The extraction of europium(III) nitrate from sodium nitrate with a series o
f tetrahexylmalonamides has been investigated. The tetrahexylmalonamides co
nsidered were N,N,N'N'-tetrahexylmalonamide (THMA), N,N,N'N'-tetrahexyl-2-m
ethylmalonamide (MeTHMA), and N,N,N'N'-tetrahexyl-2,2-dimethylmalonamide (D
iMeTHMA). This series allowed for a systematic determination of the effects
of alkyl substitution of the methylene carbon. Equilibrium modeling of the
extraction data indicates that the malonamide/Eu ratio in the extracted co
mplexes is 3 for all three malonamides investigated. This stoichiometry is
different than that determined for isolated complexes. This can be rational
ized by the formation of complexes with monodentate-bound diamide ligands i
n the extracts. The extraction constant for Eu decreases by seven-fold in g
oing from THMA to MeTHMA and a precipitous drop in the extraction constant
occurs upon substitution of a second methyl group on the methylene carbon (
i.e., for DiMeTHMA).