Transfer Free Energy of Micro-hydrated Ion Clusters from Water into Acetonitrile Solvent

Abstract The transfer free energies of micro-hydrated alkali ions ( Li + , Na + , K + , and Cs + ) and halide ions ( F - , Cl - , Br - , and I - ) clusters from bulk water to bulk acetonitrile are investigated within the framework of reaction density functional theory (RxDFT), in which the formation of micro-hydrated ion clusters containing one to four water molecules is treated as associated reaction. By analyzing the free energy of hydration and solvation, we show that the micro-hydrated ion clusters, [M(H2O)n]+, exist stably in both water and acetonitrile and a larger percentage of the total electrostatic interactions of ions is contributed by their first solvation shell water molecules. For the transfer free energy of bare ions, the calculated results agree well with the values of experimental observation. The transfer free energy significantly decreases as the size of bare ions and the number of water molecules in the cluster increase. This dependence is stronger for small ions than for large ions. Moreover, the lowest transfer free energy is correspond to the different coordinated number of water molecule (Nw). For Li + , Na + , F - , Cl - , Br - , and I - ion, the lowest transfer free energy are correspond to Nw = 4, while for K + and Cs + ion, Nw with the lowest transfer free energy are 3 and 2, respectively. Our findings provide not only the framework to predict transfer free energy but also theoretical guidance for solvent extraction, ion-selective electrodes, biological ion channels, and phase transfer catalysis.