PSI - Issue 23

Alla V. Balueva et al. / Procedia Structural Integrity 23 (2019) 173–178 Author name / Structural Integr ty P o edi 00 (2019) 000 – 000

177 5

(a)

(b)

Fig. 5. Stationary geometry of Ti(II) with oxygen (a) in [ Ti 2+ + (Ca) 2+ + PO 4

3- ]; (b) in [ Ti 2+ + 2(Ca) 2+ + PO 4 3- ]

(Ti – grey atom, O – red atom, P – orange atom, Ca – green atom)

The structure in Fig. 6 is a complex that is en route to [Ti 2+ (Ca) 2+ 2PO 4 3- ]. With the addition of another oxygen atom to the structure in Fig. 5a, the oxygen favors a bond with the calcium atom and titanium atom. In this instance the complex has a negative charge and calcium’s electron density is the most influenced out of titanium, phosphorous,

Fig. 6. Stationary geometry of Ti(II) with oxygen in [ Ti 2+ (Ca) 2+ PO 4 3- + O ] (Ti – grey atom, O – red atom, P – orange atom, Ca – green atom) and calcium. However, we suspect that, with the addition of 3 more oxygen atoms and another phosphorous, the calcium atom will resume its dominantly independent role in the complexes. 4. Conclusions Once structures with both calcium and titanium are considered, it is ev ident that calcium’s electron density becomes the least influenced as the complexity of the structures grow . This is atypical in the research thus far, as phosphorous’ electron density has typically behaved the most independently as the structures increase in complexity. It may be of interest to investigate this relationship of electron densities of phosphorous and calcium once their structures are introduced to titanium. The electron density of titanium dramatically increases with each structure, but this effect is lessened when calcium atoms are considered. This is demonstrated by the Muliken charge numbers on each atom in Figures 4-6. Figures 4a and 4b demonstrate the phosphorous being predominantly positive, while figures 5a, 5b, 6 show the charges after calcium is introduced. There is a clear trend where calcium begins to demand the least occupied orbital space, whereas phosphorous maintained that role before calcium was introduced. Additionally, the bonding strength increases between phosphorus and its neighbour oxygen atoms, as the bond lengths between Ti and O tend to decrease. This titanium-oxygen interaction may become important when considering application to osseointegration, as the titanium will be attracted to the oxygen in TCP, leaving the calcium atoms exposed to interact with human bone tissue. Since the orbital space of titanium becomes increasingly occupied with electrons from neighbour oxygen atoms, we theorize that there is a limited number of phosphate ions that can form a bond with a single titanium atom, especially in the presence of more cations such as calcium. This suggests that the phosphate-titanium (II) interactions may act as a parameter for this model, which will constrain its scope. Now, the presence of the phosphorous seems to act as a bridge from positive titanium, to negative oxygen, to slightly positive phosphorous, to negative oxygen, to positive calcium, and then to human bone.