Coordination abilities of Good's buffer ionic liquids toward europium(III) ion in aqueous solution

Good's buffer ionic liquids (GB-ILs) are new class of ILs with self-buffering capacity at the physiological pH range for biological research. GB-ILs are formed by the combination of Good's buffers as anions and various organic bases as counter ions. In this work, the complexation of europium(III) ion with tricine and tricine-based GB-ILs, tetramethylammonium tricinate, tetraethylammonium tricinate, tetrabutylammonium tricinate, cholinium tricinate, and 1-ethyl-3-methylimidazolium tricinate in aqueous solution were determined potentiometrically at T = 298.2 K and ionic strength I = 0.1 mol dm^-3 NaNO₃. The protonation constants of the studied ligands (L) and their overall stability constants (lgβ) with Eu(III) were determined. The best model that fit the potentiometric data was consisted of six main species, EuL²⁺, EuL₂⁺, EuL₃., EuH_-1.L⁺, EuH_-2L₂¯, and EuH^-3L₃^3- The lgβEu(tricine), lgβEu(tricine)₂ , and lgβEu(tricine)₃are 5.75, 9.51, and 12.79, respectively. The overall stability constants (lgβ) of tricine-based GB-ILs were found to be greater than those of tricine. The species distribution diagrams of these complexes were calculated and discussed in terms of percent Eu(III) and pH. We present a density functional theory (DFT) study to understand tricine chelating to Eu(III).