“…The possible direct interaction between the synergist and coordinated chelating molecule used for solvent extraction of metals is an important problem. 4,7,[89][90][91][92][93][94][95] The formation of an association product may occur effectively removing the free reagents from the extraction organic phase system. The interaction between acidic extractants, which belong to the pyrazolone family and neutral calixarene molecules (S1, S5) were studied at different molar ratios (respectively 1 : 2, 1 : 1, and 2 : 1) in chloroform by NMR experiments in few papers.…”