Electrolyte solutions play an important role in a wide variety of industrial and biological processes. Among the various thermodynamic properties of electrolyte solutions, the salt activity coefficient and the solvent activity belong to the most important ones. However, the reliable determination of these properties by molecular simulation is challenging and the subject of several recent studies [1,2].
In this work, a method for determining these activities by molecular dynamics simulation is presented: Simulations of the osmotic equilibrium between a solution phase and a pure solvent phase are carried out by introducing two semipermeable membranes. The solvent activity is obtained from the osmotic pressure, which is measured as the force on these membranes. The salt activity coefficient is computed from the solvent activity by the Gibbs-Duhem equation. This procedure, called OPAS, was recently assessed using different test cases [3].
OPAS simulation is applied to aqueous electrolyte solutions for which results for the solute and solvent activities are available from other recent studies [1,2]. Good agreement is observed. Then, a systematic investigation of the activities in aqueous alkali halide salt solutions in general is carried out, using ion models which were developed in our group [4,5]. While for salts of the larger ions mostly good predictions are observed, there are important deviations in several of the other systems. Therefore, a multicriteria approach for including the new data in the model development is presented. The simulations are performed with an extended version of the molecular simulation program ms2 [6].
References
[1] F. Moucka et al., J. Chem. Phys. 139 (2013) 124505.
[2] Z. Mester and A. Panagiotopoulos, J. Chem. Phys. 142 (2015) 044507.
[3] M. Kohns et al., J. Chem. Phys. 144 (2016) 084112
[4] S. Deublein et al., J. Chem. Phys. 136 (2012) 084501.
[5] S. Reiser et al., J. Chem. Phys. 140 (2014) 044504.
[6] C. Glass et al., Comput. Phys. Commun., 185 (2014) 3302-3306.
