Computer simulation study of the structure of LiCl aqueous solutions: test of non standard mixing rules in the ion interaction.

Aqueous solutions of LiCl have recently received much attention in connection with the study of the anomalies of supercooled water and its polyamorphism. From the point of view of computer simulation, there is need for a force field that can reproduce the structural and dynamical properties of this solution, and more importantly it is also simple enough to use in large scale simulations of supercooled states. We study by molecular dynamics the structure of the LiCl-water solutions with the force field proposed by Joung and Cheatham (J. Phys. Chem. B 2008, 112, 9020) appropriate for the water TIP4P-Ew model potential. We found that this force field does not reproduce the experimental ion pairing when the Lorentz-Berthelot (LB) rules are used. By incorporating deviations to the LB rules to obtain the crossed interactions between the ions, it is possible to get agreement with experiment. We have studied how the modification of the LB rule affects the structural and thermodynamic properties of the solution at increasing concentration of the solution from the low (around 2%) to medium (around 14%) concentration regimes. We also tested the transferability of the Joung and Cheatham force field to the water TIP4P/2005 model that works very well for supercooled water.