Designing Switchable-Hydrophilicity Solvents and Modelling Their Behaviour

Abstract

Switchable-hydrophilicity solvents (SHSs) are solvents that in one state forms a biphasic mixture with water but can be reversibly switched to another state that is miscible with water. In theory, this switch can be triggered in a number of ways, however we focus on dissolved CO2 as the switchable trigger. The most practical use of an SHS is for extraction of a water-immiscible liquid product from a solvent in which it is dissolved (as demonstrated by the extraction of bitumen from the oil sands, bio-oil from algae, etc). This technology provides a more environmentally-friendly means of extraction over current popular processes such as distillation. Since their discovery in 2010, more than 25 nitrogen-based SHSs have been reported in the literature. The SHS behaviour seems to be strongly correlated with hydrophilicity and basicity, not surprisingly. Indeed, amine solvents which demonstrate SHS behaviour are contained within a specific hydrophilicity and basicity range. This range can be predicted based on fundamental acid-dissociation and partitioning equations which, once developed, reveal other tuneable parameters. These parameters are intrinsic (molecular weight and density) as well as extrinsic (CO2 pressure and water:SHS volume ratio). The extrinsic parameters enable the switchable range to expand - enabling less basic and more hydrophobic solvents to act as SHSs. This is of interest since one of the goals of SHS technology is to replace volatile, smog-forming and bio-accumulating solvents with environmentally benign ones. Having a greater range to choose SHSs from increases the chances of finding more environmentally benign solvents. The other goal of SHS technology is to be as effective, if not more, than conventional solvents for extractions. Equipped with a mathematical description of the SHS process, it is a simple matter of optimizing the resulting equations in terms of the extrinsic parameters. Increasing the pressure of CO2 as well as the water:SHS volume ratio increases the amount of SHS that is extracted from the liquid product. This is true for the two-liquid system (composed of water and SHS) as well as the three-liquid system (composed of water, SHS, and a water-immisible organic liquid), though their mathematical descriptions are different.