Electrolytes induce long-range orientational order and free energy changes in the H-bond network of bulk water

Abstract

Ions induce changes in the H-bond network of water that extend by >20 nm, vary for H2O and D2O, and lead to surface tension anomalies.

Communication: Mean-field theory of water-water correlations in electrolyte solutions

“…These arguments support the hypothesis that ion-induced dipole-dipole correlations underlie the concentration dependence of both fs-ESHS and surface tension measurements for dilute electrolyte solutions. 17,32 In conclusion, we have shown that long-range, nonspecific electrolyte-induced correlations in water as recently observed in fs-ESHS experiments can be captured by a simple mean-field model that treats water molecules as noninteracting dipoles oriented by the electrostatic field of ions, which are themselves correlated following the Debye-Hückel theory. Although one can intuitively understand the orientational correlations as arising from the exponentially screened field of correlated ions, a more accurate picture, leading to quantitative predictions of MD simulations, regards them as arising from unscreened ion-dipole correlations that combine destructively when the physically relevant ion-ion correlations are included.…”

“…This expression is a natural extension to a simple Debye-Hückel model, which has been shown to qualitatively capture the concentration-dependence of the second-harmonic response, 17,19 and can be used to elucidate the nature, the range, and the energetics of the weak ion-induced ordering probed by fs-ESHS. 20 The expression provides a benchmark for a fundamental understanding of the interplay of ion-dipole and ion-ion interactions.…”

“…(7)), the RI and the SF-RI models, and the correlations computed from a MD simulation using a ∼ 20 nm cubic box with about 264 000 TIP4P/2005 water molecules. 17 All curves correspond to a salt concentration of 8 mM and a temperature of 300 K. The other physical FIG. 3.…”

“…This damping of the dipole-dipole correlations explains the saturation of the fs-ESHS signal at high electrolyte concentrations. 17 We note that the mean-field model can be further improved to include more physical effects. cos φ DH (r) diverges in the r → 0 limit because of the singularity in the electric field at the ion positions.…”

“…Molecular dynamics simulations looking directly at the orientational correlations between water molecules showed that the presence of ionic solutes has an effect on these correlations at distances of more than 1 nm. 14 Femtosecond elastic second harmonic scattering (fs-ESHS) 15,16 measurements have recently been used to probe the orientational order of water molecules in H 2 O and D 2 O electrolyte solutions, 17 revealing intensity changes that are already detectable at micromolar concentrations, and which are identical for more than 20 different electrolytes. The nonspecificity of the fs-ESHS response, its magnitude, and its onset at low concentration point to its long-range origin.…”