Sodium migration pathways in multicomponent silicate glasses: Car–Parrinello molecular dynamics simulations

Abstract

The mechanism of sodium migration in low-silica alkali-alkaline earth silicate glasses is investigated through Car-Parrinello molecular dynamics (MD) simulations. The transport of sodium to the glass surface and its subsequent release is critical for the use of these glasses in biomedical applications. The analysis of the MD trajectory, mainly through a combination of space and time correlation functions, reveals a complex mechanism, with some common features to the migration in mixed-alkali silicate glasses and several important differences. The low site selectivity of Na cations in this glass allows them to use both Na and Ca sites in the migration process. The high fragmentation and the corresponding flexibility of the silicate network enable an additional mechanism for ion migration, not favorable in the more rigid network of common higher-silica glasses, involving the creation of empty transient sites through the correlated forward-backward motion of an Na or a Ca cation. We also show that because sodium migration must involve an undercoordinated intermediate, sharing of oxygen atoms in the initial and final coordination shells is a way to reduce the energetic cost of losing favorable Na-O interactions and Na migration proceeds between corner-sharing NaO(x) polyhedra, where x=5-7. For these low-silica compositions, the present simulations suggest that due to the participation of calcium in the Na migration, the latter will not be significantly hampered by extensive mixing with less mobile Ca ions, or, in any event, the effect will be less marked than for higher-silica glasses.

The initial stages of bioglass dissolution: a Car–Parrinello molecular-dynamics study of the glass–water interface

“…We have shown that the initial Na + /H + exchange at the interface involves water dissociation at an Na + -NBO − pair, with release of the Na cation from the surface: because transport of further Na + ions from the bulk glass to the surface is thus essential to promote complete protonation of surface NBOs, additional CPMD simulations are currently addressing Na migration in these materials (Tilocca 2010b). Moreover, the effect of surface interactions on the water structure and connectivity has been investigated; the relatively low availability of favourable targets for HB interactions on the surface significantly affects water coordination in this region.…”

Structural, vibrational, and thermal properties of densified silicates: Insights from molecular dynamics

“…Vibrational 11,20,21 and elastic 22 properties of the glass at ambient pressure have also been studied and successfully compared to experimental data. Using superomputers, large scale classical simulations have recently been performed 23 , as well as ab initio Molecular Dynamics simulations [24][25][26] . However, to our knowledge, no systematic study of the evolution of the system according to pressure has been performed so far.…”

Rationalizing the Biodegradation of Glasses for Biomedical Applications Through Classical and Ab-initio Simulations

“…However, significant changes to the coordination environment of the ion itself do occur during its migration from a site to another, when the ion often visits intermediate configurations which are significantly distorted with respect to the stable initial and final states [26,48]. For instance, Fig.…”

“…Ab-initio MD (AIMD) can usually target systems of few hundreds atoms and time scales of few tens of picoseconds [26][27][28][29][30][31]. In the case of glasses, this means that it is possible to perform an AIMD melt-and-quench simulation of a system around 100-200 atoms, using quenching rates around 20-100 K/ps [31][32][33][34].…”