Separation of metals in the cationic form is the basis of hydrometallurgy. Ion‐specific separation is achieved via selective transfer between liquid phases that have been emulsified in order to be in “close” contact. We show here how the organization of water‐in‐oil (w/o) “reverse” aggregates in the solvent phase controls the free energy of transfer of cations in the form of neutral salts between phases. Indeed, all known efficient ion separation mechanisms rely on complex fluids in the Winsor II regime, i.e. when a concentrated mixed salt solution is in equilibrium with a solvent phase containing self‐assembled aggregates. Here, we point out that, in the general case of water‐poor complex fluids containing extractant molecules, long‐range interactions linked to w/o interface curvature participate in the selectivity of any multivalent ion extraction process. The free energy related to ion transfer between phases, i.e. the extraction free energy, is different from the complexation free energy. This difference is the key to the selectivity of the separation process. We give here general expressions linking complexation free energy and transfer free energy as derived from known adsorption isotherms, taking into account interfacial curvature, considered as a generalized scalar related to the packing near the interface.