Extending the Madrid-2019 force field to the perchlorate anion: Role of charge distribution and validation with experiments on Mars-relevant aqueous solutions

The perchlorate anion (ClO−4 ) has garnered increasing interest in recent years due to its presence on Martian soil (and subsoil). In this study, we extend the Madrid2019 force field (which uses a scaled charge of ±0.85e for monovalent ions and the TIP4P/2005 model for water) to the perchlorate anion. We propose two models with identical Lennard-Jones parameters but different charge distributions (i.e., the total charge of the anion is -0.85 in both models, but the partial charges assigned to the Cl and O atoms differ in each model). The experimental densities of several soluble perchlorate salts (i.e., LiClO4, NaClO4, KClO4, Mg(ClO4)2 and Ca(ClO4)2) are well described up to the solubility limit of each salt with both force fields. The viscosity of Mg(ClO4)2 and Ca(ClO4)2 solutions at 298.15 K and room pressure was also determined experimentally. The transport properties show differences due to the charge distribution; specifically, the model with a greater difference in partial charges between the ions is less viscous (thus exhibiting higher diffusion) and aligns more closely with experimental results. We also performed experimental measurements of density at ambient pressure as a function of temperature for supercooled Mg(ClO4)2 and Ca(ClO4)2 solutions and compared them against simulation results to locate the temperature at which the maximum in density occurs (TMD). The two models agree well with the experimental results although the exact location of the TMD is sensitive to the internal charge distribution within the perchlorate anion. Additionally, structural features of perchlorate solutions were calculated, finding negligible differences between the proposed models. Finally, we tested the possibility of combining the Madrid-2019 force field with the TIP4P/Ice model of water showing also excellent predictions of the experimental densities.