Study On The Thermodynamic Properties Of Microemulsion Formed In Supercritical Carbon Dioxide

As a "green-friendly" alternative solvent, microemulsion formed in supercritical carbon dioxide (W/C microemulsion) has attracted considerable interest since it appeared in 1996. The solubility of highly polar or high molecular weight molecules such as proteins, polymer, and metal ions can be improved by W/C microemulsion, which extends the application fields of supercritical carbon dioxide. In the future, W/C microemulsion should be widely applied in some chemical processes, including reaction, extraction, cleaning and nano-particle preparation. Though thermodynamic properties of supercritical CO₂ microemulsion are important, studies on this field, especially on the solubilization of polar substances, in literatures are not sufficiently for applications. In order to resolve this problem, three surfactants are selected as research object. Phase equilibrium of PFPE-NH₄/supercritical CO₂ system was studied fistly, and then the phase equilibrium of PFPE-NH₄/water/supercritical CO₂ system. Based on the results of phase equilibrium, micropolarity and solubilization characters in supercritical CO₂ microemulsion were studied.The paper is divided into five chapters. In Chapter 1, the basic theory of W/C microemulsion has been introduced. Surfactants soluble in supercritical CO₂, research methods and application fields of W/C microemulsion have been summarized.In Chapter 2, the phase equilibrium of PFPE-NH₄ and supercritical CO₂ system has been studied. A variable-volume high pressure view cell apparatus has been designed and built for measuring cloud points by visual observation. Cloud points of PFPE-NH₄, C₁₂E₉P₃ and C₁₂E₉P₂ in supercritical CO₂ were determined at different temperature and surfactant/CO₂ mass ratio. At 308.15 ～ 333.15 K and 15.62 ～ 19.84 MPa, the maximum solubility of PFPE-NH₄ in supercritical is 20.23×10^-2 g/g CO₂. The PFPE-NH₄/ supercritical CO₂ system shows a lower critical solution temperature (LCST) and a upper critical soluble pressure (UCSP) phase behavior; At 308.15 328.15 K and 15.88 ～ 32.34 MPa, the maximum solubility of C₁₂E₉P₃ in supercritical is 15.60×10^-3 g/g CO₂; At 308.15 ～ 328.15 K and 13.54 34.99 MPa, the maximum solubility of C₁₂E₉P₂ in supercritical is 15.36×10^-3 g/g CO₂; And the solubility data of three surfactants were well related by a chemical association model. A Quantitative Structure-Property Relationship (QSPR) model has been built for one type of nonionic surfactants based on the Molecular Connectivity Indexes. The QSPR model equationwas obtained as logS_ref = 0.829 -0.177^lx^v+0.198C_CH3, R = 0.969.In Chapter 3, the phase equilibrium of PFPE-NH₄, water and supercritical CO₂ system has been studied. Cloud points of PFPE-NH₄, C₁₂E₉P₃ and C₁₂E₉P₂ in supercritical CO₂ were determined by visual observation at different water content (Wo), temperature and surfactant/CO₂ mass ratio. When water content is below the maximum, cloud point pressures increase with the increase of water content, temperature and mass ratio. The formation of W/C microemulsion was proved indirectly by the comparison of solubility of water in supercritical CO₂ and in surfactant/supercritical CO₂. PFPE-NH₄ can form stable microemulsion in supercritical CO₂, C₁₂E₉P₃ and C₁₂E₉P₂ can not. At 308.15 K and when PFPE-NH₄ mass ratio is 0.025, the maximum solubility of water in W/C microemulsion is between W₀ = 25.0 and 30.0. At 308.15 K and 0.817 g/mL, the maximum correctedsolubility of water (W₀^corr ) in microemulsion is 14.89. Fendller model ofmicroemulsion was selected to calculate the dimension and aggregation number of the W/C microemulsion at different conditions. At 308.15 K and 0.817 g/mL, the maximum water pool radius, hydrodynamic radius and aggregation number are 2.68 nm, 6.98 nm and 180 respectively.In Chapter 4, the micro-polarity of PFPE-NH₄ microemulsion formed in supercritical CO₂ has been studied. Methyl orange was selected as solvatochromic probe molecular, and the absorption spectrum of methyl orange was detected by a fiber optic spectrometer connected with the view cell. It was proved that PFPE-NH₄ can form "dry" micelles without water and stable microemulsions with sufficient water in supercritical CO₂. The micro-polarity of microemulsion at different water content, temperature and pressure has been compared based on the maximum absorbance wavelength. When water content is below the maximum, the micro-polarity of microemulsion increases with the increase of water content, and decreases with the increase of temperature and pressure. At 308.15 K and 0.856 g/mL, the maximum absorbance wavelength is 430.72 nm when excessive water exists. At 308.15 K and 318.15 K, there are "free water" in microemulsion core region when W₀is not little than 20.0 (W₀^corr are 9.33 and 7.42 respectively).In Chapter 5, the solubilization characters of polar substances, such as oxymatrine, methyl orange and riboflavin in PFPE-NH₄ W/C microemulsion have been studied. An apparatus with a sampling part for the measurement of solubilization has beenbuilt. The solubilization amount of oxymatrine was determined by this apparatus at different conditions. When water content is below the maximum, the solubilization amount of oxymatrine increases with the increase of water content and temperature. Pressure has almost no influence on solubilization. At 328.15 K, 0.820 g/mL and W₀= 21.0, the maximum solubilization amount of oxymatrine is 3.938 mg/mL when PFPE-NH4/CO₂ mass ratio is 0.025. The solubilization amount of methyl orange and riboflavin were determined by a fiber optic spectrometer connected with the view cell at different conditions. When water content is below the maximum, the solubilization amount of methyl orange increases with the increase of water content, and decreases with the increase of temperature and pressure. This is caused by the micro-polarity variety of the W/C microemulsion. At 313.15 K and 0.856 g/mL, the maximum solubilization amount of methyl orange is 8.04×10^-3 mg/mL when excessive water exists. At 308.15 K, 0.850 mg/mL and W₀ = 15.0, the solubilization amount of riboflavin is 4.91 ×10^-3 mg/mL which is higher than the solubility of riboflavin in bulk water.