(453a) Prediction of Mean Ionic Activity Coefficient of NaCl-H2O System at High Concentrations Using Molecular Dynamics Simulations

Aqueous electrolyte thermodynamic models are extensively use to correlate and predict the liquid phase non-ideality for many industrial applications such as hydraulic fracturing, desalination, electrodialysis, e-waste and nuclear waste treatment, etc. Among the electrolyte thermodynamic models, Pitzer model [1] and electrolyte non-random two liquid (eNRTL) model [2] are the two most widely used models for aqueous electrolytes as the thermodynamic models for NaCl-H₂O to predict mean ionic activity coefficient. The Pitzer model is valid up to 6 molal whereas the eNRTL model claims to extrapolate up to pure salt. Specifically, for NaCl in water, the Pitzer model predicts a very sharp increase in mean ionic activity coefficient of NaCl after 6 molal. In contrast, the eNRTL model suggests the NaCl mean ionic activity coefficient should increase to a maximum and then decline with further increase in NaCl concentration. In this work, we use Molecular Dynamics simulations to calculate the mean ionic activity coefficient of aqueous NaCl system at 298.15 K and compare the predictions with those from the two thermodynamic models. The mean ionic activity coefficient is determined using the Kirkwood-Buff method [3]. The Kirkwood-Buff Force Field (KBFF) [4] parameters for NaCl are optimized to reproduce the NaCl lattice dimensions, ion to water oxygen contact distances, and Kirkwood-Buff Integral (KBI) which is related to mean ionic activity coefficient of aqueous NaCl solution. The predictions of the NaCl mean ionic activity coefficient beyond 6 molal from the Kirkwood-Buff method are found to be in line with the eNRTL model extrapolation and differ from the Pitzer model extrapolation.

References

[1] K.S. Pitzer, J.C. Peiper, and R.H. Busey, "Thermodynamic Properties of Aqueous Sodium Chloride Solutions," Journal of Physical and Chemical Reference Data, vol. 13, pp. 1-102, 1984.

[2] Y. Yan and C.-C. Chen, "Thermodynamic representation of the NaCl + Na₂SO₄ + H₂O system with electrolyte NRTL model," Fluid Phase Equilibria, vol. 306, pp. 149-161, 2011.

[3] J.G. Kirkwood and F.P. Buff, "The statistical mechanical theory of solutions. I," Journal of Chemical Physics, vol. 19, pp. 774-777, 1951.

[4] S. Weerasinghe and P.E. Smith, "A Kirkwood–Buff derived force field for sodium chloride in water," Journal of Chemical Physics, vol. 119, pp. 11342-11349, 2003.