(335e) Development of Force Field for Salts and Their Aqueous Solutions

A balance between ion-ion and ion-water interactions plays a crucial role in the crystallization and dissolution of salts. Correct description of these interactions is therefore paramount for the understanding of natural processes leading to the precipitation of minerals, as well as for the design of new technologies, such as nuclear waste separation and storage. Our motivation is to investigate salt crystallization from mixed aqueous electrolyte solutions for the rational design of complexation agents used in selective crystallization of homologous series of oxoanions.

A reliable description of molecular and ionic interactions is needed to understand atomic-scale mechanisms underlying the thermodynamics and dynamics of electrolyte solutions.  Thus, our goal is to develop a consistent force field parameterization for the study of oxoanions of general formula XO₄^2- (X=S, Se, Cr, Mo, W).
 While several force fields already exist for the description of simple alkali metal and alkali-halide series
[1], and models for individual oxoanions have been also published [2], a consistent force field for oxoions suitable for comparative studies is not yet available.

The choice of an appropriate potential model form depends on the intended applications, and must balance accuracy and computational efficiency. Since salt nucleation and crystallization occur over large time and length scales, the simplicity of the model is of high importance. While most of the above ions are highly polarizable, their mineral environment is consistently polar, justifying the use of effective pair potentials to account for those polarizable contributions. Therefore we consider pair potentials represented by a combination of point charges and Lennard-Jones interactions that are compatible with the SPC/E water model
[3]. We also test the limit of the effective description of the interactions and discuss ways to incorporate polarizability.

The resulting force field is optimized against experimental data including hydration free energy at infinite dilution
[4], chemical potential at finite concentration based on the Kirkwood-Buff formalism [5], lattice constants and energies for selected crystals, and diffusion coefficient
[4]. The potential parameters (at least 5 for each oxoion) were determined using global optimization based on the coupling parameter technique
[6].

[1] Joung et al (2008)
J. Phys. Chem. B  112(30), 9020-9041.

[2] Cannon et al. (1994)
J. Phys. Chem. 98(24), 6225-6230.

[3] Berendsen et al. (1987)
J. Phys. Chem.  91(24), 6269-6271.

[4] Marcus, Y.,
Ion properties 1997, New York: Marcel Dekker.

[5] Gee et al. (2011)
J. Chem. Theory Comput.  7(5), 1369-1380.

[6] Vlcek et al. (2011)
J. Phys. Chem. B  115(27), 8775-8784.

 Acknowledgements. This work was supported as part of the
“Center for Nanoscale Control of Geologic CO₂”,
an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, and by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy.