Towards an understanding of magnesium in a biological environment: A density functional theory study

TL;DR

It is found that only a small number of hydroxide layers are required before it is energetically more favorable for Mg(OH)2 to create bulk than to stay on Mg(0001) as single layers, providing insight into early-stage surface processes relevant for magnesium-based implant materials.

Abstract

Density functional theory is used to investigate the interactions between a layer of magnesium hydroxide, Mg(OH)2, the magnesium (Mg) surface Mg(0001), and the three amino acids glycine, proline and glutamine. The aim is to improve the understanding of Mg behavior in biologically relevant environments, such as the ones that biodegradable implants experience in the body. For a simple model of such conditions, adsorption of amino acids are studied. With the layer of Mg(OH)2 as a model of either slightly corroded Mg, or intentionally coated Mg, the interfacial interaction between a layer of Mg(OH)2 and Mg(0001) is first examined in the absence of the molecules. Then follows analyses that include amino acids on top of the Mg(OH)2 layer. We find that the Mg(OH)2/Mg(0001) interaction is weak and that the layer of Mg(OH)2 can readily slide across the Mg surface. The presence of amino acids is found to have a limited influence on the adsorption of Mg(OH)2 on Mg(0001), decreasing the binding by at most 3%, while more layers of Mg(OH)2 strengthen the Mg(OH)2/Mg(0001) binding by 13%. This is still less than the binding of Mg(OH)2 layers within its native bulk structure, and our findings indicate that only a small number of hydroxide layers are required before it is energetically more favorable for Mg(OH)2 to create bulk than to stay on Mg(0001) as single layers. This provides insight into early-stage surface processes relevant for magnesium-based implant materials.

Figures (4)

• Figure 1: Mg(OH)$_2$ on top of Mg(0001) in a $1\times1$ surface unit cell. (a) Mg(OH)$_2$ with the lower H atom above an FCC hollow site on Mg(0001) and Mg of Mg(OH)$_2$ above an HCP top site; (b) the optimal position for Mg(OH)$_2$, with H above an HCP hollow site and Mg above an FCC hollow site; (c) the lower H atom above an FCC hollow site and Mg of Mg(OH)$_2$ above an HCP hollow site; this position cannot be obtained by simply translating Mg(OH)$_2$ in (a) without rotations, the adsorption energy was separately calculated.
• Figure 2: Potential energy surface plot for a layer of Mg(OH)$_2$ translated across Mg(0001). The dashed black box illustrates the periodicity of the displacement. The energy is given relative to the optimal position. The optimal position, found in the dark-blue region close to the center of the dashed box, is the position shown in Figure \ref{['fig:mgoh2_on_mg']}(b).
• Figure 3: Illustration of the energies that go into creating a Mg(OH)$_2$ layer on Mg(0001) either from bulk Mg(OH)$_2$, (a) to (d), or from an isolated layer of Mg(OH)$_2$, (b) to (d), in an imagined separation of the process. Light blue illustrates the Mg(OH)$_2$-layer and dark blue the Mg-surface. Values at the red and green arrows indicate the energy required to go from one step to the other. All energies in meV per surface unit cell, except the (b) to (d) energy.
• Figure 4: Amino acids interacting with Mg(OH)$_2$ on Mg(0001): (a) weakly chemisorbed proline, (b) strongly chemisorbed proline, (c) weakly chemisorbed glycine, (d) strongly chemisorbed glycine, (e) glutamine. The circles in (b) and (d) indicate the position of the H atom dissociated from the amino acid.