A guide to the selection of switchable functional groups for CO2-switchable compounds

Abstract

Many CO₂-responsive species, including many of the CO₂-switchable surfactants, solvents, solutes, gels, colloids, and surfaces, rely on the ability of CO₂ to lower the pH of water. Uncharged basic groups on the CO₂-responsive species are therefore converted from a neutral state to a protonated cationic state (a bicarbonate salt), which causes dramatic and useful changes to the properties of the species. However, this switching process only works correctly if a basic group of appropriate basicity has been selected. This article presents a comprehensive guide to the selection of basic groups for CO₂-switchable species for use in water. The appropriate basicity, as measured by the pK_aH (the pK_a of the protonated compound), is a function of the concentration of the switchable species, the temperature, the pressure of CO₂, the presence or absence of an organic liquid phase, and the solubility of the neutral form of the compound.