Brønsted acidity of bio-protic ionic liquids: the acidic scale of [AA]X amino acid ionic liquids

Abstract

Amino acid ionic liquids (AAILs) [AA]X based on amino acid cations are a kind of typical “bio-base” protic ionic liquids (PILs), which are supposed to be acidic ionic liquids. The Brønsted acidity of [AA]X PILs at room temperature was systematically studied for the first time. Acid dissociation constants (pK_a) of [AA]X were determined by the potentiometric titration method. The first acid dissociation constants (pK_a1) are from 1.98 to 2.42. The actual pH values of [AA]X (0.010 mol L⁻¹) obtained from a pH meter are from 2.26 to 2.44 which are slightly higher than the calculated pH values according to the above experimental pK_a1. The Hammett method performed on UV/Vis spectra with p-nitroaniline as the indicator was used to determine the acidic strength of [AA]X. Their H₀ values (0.010 mol L⁻¹) are in the range from 2.10 to 2.44. Various frameworks of amino acid cations and five anions (including nitrate (NO₃⁻), chloride (Cl⁻), perchlorate (ClO₄⁻), trifluoromethanesulfonate (OTf⁻) and trifluoroacetate (TfA⁻) anions) were used to investigate the cationic and anionic effect on the acidity of AAILs. The Brønsted acidity of AAILs depends on the cationic structure, the type of anion and the concentration of [AA]X. In addition, the theoretical pK_a1 values were studied by using the cluster-continuum model using the density functional theory (DFT) method. The experimental and theoretical results showed that [AA]X PILs have a stronger Brønsted acidity than the common PILs prepared by one-pot syntheses.