We report a series of molecular dynamics simulations on the demixing of “homogeneous” binary water−chloroform mixtures containing species involved in the assisted ion extraction process. We consider an ionophore L (L = 1,3-alternate calix4arene-crown6), uncomplexed salts of Cs+ and the LCs+ and LNa+ cation complexes with a lipophilic (Pic-) and a hydrophilic (Cl-) counterion, respectively, as being solutes. In all cases, the liquids separate rapidly, leading to two solvent slabs separated by a well-defined interface. However, the final state is very different, depending on the hydrophilic/hydrophobic balance of the solutes:  the Cs+ and NO3 - ions of the CsNO3 salt are completely immersed in the aqueous phase, whereas Pic- anions display a strong adsorption at the interface. The LCs+ complex and the free ligand L, although more soluble in the organic phase than in water, also display a surfactant like behavior. Similar conclusions are obtained when L, LCs+, Cs+ Pic-, and Cs+ NO3 - ions are simultaneously present in the solution. On the basis of free energy perturbation calculations on LM+ complexes, we calculate a marked Cs+/Na+ recognition by L at the interface. These results have important implications concerning the mechanism of ionophore assisted liquid−liquid ion extraction and recognition processes at the interface.

According to the TATB (tetraphenylarsonium tetraphenylborate) assumption, large isosterical ions of opposite charge have identical free energies of solvation in any solvent. In this context, we present a molecular dynamics study of the solvation of tetrahedral (AsPh 4 + vs BPh 4 -) and large spherical (S + vs S -) ions in water using the recently developed TIP5P model. The results markedly differ from those obtained in TIP3P water and are in better agreement with the TATB hypothesis. According to free energy perturbation calculations, Sis better hydrated than S + , but the difference, ∆G +-, in hydration energies is much weaker in TIP5P (3.2 kcal/mol) than in TIP3P water (27.3 kcal/mol) which leads to an artifactually positive electrostatic potential at the center of the neutral S 0 species. BPh 4is better hydrated than AsPh 4 + and, contrary to the TATB assumption, ∆G +markedly depends on the details of charge distributions. The set8 charges equally diluted on all atoms lead to ∆G +of 4.3 kcal/mol only, much less than the value obtained with the set1 ESP charges (25 kcal/ mol). These values are much smaller than those obtained in TIP3P water, but still indicate some preference for the anion hydration. The differences are discussed from hydration patterns, electrostatic potentials and solute-solvent interactions. Simulations of AsPh 4 + and BPh 4at the (TIP5P)water-chloroform interface confirm the high surface activity of both ions, despite their "symmetrical structure". These results are important for our understanding of the influence of water models on calculated hydration and association of hydrophobic species in pure and mixed liquid environments.