Characterizing the Effects of a "Switchable Water" Additive on the Aqueous Solubility of Small Molecules

"Switchable water" is an aqueous solution containing a water-soluble amine additive that exhibits CO₂ -switchable properties, such as large changes in ionic strength, by forming an ammonium bicarbonate salt. Switchable water has been used to reversibly "salt-out" organic compounds from water. This study explores the salting out of several compounds in switchable water when CO₂ is present and also explores the solubility of small molecules in switchable water, compared to pure water, when CO₂ is absent. The results show that organic compounds are generally more soluble in switchable water than pure water in the absence of CO₂ , but less soluble in the presence of 1 atm CO₂ . Exceptions include carboxylic acids and phenols which, presumably due to their acidity, are more soluble in switchable water than in pure water, even when CO₂ is applied. Kirkwood-Buff solvation theory was applied to gain insights into the effects of the amine additive on the aqueous solubility of caffeine. Furthermore, the switchable properties of the additives allow for the preparation of switchable aqueous two-phase systems.