Potassium extraction by a cryptand to supercritical CO2. The role of counterions investigated by MD simulations at the water/SC-CO2 interface

摘要

We present a molecular dynamics study of the K+ extraction by the 222 cryptand, comparing different counterions X- at the supercritical-CO2/water interface: X- = picrate "Pic(-)" vs perfluorooctanoate "PFO-" vs chlorinated cobalt dicarbollides CCD- vs; Cl-. The distribution of the free K+X- salts is also investigated showing different distributions, depending on the hydrophobicity of X-. The Pic(-) and CCD- anions markedly concentrate at the interface, as do the amphiphilic PFO- anions, thus attracting K+ cations near the interface. Similarly, in the M+ CCD- series with M+ alkali cations, the less hydrophilic Cs+ cations sit closer to the interface than do Na+ or K+. The simulations confirm the strong affinity of the 222 cryptand and of the K+ subset of 222 cryptates for the interface, with marked counterion effects. The most remarkable result is the partitioning of the cryptates and CCD- anions to the CO2 phase, i.e. on the pathway to full extraction. With the other studied anions, no "extraction" proceeds. When compared to the K+ or Sr2+ extraction by 18-crown-6, the results show that the effectiveness of X- depends on its relationship with the complex: CCD- forms separated ion pairs with K+ subset of 222, whereas PFO- forms intimate ion pairs with K+ subset of 18C6 and Sr2+ subset of 18C6. Addition of hydrophobic salts like CCD- Cs+ and cryptands in excess enhances the K+ extraction. Finally, the simulations on K+ subset of 222, CCD- solutions at increasing CO2/water ratio shows the evolution from a "flat" interface to microemulsions, leading to the extraction of cryptates.