The complexation of lanthanide ions from acidic solutions was studied with cross-linked polystyrene modified with amide-type ligands: N,N,N',N' -tetramethylmalonamide (TMMA), monoamidated malonate and urea.

The TMMA resin showed preference towards Tb, Dy and Eu from highly acidic solutions. Ionic recognition is achieved through a mechanism in which two opposing processes—electrostatic attraction of M(H₂O) _xCl₄^- or M(H₂O)_x(NO ₃)₄^- by the protonated ligand and (partial) loss of the waters of hydration—dominate at different points along the lanthanide series. The proposed complexation mechanism consists of protonation of one of the carbonyls, which is stabilized by hydrogen bonding to the neighboring carbonyl oxygen, formation of iminium site and ion exchange by exchanging the chloride ion with the lanthanide chloro or nitro complex.

The importance of the substituents at amide nitrogen was probed with immobilized malonate ligands monoamidated with diethylenetriamine (DETA-MAm) and immobilized diethylenetriamine ligands with one amine nitrogen converted to urea (Urea-3). The comparison of the lanthanides complexation by the TMMA, DETA-MAm and Urea-3, was carried out under conditions in which each resin has its best performance: 6 M HCl for TMMA and 8 M HCl for DETA-MAm and Urea-3. The protonated TMMA, with two electron-donating methyl groups at the iminium nitrogen to attenuate the (+) charge (=NR₂(+)), is the weakest ligand with ionic recognition properties towards the lanthanides in the middle of the series. The reduced ligand strength makes it more responsive towards changes between the electrostatic attraction and the enthalpy of dehydration. The protonated Urea-3 showed much higher lanthanide affinities than the TMMA and DETA-MAm, and its affinity trend paralleled the trend of lanthanides ionization potentials due to the absence of methyl group to attenuate the (+) charge (=NH₂(+)). The DETA-Mam is somewhere in between the TMMA and Urea-3 because its iminium has only one −CH₂− moiety attenuating the (+) charge (=NHR(+)) and it is also a two-site interaction with the ammonium group probably contributing to the electrostatic stabilization of the chlorocomplex.