Ammonium Recognition by 18-Crown-6 in Different Solutions and at an Aqueous Interface: A Simulation Study

The complexation of alkylammonium RNH3(+) cations by 18-crown-6 (18C6) is studied by molecular dynamics (MD) and potential of mean force (PMF) simulations in different solvents (methanol, chloroform, 90:10 chloroform/methanol mixture, water) and at the chloroform/water interface. The free energies of association ΔGass, obtained with different charge models of 18C6, are compared for PrNH3(+) and K(+) cations yielding, with suitable electrostatic models, a preference for K(+) in the different monophasic solutions, as well as in the gas phase. Furthermore, for a given cation, ΔGass is markedly solvent dependent and decreases in magnitude in the order chloroform ≫ mixture > methanol > water, that is, with the (de)solvation energy of the cation and with the extent of pairing with the counterion (here, picrate, Pic(-)). Despite their macroscopic intermiscibilities at all proportions, chloroform and methanol are found to form, at the microscopic level, an inhomogeneous liquid that displays dual solvation properties toward its solutes. As a result, in the monophasic 90:10 mixture that contains mainly chloroform, the ΔGass energies for PrNH3(+) or K(+) complexation are closer to those in methanol than to those in chloroform. On the other hand, chloroform and water form a biphasic mixture and delineate an interface onto which 18C6 and the tBuNH3(+) and Pic(-) ions, as well as their complex, are found to adsorb, a feature also supported by the different free energy profiles for interface crossing. Interestingly, the complexation energy of tBuNH3(+) is found to be stronger at the interface than in pure water, demonstrating the crucial role of complexation by 18C6 at the interface to promote the cation transfer to the organic phase.