Analytical Sciences

Interfacial Behavior of Ionophoric Systems: Molecular Dynamics Studies on 18-Crown-6 and

L. TROXLER and G. WIPFF *

Laboratoire MSM, UMR 7551CNRS, Institut de Chimie, 4, rue B. Pascal, 67 000 Strasbourg, France

We report a series of molecular dynamics simulations on 18-crown-6 (18C6)uncomplexed, and on its K + Pic- complexes,at the water-chloroform interface, using an explicit representation of thesolvents. The (18C6)10 aggregate,initially at theinterface, displays after 1 ns a complex equilibrium between moleculesadsorbed at the interface, and others which havediffused to the organic phase. The (18C6 . K +Pic - )6 inclusive complexes,initially at the interface, also display after 1 ns, anequilibrium between 18C6 molecules, 18C6 . K +complexes and the Pic -counterions adsorbed at the interface, while somedecomplexed K + cations arecaptured by water. The spontaneous migration of a 18C6 . K + complex to the organic phase isalso observed, facilitated by specific water molecules and the Pic- counterions. Comparison withsimulations on the(18C6 . K + Pic - )1 and(18C6 . Sr 2+ 2Pic - )1 complexes highlights the effect of concentration and of the cationiccharge on theinterfacial behavior. We finally describe a "computer extractionexperiment", which starts with 18C6 molecules in theorganic phase and K + Pic- ion pairs in water. The resultsare discussed in relation with experimental data. We emphasizethe surfactant-like behavior of free and complexed extractant molecules,the preorganization of ionophores induced by theinterface, and the role of counterions. Surface active anions attract thecations at the interface and facilitate their captureby the adsorbed ionophores. Synergistic effects should result from theadsorption of co-extractant molecules or/and ofcounterions at the interface. (Keywords: Liquid-liquid extraction, interface, ionophores, molecularrecognition, surfactants, hydrometallurgy,adsorption, counterion effect, synergy, solvation, molecular dynamics)

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