Testing the hypothesis that solvent exchange limits the rates of calcite growth and dissolution
Nikhil Rampal,
Hsiu-Wen Wang,
Alexander B. Brady
et al.
Abstract:Using neutron scattering dynamics measurements to validate molecular dynamic simulations, it is found that the rates of solvent exchange are not likely to limit the rates of calcite crystal growth and dissolution.
The standard dissolution free energies of magnesium binding
at
each of the eight distinct calcite cation kinks were computed from
classical molecular dynamics simulations using alchemical methods
to transform magnesium into calcium at each site. The preferred calcite
kink site for magnesium binding is found to correspond to one of the
two symmetry inequivalent cation sites at the acute step along the
kink direction that forms an obtuse angle between the end of the row
and terrace (AO
a
according
to our notation system). Incorporation of magnesium at this site is
found to inhibit growth along the step edge. However, this effect
occurs not due to destabilization of the addition of the next carbonate
immediately adjacent to Mg but instead by altering the thermodynamics
of the subsequent cation. The binding free energy of this calcium
site is reduced by more than 10 kJ/mol once magnesium has been incorporated
in the step edge. Our results support and provide a mechanistic explanation
for the experimental observation that acute steps are more affected
by the presence of magnesium in the growth solution relative to obtuse
steps.
The standard dissolution free energies of magnesium binding
at
each of the eight distinct calcite cation kinks were computed from
classical molecular dynamics simulations using alchemical methods
to transform magnesium into calcium at each site. The preferred calcite
kink site for magnesium binding is found to correspond to one of the
two symmetry inequivalent cation sites at the acute step along the
kink direction that forms an obtuse angle between the end of the row
and terrace (AO
a
according
to our notation system). Incorporation of magnesium at this site is
found to inhibit growth along the step edge. However, this effect
occurs not due to destabilization of the addition of the next carbonate
immediately adjacent to Mg but instead by altering the thermodynamics
of the subsequent cation. The binding free energy of this calcium
site is reduced by more than 10 kJ/mol once magnesium has been incorporated
in the step edge. Our results support and provide a mechanistic explanation
for the experimental observation that acute steps are more affected
by the presence of magnesium in the growth solution relative to obtuse
steps.
Quasielastic neutron scattering (QENS) is an experimental technique that can measure diffusivity parameters, such as diffusion jump rate and length, of chemical species (molecules, ions, and segments) at atomic and...
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