Highly effective extraction of hydroxycinnamic acids by hydrogen-bonding-functionalized ionic liquids

In this work, imidazolium-based ionic liquids (ILs) were used to extract hydroxycinnamic acids from water. Experimental results indicated that the ILs with CF3SO3− anion had higher extraction ability for hydroxycinnamic acids because of the stronger hydrogen-bonding strength of CF3SO3−. The combination of cation and anion, both of which have strong hydrogen-bonding ability, generates a hydrogen-bonding-functionalized IL, 1-butyl-3-(11-hydroxyundecyl)imidazolium trifluoromethanesulfonate ([C4C11OHim]CF3SO3) which has strongest hydrogen-bonding ability and strongest extraction ability for hydroxycinnamic acids. Thermodynamic analysis suggested that hydrogen-bonding interaction was the major driving force; meanwhile, the influence of hydrophobic interaction and steric hindrance between ILs and hydroxycinnamic acids should be considered when interpreting the extraction behavior of ILs. The extraction ability of the ILs was strongly influenced by the aqueous pH; it decreased with increasing pH in the range of pH > pKa (dissociation constant). The stronger extraction ability of ILs at lower pH values could be interpreted by the strong hydrogen-bonding interaction between ILs and the undissociated hydroxyl and carboxyl groups of the hydroxycinnamic acids. Preliminary results suggested that [C4C11OHim]CF3SO3 may be applied to the process of the recovery of hydroxycinnamic acids from aqueous media.