Solvation of Lanthanide Ions (III) in Bifunctional Ionic Liquids: Molecular Dynamics Study

We have studied the solvation behavior of Tb3+ and Dy3+ lanthanide (Ln) ions in dry ionic liquids (ILs) and water saturated ILs via molecular dynamics simulation. The Tb3+ and Dy3+ ions were introduced into two types of ILs, which consist of an anion of bis(2,4,4-trimethylpentyl) phosphinic acid ([BTMPP]) and a cation of tetrabutylammonium ([N4444]) or trioctylmethylammonium ([N1888]). The  effects of hydrophobicity of cation, and humidity of ILs on the solvation structure of Tb3+ and Dy3+ ions were investigated. Tb3+ and Dy3+ ions were coordinated with three [BTMPP] anions in bidentate forms in dry [N4444][BTMPP] and [N1888][BTMPP] ILs, and both ions showed more stable interaction energies and longer coordination distance in [N1888][BTMPP] ILs. Dy3+ ion was more stably coordinated in both dry ILs, than Tb3+ ion. Water saturated ILs showed different solvation structures due to the hydrogen bonding between water and [BTMPP]. Dy3+ ion showed 8-coordinated complex form (Dy[BTMPP]3(H2O)2) in both [N4444][BTMPP] and [N1888][BTMPP] ILs, whereas Tb3+ ion was showed Tb+[BTMPP]2(H2O)5 in [N4444][BTMPP] and Tb[BTMPP]4- in [N1888][BTMPP]. Tb3+ had more stable interaction energies than Dy3+ in [N4444][BTMPP] ILs, while Dy3+ and Tb3+ ions showed similar interaction energies in [N1888][BTMPP] ILs. From these results, it could be postulated that hydrophobicity of cation of ILs contributed to forming stable solvation structures of Tb3+ and Dy3+ ions in dry and water-saturated ILs, and water saturated [N4444][BTMPP] ILs were suitable for Tb3+ and Dy3+ ion separations