Interfacial behavior of ionophoric systems: Molecular dynamics studies on 18-crown-6 and its complexes at the water-chloroform interface

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 the solvents. The (18C6)(10) aggregate, initially at the interface, displays after 1 ns a complex equilibrium between molecules adsorbed at the interface, and others which have diffused to the organic phase. The (18C6.K(+)Pic(-))(6) inclusive complexes, initially at the interface, also display after 1 ns, an equilibrium between 18C6 molecules, 18C6.K+ complexes and the Pic(-) counterions adsorbed at the interface, while some decomplexed K+ cations are captured by water. The spontaneous migration of a 18C6.K+ complex to the organic phase is also observed, facilitated by specific water molecules and the Pic(-) counterions. Comparison with simulations on the (18C6.K(+)Pic(-))(1) and (18C6.Sr(2+)2Pic(-))(1) complexes :highlights the effect of concentration and of the cationic charge on the interfacial behavior. We finally describe a "computer extraction experiment", which starts with 18C6 molecules in the organic phase and K(+)Pic(-) ion pairs in water. The results are discussed in relation with experimental data. We emphasize the surfactant-like behavior of free and complexed extractant molecules, the preorganization of ionophores induced by the interface, and the role of counterions. Surface active anions attract the cations at the interface and facilitate their capture by the adsorbed ionophores. Synergistic effects should result from the adsorption of co-extractant molecules or/and of counter ions at the interface.