Molecular simulations of kinetic stable calcium isotope fractionation at the calcite-aqueous interface

Abstract

The measurement of calcium stable isotope fractionation during crystal growth can provide insights into the dominant pathways and mechanisms of crystal growth, but these interpretations require a detailed understanding of mass dependent reaction rates and free energy states at the molecular scale. Here we report the results of free energy calculations used to determine the rate-limiting step controlling kinetic stable isotope fractionation during calcium ion attachment to active growth sites – kink sites – at the calcite surface. The free energy profile for attachment is combined with ion coordination states to reconstruct the multi-step molecular pathway for the ion attachment process. Dissociative desolvation of a water molecule coordinating the calcium ion approaching the kink is identified as the rate-limiting step, with a free energy barrier of ∼18kJ/mol, which is equal to the energetic cost of water molecule leaving Ca2+ coordination shell in the aqueous phase. Molecular dynamics simulations were used to investigate the dependence of this desolvation step on calcium ion mass. Both brute force MD and an analysis of mass dependent water exchange coefficients (kwex) give identical kinetic stable isotope fractionation factor values αf44/40=0.992, which corresponds to Δ44/40Ca of ∼ -8‰, originating from a mass-dependent transition frequency for the Ca-bound solvation water at the kink. This value is significantly higher than the fractionation factor associated with water exchange dynamics around Ca2+ in bulk aqueous solution determined here (αf44/40=0.999). Our results confirm that the growing calcite surface should be depleted in 44Ca with respect to the aqueous phase due to kinetically preferred desolvation of the lighter isotope. We conclude that the maximum kinetic stable isotope fractionation associated with calcium attachment to surfaces is not constant but can vary due to changes in the interfacial environment that affect local solvent dynamics during desolvation.