Further extension of the Madrid-2019 force field: Parametrization of nitrate (NO3−) and ammonium (NH4+) ions.

This article focuses on the development and validation of extended force fields within the Madrid-2019 framework for aqueous nitrate (NO3⁻) and ammonium (NH4⁺) salts using the TIP4P/2005 water model. The authors optimized interaction parameters for twelve electrolytes—eight nitrate and four ammonium salts—by fitting simulated densities, ion–ion and ion–solvent structures, and transport properties to experimental data across a wide concentration range. The resulting scaled-charge (±0.85e) nonpolarizable models accurately reproduce solution densities and structural features, including hydration numbers and ion pairing, while transport properties such as viscosities and diffusion coefficients show variable agreement depending on the ion pair and concentration. The study demonstrates the models' applicability to single and mixed salt solutions, providing a valuable tool for microscopic investigations of polyatomic ion interactions in aqueous environments relevant to biological, environmental, and industrial contexts.