Mechanism and thermodynamics of ion selectivity in aqueous solutions of 18-crown-6 ether: A molecular dynamics study

Conference · Fri Dec 01 04:00:00 EST 1995

OSTI ID:126852

Dang, L X ^[1]

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1. Pacific Northwest Lab., Richland, WA (United States)

Extensive classical molecular dynamics simulations are performed to examine the mechanism and thermodynamics for ion selectivity in aqueous solutions of 18-crown-6 ether. The free energy profiles or potential of mean force (pmf) for M+: 18-crown-6 (M+ = K+, Na+, Rb+, Cs+) complexation are computed via the free energy perturbation method. Counterion effects are considered and all long-range interactions are computed via the Ewald summation method. This is the first study of ion selectivity in aqueous solutions using the prof approach. The resultant pmfs indicate that free-energy minima for the K+ and Na+ ions are located at the crown ether center-of-mass. Potassium is selected over sodium because of the free energy penalty associated with displacing water molecules from sodium as it approaches the crown ether. This leads to a substantial increase in the activation free energy and a decrease in the binding free energy for Na+: 18-crown-6 complexation as compared to K+: 18-crown-6. The selection of potassium over rubidium and cesium is due to the cation-size relative to the crown-ether cavity. The calculated binding free energies follow the sequence K+ > Rb+ > Cs+ > Na+, which is in accordance with experimental measurements.

• OSTI ID: 126852
• Report Number(s): CONF-950402--
• Country of Publication: United States
• Language: English

Related Subjects

40 CHEMISTRY
AQUEOUS SOLUTIONS
BINDING ENERGY
CESIUM
COMPLEXES
CROWN ETHERS
FREE ENERGY
POTASSIUM
RUBIDIUM
SODIUM
THERMODYNAMICS