Hao Jiang, Shuwen Yue, Pablo G. Debenedetti, and Athanassios Z. Panagiotopoulos
Department of Chemical and Biological Engineering, Princeton University
We have used large-scale molecular dynamics simulations coupled to free energy calculations to identify for the first time a limit of stability (spinodal) and a change in the nucleation mechanism in aqueous NaCl solutions. This is a system of considerable atmospheric, geological and technical significance. We find that the supersaturated metastable NaCl solution reaches its limit of stability at sufficiently high salt concentrations, as indicated by the composition dependence of the salt chemical potential, indicating the transition to a phase separation by spinodal decomposition. However, the metastability limit of the NaCl solution does not correspond to spinodal decomposition with respect to crystallization. We find that beyond this spinodal, a liquid/amorphous separation occurs in the aqueous solution, whereby the ions first form disordered clusters [1]. Nucleation rates are much better described by polarizable models relative to non-polarizable ones [2,3]. We have also investigated the dynamic properties of alkali halide ion (NaCl, NaF, NaBr, NaI, LiCl, and KCl) solutions to evaluate non-polarizable, polarizable, and scaled-charge models in their agreement with the experimental Hofmeister series. Through high precision calculations of the shear viscosity and diffusion coefficient, we obtain the Jones-Dole B-coefficient and ion mobility to provide insight towards single ion hydration behavior. Our observations indicate that the water-ion interactions of current models are not well represented and additional physical features must be incorporated in future models.
References
[1] H. Jiang, P. G. Debenedetti, and A. Z. Panagiotopoulos, "Nucleation in aqueous NaCl solutions shifts from 1-step to 2-step mechanism on crossing the spinodal,” J. Chem. Phys., in press (2019).
[2] H. Jiang, A. Haji-Akbari, P. G. Debenedetti, and A. Z. Panagiotopoulos, “Forward flux sampling calculation of homogeneous nucleation rates from aqueous NaCl solutions,” J. Chem. Phys., 148: 044505 (2018).
[3] H. Jiang, P. G. Debenedetti, and A. Z. Panagiotopoulos, “Communication: Nucleation rates of supersaturated aqueous NaCl using a polarizable force field,” J. Chem. Phys., 149: 141102 (2018).
[4] S. Yue and A. Z. Panagiotopoulos, "Dynamic properties of aqueous electrolyte solutions using non-polarizable, polarizable, and scaled-charge models,” Molec. Phys., in review (2019).