Transport properties of CaCl2 are determined by the balance of ion-ion and ion-water interactions as revealed by molecular dynamics simulations

Aqueous calcium chloride solutions play a crucial role in many biological and technological processes. In this study we investigate the suitability of various non-polarizable force fields for modeling CaCl2 solutions at room temperature and atmospheric pressure, focusing on transport and structural properties. Molecular dynamics simulations were performed using different models for ions, where both the charge, qi, and the diameter, di, of the ions were scaled. The SPC/E and TIP4P/2005 models were used for water. The study reveals that full-charge ion models tend to produce practically non-ergodic behavior (in the sense of having slow dynamics), while scaling ionic charges and diameters can improve system dynamics. Our hypothesis is that a balance of ion-ion (II) and ion-water (IW) interactions is necessary to avoid physically unrealistic frozen structures, such as ion clusters when the II term dominates and too sticky hydration shells when the IW term dominates. A combined control parameter, the ratio of ionic charge to diameter, qi/di, is introduced to characterize the II+IW balance. We show correlations of this parameter with the agreement of simulation and experimental results for residence times, diffusion coefficients, and specific conductance. This control parameter can help to design force fields that are appropriate for the target properties. © 2025