Development of temperature and pressure variable aqueous biphasic extraction system with superheated water

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Aqueous two-phase systems (ATPS) are formed by mixing water, certain inorganic salts and water-soluble polymers and can be useful for the separation of various compounds. Polyethylene glycol (PEG) is nontoxic, nonflammable and nonvolatile so that the PEG-based ATPS can be regarded as environmentally friendly.We have developed an ATPS extraction device which enables one to carry out extraction at elevated temperature in order to enhance the separation selectivity and expand the applicability of ATPS. The distribution behavior of n-alcohols, ketones and nitroalkanes in ATPS formed from PEG and Na_2SO_4 was investigated from the viewpoints of interaction of the solutes with the polymer solution phase and the salting-out effect. The results obtained revealed that the separation selectivities exerted by the PEG-based ATPS and the PEG gel system are approximately the same. Differential scanning calorimetry studies on aqueous PEG solutions suggested that PEG polymer forms a hydration structure of which the composition is 50 % (w/w) PEG or the hydration number per ethylene oxide is 2.4 and the separation selectivity of the PEG-water systems can be ascribed to partition of solute compounds into the hydrated PEG polymer structure. Furthermore, it has been demonstrated that the separation selectivity of the ATPS depends strongly on temperature.Furthermore, in order to clarify the contribution of hydrophobic moieties of polymers on the separation selectivity of ATPS, the retention behavior of inorganic anions in reversed-phase liquid chromatography was investigated using the columns packed with alkyl bonded silica particles. From the results obtained, it has been concluded that the solvated water phase is formed on the surface of alkyl-bonded silica and exhibits an affinity to solute compounds different from that of the bulk liquid water.