Ion hydration free energies and water surface potential in water nano drops: The cluster pair approximation and the proton hydration Gibbs free energy in solution

Abstract

We estimate both single ion hydration Gibbs free energies in water droplets, com-2 prising from 50 to 1000 molecules, and water/vacuum surface potentials in pure water 3 droplets comprising up to 10 000 molecules. We consider four ions, namely Li + , NH + 4 , 4 F − and Cl − and we model their hydration process and water/water interactions using 5 polarizable force fields based on an induced point dipole approach. We show both ion 6 hydration Gibbs free energies and water surface potentials to obey linear functions of 7 the droplet radius as soon as droplets comprising a few hundred water molecules. More-8 over we also show that the differences in anion/cation hydration Gibbs free energies 9 in droplets obey a different regime in large droplets than in small clusters comprising no more than six water molecules, in line with earlier results computed from standard additive point charge force fields. Hence point charge and more sophisticated induced point dipole molecular modeling approaches both suggest that methods considering only the thermodynamical properties of small ion/water clusters to estimate the absolute proton hydration Gibbs free energy in solution are questionable. In particular taking into account the data of large ion/water droplets may yield a proton hydration Gibbs free energy in solution value to be shifted by several k B T units compared to small clusters-based approaches.

Absolute ion hydration free energy scale and the surface potential of water via quantum simulation

“…The computed effective surface potential of water, obtained from Eq. 6, is close to −0.4 V. This is consistent with previous estimates that involved cluster simulations employing polarizable classical models (68). The value is also consistent with our previous modified cluster-pair approximation result (37) and several standard models of single-ion hydration free energies that are free of interfacial potential effects (29,69).…”

Transport Models of Ammonium Nitrogen in Wastewater from Rare Earth Smelteries by Reverse Osmosis Membranes

“…The hydrated ion radius of Na + (3.58 Å) was larger than that of NH 4 + (3.31 Å) [21,23], which led to make it more difficult for the sodium ions to penetrate the membrane. This was consistent with the hydration energy of the ions, where the value of hydration energy of Na + (−454 kJ/mol) was higher than that of NH 4 + (−331 kJ/mol) [31,32]; that well explains the higher rejection for NaCl. In other words, the apparent rejection order of the two salt solutions is similar to the order of the hydration energy and hydration ion radius for the monovalent ions.…”

“…+ that was based on the hydration exclusion in the RO membrane was attributed to be the main reason [31][32][33][34]. The hydrated ion radius of Na + (3.58 Å) was larger than that of NH 4 + (3.31 Å) [21,23], which led to make it more difficult for the sodium ions to penetrate the membrane.…”

A thermodynamic view on the microsolvation of ions by rare gas: application to Li+ with argon

“…Specifically, it will be, then, possible to evaluate the role of three-body interactions for the microsolvation with polar molecular solvents. For instance in the case of cation-water solvation, it has been recently shown that accurate modeling of ion hydration thermodynamic properties needs an appropriate description of three-body terms 49 .…”