Understanding the Interaction of Boric Acid and CO₂ with Ionic Liquids in Aqueous Medium by Multinuclear NMR Spectroscopy

Boric acid is known to enhance the kinetics of CO₂ absorption by some active aqueous solutions. However, the mechanism of interaction of boric acid with CO₂ in the presence of active molecules is not yet fully understood. In this work, the interaction and dynamics of ions in aqueous solutions of functionalized choline-based ionic liquids [N_1,1,5,2OH]­[Threo] and [N_1,1,5,2OH]­[Tau] in the presence of boric acid and CO₂ was thoroughly investigated using a multinuclear NMR approach: ¹³C and ¹¹B NMR spectroscopy, ¹¹B NMR transverse relaxation, and ¹H and ¹¹B NMR diffusometry. ¹³C and ¹¹B NMR spectroscopy revealed the formation of borate-based complexes as a result of a reaction between boric acid and the anions of the ILs at ionic liquid/boric acid molar ratios larger than ca. 0.15. The formation of these complexes and their dynamics were further investigated using ¹¹B relaxation and ¹H and ¹¹B pulse-field-gradient (PFG) NMR. Plausible reaction mechanisms of boric acid with the anions of the ILs, formation of the borate complexes, and dissociation of these complexes facilitated by CO₂ molecules are suggested.