(746e) Solvent Activity in Electrolyte Solutions By Molecular Simulation of the Osmotic Pressure

Describing electrolyte solutions by molecular modelling and simulation is a promising approach for characterizing structural and thermodynamic effects of electrostatic interactions in a self-consistent way [1-3]. The solvent activity is one of the most important properties of an electrolyte solution. It is directly related to the the vapour pressure and the freezing point, and it is connected to the activity coefficient of the salt which controls the solubility limit.

However, a reliable determination of the solvent activity in electrolyte solutions by molecular simulation is challenging numerically. In the present work, a novel method for determining the solvent activity is presented. Thereby, the electrolyte solution is simulated in contact with the pure solvent. Between the two phases, there is a virtual membrane which is permeable only for the solvent. This is implemented by an external field which acts on the solutes only and confines the solute to a part of the simulation volume. The osmotic pressure can then be computed with a high accuracy from the force acting on the membrane, so that reliable data on the solvent activity are obtained [4].

The present method is validated and applied to aqueous electrolyte solutions for which results for the solvent activity are available from other recent studies [1, 2]. The simulations are performed with an extended version of the molecular simulation program ms2, which is freely available for academic users [5]. A good agreement with the literature is observed. On this basis, a systematic investigation of aqueous alkali halide salt solutions is carried out. Even though the employed series of ion models was developed without considering activity data for the model parameterization, which was mainly based on density data [3], a good agreement with experimental solvent activity data is found in many cases.

[1] F. Moucka, I. Nezbeda, and W. R. Smith, Journal of Chemical Physics 139 (2013) 124505.
[2] Z. Mester and A. Z. Panagiotopoulos, Journal of Chemical Physics 142 (2015) 044507.
[3] S. Reiser, S. Deublein, J. Vrabec, and H. Hasse, Journal of Chemical Physics 140 (2014) 044504.
[4] M. Kohns, S. Reiser, M. Horsch, and H. Hasse, Journal of Chemical Physics 144 (2016) 084112.
[5] C. W. Glass, S. Reiser, G. Rutkai, S. Deublein, A. Köster, G. Guevara Carrión, A.Wafai, M. Horsch, M. Bernreuther, T. Windmann, H. Hasse, and J. Vrabec, Computer Physics Communications 185 (2014) 3302-3306.