『Abstract
Specific effects of background electrolytes on mineral growth
and dissolution can be interpreted on the basis of the ability
of ions to modify solute hydration, in a similar way to the systematic
effects of inorganic ions on precipitation, structure and function
of organic macromolecules (i.e., the Hofmeister effect). Here,
the effect of a range of background electrolytes (sodium and chloride
salts) on dolomite (Ca0.5Mg0.5CO3) reactivity was investigated as a model system
by measuring dissolution rates using in-situ Atomic Force Microscopy.
The systematic trends found for the different ions are interpreted
in terms of characteristic parameters of background ions such
as effective hydrated radii. Entropic effects associated with
the ordering of solvent molecules induced by constituting cations
from the crystal ultimately dictate how electrolytes affect dissolution
rates. In dilute electrolyte solutions, ion-ion interactions dominate
and the stabilisation of the solvation shell of ions constituting
the crystal, by counter-ions present in solution enhances the
unfavourable entropic effect on dolomite dissolution. The tendency
for electrolytes to form ion pairs in solution reduces such an
effect, thus leading to an inverse correlation between dissolution
rates and background ion separation in solution. On the other
hand, in concentrated saline solutions the interaction between
background ions and water molecules determines the hydration of
a constituent ion immersed in an electrolyte solution. In this
case, dissolution rates correlate with the mobility of background
ions and, therefore, with their effective hydration radii. The
observed effects of background ions on growth and dissolution
could be applicable for other inorganic systems where the Hofmeister
effect has been reported.
Keywords: Dolomite dissolution; In situ AFM; Background electrolytes』
1. Introduction
2. Methodology
3. Results
3.1. Dissolution features of etch pits
3.2. Dissolution kinetics
4. Discussion
4.1. Dissolution at low ionic strength: stabilisation of
Ca2+/Mg2+ hydration shells by background
electrolytes
4.2. Dissolution at high ionic strength: impact of background
anions on water structure dynamics
4.3. Dissolution at high ionic strength: impact of background
cations and the lithium anomaly
5. Conclusions
Acknowledgments
References