A typical method of describing the surface structure of MD-simulated glass is via a z-density profile (ZDP) which counts the atomic density (in #atoms/nm3) of ions perpendicular to the surface (x-y) plane.51,73,207,208
Z-density profiles in this study were calculated as atomic densities in 2 Å slabs. Because the structure is allowed to relax in the z-direction (due to the introduction of the vacuum gap), a density change is expected as compared to the bulk structure. A “surface region” can be defined as where average overall atomic densities change abruptly, and have proved to be on the order of 5-10 Å thick for silica surfaces.50,72,73 Similar surface depths are observed for the surfaces in this work.
Figure 4.2 shows plots of these density values for the 45S5 surface simulations, both before and after creation of the surface. Because the “before”
structure is equivalent to the bulk structure, we see a fairly constant density profile;
a b
however, we see a sharp decrease in overall density at the surface of the new simulation as well as a significant relaxation of the surface (~4-5Å beyond the
“surface” created immediately on insertion of the 20 Å gap). A decrease in the density of each component as well as the overall density is noted as we approach the surface (the far right of each plot). Plots of surface ZDP are given for 55S4.3 and 60S3.8 in Figure 4.3.
Figure 4.2. ZDP for 45S5: a) before and b) after surface creation procedure.
a
b
Figure 4.3. ZDP after surface creation for a) 55S4.3 and b) 60S3.8.
While the overall density decreases at the surface, a small but noticeable increase in sodium density is observed for each glass. By normalizing the density of each component to the overall density in each 2 Å slab, we can get a better understanding of this effect (Figure 4.4). Essentially, we see the fraction of each component as compared to all components at a particular depth from the surface.
Most striking in Figure 4.4 is the large density of sodium compared to all other ion
a
b
types at the surface. Plots of these normalized number densities for the 55S and 60S simulations are given in Figure 4.5. The surfaces described by the plots in Figure 4.4 and Figure 4.5 are those used for the surface reactions in the following chapters.
A series of ten surfaces were created for each glass, with the 20 Å vacuum gap inserted at intervals of 1/10th the original simulation box size. Plots of ZDP show similar features in all cases (Figure 4.6). By taking the average over ten surfaces, we can, in essence, describe the surface of a glass with ten times the original area. Thus, as expected, we observed that many of the density fluctuations as a function of depth are decreased (e.g., oxygen concentration at the immediate surface of the 55S4.3 composition) and that the transition from the bulk to surface structure is more obvious. The surface and bulk densities begin to show differences at around 30 Å, giving a surface of about 6 Å deep for each composition.
Figure 4.4. Normalized ZDP for 45S5 surface used later in surface reactions.
One difference among the plots in Figure 4.6 is that the normalized sodium concentration is 100% at the immediate surface for only the 60S3.8 composition.
However, the overall atomic density at that depth is very small (i.e., few ions exist in that 2 Å slice; in fact, Na ions exist in that slice for only two of the 60S3.8 surfaces),
so that it appears as though the 60S3.8 surface is more enriched in sodium than the other two surfaces, when there is only a small difference. Hence, if we disregard the final data point at 38 Å, the normalized ZDP of all three compositions (averaged over ten surfaces) are very similar. This is a drawback to using the normalized ZDP exclusively, so it should be considered in tandem with the unnormalized ZDP.
Figure 4.5. Normalized ZDP for a) 55S4.3 and b) 60S3.8 surfaces used later in surface reactions.
a
b
The presence of sodium on the surface of glass following fracture has been observed.211 Also, other MD studies have shown the presence of excess sodium at the surface of simulated silicates created at 300 K (it was found for Na and K, but not Li).46,173,212 The effect was enhanced for surfaces reheated to 1500 K and then requenched to 300 K, as following the procedure to create the surfaces in this study.
The presence of sodium ions at the surface may be partly attributed to the presence of negatively charged NBO, some of which existed in the bulk glass, and some of which are created during the surface creation process. This presence of NBO at the immediate surface is confirmed via Figure 4.7, which shows the ratio of NBO to BO as a function of depth. Again, around 30 Å, the surface becomes characteristically different from the bulk as the proportion of NBO increases quickly from the relatively constant bulk ratio. The curves do not continue completely to the surface at around 36 Å because no BO exist there (i.e., there is an infinite [NBO]/[BO]).
Figure 4.1 shows a side view snapshot graphic of a single simulated 45S5 surface, both before and after surface relaxation. Sodium ions have accumulated on the immediate surface during the surface creation. This proves to be the case for all three compositions. Figure 4.8-Figure 4.16 show the surface structure of these glasses to three depths (9, 6, and 3 Å), with and without modifier present. As expected from the network fragmentation observations (Chapter 3), a more connected network structure is observed for the compositions with higher silica content. Also, the immediate surfaces show evidence of continuous “channels” for ease of water or sodium diffusion, most evident in the 45S5 case. This is evident from “holes” in the sections where no network formers exist. The channels of alkali observed for glasses that can be described by Greaves’ MRN are likely routes for alkali diffusion, ion exchange, and infiltration of molecular water.117,213,214
An increased sodium diffusion to the surface may aid in the rate of HCA formation (i.e., in increasing bioactivity) via an increased ion-exchange rate.
Figure 4.6. Normalized ZDP for a) 45S5, b) 55S4.3, and c) 60S3.8, averaged over ten surfaces.
a
b
c
Figure 4.7. [NBO]/[BO] as a function of depth.
The accepted set of surface reactions that takes place when bioactive glasses come in contact with body fluid begins with an ion exchange of H+ or H3O+ for Na+ in the surface of the glasses.23 Clearly, these simulations have shown a “natural”
enrichment of sodium ions at the glass surface. One explanation for this observation is that after the bulk is “fractured,” some BO become NBO at the immediate surface and sodium ions can easily compensate the negative charge created. This accumulation of sodium would leave the surface open to a quick leaching (i.e., ion exchange)in vivo, which is indeed observed.
The results of these surface simulations are promising from the standpoint that the sodium in the glasses, which is known to leach out, is preferentially on the surface. This sets the stage for further simulations involving these surfaces in an aqueous environment. However, it must be noted that the surfaces in this study were createdin vacuo and that this may have effects on the structure (including perhaps the observation of Na on the surface) of the resulting surfaces. Nonetheless, it is tempting to say that we observe an increased density of sodium at the glass surface that represents the true surface of these glasses, rather than solely ones created in vacuo.
95
Figure 4.8. Side view of surface structure of 45S5: a) entire simulation, b) only pieces of network.
a b
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Figure 4.9. Top and side view of 45S5 surface simulation at three depths (9, 6, and 3 Å).
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Figure 4.10. Top and side view of 45S5 surface simulation at three depths (9, 6, and 3 Å), including only pieces of the network.
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Figure 4.11. Side view of surface structure of 55S4.3: a) entire simulation, b) only pieces of network.
a b
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Figure 4.12. Top and side view of 55S4.3 surface simulation at three depths (9, 6, and 3 Å).
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Figure 4.13. Top and side view of 55S4.3 surface simulation at three depths (9, 6, and 3 Å), including only pieces of the network.
101
Figure 4.14. Side view of surface structure of 60S3.8: a) entire simulation, b) only pieces of network.
a b
102
Figure 4.15. Top and side view of 60S3.8 surface simulation at three depths (9, 6, and 3 Å).
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Figure 4.16. Top and side view of 60S3.8 surface simulation at three depths (9, 6, and 3 Å), including only pieces of the network.