The 222 cryptand and its cryptates at the water/“oil” interface: molecular dynamics investigations of concentrated solutions

Abstract

Molecular dynamics simulations on concentrated solutions of the bicyclic 222 cryptand and its K⁺ and Ba²⁺ cryptates reveal that, despite their “spherical” shape, these species are highly surface active at the water/“oil” interface, where “oil” is modelled by chloroform. This is found for two forms of 222 uncomplexed, which are, respectively, “hydrophobic” and more “hydrophilic”. For the cryptates, a comparison of picrate (Pic⁻) vs. Cl⁻ as counterions reveals marked counterion effects. The Pic⁻ anions are surface active and display π-stacking interactions near the interface, exchanging from dimers to pentamers. They have no direct contact with 222·K⁺ which forms aggregates on the “oil” side of the interface. Spontaneous diffusion of a few 222·K⁺ complexes from one interface to the other via the “oil” phase is observed during the dynamics. The hydrophilic Cl⁻ counterions are fully dissociated from 222·K⁺ and immersed in water. The 222·Ba²⁺ cryptates also adsorb at the interface, but more on the water side. About half of them make short contacts with the counterions, behaving therefore as a + 1 charged species. The remaining Cl⁻ anions sit in water, while the other Pic⁻ anions stack at the interface. These results are important for the mechanism of assisted cation extraction from water, as well as for electrical properties of the interface. They demonstrate how solvation at the inherently asymmetrical interface induces supramolecular organization of large “spherical” solutes.