Pulido-Ayazo, Juan C.

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2003 - 20102
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    Transport properties of proton-exchange membranes: effect of supercritical-fluid processing and chemical functionality
    (2010) Pulido-Ayazo, Juan C.; Suleiman-Rosado, David; College of Engineering; Torres-Lugo, Madeline; Estévez De Vidts, L. Antonio; Benítez Rodríguez, Jaime; Department of Chemical Engineering; Banerjee, Jayanta
    Nafion® membranes commonly used in direct methanol fuel cells (DMFC), are tipically limited by high methanol permeability (also known as the cross-over limitation). These membranes have phase segregated sulfonated ionic domains in a perfluorinated backbone, which makes processing challenging and limited by phase equilibria considerations. This study used supercritical fluids (SCFs) as a processing alternative, since the gas-like mass transport properties of SCFs allow a better penetration into the membranes and the use of polar co-solvents influenced their morphology, fine-tuning the physical and transport properties in the membrane. Measurements of methanol permeability and proton conductivity were performed to the Nafion® membranes processed with SCFs at 40ºC and 200 bar and the co-solvents as: acetone, tetrahydrofuran (THF), isopropyl alcohol, HPLC-grade water, acetic acid, cyclohexanone. The results obtained for the permeability data were of the order of 10-8 - 10-9 cm2/s, two orders of magnitude lower than unprocessed Nafion. Proton conductivity results obtained using AC impedance electrochemical spectroscopy was between 0.02 and 0.09 S/cm, very similar to the unprocessed Nafion. SCF processing with ethanol as co-solvent reduced the methanol permeability by two orders of magnitude, while the proton conductivity was only reduced by 4%. XRD analysis made to the treated samples exhibited a decreasing pattern in the crystallinity, which affects the transport properties of the membrane. Also, SAXS profiles of the Nafion membranes processed were obtained with the goal of determining changes produced by the SCF processing in the hydrophilic domains of the polymer. With the goal of searching for new alternatives in proton exchange membranes (PEMs) triblock copolymer of poly(styrene-isobutylene-styrene) (SIBS) and poly(styreneisobutylene-styrene) SEBS were studied. These sulfonated tri-block copolymers had lower methanol permeabilities, but also lower proton conductivity, even with blends of these and blends with Nafion membranes. Other alternative studied was the functionalization of the membranes SIBS with metallic cations, which decreased the methanol permeability in the membranes containing the cations Mg2+, Zn2+ and Al3+, while the proton conductivity was maintained more or less constant. The permeation of methanol vapor was investigated and the behavior through the membranes studied followed a pattern of Fick’s Law, while the pattern shown by the permeation in liquid phase was non-Fickian.
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    Solubilities of anti-cancer and anti-AIDS drugs in supercritical carbon dioxide
    (2003) Pulido-Ayazo, Juan C.; Suleiman-Rosado, David; College of Engineering; Romañach, Rodolfo; Velázquez Figueroa, Carlos; Department of Chemical Engineering; Rivera, Luis
    Solubility data for pharmacological drugs in supercritical fluids (SCFs) is of great importance, because there is an environmental and cost effective need for alternative specialty separation methods. This research was focused on the study and determination of solubilities of some anticancer (e.g., Taxol, 5-Fluorouracil) and anti-aids drugs (e.g., Azodicarbonamide, Thymidine and 2-Phenyl-4H-3,1-benzoxazin-4-one) in supercritical carbon dioxide. These measurements were made using a Supercritical Fluid Chromatograph (SFC) coupled to a high pressure UV detector online. The solubility of these drugs were studied as a function of temperature (35.1°C ñ 55.1°C) and pressure (100 ñ 300 bar). This technique was initially validated using phenanthrene and compared with the data of several other investigators. The technique proved to be fast, reliable and reproducible. The order of magnitude of the obtained solubilities was 10-6 to 10-4 mole fraction. The drug with the highest solubility was 2-Phenyl-4H-3,1-benzoxazin4-one and the less soluble was taxol. These results correlated well with the volatility of the drugs (indicated by their melting point). This research also studied the effect of pressure (100 ñ 300 bar) and temperature (35.1°C ñ 55.1°C) on the solubility of the drugs. The effect of pressure on the solubility of the drugs followed the expected trend of increasing solubility with an isothermal increase in the pressure for all temperatures studied. This is explained since as pressure is increased, carbon dioxide density increases, and the intermolecular mean distance of carbon dioxide molecules decreases; thereby, increasing the specific interaction between the solute and solvent molecules. The temperature effect always showed a proportional effect in solubility. This indicated that the temperature effect in solute volatility (proportional effect) was more significant than the temperature effect in solvent density (inversely proportional effect). This study showed that it is possible to determine relatively fast a large number of solubility measurements for the studied systems by retention in SFC.