Solvation of M3+ lanthanide cations in room-temperature ionic liquids. A molecular dynamics investigation

Abstract

We report a molecular dynamics study of the solvation of M³⁺ lanthanide cations (La³⁺, Eu³⁺ and Yb³⁺) in two room-temperature ionic liquids: [BMI][PF₆] based on 1-butyl-3-methyl-imidazolium⁺,PF₆⁻ and [EMI][TCA] based on 1-ethyl-3-methyl-imidazolium⁺,AlCl₄⁻. They reveal specific solvation as a function of the cation size and on the solvent. In [BMI][PF₆] solution, the three studied M³⁺ cations are surrounded by six PF₆⁻ anions, while in [EMI][TCA] solution, they are surrounded by eight AlCl₄⁻ anions. The precise binding mode of the AlCl₄⁻ or PF₆⁻ solvent anions (denticity, number of coordinated halides and dynamics) depends on the M³⁺ cation size. The first shell PF₆⁻ anions rotate markedly during the dynamics while AlCl₄⁻ do not. In both liquids, this first ionic shell is surrounded by about 11–13 imidazolium cations. The solvation of the neutralizing NO₃⁻ counterions is similar in both liquids, and involves mainly imidazolium⁺ cations in the first shell (≈5.0 in [BMI][PF₆] and 4.3 in [EMI][TCA]). When simulated in the gas phase, the first anionic shells are similar, but more rigid than in the solutions. Finally, free energy calculations are performed to compare the Eu³⁺/La³⁺ and Yb³⁺/Eu³⁺ solvation. In both ionic liquids, the smaller cations are better solvated, but the differences from one cation to the other are weaker than in water.