Molecular dynamics simulation on Na+−F− ion-pair association from ambient to supercritical water

Abstract

Constrained classical molecular dynamics simulations were carried out to investigate the sodium fluoride associations in dilute aqueous solutions from ambient to supercritical thermodynamic condition. Solvent mediated potential of mean force (PMF) of Na+−F− ions pair was calculated by thermodynamic integration with five different force fields and the SPC/E water model. The corresponding association constants were also calculated from the PMFs. The structure and dynamics of Na+ and F− in water calculated from these force fields were compared with the results from ab initio molecular dynamics at 298 K. The results show Na+−F− ion-pair tends to associate with increasing temperature and dissociate with increasing pressure. The association constants of the ion-pairs (Kall) including contact ion-pair (CIP), solvent shared ion-pair (SShIP) and solvent separated ion-pair (SSIP) from different force fields agreed well with each other within 0.4 log unit in the whole thermodynamic condition, and they are also in accordance with the recently published data at 298 K. But in the sub-supercritical region, significant deviations occurred between the contact ion-pair association constants (KCIP) and Kall for each force field and also between KCIP from different force fields. These two kinds of deviations decreased at higher temperature when CIP became the predominated configuration. Comparison of radial distribution functions (RDFs) characteristics from the five force fields with that from AIMD calculation shows that the force field parameters developed by combing single ion properties with mineral lattice energy or ion-pair interaction properties is better.