Quantifying the hydration structure of sodium and potassium ions: taking additional steps on Jacob’s Ladder

Abstract

<div><div><div><p>The ability to reproduce the experimental structure of water around the sodium and potassium ions is a key test of the quality of interaction potentials due to the central importance of these ions in a wide range of important phenomena. Here, we simulate the Na+ and K+ ions in bulk water using three density functional theory functionals: 1) The generalized gradient approximation (GGA) based dispersion corrected revised Perdew, Burke, and Ernzerhof functional (revPBE-D3) 2) The recently developed strongly constrained and appropriately normed (SCAN) functional 3) The random phase approximation (RPA) functional for potassium. We compare with experimental X-ray diffraction (XRD) and X-ray absorption fine structure (EXAFS) measurements to demonstrate that SCAN accurately reproduces key structural details of the hydra- tion structure around the sodium and potassium cations, whereas revPBE-D3 fails to do so. However, we show that SCAN provides a worse description of pure water in comparison with revPBE-D3. RPA also shows an improvement for K+, but slow convergence prevents rigorous comparison. Finally, we analyse cluster energetics to show SCAN and RPA have smaller fluctuations of the mean error of ion-water cluster binding energies compared with revPBE-D3.</p></div></div></div>

What is the fundamental ion-specific series for anions and cations? Ion specificity in standard partial molar volumes of electrolytes and electrostriction in water and non-aqueous solvents

“…Progress is still being made on theoretical approaches for explaining the true origins of specific-ion effects, 37–41 though unanimity is still to be achieved. 28,42–46 The variations observed in the ordering of specific-ion effects between different experiments and even the same experiment under different conditions hinders theorists' attempts at achieving a physical description and a mathematical formulation for specific-ion effects.…”

Water Structure in the Electrical Double Layer and the Contributions to the Total Interfacial Potential at Different Surface Charge Densities

“…For the Stern layer, increasing evidence from experiment and theory points to their substantial contributions of net-aligned dipoles to the total interfacial potential assumptions. [8][9][10][11][12][13][14][15][16][17][18][19][20][21] Explicit thermodynamic treatments of solvent dipoles at charged interfaces are reviewed in Habib's classic work. 22 The relative permittivity of Stern layer water is challenging to determine, with experimental estimates [23][24] ranging as low as 2 to below 30 and to a point-estimate of 43 from linear regression analyses of proton surface charge data for oxides.…”

Prediction of the Osmotic/activity Coefficients of Alkali Hydroxide Electrolytes

“…These can be attributed to the fact that it is know that uncorrected revPBE-D3 is too repulsive on the first hydration shell around the sodium cation. 23,24 This results in the water molecules residing further away from the sodium. This explains why the peaks in the NaOH PMF are shifted by a similar amount and why too much contact pair formation is observed relative to the NN-MD calculation.…”