Improvements in the Performance and Understanding of Switchable-Hydrophilicity Solvents
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Switchable-hydrophilicity solvents (SHSs) are amine and amidine solvents that can be reversibly switched between two forms: one that forms a biphasic mixture with water and another that forms a monophasic mixture with water. The addition or removal of CO2 from SHS-based systems acts as the trigger to switch between these two forms. SHSs have attracted attention as alternative solvents for a variety of applications because their switchable behaviour allows them to be used in energy- or material- efficient ways. One such application is distillation-free solvent-solute separations. The present research builds an understanding of the properties of amines that determine whether they are SHSs or not. It also applies this understanding to the development of new SHSs with fewer health and environmental hazards, such as decreased acute toxicity and decreased volatility, as well as improved performance with respect to solvent-solute separations. The relationship between the pKaH and log Kow of an amine and the ability of the amine to act as a SHS is established. A mathematical model is described that can be used to predict whether an amine acts as a SHS based on these two properties. Furthermore, the influence of CO2 partial pressure and the water/amine volume ratio on these pKaH and log Kow requirements are investigated. The model acts as a guide when identifying new SHSs. Many new SHSs are identified with a variety of beneficial properties. SHSs that have oxygen-containing functional groups have decreased environmental, health, and safety risks than trialkylamine SHSs. Secondary amine SHSs are shown to switch to their hydrophilic state more rapidly than tertiary amine SHSs. Diamine SHSs can be more completely separated from solutes than monoamines, resulting in a more pure isolated solute after a SHS-based separation. A virtual screening approach to the design of SHSs is presented that can be used to rapidly screen thousands of compounds to identify those that are estimated to be SHSs with few environmental, health, and safety risks. The identification of new SHSs increases the options when selecting a solvent for a SHS-based process and provides data to improve the understanding of SHSs.