Numerical Prediction of NaCl Solubility in Water Using the TIP4P/05 Water Model NaCl/Madrid

Abstract

The solid-liquid equilibrium and solubility of NaCl in water were investigated using molecular dynamics simulations with the NaCl/Madrid force field and the TIP4P/05 water model. Particular attention was given to the influence of the intermolecular cut-off radius, rcut, used to identify ions belonging to the crystalline phase. Simulations were initiated from a NaCl crystal immersed in an aqueous NaCl solution, following the configuration proposed by Manzanilla-Granados et al. After equilibration, the system was analysed over an extended simulation trajectory. The radial distribution function, g(r), was used to establish a physically motivated criterion for distinguishing the crystalline and aqueous phases. Specifically, the first minimum following the principal Na-Cl correlation peak was used to characterise the ionic coordination environment and constrain the range of meaningful cut-off radii. The number of ions remaining in the crystalline phase was then evaluated as a function of rcut, allowing the corresponding solubility to be estimated. The resulting curve exhibits a well-defined inflection point, providing a criterion for identifying the transition between predominantly crystalline and dissolved ionic populations. This approach offers a structural basis for determining NaCl solubility directly from the evolution of the solid-liquid system, avoiding the arbitrary selection of the cut-off distance. The results demonstrate that combining radial distribution functions with cut-off-dependent analysis provides a useful framework for characterising solid-liquid behaviour and estimating the solubility of ionic crystals through molecular dynamics simulations.