Revisiting the hydration structure of aqueous Na+

Abstract

A combination of theory, X-ray diffraction (XRD) and extended x-ray absorption fine structure (EXAFS) are used to probe the hydration structure of aqueous Na. The high spatial resolution of the XRD measurements corresponds to Q = 24 Å while the first-reported Na K-edge EXAFS measurements have a spatial resolution corresponding to 2k = Q = 16 Å. Both provide an accurate measure of the shape and position of the first peak in the Na-O pair distribution function, g(r). The measured Na-O distances of 2.384 ± 0.003 Å (XRD) and 2.37 ± 0.024 Å (EXAFS) are in excellent agreement. These measurements show a much shorter Na-O distance than generally reported in the experimental literature (Na-O ∼ 2.44 Å) although the current measurements are in agreement with recent neutron diffraction measurements. The measured Na-O coordination number from XRD is 5.5 ± 0.3. The measured structure is compared with both classical and first-principles density functional theory (DFT) simulations. Both of the DFT-based methods, revPBE and BLYP, predict a Na-O distance that is too long by about 0.05 Å with respect to the experimental data (EXAFS and XRD). The inclusion of dispersion interactions (-D3 and -D2) significantly worsens the agreement with experiment by further increasing the Na-O distance by 0.07 Å. In contrast, the use of a classical Na-O Lennard-Jones potential with SPC/E water accurately predicts the Na-O distance as 2.39 Å although the Na-O peak is over-structured with respect to experiment.

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

“…In Figure 5 we compare the k 2 weighted fine structure, k 2 χ(k), vs. k (inÅ −1 ), generated in this manner to the experimental measurement. 20,57 For both K + and Na + we observe nearly quantitive agreement with the measured signal.…”

“…bound water molecules and a more diffuse sixth water molecule that occupies both the first and second hydration shells. 20 In contrast, water molecules beyond the first shell show qualitatively different behaviour for the two ions. With the seventh being more localised for sodium whereas the sixth, seventh and eighth are all less localised for potassium.…”

Understanding the scale of the single ion free energy: A critical test of the tetra-phenyl arsonium and tetra-phenyl borate assumption

“…31 DFT has also been demonstrated to reliably model the air-water interface, where the inclusion of dispersion corrections is a key advance. 32,33 Here, we set out to confirm the spectroscopic observations of Scheu et al 26 by simulation of the TA + and TB ions in water in addition to the neutral tetra-phenyl carbon (TC 0 ) solute. To this end, we establish connection with recent spectroscopic experiments in addition to computing average ion-water binding energies and net potentials demonstrating that there are substantial charge asymmetries between the TB and TA + providing strong evidence that the TATB approximation is not reliable.…”

Strong attractions and repulsions mediated by monovalent salts

“…The widespread use of both dispersion and flocculation of NPs by adding high concentrations of monovalent salt suggests the possibility of an underlying robust principle responsible of these phenomena. Here, we investigate interactions between neutral and charged NPs in aqueous solutions at high monovalent salt concentrations, where steric interactions among ions (8) as well as hydration effects cannot be ignored (9,10), to elucidate the origin, range, and strength of the attraction and/or repulsions between identical NPs as the NP surface charge density increases.…”