Ammonium recognition by 18-crown-6 in different solutions and at an aqueous interface: a simulation study

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Supramolecular Chemistry

Background:

• Crown ethers like 18-crown-6 (18C6) are known for their ability to selectively bind cations.
• Understanding cation complexation in different solvent environments is crucial for applications in separation science and drug delivery.

Purpose of the Study:

• To investigate the complexation of alkylammonium cations with 18-crown-6 (18C6) across various solvents and interfaces.
• To determine the influence of solvent properties and cation-solvent interactions on the thermodynamics of complexation.

Main Methods:

• Molecular dynamics (MD) simulations were employed to study cation-18C6 complexation.
• Potential of Mean Force (PMF) calculations were used to determine free energies of association (ΔGass).
• Simulations were conducted in methanol, chloroform, a chloroform/methanol mixture, water, and at the chloroform/water interface.

Main Results:

• Potassium cations (K+) showed a preference over propylammonium cations (PrNH3+) for 18C6 complexation in most environments.
• Complexation free energy (ΔGass) was highly solvent-dependent, decreasing in the order: chloroform > mixture > methanol > water.
• The chloroform/water interface facilitated adsorption of 18C6 and cations, with stronger complexation of tert-butylammonium (tBuNH3+) at the interface compared to pure water.

Conclusions:

• Solvation energy and counterion pairing significantly impact cation-18C6 complexation thermodynamics.
• The chloroform/methanol mixture exhibits unique solvation properties due to microscopic inhomogeneity.
• 18C6 complexation at the chloroform/water interface enhances cation transfer into the organic phase, highlighting interfacial complexation's importance.