Synergism:

In synergism, there are two extractants which work together to increase the extraction by a huge factor. The extracting power of the mixture exceeds the sum of the extracting power of the individual extractants. In synergism, one of the two extractants is a ligand or an organic acid (HA) which neutralizes the charge on the metal ion and the other, a solvating molecule (S) which either replaces the water molecule from the coordination sphere or forms an adduct which is less hydrophilic.