Applications of CO2-Switchable Solvents

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Boyd, Alaina
microalgae , spiropyran , switchable hydrophilicity , switchable polarity , Switchable Solvents , botryococcus braunii
Solvents are ubiquitous in chemistry. They can drastically impact the outcome of a process or reaction, and are usually the largest material component, greatly affecting environmental impact. Choosing the best solvent usually means optimizing for the best yields, but as safety standards and environmental regulations increase there is increasing need to optimize for these as well. There have been many advances in green chemistry, including the identification of alternative solvents. This thesis discusses the application of two CO2-switchable solvents. Switchable polarity solvents (SPS) were studied to control the lifetime of a photoswitchable dye. The dyes used are spiropyrans, a class of dyes that change from colourless to coloured upon irradiation by ultraviolet (UV) light, and spontaneously revert to their colourless form. The rate of this reversion is dependent on the solvent polarity, where the coloured form is stabilized by polar solvents. To control this reversion, solvents with switchable polarity have been investigated; amidine- (or guanidine-) alcohol mixture, where the introduction of CO2 causes an ionic liquid to form, raising its polarity. DBU/alcohol has been used in particular in this research. Switchable hydrophilicity solvents (SHS) were investigated for the extraction of bio-oils from microalgae. Microalgae have been identified as a potential source of biofuel. They have several benefits compared to terrestrial crops, including increased oil production. Current methods employ highly volatile solvents to ensure low energy solvent removal, however, this also leads to increased safety demands and risk of exposure. SHS have the unique ability to change their behaviour toward water with the addition of CO2 at atmospheric pressures. In the absence of CO2 the solvent is poorly miscible with water, and with the addition of CO2 becomes miscible with water in all proportions. This property allows for easy separation of extracted material. In the hydrophobic form the solvent is excellent for the extraction of non-polar materials, and can be easily separated from those materials with the addition of carbonated water.
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