Ion association in lanthanide chloride solutions

A better understanding of the solution chemistry of the lanthanide (Ln) salts in water would have wide ranging implications in materials processing, waste management, element tracing, medicine and minerals processing ‐ particularly given governmental concerns about Ln security of supply. Despite much effort, even in simple systems, the mechanisms and thermodynamics of Ln(III) association with small anions remain unclear. In the present study, molecular dynamics (MD), using a newly developed force field, provide new insights into LnCl₃(aq) solutions. (Adaptive) bias MD simulations show that the mechanisms for ion pairing change from dissociative to associative exchange depending upon cation size. Thermodynamics of association reveal that while ion pairing is favorable, the equilibrium distribution of species at low concentration is dominated by weakly bound solvent shared and solvent separated ion pairs cf. contact ion pairs, reconciling a number of contrasting observations of Ln(III)‐‐Cl association in the literature. In addition, we show that the thermodynamic stabilities of a range of inner sphere and outer sphere LnClχ coordination complexes are comparable and that the kinetics of anion binding to cations may control solution speciation distributions beyond ion pairs. The techniques adopted in this work provide a framework with which to investigate more complex solution chemistries of cations in water.