Phase behavior of CO 2–perfluoropolyether oil mixtures and CO 2–perfluoropolyether chelating agent mixtures

Abstract

Perfluoropolyethers and functionalized perfluoropolyethers have been shown to exhibit high solubility in liquid and supercritical carbon dioxide. A perfluoropolyether oil, Krytox 16350, molecular weight (MW) of 11 350, was fractionated into 14 monodisperse fractions using carbon dioxide at 373 K at pressures between 20.69 MPa and 34.47 MPa. The molecular weights of these fractions, determined using 19F NMR, ranged between 8160 and 29 252. The phase behavior of mixtures of carbon dioxide with each fraction was determined at 298 K. The two-phase boundary shifted to higher pressures as the MW of the perfluoropolyether oil fraction increased. The high solubility of these oils in carbon dioxide led to the development of a series of carbon-dioxide-soluble chelating agents. These amphiphilic compounds were composed of a CO 2-philic perfluoropolyether tail to enhance the solubility of the chelating agent in CO 2 and a hydrophilic chelating head group for metal ion extraction. CO 2-chelating agent phase behavior at 298 K was determined for chelating agents with bis(picolyl amine) or dithiol head groups and fluoroether tails with molecular weights of 2500, 4000 and 7500. As the MW of the tail increased, the overall MW of the chelating agent increased, tending to decrease solubility in CO 2. The increasing MW of the fluoroether tail also increased the chelating agent's CO 2-philicity, thereby enhancing the solubility of the compound in CO 2. These competing effects resulted in an optimum MW of the tail, 4000, for monofunctional chelating agents at 298 K, that minimized the pressure requirement for complete miscibility of the CO 2 and chelating agent.