Electrical conductivity, ion pairing, and ion self-diffusion in aqueous NaCl solutions at elevated temperatures and pressures

Abstract

We have performed classical molecular dynamics (MD) simulations of aqueous sodium chloride (NaCl) solutions from 298 to 674 K at 200 bars to understand the influence of ion pairing and ion self-diffusion on electrical conductivity in high-temperature/high-pressure salt solutions. Conductivity data obtained from the MD simulation highlight an apparent anomaly, namely, a conductivity maximum as temperature increases along an isobar, which has been also observed in experimental studies. By examining both velocity autocorrelation and cross-correlation terms of the Green-Kubo integral, we quantitatively demonstrate that the conductivity anomaly arises mainly from a competition between the single-ion self-diffusion and the contact ion pair formation. The velocity autocorrelation function in conjunction with structural analysis suggests that diffusive motion of ions is suppressed at high temperatures due to the persistence of an inner hydration shell. The contribution of velocity cross-correlation functions between oppositely charged ions becomes significant at the onset of the conductivity decrease. Structural analysis based on Voronoi tessellation and pair correlation functions indicates that the fraction of contact ion pairs increases as temperature increases. Spatial decomposition of the electrical conductivity also indicates that the formation of contact ion pairs significantly decreases the electrical conductivity compared to Nernst-Einstein conductivity, but the contribution of distant opposite charges cannot be ignored except at the highest temperature due to unscreened long-range interactions.

Electrical Conductivity of Aqueous NaCl at High Pressure and Low Temperature: Application to Deep Subsurface Oceans of Icy Moons

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Selective recovery of critical materials in supercritical water desalination

“…Above 243 • C, a decrease in the dielectric constant of water stabilizes ion pairs. The temperature where the free ion concentration starts to decrease is close to the threshold temperature found using molecular dynamics (MD) simulations and separate experimental measurements [31,67] There are two possible contributions to the double-sigmoidal behavior in c D Nd profile (Run c3) at high temperatures. First, as given in Figure 2 (a), the solution density in Run c3 is much higher that in other Runs.…”

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“…If the movement of a given ion is on average in the same direction as that of another ion, then the integration of this correlation function becomes positive. If the motion of the other ion is in the opposite direction, then it becomes negative, and the integral of this correlation yields zero when there is no relation in the motion of the two ions . As we can see in Figure , both Na + –anion and K + –anion correlation functions are consistently more positive than that of the corresponding Li + –anion values.…”