of molar ratio of water to surfactant. The comparison of the cloud point pressure versus temperature curves of Ls-54/CO2/H2O, Ls-45/CO2/H2O and Ls-36/CO2/H2O systems demonstrated that the structure of surfactant has significant impact on the stability of the microemulsion. The surfactant that contains more CO2-philic PO groups was favorable to the formation of microemulsion at lower pressure and the surfactant that contains more hydrophilic EO groups was favorable to the stability of the microemulsion.(2) The phase behavior of Ls-mn/CO2/H2O systems and Ls-mn/CO2/H2O/1,3-PDO systems were investigated. The results demonstrated that thermodynamically stable microemulsions with Ls-mn as surfactants can be used to selectively extract the1,3-PDO from its dilute aqueous solution by controlling the operation pressure and temperature. Moreover, the results revealed that the surfactant concentration and water content had significant influence on the solubilization capacity for1,3-PDO using these microemulsions. The increase of Ls-54concentration (>0.0051mol·L-1) or W0value (>10) could improve the solubilization capacity of the microemulsion and increase the controllable pressure for selectively solubilizing1,3-PDO from its dilute aqueous solution. Besides, the controllable pressure for selective solubilization of1,3-PDO from its dilute aqueous solution using the scCO2microemulsion with Ls-45as surfactant was high even at low Ls-45concentration and low W0value. For example, when the Ls-45concentration and W0value were0.0050mol·L-1 and6.8, respectively, the maximum controllable pressure for the selective solubilization of1,3-PDO from2.5wt%,5.0wt%and7.5wt%1,3-PDO dilute aqueous solutions reached1.48MPa (at44.0℃),1.02MPa (at38.0℃) and0.98MPa (at38.0℃), respectively.(3) The selective solubilization of1,3-PDO using the SCCO2microemulsion with Ls-36as surfactant can be only realized at relative high Wq value. Moreover, the addition of small amount of co-solvent ethanol could significantly improve the solubilization capacity of this microemulsion. The controllable pressure range for selective extraction of1,3-PDO was higher at higher Ls-36concentration.(4) Small-angle X-ray scattering measurement was employed to analyze the micro-structure of the Ls-mn/CO2/H2O/EtOH quarternary system and Ls-mn/CO2/H2O/EtOH/1,3-PDO quinery system in order to understand the structure of the scCO2microemulsions with Ls-mn as surfactants. The results indicated the aggregations with the structure similar to sphere were formed in the microemulsions with Ls-mn as surfactants. The formation of scCO2microemulsion with Ls-54was further proved by the results of molecular dynamic simulations on Ls-54/CO2/H2O ternary system and Ls-54/CO2/H2O/1,3-PDO quarternary system. Through analyzing the radial distribution function of the components and surfactant tail, it was found that this microemulsion could be used to densify the1,3-PDO dilute aqueous solution. Based on analyzing the simulation process Ls-54/CO2system, CO2/H2O system and Ls-54/CO2/H2O system, a new possible formation mechanism of SCCO2microemulsion based on Ls-54surfactant was proposed. The existance of water clusters was thought to be the necessary condition for the formation of SCCO2microemulsion.