(377j) Bridging Two-Liquid Theory with Molecular Simulations for Electrolytes: An Investigation of Aqueous NaCl Solution

Aqueous electrolyte solutions are present in many industrial, environmental, geological, and biological systems [1-3]. Extensive thermodynamic modeling efforts have been reported in the literature to calculate phase equilibria and thermodynamic properties of aqueous electrolytes [1]. Among them, the Pitzer model [4] and the electrolyte Non-Random Two-Liquid (eNRTL) model [5] are the two most widely practiced thermodynamic models for aqueous electrolytes. A recent molecular dynamics simulations study suggested the eNRTL model extrapolation to higher concentrations (i.e., above 6 molal) is more reliable than that of the Pitzer model [6].

The eNRTL model requires two adjustable binary interaction parameters per molecule-electrolyte pair (Ï„_ca,m and Ï„_m,ca), commonly identified from regression of experimental data. However, quantifying the binary interaction parameters from regression may not always be practical since it requires availability of experimental data, substantial experience in data treatment, and necessity of properly defining the optimization problem. Recently, Ravichandran et al. [7] presented a novel methodology to predict the NRTL binary parameters from molecular dynamics (MD) simulations for nonelectrolyte solutions. Hence, it is of great interest to explore the feasibility of quantifying the binary interaction parameters of the eNRTL model from MD simulations for aqueous electrolytes.

In this study, we introduce a methodology to quantify the eNRTL binary interaction parameters for molecule-electrolyte pairs, (τ_ca,m and τ_m,ca), by combining the statistical mechanic framework of two-liquid theory for electrolytes with MD simulations. τ parameters are formulated as a function of species diameters (σ), effective interaction strength (∈), and the first neighbor shell radii around the center species (R), which are calculated from the MD simulations. Aqueous NaCl solution is selected for the case study due to availability of proven force field parameters [6] and its ubiquitous presence in nature. To investigate the composition dependence of the binary interaction parameters, the simulations are carried out in the concentration range of 0.11 to 16 mol.kg^-1 NaCl, at 298.15 K and 1 bar. Furthermore, we obtain non-randomness factor parameter (α) from the MD simulations following the definition introduced by Brandani and Prausnitz [8] from the total coordination number (z) around a center species. The binary interaction parameters determined from the MD simulations are consistent with those obtained from regression. Future studies will investigate the applicability of the proposed approach for a broader range of electrolytes such as potassium and magnesium chlorides.

References:

[1] S. Honarparvar, S. H. Saravi, D. Reible, and C.-C. Chen, "Comprehensive thermodynamic modeling of saline water with electrolyte NRTL model: A study on aqueous Ba²⁺-Na⁺-Cl^−-SO₄^2− quaternary system," Fluid Phase Equilibria, vol. 447, pp. 29-38, 2017.

[2] D. M. Sherman and M. D. Collings, "Ion association in concentrated NaCl brines from ambient to supercritical conditions: results from classical molecular dynamics simulations," Geochemical Transactions, vol. 3, pp. 102-107, 2002.

[3] W. Habermann and E. H. Pommer, "Biological fuel cells with sulphide storage capacity," Applied microbiology and biotechnology, vol. 35, pp. 128-133, 1991.

[4] K. S. Pitzer, “Thermodynamics of electrolytes. I. Theoretical basis and general equations,” Journal of Physical Chemistry, vol. 77, pp. 268-277, 1973

[5] Y. Song and C.-C. Chen, "Symmetric electrolyte nonrandom two-liquid activity coefficient model," Industrial & Engineering Chemistry Research, vol. 48, pp. 7788-7797, 2009.

[6] N. Hossain, A. Ravichandran, R. Khare, and C.-C. Chen, “Revisiting Electrolyte Thermodynamic Models: Insight from Molecular Simulations”, submitted to AIChE Journal, 2018.

[7] A. Ravichandran, R. Khare, and C. C. Chen, "Predicting NRTL binary interaction parameters from molecular simulations," AIChE Journal, 2018. https://doi.org/10.1002/aic.16117

[8] V. Brandani and J. M. Prausnitz, "Two-fluid theory and thermodynamic properties of liquid mixtures: General theory," Proceedings of the National Academy of Sciences, vol. 79, pp. 4506-4509, 1982.