Abstract
Nanoporous silica-rich gel formed on silicate glass surfaces during dissolution in aqueous environment is critical in elucidating the corrosion mechanisms and the long-term residual dissolution behaviors. Silica gel models were created using two types of methods with reactive force field-based molecular dynamics simulations. The results show that the remnant silica gels created from the ISG bulk structure have a more isolated and closed pore morphology and slightly higher glass network connectivity. This contrasts with the gel structures created by hydrogarnet defect formation that exhibit more connected pore morphologies. The remnant gel structures show lower water diffusivity which was explained by the nano-confinement effect of water molecules due to frequent interactions of water molecules with adjacent silica walls and the more isolated pore morphology in the remnant gel structures. These results reveal the complexity in terms of micro and atomic structures of these silica gels, and both structure features have impact on water transport in the gel layer hence the passivating effect that controls the long-term dissolution behavior of these glasses.
Highlights
Silica gels are hydrated nanoporous silica systems commonly found in the fields of biomaterials, including silicate-based bioactive glasses,[1,2] carbon sequestration,[3,4] liquid chromatography,[5,6] and catalysis.[7,8] Silica gels form during dissolution and consist of several alteration layers, including a hydrated glass and a crystalline layer.[9,10] Experimental elemental profiles have shown that the alteration layer is deficient in soluble species, including sodium, boron, and calcium but rich in silicon.[9]
Silica gels were theorized to form from the precipitation of silica from an oversaturated solution,[18] but studies using isotopically tagged samples indicated that only 1:600 silicon atoms in the gel structure had been deposited through condensation.[11]
Nanoporous silica models formed through processes that mimic sol–gel[39,40] or chemical vapor deposition (CVD)[41] methods have been created by classical molecular dynamics (MD) or Monte Carlo simulations
Summary
Silica gels are hydrated nanoporous silica systems commonly found in the fields of biomaterials, including silicate-based bioactive glasses,[1,2] carbon sequestration,[3,4] liquid chromatography,[5,6] and catalysis.[7,8] Silica gels form during dissolution and consist of several alteration layers, including a hydrated glass and a crystalline layer.[9,10] Experimental elemental profiles have shown that the alteration layer is deficient in soluble species, including sodium, boron, and calcium but rich in silicon.[9]. Direct development of silica gel models is relatively rare, and instead nanoporous silica structures are created and hydrated to form gels. Nanoporous silica models are typically created by removing blocks of atoms from silica, creating highly ordered pore structures that do not represent the complexities of experimental systems.[36,37,38] Alternatively, nanoporous silica models formed through processes that mimic sol–gel[39,40] or chemical vapor deposition (CVD)[41] methods have been created by classical molecular dynamics (MD) or Monte Carlo simulations. Computational models of silica gels formed from an initial multicomponent glass composition are rare in the literature, possibly due to the complexity of the multicomponent borosilicate glass system, but these structures will allow for detailed description of the structure and properties of dissolution-based silica gels.
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