Alternative (Potentially Green) Separations Media: Aqueous Biphasic and Related Systems Extending the Frontier Final Report For Period September 1, 2002 January 31, 2007 (2007)

Through the current DoE-BES funding, we have extended our fundamental understanding of the critical phase separation of aqueous polymer solutions at the molecular level, and have developed a similar understanding of their application as novel solvent systems. Our principal aims included mode of delivery of the aqueous biphasic system (ABS) solvent system and the application of this system to problems of reactive extraction. In the former case we have developed novel solid phase analogues, in the form of cross-linked polyethylene glycol hydrogels, and in the latter case we have examined the role that ABS might play in reaction engineering, with a view to greener, simpler, and safer processes. We have also developed a new salt/salt ABS and have extended our understanding of this system as well. The major outcomes are as follows: (1) Through the use of variable temperature phase diagrams, coupled with differential scanning calorimetry (DSC) measurements of the phases, a better understanding of the thermodynamics of phase formation was obtained. Evidence to the existence and role of an upper critical solution temperature (UCST) or lower critical solution temperature (LCST) (or both) in these systems was gained. With variable temperature solute partitioning, thermodynamic parameters were calculated, and inter-system comparisons were made. Through the use of Abraham's linear solvation energy regression (LSER) the solvent-solute properties of liquid/liquid ABS were examined. We have shown that ABS are indeed very tunable and LSERs have been used as a tool to compare these systems to traditional organic/water and other liquid/liquid systems. (2) We have successfully shown the development of novel reaction media for chemical synthesis and reaction; Aqueous Biphasic Reactive Extraction (ABRE). As a proof of concept, we have shown the synthesis of adipic acid from cyclohexene in an ABS, which represents an important development in the exploitation of this technology. Previous oxidations of this type have relied on the use of phase transfer catalysts, which are expensive to produce and difficult to recover. In this reaction the polyethylene glycol (PEG) phase seems to function simultaneously as the phase transfer catalyst, the reaction solvent, and to provide the reaction driving force. (3) PEG hydrogels may be used as probes for their macroscopic analogues by which the molecular events underlying the phase behavior of polymer-salt systems can be investigated. The properties of covalently cross-linked PEG hydrogels have been studied. It was demonstrated that these hydrogels could be thought of as analogous to polymer/salt ABS without phase separation. The salts examined cause collapse of the hydrogel, and there is a physical limit to the degree of collapse that can be achieved. In addition, salts bringing about significant collapse are only prevented from reaching this limit by the limits of their own solubility. This lead to our discovery that PEG will phase separate with KSCN at high enough concentration of polymer and salt. We have also successfully shown the development of an IL-PEG hydrogel as well as a Si-modified PEG hydrogel. We have also demonstrated for the first time that this cross-linked PEG matrix has been used to gel non-aqueous solvents. (4) The use of hydrophilic ionic liquids (ILs) in separation schemes has been accomplished via a 'salting out' technique using inorganic, kosmotropic salts that is applicable to many classes of these materials. We have begun to obtain a deeper knowledge about the role that each component plays in the process, including that of the ionic liquid cation and anion, the kosmotropic salt cation and anions, as well as the distribution of water in the system. This is allowing us to design separation systems with desired properties. In addition, temperature studies on these aqueous biphasic systems are revealing thermodynamic data for the first time, so that we can quantitate the importance of entropic and enthalpic contributions to the biphase-forming process.