Molecular dynamics simulations of Na+-Cl− ion-pair in supercritical methanol

• Solvation of Na+-Cl− ion-pair in supercritical methanol is studied by MD simulations.
• Stability of CIP is found to increase with temperature at a particular pressure.
• PMFs are dominated by CIPs and the SAIPs are replaced by a shoulder region.
• Methanol molecules in supercritical conditions retain significant amount of hydrogen bonding.
• Ion-pair formation in supercritical methanol is endothermic and driven by entropy.

Constrained molecular dynamics (MD) computer simulations are presented for the solvation of Na+-Cl− ion-pair in supercritical methanol in an isothermal–isobaric ensemble (NPT) to understand the changes in the solvation structure of the ion pair as temperatures and pressures change from ambient to supercritical conditions. Potentials of mean force (PMF) of the Na+-Cl− ion-pair are calculated as a function of temperature and pressure. Contact ion-pair (CIP) is found to be the most stable and dominant in supercritical conditions. Over the temperature and pressure ranges investigated, we observe that methanol molecules retain significant hydrogen bonding. From calculations of energies and entropies through temperature derivatives of PMFs, we have found that Na+-Cl− ion-pair association in supercritical methanol is endothermic and is driven by entropy.

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