The effect of solvent heterogeneity on the solvation and complexation of alkali cations by 18-crown-6: a simulation study in the 90 : 10 chloroform/methanol mixture

Abstract

We studied by molecular dynamics ''MD'' and potential of mean force ''PMF'' simulations the complexation of M + alkali picrates by 18C6 in the 90 : 10 chloroform/methanol mixture and in its neat solvent components, for comparison. The mixture is found to be microscopically heterogeneous, owing to self-aggregation of methanol molecules into labile nano-domains. It thus displays dual ''amphiphilic'' solvation properties: the free or complexed M + and Pic À ions are mainly solvated by the methanol molecules, while 18C6 prefers chloroform. As a result, although chloroform is in excess in the mixture, the predicted thermodynamics of complexation (absolute binding free energies and selectivities) is more methanol-like than chloroform-like, with the selectivity order K + 4 Cs + 4 Na + . In pure chloroform, it would be: Na + 4 K + 4 Cs + , as in the gas phase. The results point to the analogy between preferential solvation in the studied mixture and at water/oil interfaces. Beyond the prototypical case studied here, understanding the subtle factors that determine the thermodynamics of binding and recognition in solvent mixtures, from traces to co-solvents, is crucial in many processes in chemistry and biology.

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“…The recent MD study of Benay and Wipff investigated solvation and complexation of alkali cations ( picrate salts) by 18-crown-6 in a 90 : 10 chloroform/methanol mixture and found that the ions and the highly polar moieties are surrounded mainly by the polar and protic methanol molecules. 30 Accordingly, they found that the calculated free energy change of complexation in the solvent mixture was very much like in pure methanol as intense interaction energies of polar and ionic groups dominated the energy change of the process. While specific MD studies are beyond the scope of the present work, these findings allow a realistic treatment of the complexation in methanol/dichloromethane and methanol/chloroform 50/50 solvent mixtures even with an implicit solvent model if pure methanol solvent is assumed.…”

“…† Molecular dynamics (MD) studies with explicit solvent molecules and accurately parameterized solvent-solute interaction are needed to account for such a complex environment. 30 Implicit solvent models do not allow the realistic treatment of solvent mixtures, especially in the case of different solvent types (e.g., protic vs. aprotic, highly polar vs. slightly polar/apolar).…”