Impact of magnetic and electric fields on the free energy to form a calcium carbonate ion-pair

Electromagnetic fields are used in water treatment and desalination to regulate scale formation and extend the lifetime of membranes. External electric and magnetic fields can promote or suppress mineral nucleation and growth. However, the molecular-scale mechanisms of such processes remain unknown. Computing the free energies needed to form ion pairs under external fields provides important insights into understanding the elemental steps during the initial formation of mineral scales. In this paper, we used molecular dynamics combined with metadynamics simulations to investigate the free energies of forming the [Ca–CO₃]⁰ ion pair, a fundamental building block of carbonate scales, under a range of magnetic (up to 10 T) and electric (up to 10 V m^–1) fields in water. The presence of constant magnetic or electric fields favored the ion pairing reaction and lowered the free energies by up to 3% to 6%. The internal energy and entropic components of the free energy showed significant changes and exhibited non-linear behavior with increasing field strength. The [Ca–CO₃]⁰ ion pairing is an entropy-driven process in the absence of an external field, but the mechanism shifts to an internal energy-driven process under selected external fields, suggesting possible changes in the nucleation pathways.