The effect of a solvent modifier in the cesium extraction by a calix[4]arene: a molecular dynamics study of the oil phase and the oil-water interface.

We report a molecular dynamics (MD) study of the effect of a fluorinated lipophilic alcohol (referred to as "cs3" by Delmau et al, Solvent Extr. Ion Exch., 2005, 23, 23) used as a phase modifier in the solvent extraction of Cs(+) NO(3)(-) by a calix[4]arene. It is shown that adding cs3 to a chloroform phase improves the solvation of all partners of the extraction system, i.e. the free calixarene ligand L, its LCs(+) complex and its counterion. This effect is most pronounced for the NO(3)(-) anion that is H-bonded to the -OH and terminal -CF(2)H protons of cs3. On the average, the dissociated nitrate interacts with 2 to 4 cs3 molecules, whereas the associated nitrate (LCs(+)NO(3)(-) complex) interacts with one cs3 dimer. The remaining modifier molecules are mainly dissolved in the solution as dimers or trimers and, to a lesser extent, as monomers and tetramers. Insights into the question of ion pairing in the organic phase are obtained via free energy perturbation (FEP) calculations, showing that the LCs(+)NO(3)(-) paired complex is more stable than its analogue with the dissociated anion. Furthermore, the nitrate dissociation energy is ca. twice as small in the cs3-modified solution than in a pure chloroform phase. We also simulated chloroform/cs3/water ternary systems, showing that the modifiers are surface active and stabilize the formation of water nanodroplets, while other modifier molecules drag some water to the organic phase. Calixarene complexes also adsorb at the aqueous interface in the presence of modifiers, confirming the importance of interfacial phenomena in the assisted cation extraction process. The microscopic insights obtained by the simulations are consistent with experimental results and allow us to better understand why the extraction is enhanced after addition of modifiers to the organic phase.