The mechanisms by which amorphous silica dissolves have proven
elusive because noncrystalline materials lack the structural order that
allows them to be studied by the classical terrace, ledge, kink-based
models applied to crystals. This would seem to imply amorphous
phases have surfaces that are disordered at an atomic scale so that the
transfer of SiO4 tetrahedra to solution always leaves the surface free
energy of the solid unchanged. As a consequence, dissolution rates of
amorphous phases should simply scale linearly with increasing driving
force (undersaturation) through the higher probability of detaching
silica tetrahedra. By examining rate measurements for two amorphous
SiO2 glasses we find, instead, a paradox. In electrolyte
solutions, these silicas show the same exponential dependence on
driving force as their crystalline counterpart, quartz. We analyze this
enigma by considering that amorphous silicas present two predominant
types of surface-coordinated silica tetrahedra to solution. Electrolytes
overcome the energy barrier to nucleated detachment of
higher coordinated species to create a periphery of reactive, lesser
coordinated groups that increase surface energy. The result is a
plausible mechanism-based model that is formally identical with the
classical polynuclear theory developed for crystal growth. The model
also accounts for reported demineralization rates of natural biogenic
and synthetic colloidal silicas. In principle, these insights should be
applicable to materials with a wide variety of compositions and
structural order when the reacting units are defined by the energies
of their constituent species.
%0 Journal Article
%1 dove2008
%A ?,
%D 2008
%J PNAS
%K 2008 SiO2 dissolution experiments kinetics mechanism quartz silica
%N 29
%P 9903–9908
%T Kinetics of amorphous silica dissolution and the paradox of the silica polymorphs
%V 105
%X The mechanisms by which amorphous silica dissolves have proven
elusive because noncrystalline materials lack the structural order that
allows them to be studied by the classical terrace, ledge, kink-based
models applied to crystals. This would seem to imply amorphous
phases have surfaces that are disordered at an atomic scale so that the
transfer of SiO4 tetrahedra to solution always leaves the surface free
energy of the solid unchanged. As a consequence, dissolution rates of
amorphous phases should simply scale linearly with increasing driving
force (undersaturation) through the higher probability of detaching
silica tetrahedra. By examining rate measurements for two amorphous
SiO2 glasses we find, instead, a paradox. In electrolyte
solutions, these silicas show the same exponential dependence on
driving force as their crystalline counterpart, quartz. We analyze this
enigma by considering that amorphous silicas present two predominant
types of surface-coordinated silica tetrahedra to solution. Electrolytes
overcome the energy barrier to nucleated detachment of
higher coordinated species to create a periphery of reactive, lesser
coordinated groups that increase surface energy. The result is a
plausible mechanism-based model that is formally identical with the
classical polynuclear theory developed for crystal growth. The model
also accounts for reported demineralization rates of natural biogenic
and synthetic colloidal silicas. In principle, these insights should be
applicable to materials with a wide variety of compositions and
structural order when the reacting units are defined by the energies
of their constituent species.
@article{dove2008,
abstract = {The mechanisms by which amorphous silica dissolves have proven
elusive because noncrystalline materials lack the structural order that
allows them to be studied by the classical terrace, ledge, kink-based
models applied to crystals. This would seem to imply amorphous
phases have surfaces that are disordered at an atomic scale so that the
transfer of SiO4 tetrahedra to solution always leaves the surface free
energy of the solid unchanged. As a consequence, dissolution rates of
amorphous phases should simply scale linearly with increasing driving
force (undersaturation) through the higher probability of detaching
silica tetrahedra. By examining rate measurements for two amorphous
SiO2 glasses we find, instead, a paradox. In electrolyte
solutions, these silicas show the same exponential dependence on
driving force as their crystalline counterpart, quartz. We analyze this
enigma by considering that amorphous silicas present two predominant
types of surface-coordinated silica tetrahedra to solution. Electrolytes
overcome the energy barrier to nucleated detachment of
higher coordinated species to create a periphery of reactive, lesser
coordinated groups that increase surface energy. The result is a
plausible mechanism-based model that is formally identical with the
classical polynuclear theory developed for crystal growth. The model
also accounts for reported demineralization rates of natural biogenic
and synthetic colloidal silicas. In principle, these insights should be
applicable to materials with a wide variety of compositions and
structural order when the reacting units are defined by the energies
of their constituent species.},
added-at = {2009-09-30T17:00:11.000+0200},
author = {?},
biburl = {https://www.bibsonomy.org/bibtex/2b2a5e0eb186358254650f8c6d804550a/schepers},
interhash = {f0b69415ff717ed0b8805d41375e29ed},
intrahash = {b2a5e0eb186358254650f8c6d804550a},
journal = {PNAS},
keywords = {2008 SiO2 dissolution experiments kinetics mechanism quartz silica},
number = 29,
pages = {9903–9908},
timestamp = {2009-09-30T17:00:32.000+0200},
title = {Kinetics of amorphous silica dissolution and the paradox of the silica polymorphs},
volume = 105,
year = 2008
}