Elucidating the Atomic Structures of the Gel Layer Formed during Aluminoborosilicate Glass Dissolution: An Integrated Experimental and Simulation Study

Altered glasses produced during the
aqueous dissolution of silicate
and borosilicate glasses are among the most complex structures to
understand at the atomic level due to their amorphous nature, random
porosity, and various levels of hydration. In this study, we gained
insights into the complex atomic structures of altered aluminoborosilicate
glasses by combining a range of experimental and computational approaches.
The altered glasses were prepared by the dissolution of three glasses
with varying levels of alumina in an acid for 7 days. A comprehensive
set of experimental [elemental analysis, high-energy X-ray diffraction, 29Si and 27Al solid-state nuclear magnetic resonance
(NMR), and O 1s X-ray photoelectron spectroscopy (XPS)] and modeling
(molecular dynamics (MD) simulations using nonreactive and reactive
force fields) approaches were used to study the atomic structures
of these altered glasses. Elemental analysis showed that most of the
B in the pristine glasses was leached into the solution and was not
contained in the altered glass. The 29Si and 27Al solid-state NMR spectra revealed that the altered glasses have
more polymerized silicate networks as compared to those in the pristine
glasses due to the reformation of linkages among Si and Al oxygen
polyhedral in the altered glasses. The bridging and nonbridging (or
hydroxyl O) atoms in the altered glasses were also quantified from
their O 1s XPS spectra. Atomic structure models of the altered glasses
were constructed using MD simulations using the reactive force field
based on the compositional information obtained from experiments.
Various pore structures were generated using the charge-scaling (CS)
method using different initial densities and CS temperatures; the
best CS parameters of each altered glass were then determined by comparing
with the experimental structure factors obtained from high-energy
X-ray diffraction. Pore and atomic structures and vibrational properties
around these pore surfaces were analyzed. These results from this
comprehensive study thus provide a realistic insight into the pore
morphology, atomic structure, and vibrational properties of altered
glasses.