Cation extraction by 18-crown-6 to a room-temperature ionic liquid: The effect of solvent humidity investigated by molecular dynamics simulations

Abstract

We report a molecular dynamics study of the solvation of 18-crown-6 (“18C6”) and of its K+, Cs+ and Sr2+ complexes in a room-temperature ionic liquid (IL) based on 1-butyl-3-methyl-imidazolium+, PF6−. “Dry” [BMI][PF6] versus “humid” [BMI][PF6][H2O] models of the IL are compared, demonstrating the importance of solvent humidity on the solvation properties. Upon “dissolution” of a piece of crystal, 18C6 is found to undergo a conformational change from Ci to D3d, mainly due to enhanced interactions with the BMI+ solvent cations and H2O molecules, when present. The complexes were first studied with dissociated counterions. In the dry IL, the complexed K+ and Sr2+ cations are locked at the center of the crown by 1 + 1 (K+), 1 + 2 or 1 + 3 (Sr2+) PF6− anions in facial positions, respectively. The Cs+ cation is perched over the crown, solvated by 3 PF6− anions. In the humid IL, the complexed K+ also binds to 1 + 1 PF6− facial anions only (no water), whereas Sr2+ is asymmetrically coordinated to at least 3 H2O molecules. When co-complexed with Cl− or NO3− counterions, Sr2+ is shielded from the dry IL, but coordinates up to 3 additional H2O molecules in the humid IL, while K+ is not hydrated. The solvation of the “naked” K+, Cs+ and Sr2+ ions also markedly depends on the solvent humidity. K+ is coordinated to 4 PF6− anions in the dry IL and by 2 PF6− plus 3–5 H2O in the humid IL. The most spectacular difference concerns Sr2+, whose first shell is purely anionic (5 PF6−) in the dry IL, but all neutral (8 H2O) in the humid IL. According to an energy component analysis, the 18C6 crown, the cations and their complexes are better solvated by the humid than by the dry IL. Finally, we report simulations of 18C6 and on its Sr ⊂ 18C6(NO3)2 complex at the aqueous interface with the ionic liquid, showing enhanced solvent mixing, compared to the interface with classical organic liquids. The microscopic views obtained by these simulations show the active role of the ionic and aqueous components of the liquid on the solvation of the free crown, the free cations and their complexes.