Solvation of Lanthanide Ions (III) in bifunctional ionic liquid: Molecular Dynamics Study

Abstract

The effects of hydrophobicity of ionic liquid (IL) cation on the solvation structure of Tb3+ and Dy3+ ions were investigated via molecular dynamics simulation, under dry and humid IL condition, respectively. Lanthanide 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]). Solvation structure was analyzed using radial distribution function and stability of solvation shells was examined by calculating interaction energy between ion and ILs. Under dry condition, lanthanide ions had similar solvation structure in both type of ILs. Solvation shell consisted of three IL anions, each of which had bidentate coordination with lanthanide ions. Hydrophobic [N1888][BTMPP] had longer coordination distance and more stable interaction energy than less hydrophobic [N1444][BTMPP]. Under humid condition, solvation structures were altered due to the hydrogen bonding between water and [BTMPP]. Tb3+ ion exhibited Tb[BTMPP]2(H2O)5+ form in [N4444][BTMPP] and Tb[BTMPP]4- form in [N1888][BTMPP], whereas Dy3+ ion showed 8-coordinated complex form (Dy[BTMPP]3(H2O)2) regardless of ILs. Same as dry condition, interaction energy was more stable when ions were solvated in hydrophobic [N1888][BTMPP]. Independent of humidity conditions, IL containing hydrophobic cation displayed more stable solvation structure than less hydrophobic IL. Thus, we concluded hydrophobic ILs were more suitable to extract Tb3+ and Dy3+ ions than hygroscopic ILs.