Ion Pairing and Multiple Ion Binding in Calcium Carbonate Solutions Based on a Polarizable AMOEBA Force Field and Ab Initio Molecular Dynamics

21 February 2020, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

The speciation of calcium carbonate in water is important to the geochemistry of the world’s oceans and has ignited significant debate regarding the mechanism by which nucleation occurs. Here it is vital to be able to quantify the thermodynamics of ion pairing versus higher order association processes in order to distinguish between possible pathways. Given that it is experimentally challenging to quantify such species, here we determine the thermodynamics for ion pairing and multiple binding of calcium carbonate species using bias-enhanced molecular dynamics. In order to examine the uncertainties underlying these results, we have derived a new polarizable force field for both calcium carbonate and bicarbonate in water based on the AMOEBA model to compare against our earlier rigid-ion model, both of which are further benchmarked against ab initio molecular dynamics for the ion pair. Both force fields consistently indicate that the association of calcium carbonate ion pairs is stable, though with an equilibrium constant that is lower than for ion pairing itself.

Keywords

CaCO3
Molecular Dynamics
Free Energy
Multiple Ion Binding
AMOEBA
Meatadynamics
DFT

Supplementary materials

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